01 paper machine steam & condensate systems.pdf

r *X CONFIDENTIAL:

MINNESOTA TOBACCO LITIGATION

Paper Machine Steam and Condensate Systems

Fourth Edition, Revised

A PROJECT OF THE WATER REMOVAL COMMITTEE

OF THE ENGINEERING DIVISION

CA4843

EDITED BY ROBERT D. PERRAULT

it$SI856fbi

V. 2Q30-W7ft)f J

2030307401 http://legacy.library.ucsf.edu/tid/tbh48h00/pdf

CONFIDENTIAL: MINNESOTA TOBACCO LITIGATION

The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or dwa, including but not limited to any liability or responsibility under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published. Within the context of this work the authors may use as examples specific manufacturers of equipmeni. This does not imply that these manufacturers are the only or best sources of the equipment or that TAPP1 endorses them in any way. The presentation of such material by TAPPI should not be construed as an endorsement of or suggestion for any agreed upon course of conduct or concerted action.

International Standard Book Number 049952-5Q4-7

Library of Congress O t a l o f i n i - f r Publication Data Paper machine steam and condensate systems: a project of

the Water Removal Committee of the Engineering Division / edited by Robert D. Perrault - 4th ed., rev.

p. cm. < ^ * y ISBN 0-39852-504-7 T ^wt fO I. Papermaking machinery. 2. Drying apparatus. V \ t " *

I. Perrault. Robert D. TSIII8.D7P37I990 67e'.232-dc20 90-41193

CIP

Copyright 1990, 1982, 1977,1970 by TAPPI Technology Park/ Atlanta, P.O. Box 10S113 Atlanta, G A 30348-5113

All rights reserved.

Permission of TAPPI is granted to photocopy items for internal or fm\J 3 UuU'^vtf personal use of specific clients, for libraries or other users provided that the copying organization pay the ease Zee of SIM VS. per copy, plus S.50 U.S. per page directly to the Copyright Clearance Center, 27 Congress Street, Sakm, MA, 01970, U.S.A. 089852-504-7 51.00 +$.50 pp. Printed in the United States of America R0 96



FOREWORD CtK-4 The purpose of this book is to provide papermakers and those called unA^to* solve drying-related problems a basic understanding of the paper machine steam and condensate systems.

The original book was published in 1970 as a result of a Pressing and Drying Committee assignment. The book was revised in 1977 and again in 1982. This fourth edition published in 1990 has been expanded to include the new developments and innovations of the past few years.

The material in this book is not intended to cover the theory of drying or the operations and theory of individual pieces of equipment There are many excellent papers available on these topics, and they will not be duplicated here.

This book is reviewed every five years by the members of the Water Removal Committee of the TAPPI Engineering Division. The committee welcomes your comments and suggestions. Please send your suggestions to the attention of the editor for consideration at the next revision.

2030307403


- N ^ S N T J A L .

CONTRIBUTORS TO THIS AND PREVIOUS EDITIONS . C^tri^

Nick De'vich, A bitibi-Price Inc., Sheridan Park, Mississauga, Ontario, Caii^w

Horace P. Fish wick, Retired /Consultant, Norwood, MA

Thomas A. Gardner, Gardner Systems Corp., Neenah, WI

Stanley P. Garvin, Beloit Corporation, Beloit, WI

Alan F. Hartwig, Champion International, Hamilton, OH

Edward D. Hoyle, Stone & Webster Eng. Corp., Boston, MA

Robert B. Hurm, Retired, Beloit, WI

Lawrence J. McDonough, Retired/Consultant, Three Rivers, MI

John P. O'Donncll, Ametek, Schutte & Koerting Inc., Bensalem, PA

Ivan 1. Pikulik, Pulp & Paper Research Institute of Canada, Pointe Claire, Quebec John S. Porter, Jr., Milton J. Wood Co., Jacksonville, FL

Richard A. Reese, CRS Sirrine Co., Greenville, S.C.

R. Jerry Retter, Valmet Enerdry, Norcross, GA

Travis Sizelove, Retired/Consultant, South Beach Haven, NJ

Harry J. Stratton, Retired/Consultant Cheltenham, PA

Gregory L. Wedel, Beloit Corporation, Beloit, WI

Robert E. White, Villanova University, Villanova, PA

Many of the contributors to this manual could easily write their own books covering dryer drainage systems. It was not possible to get all the contributors to agree on all points covered in this manual. In those cases where there was disagreement, the majority opinion as interpreted by the editor is stated. 2030307404

A very special thanks to all the contributors who spent many long hours proofing and advising in the preparation of this book.


CONFIDENTIAL-MINNESOTA TOBACCO U T O A V I O N

CONTENTS Foreword / Hi Contributors / iv Introduction / vii

1 Steam Control and Condensate Evacuation Systems Deflgn/1

1.0 General/ 1 1.1 Basic steam pressure control system / 1 1.2 Multiple pressure control sections / 1 1.3 Automatic differential pressure control / 2 1.4 Dryer temperature control using a vacuum system / 3 1.5 Pressure and temperature control for each dryer section / 3 1.6 Simple three-section cascade dryer section / 5 1.7 Positive pressure control loops for wet end dryers / 6 1.8 Low pressure wet end dryer / 6 1.9 Temperature control of wet end dryers / 7 1. 10 After size drying control / 8

1.11 Single felted dryer section / 8

2 Thermocompressors (THC) / 9 2.0 General / 9

2.0.1 Construction and operation / 9 2.0.2 Advantages and disadvantages of thermocompressor systems / 9 2.0.3 Performance/ 10

Thermocompressor Systems / 13 2.1 Thermocompressor pressure control system / 13 2.2 Thermocompressor pressure control system with differential control valve / 13 2.3 Dryer differential control using the thermocompressor spindle / 14 2.4 Thermocompressor cascade system / 14 2.5 Yankee dryers/ 15

3 Blow Through (Flow) Control for Dryer Drainage Systems /17 3.0 General / 17 3.1 Blow through control principles / 17 3.2 Blow through control installation / 18 2030307405 3.3 Yankee or single dryer blow through control system / 19

4 Mechanical Vapor recompression (MVR) / 21 4.0 General / 21 4.1 Advantages/ 21



5 Dryer Drainage System Controls and Equipment / 23

6

S.O feneral / 23 5. t Pressure control / 23 5.2 Differential pressure control / 23

5.2.1 Transmitter installation / 23 5.3 Separator control / 24 5.4 Vacuum system / 24

5.4.1 Vacuum pump / 25 5.4.2 Condenser (heat exchanger) / 25

5.5 Effect of air in steam / 26 5.5.1 Noncondensable bleeds / 27

5.6 Syphons for paper machine dryers / 27 5.7 Effect of centrifugal force / 28 5.8 Drive horsepower and dryer condensate load / 29

^


Introduction

The main objective of paper machine steam and condensate systems is to provide control of steam pressure in the dryers and optimal drainage of the dryers over the range of machine speeds and production rates, under all operating and upset conditions.

To achieve this objective, it is necessary to provide effective and efficient removal of condensate and noncondensible gases. The intent is to provide the highest heat transfer rate possible for a given pressure by the most economical and optimum means.

The drying of a sheet of paper is a complex process. Experience has shown that all grades should be dried with surface temperatures commensurate with what the sheet at the first stages of drying can withstand without loss of quality. Very hot dryers right after the press section can create nonuniformities in the sheet, such as cockling, curling, picking, surface sealing, grainy edges, rough sheet surface, and loss in drying rate.

The ever-increasing speed of paper machines has created new problems with respect to effective condensate removal. These challenges have prompted the development of new, more effective dryer drainage systems, better controls, more efficient syphons, and improved design standards and criteria. -.

This revised book covers the basic design, operation and control of equipment necessary fopjy the proper operation of efficient steam control and condensate removal systems.

^ \ j t \ r I D t z N T A L ' MINNESOTA TOBACCO LITIGATION

Steam Control and Condensate Evacuation Systems Design

1.0 General

Each dryer drainage system should be designed to fulfill the specific requirements of the machine and of the various grades of paper. There are no two systems exactly alike. Older systems are rarely designed properly and adequately in view of recent developments and practice. Over the years, lines and valves are often added that can adversely affect their operation. Changes to a dryer drainage system should be made only after a thorough investigation, and they should be made only by persons or companies qualified and having the necessary design formulae and experience.

Please note that all symbols used on figures are defined in the Legend for Figures, p. 40.

1.1 Basic steam pressure control system

The evolution of dryer drainage systems has been improving with developing technology and the need for steam economy and better control of the drying system.

The simplest but least efficient steam and condensate system consists of a pressure-controlled steam header

which supplies steam tA\f the dryers and dumps condensate to the sewer as shown on Fig. 1.1. A fixed restriction can be installed in the drain line from each individual dryer to limit blow through steam. Some cylinder board machines are still operating this way.

