Pulp and Paper –A Guide to Steam Conditioning

2 CHP Pulp and Paper Process

CHP in Pulp and Paper 2

Requirement of Steam 2

Steam Conditioning Applications 3

Pulp and Paper, Recovery Cycle 4

Paper Making 5

Key Products for Severe Service Applications:

VST-SE 6

VLB 6

DRAG® 7

Desuperheaters 7

Introduction, What is CHP in Pulp and Paper Industry

CHP (Combined Heat and Power) is an efficient technology for generating electricity and heat together.

A CHP plant is an installation where there is simultaneous generation

of usable heat and power in a single process. Figure 1 shows a possible

configuration for a CHP plant. The heat source can be established from many

different sources. Waste heat from process (e.g. black liquor recovery) and

waste heat from gas turbine (also electricity generator) by a heat recovery

steam generator (HRSG). This heat is used to provide process steam which

is required for the production of pulp and the paper machine. Availability of

steam is of the utmost importance, electricity can be bought via the grid, but

non availability of steam means that production (revenue) will stop.

CHP provides a secure and highly efficient method of generating electricity

and steam at the point of use. Due to the utilization of heat from electricity

generation and the avoidance of transmission losses because electricity

is generated on site, CHP typically achieves a 35% increase in efficiency

compared with power stations and heat only boilers. This can allow

economic savings where there is a suitable balance between the heat and

power loads.

Why is Steam Required and at What Degree of Superheat?

Steam is required for the paper machine and evaporators at a condition close

to saturation owing to the excellent heat transfer properties of saturated

steam. Paper making, typically requires steam at 3.5 bar a at 145 C. If there is

too much superheat in the steam, then there the heat transfer at the process

will be inefficient and the paper run can be ruined.

Steam is therefore normally available from the steam turbine or its bypass

valve or a combination of both. It should be noted, the requirement of

the power plant is primarily to provide steam for the process (industry)

and generating electricity is merely a benefit as the electrical needs can be

imported if necessary. Steam supply at the correct pressure and temperature

and not electricity is of the utmost importance.

Figure 1: Typical simple CHP scheme with black liquor recovery boiler and steam turbine.

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Steam Provided by Steam Turbine

Steam from the (boiler or HRSG), normally high pressure and superheated,

will pass through the steam turbine. For example, steam to the relevant

process can be taken from extraction or the exhaust of a steam turbine of the

backpressure design (refer to Figure 1).

Turbine Extraction/Exhaust

The outlet steam temperature from extraction or exhaust varies depending on

the steam going through the steam turbine(refer to Figure 2). For example,

considering exhaust steam, as the steam flow through the turbine decreases,

the outlet temperature increases. Depending on the exhaust flow in general

as the extraction flow reduces, the extraction steam temperature increases.

This means to obtain a constant set temperature downstream, the proportion

of spraywater required at low flow is higher than compared to at full flow

where the requirement will be small if any at all.

The exhaust steam supply pipe to paper mill will be large in diameter and

combined with the conditions as detailed above and potentially low flow,

providing good temperature control to the process close to saturation will

need special consideration.

Steam Provided by the Bypass Valve

If the steam turbine is not available, then the bypass valves are utilized to

condition the steam to the exact conditions required for the process (refer

to Figure 3). When the steam flow through the turbine does not meet the

process demand the bypass valve must make up the difference between the

process demand and that being supplied by the steam turbine. Availability of

turbine bypass valve is therefore critical to production.

Steam Turbine Bypass to Extraction/Exhaust for Back Pressure Turbines

The steam turbine bypass reduces the pressure and temperature of the steam

to match the appropriate extraction/exhaust conditions. They are used during

startup, in the event of a turbine trip, non availability of the steam turbine

or supplementing steam to process that may not be available from the steam

turbine.

The bypass valve should:

Be suitable for severe thermal

shock (up to 300 C)

Modulate in 2-3 seconds or less.

Snap action in this time is not

acceptable as the boiler will trip.

Have high rangeability to

maximize turndown

Provide repeatable tight shutoff

Inline repairability

Be of low noise design

Reliability of this equipment is of the utmost importance. Non availability

means loss of production. The valve illustrated in Figure 4 (VST-SE) meets all

the above criteria. CCI with extensive experience and knowledge can provide

installation guidelines in conjunction with the correct product selection for

the optimum system solution.