In the system illustrated in Fig. 1.1, all dryers operate at the same pressure. Unless the operating pressures and resultant surface temperatures are low, sheet picking and sticking and other related problems can result on the wet end dryers. Production is directly related to dryer operating pressure. Therefore, if pressures are reduced to eliminate the above mentioned wet end problems, the capacity of the machine will be limited.

In addition, the treated boiler water (condensate) and its heat value along with the blow through steam are lost to the sewer, making this system thermally inefficient

1.2 Multiple pressure control sections

The first logical step is to isolate several of the wet end dryers into a separate dryer section with its own steam supply and control loop. The drying pressure and resultant surface temperature of these wet end dryers may

STEAM SUPPLY

rO PC PCV )2-|> I

PRESSES DRYER SECT I Of J REEL

SEWER 2030307408

Fig. l.l Basic steam pressure control system.


2/ P$per Machine Steam and Condensate System \ ^ \ J I M I U C I M l # * L - -MINNESOTA TOBACCO LITIGATION

STEAM SUPPLY

PCV tf

PC

PCV PCV Ltf PC i PRESSES WET ENo

SECTION NTEftMEBIATE SECTION

M A I M SECTION

REEL

T SEWER

Fig. IJ Multiple pressure controlled sections.

then be reduced without significantly reducing the drying capacity of the machine. Should the required drying pressure be high, it could be necessary to add a third dryer section which would operate at some intermediate pressure between the wet end and main dryer sections (see Fig. 1.2.).

Dryer outlet pressures will always be above atmos-pheric pressure in these systems, since the dryers are continually discharging to the sewer from the condensate outlet lines from the dryers. As dryer operating pressures are raised, blow through steam quantity and velocity will increase. In addition to the waste of steam due to excessive blow through, the increase in velocity will accelerate the rate of erosion of dryer syphons and piping.

The fixed restrictions located in the condensate outlet lines from the dryers may be replaced by adjustable restrictors which are manually adjusted to compensate for changes in dryer operating pressure. Any substantial increase in dryer operating pressures would require that the amount of restriction be increased to limit blow through. Conversely, a decrease in operating pressure must be compensated for by opening the restriction to ensure sufficient flow of blow through steam. These devices should be utilized only on dryers whose range of operating pressure is somewhat limited. The major disadvantage of adjustable restrictors lies in the difficulty in obtaining and maintaining correct settings to ensure proper blow through rates.

For improved drying performance, machines having dryer sections equipped with fixed or adjustable orifices should remove all such restrictions and install automatic differential controls when possible.

1J Automatic differential pressure control

The next logical addition to this simple steam system is the direct control of the differential pressures between the supply headers and the condensate headers. This addition is called differential pressure control.

< & < & *

The term "differential pressure," or "DP," can be confusing. This term is defined as the difference in pressure between the dryer steam supply header and dryer condensate header. Other differential pressures will be qualified, such as "differential pressure across the steam joint" at the dryer or "differential pressure between cascading sections."

The basic method of automatically controlling differential pressure is shown in Fig. 1.3. This control consists of a differential pressure transmitter (DPT), which is connected to pressure taps in the steam and condensate headers and measures the differential pressure between the condensate header and the steam header. It then sends a pneumatic or electrical signal to a DP controller (DPC) which adjusts a control valve to maintain the condensate pressure at a value lower than that in the steam header by a set fixed amount.

The DPT measures the differential pressure between the headers, and is used as part of the control system to maintain the necessary DP. The most important differential, however, is the one across the steam joint The specified DP should be simply the pressure drop across the steam joint. This DP may be considerably less than the DP between headers, especially if the connecting pipes are small. For this reason, pressure gauges should be installed on one dryer in each steam section, in order to relate the header DP to the specified steam joint DP required.

The header DP includes the piping losses to and from the steam joint, losses within t ^ f t ^ B f r o Q M j Q Q K losses, dynamic losses or ga ins^Mw^tnfu^fforce , and potential kinetic energy of the condensate. Note that piping losses from the inside of the dryers to the drain manifold involves two-phase flow.

The differential pressure between cascading sections (these will be discussed later) will be more than that measured across headers because of the additional pressure drop which occurs in the separator, valves and piping between cascading or recirculating sections. That

2080301449 2030307409 http://legacy.library.ucsf.edu/tid/tbh48h00/pdf

O U I M r ILJtZ I M I | / \ L " Starn Control and Condensate EvMcuMtion Systems Design/3 MINNESOTA TOBACCO LITIGATION

STEAM SUPPLY *

PRESSES WET ENO SECTION

~T -0-XDPCV

INTERMEDIATE SECTION

-CHfePCV

TPCV ff

MAIN SECTION REEL

-O-XOPCV

r SEWER

y 4 ^ ^ J

Fig. 13 Automatic differential pressure control.

is, the pressure in a secondary dryer section will be less than the pressure in the primary section minus the previous section differential pressure.

1.4 Dryer temperature control using a vacuum system

Some grades of paper generally require low dryer surface temperatures, especially at the wet end of the dryer section. One way to reduce the temperature of steam is to introduce air into the steam. This can result in uneven drying by creating pockets of air inside the dryer due to ineffective mixing of steam and air. Furthermore, it causes a major loss in heat transfer as a result of accumulating air in the film at the steam-condensate interface. Purposely admitting air in steam is not a common practice and is generally not recommended.

The most widely accepted method for reducing dryer surface temperature is to reduce dryer pressure below atmospheric pressure with the help of a vacuum system as shown on Fig. 1.4. The resulting dryer pressure and steam temperature available depends on the vacuum created in the system.

Blow through and flash steam from the wet end dryers is condensed by the heat exchanger. This becomes a closed system, with the condensing of the steam creating a vacuum or negative back pressure. Noncondensable gases and some water vapors are removed from the system through the use of the vacuum pump.

If economy of operation was not a concern, the system design could be considered complete with the arrange-ment shown in Fig. 1.4. The system gives complete control of steam and condensate pressures. The evacuation of condensate, air and other noncondensable gases is assured over the complete range of operating conditions: run, start-up or break. Additional dryer sections might be required for control from a breaker stack or size press, for graduated pressure (temperature) control or for trimming; however, the control of these

4/ Paper Machine Steam and Condensate Systems AAMrirvi-i . . . - . CONFIDENTIAL: MINNESUIA rOBAfcftp I iTiriJriqa STEAM SUPPLY -

)DPC

PRESSES

r O c rOPC

WET END SECTION

* 5 DPCV

-QPC -QoPC

iNTERMEbiATE) SECTION

MANIFOLD I

VB

t

Vi/vJIMlf L/CHI N I Ir\L-B %^^^oam^MndCoadtasMteEyaemHonSj^msDesittt/5 MINNESOTA TOBACCO LITIGAT)

is reversed. This causes the denser condensate 1o separated from the steam. Level of condensate in t separator tank is maintained through the use of a level controller which positions a level control valve to throttle the discharge of the condensate pump. Control of condensate level within the tanks ensures that the system will remain sealed and that sufficient steam space volume is maintained above the liquid level to ensure separation of condensate and steam.

In order to have a truly economical system, some use i must be found for the blow through steam without

limiting flexibility of operation within the requirements of the machine, or sending this blow through steam to the atmosphere or heat exchanger. Wet end steam showers or hood heaters can be two good uses for flash and blow through steam.

1.6 Simple three-section cascade dryer section

The cascade method of reusing blow through steam can provide a very efficient dryer drainage design. There are many variations of cascade systems. A simple three-section cascade dryer section is illustrated in Fig. 1.6. Cascade systems are popular and used where steam economy is important. As a rule of thumb, blow through from two to three dryers can be discharged or cascaded into one dryer, normally from the dry end of the machine to the wet end. The last stage at the wet end usually discharges to a vacuum condenser or other mill process

3 requiring low pressure steam. Discharging to the atmosphere is a last resort. Cascading ratios are designed so that all the blow through steam can be condensed in the next dryer section under all operating conditions with a sheet on the machine. Otherwise, it would not be possible to control differential without wasting steam.

Simple cascade systems such as the one shown in Fig. 1.6 provide little flexibility of operation. Pressure in the cascading sections is dictated by the differential required in the preceding sections to properly evacuate the condensate. Changes in pressure or drying have to be made with the main dryer section, and all of the dryers in the cascaded sections will follow this pressure change. It is difficult to make minor changes in drying rate. This is particularly a problem for machines that are dryer limited because of the need to maintain DP's between cascaded sections. Another limitation of cascade systems is that all of the dryers cannot be operated at maximum pressure. This results in loss of potential production. Cascading systems may require more dryers to obtain the same production as non-cascading systems.