Figure 2: Turbine extraction/exhaust desuperheating

Figure 3: HP turbine bypass to process

Figure 4: VST-SE bypass valve for fine control of steam to paper machine

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Pulp Manufacture and Recovery Cycle

The details below are typical for a large pulp and paper mill.

Received wood is cleaned, processed and made into small chips. The bark

that is stripped will be used to burn in the bark boiler that produces steam

at the same pressure as he black liquor recovery boiler described later.

Wood chips are cooked in the digesters. White liquor, hydroxide sulfide

is used in the boiling process to separate fibres from the chemicals in the

wood that bond the fibres together. The remaining water and chemical

mixture is known as white or feed liquor. The mixture is 15% white

liquor and 85% water.

The pulp is refined in the respective fibreline and includes screening,

washing and possibly bleaching (for production of white paper)

Unbleached pulp will be in its natural color and produces cardboard and

brown paper bags etc. The pulp can be packed ready for export or used

for the paper mills on site.

The white liquor goes to the evaporators where water is evaporated and

the chemicals become concentrated until it constitutes over 70% of the

mixture which is known as black liquor and is used in the recovery boiler

for combustion. The ash created is called smelt and is fluid at about 1100 C.

Water is added to the smelt which is known as green liquor and is then

mixed with chalk and becomes white liquor and can be used again at the

beginning of the process with treatment (addition of chemicals.)

The recovery boiler produces 350 T/hr steam at 60 bar and 485 C. There

is also a bark burning boiler which at the same pressure and temperature

produce about 70-80T/hr. There can be 4 headers, 60 bar, 35 bar, 10 bar

and 3 bar. Some of the 60 bar steam is used for the breaking of wood at

full temperature. The evaporators will take steam from the 10 bar header

(approx 70 T/hr) which will be let down to 3 –5 bar for evaporating the

water from the white liquor to produce black liquor. 60 bar superheated

steam is used in the flash dryers in the fibreline process, to remove moisture

from the damp pulp. The 3 bar header is used to supply the paper mills and

the 10 bar header may also be used in some of the paper machines.

Figure 5: Manufacture of pulp and the recovery cycle

Figure 6: Pulp and paper mill incorporating recovery cycle

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Paper Making

It is important that steam for the paper mill is provided at a condition close

to saturation. Steam that has too high a degree of superheat will result in

possible damaged product, but also reduced output owing to the fact that

superheated steam has poor heat transfer capabilities compared to saturated

and steam and therefore throughput of paper is reduced.

The pulp is blended with water and other chemicals, separated and fed to

the headbox at the right consistency. The raw materials (pulp) material fibres

(99% water) are pumped into head box. The stock is fed evenly onto wire

mesh. As the paper stock flows from the head box onto the wire the water

drains away leaving the tiny fibres as a mat on the mesh. When the mat has

reached the end of the wire section, it has become a sheet of paper, although

very moist and of little strength. It then passes over the press section.

The press consists of a number of cylinders which squeezes moisture from

the paper and the water is drawn away by suction. The Paper then passes to

the drier section, which consist of a large number of steam heated drying

cylinders. The cylinders temperature is normally slightly over

100 C. Synthetic drier fabric carrys the web of paper round the cylinders

until the paper is completely dry. Part of the way through the cylinders, is a

size press, where a solution of water and starch can be added to improve the

surface for printing. For tissue machines, a yankee drum can be used which

is a rely large diameter and relies on the large surface area to dry the tissue

paper. Steam quality at this stage is paramount to the quality and speed the

machine can operate at.

At the end of the process, the paper is smoothed using an, ironing method,

which consist of polished iron rollers. This helps to consolidate, polish and

glaze the surface of the paper. The paper is then reeled and ready for

uss/distribution.

High quality papers can be additionally coated by using clay and other

pigments. These coated papers are usually done on a separate machine, ut in

some cases may also have an on-machine coater for precoating before

being reeled.

The board machine is a similar process, but has several wet ends (headboxes

and wires) producing multiply sheet.

Figure 7: Typical paper machine, showing heating cylinders

Figure 8: Paper machine using low pressure steam close to saturation

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VST-SE

The VST-SE was designed as a steam turbine bypass to process conditioning

valve. The requirements are to open and close very quickly in response to a

turbine trip, startup or to provide additional steam flow to the process.