An incidental advantage of cascade systems is that condensate pumps on the cascading separators can often be eliminated as shown on the main section in Fig. 1.6. The differential pressure created is sufficient to push condensate to the next section. This may require a slightly larger intermediate separator and condensate pump, but it eliminates a pump and motor.

In this system (Fig. 1.6), the vacuum system should

STEAM SUPPLY*, iOPC

)PT

PRESSES

1DPCV

WET END SECTION

iDPCV

INTERMEDIATE SECTION

VB

CHV

Ore

r O ' 1 MAIN SECTION REEL

>*W- , DPCV

r4 -&r& ^znk C? CP

LC,

2030307412 LCV

SEWER

Fig. 1.6 Simple three-section cascade dryer system.


CONFIDENTIAL 6 / Paper Machine Steam and Condensate Systems N N E S ^ O B A C C O LITIGATION be sized to condense all blow through from the w w e # 7 taction dryers of the system shown in Fig. 1.7 and intermediate sections. It is not economical $ # as low as 15 psig and, on high speed machines 1 necessary to design die condenser to condense blow through from the main section.

1.7 Positive pressure control loops for wet end dryers

The main disadvantage of the system indicated in Fig. 1.6 is that operating pressures for the wet end and intermediate dryer sections are wholly dependent on the pressure being carried in the main section dryers. A section which is on automatic differential control will always operate at a pressure less than the cascading section by an amount approximately equal to the differential required across the cascading dryer section.

A partial solution to this problem is shown in Fig. 1.7. A pressure control loop is furnished for the wet end dryers. Wet end pressures may be reduced independently of the pressure required to maintain correct differential across the intermediate section dryer syphons. Differen-tial control of the intermediate section remains unchanged. This control gives the machine operator the option of controlling the wet end dryers at a low pressure or, by raising the set point of the wet end pressure controller to fully open the pressure control valve, operating the system as a straight three-section cascade.

Group control of several wet end dryers may be accomplished as described above.

Minimum available operating pressures in the main

could be requiring

relatively high differential pressures, probably closer 10 24 to 28 psig.

While minimum pressures of 24 to 28 psig are satisfactory on many machines, some, producing fine papers and light weight sheets, require lower pressures in all dryers. A method of obtaining these low wet end pressures is illustrated in Fig. 1.8.

1.8 Low pressure wet end dryer

Several grades of paper may require one or more individually controlled dryers. A group of dryers would have to operate all the dryers at the lowest pressure required not to pick or damage the sheet during the early phases of drying. For this reason, individual dryers can be controlled, as shown in Fig. 1.8. This arrangement permits a gradual increase of dryer pressures and sheet temperature as required.

Individually controlled wet end dryers may reduce picking on the wet end dryers. They also reduce the possibility of high dryer surface temperatures which can also cause the sheet to float off the dryer surface in whole or in part.

This control gives the machine operator the option of controlling these dryers on either "three-section cascade" or "two-section cascade."

STEAM SUPPLY *

PRESSES

r i -feii zr

REEL

2030307413

VP SEWER

CP CP

Fig. 1.7 Pressure control for wet end dryers.


CONFIDENTIAL: M I N N E S O T A T O B A C C O

L m G A T l O ^ * ~ - -

On "three-section cascade," control is sirrujar^oVBT^ described for the system shown in Figure 1.7. %a$&P steam for the wet end dryers is supplied from thesteam header of the intermediate section, however, this has no

Systems Dtsiga / 7

effect on the control. The valve in the line between the headers throttles makeup steam when required. When blow through from the intermediate section exceeds the requirements of the wet end dryers, the makeup valve will close, and the excess steam will be throttled to the vacuum system-

To go to "two-section cascade," the selector switch is set to block the output of the intermediate section differential controller and to direct a full range signal to fully open the makeup control valve. The system headers for the first two dryer sections are now common and, except for a slight loss across the makeup valve, will operate at the same pressure. Intermediate section differential will be maintained by throttling all blow through steam directly to the vacuum system. Reverse flow is prevented through the use of a check valve. Main section presure may now be reduced by an amount approximately equal to the differential pressure required across the intermediate dryer section.

The control is of use not only in meeting the requirements of drying the sheet but also as an aid in rapid evacuation of the dryers during startup due to the elimination of one cascade section.

The basic three-section cascade system and some variations to the system have been covered. There are

many variations of the systems shown here. The cascade system is the most efficient system from

the steam usage standpoint, providing that flexibility of operation is not a prime requisite. If an increasing temperature or pressure gradient is to extend the length of the machine, the simple cascade system will usually suffice, and steam consumption per ton of paper will be at a minimum.

If, on the other hand, some machine flexibility is required and a modified cascade system is used, both equipment requirements and steam usage will increase.

The cascade system has been utilized successfully for many years; however, it does have limitations. If these limitations are recognized, a machine employing this system will operate efficiently with minimum expenditure for dryer drainage equipment

1.9 Temperature control of wet end dryers

Temperature control of dryers is largely obsolete and generally not recommended. Some older machines still use this method. There are many problems, and poor accuracy is obtained with temperature control.

For those grades such as glassine that require very accurate, low, and graduated dryer surface temperatures, pressure control instead of temperature control of each individual dryer is recommended.

(TIP) STEAM SUPPLY*-

Q " Q Q Q rvn

VP CP

PCV f J? PC

MA IK SECTION REEL

2030307414

LCV CP CP

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Fig. 1.8 Low wet end dryer pressure.


81 Paper Midline Steam and Condensate Systems

1.10 After-size drying control

IVM

CONFIDENTIAL-ig^SOTA TOBACCO LITIGA I i w

a^ . F

After-size drying control is generally along the sanfclti& as the main section. Ifesic concepts that are generally agreed upon include several individually controlled dryers to warm up the sheet and reduce dryer surface picking, followed by separate control of top and bottom dryers for sheet curi control. Steps to conserve steam are also important, and some method of reusing blow through steam by cascading or use of thermocompressors is common. (A detailed discussion of thermocompressors can be found in the next section.) For maximum reuse of blow through steam, some mills install thermocom-pressors on individual top and bottom dryer groups. Figure 1.9 shows a typical after-size drying control with individual control dryers, cascading, and top and bottom dryer pressure control for curl control.

Other variations of the typical arrangement shown by Fig. 1.9 are as follows:

(a) Top and bottom dryers with recirculating THCs instead of cascading as shown.

(b) Top and bottom sections can discharge optionally into one separate tank system, instead of two as shown. This would require one DPCV for the top section and one DPCV for the bottom section. This system is difficult to control and is not usually recommended.

J&J^XI Single felted dryer section

There are many special cases requiring customized condensate removal design. Such a case is the single-felted dryer section in which the top felt follows the path of the paper web in the dryers, normally in the first wet end group of dryers. This creates a special condition because the top dryers which directly contact the sheet have high condensing loads, while the bottom dryers that only contact the felt have small condensing loads.

Excessive blow through rates result from the small condensing loads of the bottom dryers because there is not enough liquid in the syphons to impede the flow Many mills have shut off steam to the fabric side (bottom) dryers or disconnected the steam and condensate connections altogether.

Other mills have reduced the syphon pipe size to reduce blow through steam to less than half of previous rates. Another option is to provide separate or individual controls to the top and bottom dryers similar to after-size dryer controls. In this case the bottom dryers are set to a lower pressure than top dryers and usually not changed from this setting. The best option is separate control of top and bottom single-felted dryer sections.

MAIN STEAM SUPPLY

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,111,111,111, HE

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03 &07

Fig. 1.9 Typical after-size drying control with individual control dryers, cascading, and top and bottom dryer pressure control for curl.


-SSKSBgffib, Thermocompressors (THC)

2.0 General G^AjW&fgned unit can waste high pressure motive steam and "

19 / Paper Machine Steam and Condensate Systems CONFIDENTIAL: MUNMFBnTA T D R A r m LITIGATION

HIGH PRESSURE MOTIVE STEAM

P , 0 R P M W t O R W i i

PISTON ACTUATOR NOZZLE

ItSM L POSITIONER

SPINDLE BODY

DIFFUSER

SUCTION STEAM LOWER PRESSURE

P 3 0 R P s W 3 0 R W S

DISCHARGE STEAM

P2 0RPD W j 0 R W D

^ ^

conditions, and a design is determined. The final design is usually determined by the minimum operating condition. An example of initial requirements is shown in Table 2.0.3. In this example, the motive flow and size were based on the design for minimum conditions.

Additional calculations are made to evaluate thenno-compressor performance at other operating and upset conditions. Special thennocompressor curves such as those in Figs. 2.0.3.1, 2.0.3.2 and 2.0.3.3 have been developed for this purpose.

Compression pressures of 4,8, and 12 psi were selected in these curves. The motive flow (Wi) curve moves toward and crosses the suction flow curve (W,) as compression increases and R* decreases. This shows the required increase of nozzle energy to achieve higher

Table 2.03 Example of initial requirements for a thenno-compressor

Fig. 2.0.1 Basic automatic thennocompressor.

E. Have limited turndown control with differential pressure control.

2.03 Performance

Thennocompressor performance is of two types, critical and noncritical, based on compression ratio. Compres-sion ratio is defined as the discharge absolute pressure divided by the suction absolute pressure. Ratio * P2/ P (absolute units).

In general, when the compression ratio is two or more, performance is termed critical. This type of performance produces sonic velocity in the throat of the diffuser. While this does not often occur in thermocompressors used on paper machines, it can occur if wide ranges of operation are required. An example would be a dryer section operating at IS psig, with a 15 psi differential.

The other type of performance is termed noncritical and does not require sonic velocity in the diffuser to achieve the desired compression. Most dryer drainage systems utilize noncritical compressors, and the suction capacity varies directly with motive flow at a given discharge pressure. If the motive flow increases and additional suction flow is not available, differential pressure will increase until equilibrium is established.

The performance of a thennocompressor is generally evaluated for several anticipated operating conditions, but normally a minimum and a maximum operating point are sufficient. Design data required by the thennocompressor manufacturer to properly size the unit includes motive pressure and temperature, suction pressure, discharge pressure, suction flow, and condens-ing load for the specified operating conditions. The calculations are then made on maximum-minimum

Motive Steam, psig (Pi) Temperature, *F

Min.

130 355

Discharge Pressure, psig (P2) 10 Suction Pressure, psig (P.) Compression, psi (P2-P.) Suction Flow, pph (W.) Condensing Load, pph Motive Flow, pph (Wi) Size, in.

Max.

130 355 95

zo&mwft 3450 8000 5600

6

5000* 22,210 12,000

6

* Maximum thennocompressor suction flow with spindle fully open would be 10,300 pph, and motive steam requirements would be 15,000 pph.


CONFIDENTIAL: MINNESOTA TORAnCQ LITIGATION

MOTIVE PRESS. 130 psig COMPRESSION PRESS. 4.0 psig

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12 / Ptper Machine Steam and Condensate System CONFIDENTIAL: tiiMMFsnTA TORAcnn LITIGATION

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ao

a

Fig. 2.0J J Motive flow, suction flow, and entrainment curves at 12.0 psi differential preasurt:

compression resulting in less nozzle energy available to entrain suction steam. Note that the amount of compression pressure includes the dryer differential pressure plus separator and line losses from the THC discharge to the supply manifold, from the drain manifold to the separator, and from the separator back to the suction side of the THC.

The practical operating limits of a thermocompressor depends on the motive steam pressure available. As an example, the motive steam pressures are assumed to be 100, 150, and 200 psig. A compression pressure of 12 psi is to be maintained between the thermocompressor suction and discharge. The minimum dryer section pressure is to be 5 psig. From Fig. 2.0.3.4, it can be seen that maximum efficiency of the 200 psig steam is obtained at 70 psig, and from there to 125 psig entrainment decreases. Likewise, with 150 psig steam, maximum efficiency is reached at 55 psig, and with 100 psig steam, at 35 psig. From the maximum points shown, the decrease is rapid. A lower differential across the thermocompressor will result in increased entrainment. At the top end of the curve, Rw is limited by the approach of the discharge pressure to the motive pressure, and at the low end of the curve the Rw is limited by the increase in compression ratio (Pa/Pi).

IJO

I -a ro

>

M

M i

t

\ 200 pif V

K I50f*i| \

V ' " N

N

^

^

\

\ \

s / '

\, \ \

t

/

\

I 4P 12 paif

> J * J A .1 M f M .t LO LI 14 U 14

Fig, 2,03.4 Entrainment curves for 100-, ISO-, an psig motive steam pressures. r H "^0** '** **" *


Thermocompnsson (THC) /13

THERMOCOMPRESSOR SYSTEMS

2.1 Thermocompressor pressure control system*'' / ^ ^ '

The thermocompressor pressure control system was one of the original applications of a thermocompressor in the paper industry and is still used often in connection with yankee dryers. However, it is one of the most uneconomical. With the basic system shown in Fig. 2.1, the thermocompressor spindle is controlled by the pressure controller. With this method of control, the thermocompressor is always operating in the wide open condition (3-9 psi), and additional makeup steam is controlled with the pressure control valve (PCV, 9-15 psi). With this method of operation, there is no control of dryer differential. The differential is dictated by thermocompressor design capacity, which is usually much more than is required over most of the range of operation. Excessive differentials cause unnecessary erosion of condensate piping and also a waste of high energy motive steam, unless the makeup steam pressure is the same as the motive steam supplied. Should the thermocompressor not be capable of controlling the set differential, the only option left to generate the additional differential required is to open the atmospheric or vacuum condenser valve (DPCV).

A check valve (CHV) is extremely important in all thermocompressor suction steam lines. Reverse flow of steam from the thermocompressor can occur under certain conditions, such as when critical compression ratio is involved (Pa/Pt > 2). Without a CHV, a thermocompressor becomes no more than an elbow in the pipeline if the motive steam is turned off. Thus, on failure of motive supply or of the control signal, the pressure in the dryer supply and drain manifolds would equalize, and differential pressures would become zero. When the differential pressure is zero, makeup steam flows from the makeup valve (PCV) to the blow down valve (DPCV), without a CHV in the line.

2.2 Thermocompressor pressure control system with differential control valve

A significant improvement over the thermocompressor pressure control system is the simple addition of a differential pressure control valve (DPCV) as shown in Fig. 1 1 This provides better control of the required dryer differential. The thermocompressor operates in the wide open condition with the sheet on the dryers and is split range operation with the makeup valve.

The differential pressure is controlled by a split range controller using the differential pressure control valve DPCV with a 9-15 psig A/C signal and the blow down valve DPCV with a 3-9 psig A/C signal. Valve DPCV-1 closes first with a 3-9 psig signal and does not normally open during normal operation, but it is open on sheetbreak and start up conditions. Valve DPCV closes

j&*P MAKEUP STEAM MOTIVE STEAM

Fig. 2.1 Thermocompressor pressure control system.

MAKEUP STEAM

MOTIVE STEAM

SYSTEM

8

DPftC

O THC

l2f ' OPCV

: t e

PRC

o PCV

PIT i PAPER DRYERS

OPCV-I

rQ\Z_ 30B< 203(ft0?420

Fig. 12 Thermocompressor pressure control system with differential control valve.

next with a 9-15 psig signal, and this valve generally provides the operating control of the dryer section differential pressure.

y ^ - v T'^'-V

iVilNNESOTA TOBACCO UTIQAriON 2030307420 http://legacy.library.ucsf.edu/tid/tbh48h00/pdf

14 / Paper Machine Steam and Condensate Systems

13 Dryer differential control using the thermocomp"resj^jj$a which they must operate to assure proper sheet warm-sor spindle yp An improved system * wide use that uses only enough high pressure motive steam to control the set differential is shown in Fig. 2.3. This system is also called the low pass control loop.

The thermocompressor is supplied with a 3-9 psi split range positioner and PCV with a 9-15-psi positioner. Valve DPCV, which is air-to-close, is supplied with a 9-15-psi reverse acting positioner so that it will function as a normal air-to-open valve and yet fail open should there be an air failure. The item shown as SR is a low pass selecting relay which will select the lower of the signals from the pressure controller or the differential controller to modulate the thermocompressor. This is extremely important so as not to overpressurize the dryer above coded pressure with high pressure motive steam during a sheet break or similar loss of condensing load. The instrument DPC will be initially set to maintain the required differential across the steam joint and syphon pipes.

During normal operation, makeup steam will always be required; therefore, the PCV will be throttling on a pressure instrument output of 9-15 psi. Differential will be maintained by modulating the thermocompressor spindle. The lower signal from DPC (3-9 psi) will pass to the thermocompressor, and the higher output signal (9-15) from PC will be blocked by the relay SR. The higher signal (9-15 psi) will throttle the makeup valve.

On a paper break, pressure in the steam header will rapidly increase because the condensing load in the section is drastically reduced. To maintain the set pressure, the signal from PC will decrease, tending to close PCV. The quantity of motive steam to the thermocompressor will still exceed the low condensing load on the dryer section, and the output signal from PC will continue to decrease. When the output signal becomes less than the output of DPC, it is automatically transferred by means of the selector relay to control the THC. The signal from the DPC will now be in the range of 9-15 psi and will throttle the differential valve DPCV to maintain the required differential across the syphons. The selector valve SV permits the thermocompressor to be shut off during initial machine warmup so that air can be evacuated from the dryers. This is accomplished by blocking the supply air to the thermocompressor positioner. With the THC closed, steam is admitted through the PCV, while blow through and nonconden-sables will be evacuated through the DPCV. Blowing the machine down while running can be accomplished in the same manner.