The bypass to process with VST-SE will benefit from:

Reliable operation: suitable for up to 300 C thermal shock. Fully

machined circular section valve body.

More revenue owing to higher electrical production: this is achieved by

providing high turndown capability with regard to steam flow by means

of steam atomization.

High performance and stable control: solved by integral water

proportioning.

Reduced maintenance cost & downtime: provide repeatable tight

shutoff despite exposure to thermal shock having unique two piece seat

providing flexibility.

Maximize plant flexibility: the VST-SE provides modulating steam

atomization. Generally standard systems provide on/off atomization

VLB

The VLB was designed as a steam turbine bypass valve and is widely used for

bypass or dump to condenser.

The bypass system with VLB will benefit from:

Reliable operation: suitable for up to 300 C thermal shock. Fully

machined circular section valve body.

High performance and stable control: system stability despite

pressure, flow and temperature transients with CCI total system

understanding implemented.

Reduced maintenance cost & downtime: provide repeatable tight

shutoff despite exposure to thermal shock having unique two piece seat

providing flexibility.

Excellent evaporation of water: ensuring trouble free operation when

bypassing to condenser owing to special multiple nozzle configuration

around valve outlet.

Pressure sealed bonnet: maintains tightness regardless of temperature

transients and allows quick and easy access to valve internals.

Accurate control of final steam conditions to condenser: preventing

condenser damage owing to overspray and vibration.

Low noise (DRAG® dump tube used if noise requirements are onerous.)

Custom design of bypass valve: inlet/outlet connections to suit

application.

Figure 9: VST-SE steam atomizing and water proportioning

Figure 10: Typical VLB

Figure 11: Bypass to water cooled condenser

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DRAG® — Velocity Control Technology

High velocity fluid or steam as a result of high pressure drop or large change

in pressure ratio creates velocity, which if to high causes cavitation and or

erosion resulting in valve failure.

CCI DRAG® solution is unique in solving this, utilizing multi flow paths and

introducing the required number of pressure reducing stages. Refer to CCI

DRAG® brochure.

CCI DRAG® Benefits

Low noise: depending on application, noise levels of >85 dBA at 1 m are

possible. Working with CCI can provide reduced total system noise.

Reliable operation: by controlling velocity.

Longer valve life: controlling velocity and pressure head

More revenue owing to higher electrical production: will reduce or

eliminate maintenance activity or process shut down owing to

equipment failure.

High performance and stable control: disk stack can be custom

characterized to suit particular application, such as boiler level control

valve (feedwater control valve.)

Reduced maintenance cost & downtime: provide repeatable tight shutoff

utilizing MSS-SP61 shutoff by pressurized seat design.

Reduced installation cost: valve custom designed including connections

to suit application.

Desuperheating of Process Steam

Controlling desuperheating of extraction and exhaust steam is challenging

owing to the following:

Low velocity at startup

Insufficient coverage

Large piping diameters don’t encourage mixing

Set temperature close to saturation

Desuperheaters subject to transient conditions

Key components for successful desuperheating:

Small Diameter + High Velocity = Good Mixing

Hotter water (up to 120-130 C) smaller water droplet dia.

Higher P means better atomization of water (smaller water drop dia.)

Smaller water droplet diameter = quicker evaporation

Even distribution (across the area of the steam) of the spraywater

Control of downstream temperature

Installation considerations CCI have several innovative styles of desuperheaters, review and advice of the system is necessary. Aspects such as liners, enthalpy control, reduced sections of piping, installation are all aspects necessary to meet performance requirements.

Figure 12: Uncontrolled velocity – a control valve’s worst enemy

Figure 13: DRAG disk multi-trim/flow path

Figure 14: Multi nozzle DAM desuperheater

CCI will provide the correct total system solution for the application.

CCI World Headquarters—CaliforniaTelephone: (949) 858-1877Fax: (949) 858-187822591 Avenida EmpresaRancho Santa Margarita,California 92688 USA

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DRAG is a registered trademark of CCI.©2003 CCI 563 3/03 4K

Throughout the world, companies rely on CCI to solve their severe service control valve problems. CCI has provided custom solutions for these and other industry applications for more than 80 years.

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