An important consideration when designing any thermocompressor system is that the condenser must be sized to condense all blow through steam from the dryer section(s) and the wet end dryers at the vacuum which will evacuate the wet end dryers at the minimum pressure

up and drying. That is, if the condenser is sized to condense at the minimum required pressure based on blow through and flash steam from the wet end (and lead in after size dryers, if applicable) only, the addition of condensing blow through from the main and intermediate section dryers will overload the condenser to the point where it will be incapable of evacuating the dryers at the low pressures required by the lead dryers.

MAKE UP STEAM

MOTIVE STEAM

SYSTEM

PAPER DRVERS

OR OPCV

0

LCV y

Fig, 23 Thermocompressor using only enough high-pressure steam to recirculate blow through steam.

2.4 Thermocompressor cascade system

This system is not very popular and rarely recommended. It is difficult to operate and has had many reported problems. It is discussed here because there are several still in operation.

This system uses a thermocompressor in a convention-al cascade system. In this combined system, the blow through steam pressure is increased by a thermocom-pressor and then cascaded to another stcam^eawnjin the machine. The receiving J ^ o j O S O * **w*d anywhere in the dryer section as long as its condensing load is more than the thermocompressor discharge flow, and its operating pressure is equal to or lower than the thermocompressor discharge.

One advantage of this system is that it minimizes the collection and buildup of noncondensable gasses in the dryer sections. Unfortunately, the thermocompressor will generally be wide open, using the maximum quantity of

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re

Thcrnocomprtssors (THQ /15

high pressure steam at all times in normal operation.'If ,^ .2.5 Yankee dryers the motive steam to the thermocompressor and the*5 supply to the makeup valve are from the same steam supply header, it doesTlot matter whether the steam to the section is supplied by the thermocompressor or the makeup valve. If, however, a high pressure source is used for the thermocompressor and a low pressure source is available for the makeup valve, it would generally be more economical to use the low pressure source for makeup.

A second aspect of the cascading thermocompressor system is that if the receiving section is operated at a lower pressure than the cascading section; the wide open thermocompressor tends to choke, and the differnetial control is forced to dump steam to the condenser to maintain differential. On the other hand, if the receiving section is at the same or higher pressure than the controlled section, the compression is greater than required by a recirculating THC because of the additional pressure drop in the DPCV and may be much greater for increased pressure. This usually results in loss of steam to the condenser. In paper machines that use the cascade method, it is normal to find most of the DPCV valves dumping steam to the vacuum condenser.

A further improvement of this system is to place the thermocompressor on differential control as was shown in Fig. 2.3 while still maintaining the cascading feature and overriding pressure relay.

The biggest disadvantages of the cascading thermo-compressor system are the loss of flexibility to operate each section independently and the waste of steam heat to the condenser.

MAKEUP STEAM

prcv

&] fo.T,,

r* P STEAM

M*t> MTCRS

s * ~:

$

Fig, 2.4 Thermocompressor cascade systems.

The drying section of a paper machine making tissue, crepe wadding, toweling, and other light sheets usually consists of one large dryer typically 10 to 20 ft in diameter. This is commonly called a yankee dryer. This large dryer may be used in conjunction with predrying and/or after drying sections with conventional dryers.

Operating range of yankee dryers may range from low pressures to as high as 16X1 psig, with machines being balanced for speeds up to 7000 ft/ min. These high speeds, combined with large dryer diameters, require large differentials to be carried across the steam joints. These differentials are typically in the range of 12 to 18 psL

Use of yankee dryers introduces some special control requirements. One of these requirements is gradual warmup of the dryer during the startup period. Rapid inflow of steam can cause serious damage to a cold dryer because of thermal stresses caused by unequal distribu-tion of heat through the shell. Therefore, it is desirable that some provision be made to guard against accidental shortening of the warmup period.

Steaming and drainage control used on a yankee dryer can vary from a simple pressure control instrument throttling a steam supply valve to a complex system employing pressure, temperature, and timing devices interlocked for maximum safety and operation efficiency.

If a jet compressor is used in a single dryer machine, all of the steam blow through must be recirculated. If the quantity of blow through steam is too great, motive steam requirements could exceed that being condensed in the dryer, thereby choking the compressor and causing loss of differential, which would result in a flooded dryer.

A relatively simple system which meets the require-ments for warmup, Sunday drive, and run operation is shown in Fig. 2.5. In the warmup position, the selector valve passes a 20-psig air supply through three-way valve, item TCV-1A, to the diaphragm operator of the valve PCV-IA which limits initial steam flow to the yankee dryer. Opening of this valve is limited by a stop which has been set for an input rate which will warm the dryer over a two- to three-hour period. Setting of the limit stop is determined by testing at the time of initial startup.

When the safe warmup period has been completed, the temperature controller, TC, will position TCV-1A to block the 20-psig air and direct output of the Sunday drive pressure controller, PC-I A, to warmup valve PCV-IA. The warmup valve will tbg^iitrtri>iffliTOpjtain some low pressure, as set o

16 / Paper Machine Steam and Condensate Systems

The three-way block valve, item TCV-1, prevents air supply from reaching the positioners of the jet compressor and steam makeup valve should the selector valve be switched to tfc run position before warmup has been completed. The system will remain in Sunday drive until the selector is placed in the run position.

While it is obvious that this sytem can be intentionally bypassed, this is true for most, if not all, of the more complicated safety systems with the same potentially disastrous results. The advantages of the system are that it cannot be accidentally bypassed and that it is simple.

m& means that there are fewer devices to fail, making the detection of tampering much easier.

In the blowdown position, supply air to the positioner of the jet compressor is blocked, and the compressor will close. Pressure and differential control will be as described for the system shown in Fig. 2.3. The run position also operates with this same basic control.

Flow control is used with considerable benefit on yankee tissue machines. Flow control is covered in the next section.

OOPC QfC

< * Blow Through (Flow) Control for Dryer Drainage S^f4mv ov

i

3.0 General

Blow through control is often called flow control. The blow through control system is a different way of controlling dryer drainage than the DP control system outlined in the previous sections. Instead of controlling the flow of blow through steam to maintain a fixed DP between the supply and condensate headers, the controller maintains a fixed DP across an orifice or restriction in the blow through line, resulting in constant flow of blow through steam at any given pressure.

This approach causes the DP to adjust to system requirements rather than a fixed maximum. It automat-ically maintains the same rate of drainage regardless of changes in speed, condensing load, and pressure. For example, when pressure is increased, condensing rate increases, and the quantity of blow through steam increases proportionally due to the increased density of the blow through steam.

3.1 Blow through control principles

In its simplest form, the blow through system replaces the normal header DP as the control input with the pressure drop across an orifice plate. This system is shown in Figure 3.1. This can be compared to the normal DP control system shown on the main section in Fig. 1.5.

Blow through control can be used in most of the previous systems in place of DP control. It works well in both cascading and thermocompressor arrangements.

With the blow through control system, blow through flow is maintained at a preset value. If the dryer DP is inadequate to evacuate one of the dryers, that dryer will begin to fill up with condensate. This reduces the quantity of blow through steam, causing the blow through valve to open. This tends to increase DP and promotes evacuation of that dryer. With the normal DP control system, the valve would tend to close and aggravate the flooding.

Flow control is especially advantageous during a sheet break. During sheet break, the condensing load drops

Fig. 3.1 Basic flow control system.

to roughly ten percent of normal running load. With the conventional method of differential control, the blow through rate can increase by as much as twice or more than the normal running requirement. This is due to a temporary loss of syphon resistance caused by a loss of condensing load. The differential valves and/or thermocompressors then go wide open in an attempt to maintain differential pressure. This is usually not enough, and the atmospheric or heat exchanger valves open. With a blow through control system, the blow through remains constant during a sheet break, and the dryer DP automatically drops. That allows the thermocompressor to close due to the lower differential requirements. This reduces motive steam supply so that even with lower condensate load demand the U

18 / Paper Machine Steam and Condensate Systems ^ . ^ 0

J , - - . - , ' t r e a s u r e the resulting differential across the steamfit remains essentially constant, following line BB.

DIFFERENTIAL CONTROL FOLLOWS LINE ~A-A" AND BLOW THROUGH CONTROL FOLLOWS LINE "B-B"

' I I I I I I I I OFFEWNTUL PRESSURE PSQ ACHOSS THI SRAM JOMT

^y^When there is sufficient differential to evacuate * condensate to the condensate separator, the condensate

drops out, and the blow through steam returns to the thermocompressor for recompression.

The flow control orifice plate is usually designed for a differential pressure of about 1 psi at maximum flow (approx. 27 in. of H20). Of course, the actual line loss is little more than half the differential across the orifice plate. Straight runs of pipe ahead of the orifice are not necessary. The orifice plate may be installed in very short pipe sections near elbows. This is because exact gravimetric flow measurement is not required. The relative pressure drop across the orifice can be used for control.

Flow through the orifice plate is measured by a differential pressure transmitter (DPT), which feeds a differential flow controller (DFQ. The flow controller then controls the thermocompressor and valve DV, if required. The flow required to entrain and evacuate the condensate for the entire range of operating conditions is then set and maintained regardless of speeds, condensing load, sheet break or other upset conditions.

The sizing of piping and orifice bores is critical and should only be done by experienced specialists. An error in the bore affects blow through flow to the square of the bore, and flow affects differential by the square. Accordingly, differential is affected by the fourth power of the bore. Trial and error method is not recommended.

Fig. 3.1.1 Differential pressure, control line UA-Anand blow through control line "B-B".

3.2 Blow through control installation

Figure 3.2 shows a standard blow through control system for a THC loop system, in the arrangement shown, the dryer section DP is monitored as a good troubleshooting advantage. The DPT measures the differential between the steam and condensate headers.

The first order of control is with the thermocompressor (THC). When the thermocompressor is wide open and more differential is required to maintain set point, valve DV opens. Some machines may have a lower pressure steam header to discharge into, and blowing to the atmosphere will not occur when more differential is required than can be provided by the thermocompressor. By measuring differential between the steam inlet and condensate headers, the resulting differential pressure across the steamfit is not known. This will vary with each system, depending on connecting pipe sizes and lengths. Many mills have added pressure gauges on the steamfit

Fig, 33 Standard thermocojaar^or^stam/iMttt^low through control. 41)01)31/74^0

CONFIDENTS MINNESOTA TOBACCO LITI \Ji.


Bbw Through (Flow) Control for Dryer Damage System /19

This system is probably the most common method of draining dryers being supplied today for new and rebuilt systems. With blow through control, the thermocompres-sor spindle is opened only as much as required to maintain set blow through. This conserves high pressure motive steam. The low pressure override is used so that on sheet break the thermocompressor converts to pressure control if necessary. This is not likely to occur with flow control as compared to differential control. During sheet breaks, the condensing load is greatly reduced so that if the THC is not cut back, more steam is injected into the dryers than can be condensed, causing the pressure to rise above the set point.

With loss of paper on the dryers, the condensing load drops, and the flow of blow through steam increases if differential pressure is maintained constant With blow through control, the flow of blow through steam is maintained at the preset rate and the differential pressure drops. Thus, the thermocompressor has less work to do on a break. The motive steam is therefore reduced, and the amount of steam discharged to the atmosphere or condenser is reduced or eliminated. Following a break, the blow through rate continues to be maintained at the preset rate, and differential pressures automatically return to normal levels.

On most modern systems, the blowdown valve that dumps to atmosphere or HE rarely opens during run or break, operations. During dryer warmup, the THC^tf is turned off, causing the blowdown valve to open and^ to discharge air to the atmosphere. This provides rapirf4 ' and effective elimination of noncondensables, which is desirable for all steam sections.

Perhaps the most important advantage of blow through control in a THC system is the steam pressure turn down capacity. Turn down capacity is extremely important for most fine and groundwood papers. With ordinary DP control, as dryer pressure is lowered, blow through flow increases, causing an exponential increase in THC work, and the THC typically becomes unable to maintain DP below pressures of 15 to 20 psi. With blow through control, the percentage of blow through stays constant, and the DP drops with lower dryer pressure. Thus the THC is able to work in its normal way at exceptionally low dryer pressures, often as low asOpsig.

re. "o Qa

I u n n n n r~i r~i n r**i

MMUP9UM) MM tLON

Fig. 33 Yankee or single larger dryer blow through control.

0 *& r"

33 Yankee or single large dryer blow through control system

Figure 3.3 is basically the same system as described in Fig. 2.5 except that flow control has been added. This is one of the more popular methods of yankee dryer control.

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'*;

Mechanical Vapor Recompression (MVR) v * flP

4.0 General

Much interest has been given recently to this new and developing technology. This is just a short introduction covering the basic system for paper machine condensate systems.

Mechanical vapor recompression has been used in the paper and other industries for many years. There are several hundred evaporators in the paper industry using a positive displacement pump (PDP) to create MVR very economically. The only difference between the evapor-ator pumps and those used for paper machine dryer drainage systems is the pump casing coded pressure. Evaporators generally operate at low steam pressures, while paper machine dryers operate up to 160 psig. PDP can operate from -8 psig to 160 psig and create differentials up to 20 psig.

4.1 Advantages

A positive displacement pump has all the advantages of blow through control. It maintains constant flow and variable differential to automatically compensate for upset conditions that may occur, including sheet breaks and changes in speed and condensing load.

A PDP replaces the thermocompressor as shown on Hg. 4.1 and eliminates the use of high pressure motive steam. This means that the high pressure steam tine from the power house may be eliminated and more electrical energy can be generated.

Steam loss to the atmosphere or heat exchanger can be reduced or totally eliminated in many cases. This is especially noticeable during sheet breaks. Heat ex-changers can be eliminated from some dryer drainage systems, resulting in less hot water being generated. Most paper machines generate more hot water than they use.

The PDP is over 90% efficient. Much of the horsepower input to drive the PDP is recaptured in the form of heat of compression. This is in the form of superheat which creates steam from the condensate inside

S / . ^" jljjetdryer. This relates to less steam that has to be made ^up to the dryer sections. Horsepower varies with the

required differential. At low speeds, low condensing loads or sheet break conditions, very little horsepower is required to maintain set flow.

A PDP allows all dryers to operate at the maximum pressure without venting steam. A simple cascading system would allow the pump to idle ^hen enough differential between sections exists, and the horsepower input will increase as the differential pressure requirement increases. The no load PDP still acts as a constant metering device to maintain set flow, even if larger than set differential exists between sections.

A PDP will handle up to 10% condensate by volume or a flooding separator without any damage to the pump. It is often recommended that 10% condensate be piped into the pump. This will serve a dual purpose of creating better seals and helping to reduce superheat leaving the pump.

Fig. 4.1 Thermocompressor-mechaniad vapor recom-pression flow control system.

21

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Dryer Drainage System Controls and Equipment

5.0 Genera!

Controls for a basic dryer drainage system are very simple. The three major control loops are pressure, differential (flow), and separator level

It is important that all valves fail-safe in case of a power or air failure. This means that all valves admitting steam to a dryer section will fail shut (A/ O, air-to-open valves), and all valves on the discharge, condensate side of a dryer will fail in the open position (A/C, air-to-close valves).

Major lines on a dryer drainage system should be equipped to measure flow rates. This should include steam supply, blow through, and condensate flows for each section, along with the total steam to the paper machine.

5.1 Pressure control

Pressure is normally automatically controlled for a steam header feeding multiple dryers, using one or more control valves. Multiple valves are normally split-ranged so that they open one at a time to satisfy steam requirements starting with the steam to be used first. Thermocompres-sors may also be split-ranged with makeup valves.

A safety relief valve must be installed on each dryer steam section supply header to allow dryers to operate as close as possible to coded pressure and prevent the dryers from operating above dryer coded pressure. A pressure relief valve on the main machine header or further back in the system may result in lower than coded maximum desired pressure in the dryers.

5.2 Differential pressure control

Control of the pressure drop across the dryers is used to insure proper evacuation of condensate from the dryers. This pressure drop is normally measured from the steam supply header to the condensate header. Normally, most of the pressure drop is taken across the steam joint, syphon shoe, and syphon pipes in the dryer.

On&^v&veral split-range valves and/or thermocompres-sort may be used to control the differential. Blow through steam normally cascades to a secondary section or is recirculated back into the same section. Thermocompres-sors are often used on dryer limited machines to obtain the necessary differentials while operating most of the dryers at maximum pressure. Differential controls are usually set in order of priority so that blow through steam is reused in the dryer as much as possible and only dumped to a condenser or to atmosphere as a last resort Differential must be controlled over the entire range of speed, pressure, and condensing loads.

5.2.1 Transmitter instillation

It is usually preferred to have differential pressure transmitters mounted above both the steam and condensate headers. If the transmitters are not mounted above the headers, then constant equal pressure has to be created with equal water legs on each side of the transmitter. This is accomplished by using seal pots, as shown in Fig. 5.Z1. Most older installations are designed that way. Without these artificial water legs, there could be twice as much differential from set point or twice as little. It is difficult to be sure that these water legs are full at all times. Seal pots must be installed at the same elevation above both headers, unless transmitters can be zeroed or suppression kits are provided to allow mounting seal pots at different elevations. Newer installations mostly use zeroing transmitters. The pressure transmitter should not be mounted at the bottom of the steam side water leg, as is often the case, without using a transmitter that can be zeroed. This gives a higher pressure reading than is normally being used by the height of the water leg. The pressure transmitter should be mounted in the pressure header without creating a drop leg that can S^RCff^Q^SMJ^ See

2O3O30Tr2 23


24/ Piper Machine Steam nod "CONFIDENTIAL:

""ffifWlESOTA TOBACCp jjTIGATION If transmitters are located above the headers, risers

must be one-inch pipe, no horizontal runs, no pockets or sumps, and a minimum slope of 45*.

5J Separator control

The third major control loop is the condensate separator tank. The main purpose of this tank is to separate entrained condensate from blow through steam. Condensate is returned to the power house, and blow through steam is reused in the most efficient manner. It is important to have sufficient controls and alarms on the separator tank to warn operators in case of problems. The two most important alarms are low and high condensate level. If the pressure inside the tank is sufficient to push the condensate out to the central collection tank without pumping, the condensate pump should not run until a preset high level is reached.

Separators may vary greatly in water separation efficiency depending on entering steam velocity and arrangement of internal baffles. Not uncommonly, a large percentage of the condensate entering with the blow

4hreAigJ^eam is earned right on out without separating. Thmw&ns that the dryers supplied with this wet blow through steam may be much more difficult to drain.

Older low capacity separator efficiency may be optimized by running the level set point as low as possible to minimize blow through velocity. Install a 0,5-in. (12.7-mm) line recirculation from the pump discharge back to the separator tank below the condensate level to prevent dead shut off of pump. Provide a 0.75-in. (19-mm) line from the pump suction up to the separator steam compartment to relieve steam collected in suction line and reduce pump cavitation.

5.4 Vacuum system

On most machines, high dryer surface temperatures on wet end dryers can cause dusting, picking, cockles, case hardening, reduction of drying rate, sheet blisters, discoloration, web flutter and other related problems. These machines require a vacuum system to achieve low dryer operating pressures.

The vacuum system normally consists of a vacuum

i t - O d S

PT

* - 1 " riser pipes. No pockets or horizontal runs. Minimum 45* slopes.

DPT A Steam I [ (ybondensatep

Transmitters mounted above headers

rp ipe

7 Vent valves 7 Seal D Q Pots

Transmitters Drain below headers

Fig. 5.2.1 Scat pot installation.

TO PUT IN SERVICE A. With all valves closed, proceed as follows:

1. Open seal pot vent valves, high & low pressure manifold valves, transmitters & equalizing valves.

2. Crack vent screws on high & low pressure diaphragm housing of D/P transmitter.

3. Fill system with fresh water through either vent valve until all air is vented. (Steady air-free flow from second vent valve,)

4. Close vent screws on D/P transmitter. 5. Close seal pot vent valves. 6. Close equalizing valves. 7. Slowly open main condensate shut-off

valve. a Slowly open main steam shut-off valve.

B. In order to prevent sweeping condensate out of the chambers, the equalizing valves should never be opened while the main steam & condensate valves are open. .

C. To check the value of the transmitter out-put at zero differential, open the equaliz-ing valves one at a time.

Q Check "HIGH PRESg)lS363W429 PRESSURE" and "DIFFERENTIAL PRESSURE" readouts.

203030^^9


^MrinCKlTI Al * ^ ^ DryerDnimpSystem ControlsutdEquipment/25 ONFIDEN I I M U . ._, jfc*

Steam-out

Level controller!

Level control valve Condensate line

Bypass valve piping

Fig. S3 Condensate separator

pump and condenser (heat exchanger). Dryers discharg-ing to the atmosphere have to operate at 10 psig or higher, depending on speed, to provide sufficient pressure differential to evacuate the condensate. Below these pressures, a vacuum system is required.

Vacuum systems are usually designed to generate vacuums of 15 to 25 inches of mercury. Vacuums above 20 in. of Hg are usually diflicult to maintain because of air leakage into the system from steam joints, flanges, fittings, etc.

5.4.1 Vacuum pump

Any type of vacuum pump capable of operating at the design vacuum level may be used. The pump is generally designed to handle at least 5% of the condensing load volume as noncondensables. The vacuum pump does not control the vacuum level and is not be designed to do this. The vacuum is normally controlled by the flow of cooling water through the condenser.

Machines operating well above atmospheric pressure do not need vacuum pumps. This is especially true for high pressure recirculating thermocompressor systems.

5.4.2 Condenser (heat exchanger) There are several types of condensers. The two most popular types used on dryer drainage equipment are "IT tube and straight tube designs, with or without condensate impingement plates. If condensate is sent directly to the heat exchanger along with blow through steam to be condensed, impingement plates mounted inside the heat exchangers will be required to deflect condensate away from the condenser tube bundle. Condensate impinging directly on the condenser tubes will cause severe erosion and short life for the tube bundle. It is often better to separate steam and condensate before sending the blow through to the heat exchanger. When mounting heat exchangers, space must be reserved to pull out the tube bundle for inspection or replacement.

Condensers should be selected to condense all of the blow through steam at minimum and maximum operating pressures at the designated vacuum level. Cooling water temperature ^ r f f e f P t o & l S Q 1 6 condenser required. A fouungTanof\WXoTto 0.002 is normally allowed for the water side.

The economics of collecting the blow through steam with a heat exchanger for systems operating above atmospheric pressure should be weighed against the cost

20303CT743O


26/ Paper Midline Steam and Condensate Systems

X w Vacuum relief valve

Cooling water control valve

Blow through steam from dryers

Level controller LC

CMS Condensate

vacuum pump and motor Condensate pump and motor

Fig. 5.4 Condensing equipment

of operating and maintaining condensing equipment, treating additional water at the power house, and the demand for hot water generated.

5.5 Effect of air in steam

It is well known that air in steam reduces its condensation temperature. The question is: what is the net effect of a given quantity of air in steam? It would be desirable to know how to reduce the level of noncondensables to that which would not appreciably reduce drying rates. Air not only reduces the partial pressure of the steam in the mixture but literally poisons the condensing film heat transfer. Dryer steam temperature will be lowered proportionally to the amount of air in the steam mixture, and dryer surface temperature will drop a great deal more as a result of loss in heat transfer.

Figure 5.5 shows the effect that air has in reducing overall steam temperature in an ideal, uniform mixture of air and steam in a range from 10 to 50% air. This has a direct effect on the resulting dryer surface temperature and production, but the effect of loss of heat transfer is even greater. The loss in surface temperature is substantial even for small quantities of air. Loss in

production can be estimated for any drop in effective temperature from the TAPPI drying curves for each grade of paper. For example, a 10* F drop in surface temperature on dryer limited linerboard grades will result in approximately an 8% production loss.

High pressure dryers are less affected by insufficient purging of noncondensables at startup. For example, with a dryer at atmospheric pressure compressed with steam to 150 psig, the original specific volume of 28.6 ft3/ lb is reduced to 2.75. Therefore, with no purging there will be approximately 10% air in this final dryer mixture and a substantial loss in production. If time is taken to purge to 10% or less air before compressing to 150 psig, approximately 1% air will be in the final dryer steam mixture. This 1% air is considered negligible in terms of air content When compressing to lower pressures, proper purging becomes mutfQftfQr3Q17431.

High-speed videos inside dryers nave shown very high turbulence, indicating good mixing and relatively uniform air-steam mixture.

The purging cycle normally requires a minimum of one to three hours at 10 psig or less. Machines that have full scale purging during normal operations, such as a cascading system or blowing to a lower pressure header,

2030307*431 CONFIDENTIAL:

MINNESOTA TOBACCO LITIGATION 2030307431 http://legacy.library.ucsf.edu/tid/tbh48h00/pdf

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There are two general types of syphons: rotating and stationary. Rotating syphons revolve with the dryer cylinder, and stationary syphons remain fixed relative to a point outside the dryer.

0020207*132.


2$ I Paper Machine Stem and Condensate Systems

Differential pressure requirements necessary for condensate flow from a dryer is a function of syphon type, size, condensate load, speed, and dryer diameter Stationary syphons vftth scoops require less differential pressure than rotating syphons under similar operating conditions since the velocity energy aids drainage. Plain end stationary syphons need approximately as much DP as rotary syphons for condensate evacuation.

Blow through steam, mixed with condensate, results in a two-phase flow with an average density much lower than liquid condensate. Therefore, the differential pressure requirements to overcome centrifugal force in a rotary syphon are much lower than the theoretical differential pressure required for a solid water column.

The rotary syphon is best suited for speeds between 500 and 4000 ft/min, where rimming of condensate is the rule. Above about 3000 ft/min, a flooded dryer normally cannot be evacuated with the available differential pressures. Stationary syphons with simple vertical pipe are satisfactory for speeds below 500 ft/ min because the syphon operates in a condensate puddle for good evacuation of condensate and excellent heat transfer. The major problem with stationary syphons is maintaining the desired clearance between the scoop pickup shoe and dryer inner surface. Thermal distortion, impact of condensate, and inadequate external support contribute to problems with stationary syphons. However, the rigidity of supports has improved, and it is now easier to maintain set clearance.

Grooves are sometimes machined in the dryer bore, under the syphon shoes to minimize condensate thickness. This usually compensates for the overall increase in shell thickness, equivalent to the groove thickness, which is required to maintain code require-ments. Grooves are generally placed outside the sheet contact area because of the surface temperature difference a groove produces. Syphon grooves tend to increase dryer shell length. Sometimes syphon grooves are alternated front and back and are left inside the sheet edge at the wet end. As the sheet shrinks during drying, it moves away from the syphon grooves.

The use of dryer bars inside dryers improves heat transfer so that maintaining a thin condensate layer for best or optimum heat transfer rates is not as critical. However, there is now good evidence that excessive condensate load (above design values) does in fact cause loss in heat transfer even with dryer bars. This does allow stationary syphons to be used without a groove, but syphon clearance should be kept at the designed clearance. This minimizes the chance of gouging damage on the dryer inner surface and also reduces required shell thickness. Higher condensate volumes in dryers equipped with dryer bars require a higher torque load or drive horsepower to get the condensate to rim. This often results in dryer drive overload and kick out. This problem is usually solved by rotating all of the syphons to the same location in the puddle before startup, or the dryers

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can be rotapd ^ctyjfr until the condensate is evacuated. A major proUftnwith heavy condensing load dryers,

such as those at the wet end of a paper machine, is that the perimeter of the shoe may be too small to handle the condensing load. Several mills increased the diameter of the vertical syphon pipe and still had evacuation problems until the shoe diameter was also increased. This is true particularly at low operating pressures.

S.7 Effect of centrifugal force

The effect of centrifugal force is especially critical for rotating syphons when a dryer is flooded. Very high differential pressure may be required to evacuate dryers once they are flooded. The pressure acting on a solid water column at various speeds is noted on Figure 5.7. The pressure indicated in Fig. 5.7 is the theoretical minimum required to raise a solid water column out of the dryer. The friction drop for the quantity of flow being evacuated needs to be added to this value. (There is about a 10% difference on the effect of centrifugal force for cold and hot condensate at 0 psig and 150 psig as shown.)

Dryers running below 1800 ft/ min usually do not have problems with insufficient differential to cover any situation including flooding dryers. It should be noted that the differential shown in Fig. 5.7 is that which is required across the steamfit and not the headers where most differentials are measured on paper machines. Steamfit differential will vary between 40-80% of the measured header differential, depending on pipe sizes and restrictions in each system. There are many opinions as to what the required differential across headers or steamfits should be (see TAPPl TIS 0404-31 "Recom-mended Dryer Differential Pressures**). However, professional designers have computer software programs that accurately project syphon performance and the relationship of blow through steam to DP. If a machine runs slow enough and can maintain sufficient differential across the steamfit to evacuate the dryer when the syphon shoe is flooded, the dryers will not flood. For speeds over about 1800 ft/min, it becomes impractical to maintain sufficient differential across the steamfit to cover flooding conditions. Over 1800 ft/min, the high differential required across the steamfit to evacuate a solid water column causes excessive blowdown or steam loss, as well as excessive erosion problems and high motive steam usage on thermocompressor systems. This does not mean that high controlled differentials should not be used for short periods of time to get out of a problem. In normal practice on high-speed machines, the steam system is designed to S @ J C O S K l W 4 0 9 n v condition. ^ ^

A small 0.25-in. (6.4-mm) aspirator hole on the syphon shoe, 2 to 3 in. (51 to 76 mm) above the dryer shell had been proven helpful in maintaining two-phase flow by breaking up a solid water column on some installations.

M , N . 9 O 0 N F I D E N T I A L : MINNESOTA TOBACCO LITIGATION 2030307433 http://legacy.library.ucsf.edu/tid/tbh48h00/pdf

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6 ^ Troubleshooting, Check-out, Startup, and Shirtrjpwn of Dryer Drainage Systems

6.0 Equipment Check-out

6.1 General installation

A. Physically trace all piping from mill supply to central condensate receiver to make sure there are no construction errors and that the proper equipment is in the proper place. Piping configurations which could cause a buildup of condensate should have water traps. Note any piping changes (tie-ins, additions, deletions) that may have been done over the years, and determine their effect over the entire range of operation.

B. Check to see that pressure relief valves are installed on all individual headers. Dryers must not exceed coded pressure at any time.

C. Locate check valves in each line that should have one, and check direction of flow.

D. Be sure the main steam valve is shut and tagged before stroking or opening any downstream valves for check-out purposes.

E. Check out special dryers such as swing dryers. Bypass interlocks, and run the system completely before startup, to insure everything is functioning properly.

F. Check pipe sizes and valve sizes for excessive velocities and excessive pressure drops and differen-tials. Size the condensate lines for two-phase flows.

G. Open all hand valves in each section: dryer valves, transmitter valves to process, valves to traps, control isolation valves, etc.

6.2 Control valves

A. Check all control valves for direction of flow, specific size, location for ease of bonnet removal, positioner, if required, and nameplate data, generally assuring the proper valves are in the proper locations. If an actuator is horizontal and large (4* control valves or larger) it may need an external support Check

for shutoff and bypass valves required with each control valve. See Fig. 6.1.

B. Check that valve position and positioner output pressure are correct for varied controller outputs. (Observe when stroking valve). Each valve must be checked. Improper valve action is a common error.

C. Set the supply to all positioners as specified by suppliers.

D. The percent opening of a valve is not an indication of its percent of capacity. Valve curves must be checked for capacity versus opening. This varies greatly for different valve designs.

E. All valves should be installed to failsafe in case of air or electrical failure. All inlet steam valves must fail shut or require air to open. All downstream valves must fail open or require air to close.

Fig. 6.1 Typical control valve piping arrangement

31


32 / Paper Machine Steam and Condensate System

Table 6.2 POSITIONER OUTPUT PRESSUftft^ ^ B H P ' N P S H M o t o r H P M o t o r R P M < a n d AND VALVE TRAVEL % FOR T Y J P i e S I ^ - rotation). VALVES WITH POSITIONERS ^ ^ ^

Controller range, output

pressure

3 psi 6 psi 9 psi

12 psi 15 psi

"Full range valve, 3-15 psi

0% 25% 50% 75%

100%

Split range valve, 3-9 psi

Split range valve, 9-15 psi

% Valve Opening 0%

50% 100% 100% 100%

0% 0% 0%

50% 100%

D.

E. F.

G. H. 1

J.

63 Separators

A. Check the drain valves to carry blowdown steam away from the condensate pump and motor (usually piped to sewer). Drain lines should be anchored securely. Check general steam and air piping.

B. Check liquid level controller displacer for proper elevation on separator (suitably marked with an arrow on the displacer housing). Too high a level may cause condensate carryover.

C. Make sure the level glasses on the separator are tight. D. "Stroke" the separator level control valve by means

of the surface mounted level controller (after supply air to controller has been turned on).

E. Set the pressure switch to turn condensate pumps on at 9 psi output from controller and off at 14 psi. (Low level at 14 psi). Pressure inside the separator may be sufficient to evacuate the condensate with the condensate pump off.

F. Check and/ or calibrate the gauges on the separator. Make sure that gauges read zero when there is no pressure in separator. Gauges need replacing periodically.

G. Set controllers to minimize control valve cycling at startup (readjust after startup as necessary).

H. Set level control dial inside controller on mid-point of float travel.

I. Action of level controller should be reverse acting for a decreasing output signal with a rising level.

J. Replace separator sight glasses if dirty and it is difficult to see condensate levels.

K. Each separator should have a high level flooding alarm.

L. Each separator should have a low level alarm.

6.4 Condensate pumps and motors

A. Check for correct pump and motor (GPMt TDH,

Check piping for bypass and piping stresses. Check that pumps are mechanically free (rotate by hand).

D. Check to see that packing glands are not cocked. Hand-tighten packing evenly, just enough to prevent leakage when running. Check that pumps are properly lubricated. Check that pump spillover lines are omitted or have shutoff valves. Check that discharge gauges are properly installed and calibrated and have proper range. Check pump rotation (start and stop pump quickly). Pump casing vents should not be piped to the top of the separator on vacuum receiver. Check secondary piping (seal water and source, cooling water and source). If condensate is used for seal water, a condensate c

01 paper machine steam & condensate systems.pdf

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