THE APPLICATION OF STEAM TURBINES TO THE CANE SUGAR

INDUSTRY J -,.

S. van der Linden

Regional Manager, Worthington International, Inc., Sun Juan, Porto Rico

INTRODUCTION

Sugar Mills and Factories are large users of steam and how this steam is used in providing power is an important factor in the plant steam balance and economics.

When considering steam turbines for driving sugar mills or generators for power, or mechanical drives for pumps, fans, knives, and shredders, many questions usually come to mind. What horsepower should be specified? What type of turbine, governor, speed and gear ratios should be selected? How can one be sure that theheat balance of the factory will not be upset by the new installations? Will the turbine operate with the existing steam pressure and temperature? Would more boiler capacity be required? What safety devices should be included? Nearly twenty years later, we can safely say that turbines are ideally suited as priine movers for the Sugar Industry.

During this period of time turbines have become a'ccepted as the modern pro- ducer of power and there are advantages of using modern steam turbines as compared to reciprocating steam engines, most important ones being oil free exhaust process steam, better plant steam balance, less maintenance, greater flexibility of operation, less space required, easier installation and simpler foundations.

A. THERMODYNAMIC CONSIDERATIONS

All sugar factories use large quantities of low pressure steam as a heating tool in the various processes. The steam systems vary widely depending upon the steam-power ratio in the factory or mill and the surp ie or otherwise bagasse. The steam consump- tion per ton of cane crushed varies according to the degree of steam economy achieved in the process, in other words, multiple effective evaporators and the amount of vapor being used.

A Sugar Mill making raw sugar without using electricity, even for pumps, use 0.6 .to 0.7 tons of steam per ton of cane. A modern factory with a turbine generator supplying all the motor driven pumps and small auxiliaries, with careful attention to steam will reduce this figure to 0.5 to 0.6 tons of steam per ton of cane.

With quintuple effect evaporation under pressure and full vapor bleeding, and with superheat steam at high pressure, the steam consumption can be reduced to 0.4 to 0.5 tons per ton of cane.

For example, a 5000 ton per day mill could be generating Z ~ O O O O lbs. of steam per hour. This amount of steam at 150 psig dry and saturated conditions and expanded to 15 psig for process according to theoretical steam rate tables is capable of 9500 1cW

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or alternatively if at 250 lbs. equalivant of 12000 kW, and in terms of a modern power plant the quarter million pounds of steam represents 33000 kW.

The electrical demand of the Sugar Mill naturally varies, depending upon the degree of plant electrification. Typical figures are 8 to 12 kW per ton of cane for partially electrified factories and I5 to 24 kW per ton of cane for totally electrified factories.

I . Process Steam Powev - Economic Answer

The large quantity of low pressure steam used as a heating tool in the various processes is an ideal source of power to meet the mill demands for m~chanical and electrical power.

The total, power requirements can be met by the back pressure steain turbine which passes all its exhaust steam heat on to the process. The high pressure steam is expanded through several stages and the kinetic energy of expansion is converted by the turbine into electrical or mechanical power.

The back pressure turbine generates power at nearly ideal efficiency, recovering "power heat" and passing on latent heat or "heating heat" to the process where it is released at constant temperature. The increased power generated more than offsets the cost of additional fuel required to create the heat drop through the turbine. Power thus produced as a by-product of the process steam is at a lower cost than purchased power from the public electricity supply power.

When considering power as a by-product of process steam the same fundamen- tal law applies to steam turbines that applies to all other heat engines; i.e., heatin steam entering turbines = heat in steam leaving turbines + heat equalivant of energy used in turbine.

The recoverable power with a given flow of steam from a back pressure turbine which generates power at near ideal efficiency is rigidly determined by: (a) supply steam pressure and temperature, (b) exhaust or process steam pressure, (c) turbine internal efficiency.

The process heating temperature determines the exhaust pressure. The plant designer selects a boiler pressure and temperature that will provide sufficient heat drop, from the available process steam flow, to generate enough power to meet the factory electrical demands.

Steam tables show that as the steam pressure falls, the latent heat increases and the sensible or water heat reduces. As an example of steam at 50 psig has a latent heat of 912 BTU/lb. while steam at 5 psig has a latent heat of g61/lb. This lower steam pressure has 5.3% more BTU/lb of latent heat which it can readily give up to the process. This means if a plant can accept heating steam at 5 psig instead of 50 psig it would use 5% less steam. I t follows that every effort should be made to use steam at the lowest possible pressure, not only gaining "heating heat" but also power recovera- ble from larger heat drop. The familiar Mollier diagram (Fig. I), displays the basic thermodynamics involved. Note that for equal pressure increments, vertical distances between the constant pressure lines rapidly increase as one descends the pressure scale.

I t is interesting to note that a pound of steam possessess more potential work between the pressures of 10 psig and 2" Hg than it does between 400 psig and 10 psig.

. - ---

1 5. VAN DER LINDEN

An increase in the ratio: supply steam pressure/exhaust steam pressure gives an increase in available power, thus increasing the pressure ratio. With the installation of a higher pressure boiler increases the available power but a greater gain can be made by reducing the exhaust pressure when not limited by existing plant. To illus- trate this point further we can use the example of a turbine designed to pass 120000

lbs. per hour and to give a constant zoo F superheat in exhaust steam. At a condition of 400 psig 630" and exhaust pi-essure of 40 psig or an absolute pressure ratio of 7.55

100 400 psig

300 psig (96%)

90 200 psig (90 %)

80 100 psig (80%)

s 3 70 a, C 0

0 - n a 60 20 psig (59%) - 0 > a 10 psig (54%)

50

0 psig (45%)

40

30

Relative available energy 400' psig o0sh to 28' Hg vac.

A B

Fig. I . Part of Mollier Diagram showing the large amount of energy available in low pressure steam. Note that steam possesses more potential work between the pressures of 10 pslg and 2 inch Hg than between the pressures of 400 and 10 psig.

the turbo-generator will have an output of 4450 1cW however by increasing the inlet pressure to 600 psig, or an absolute pressure ratio of 11.2 the output increased to 5450 kW. With inlet pressure kept at 400 psig exhaust reduced to 10 psig absolute pressure ration increases to 11.6 and output increases to 6200 kW.

The power available from the turbine from a given pressure ratio is proportion- al to the inlet steam temperature, and further as the inlet pressure increases with a fixed exhaust pressure, the inlet temperature also increases to maintain the same, exhaust condition. The ratio : steam supply temperaturelsteam exhaust temperature,

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remains constant for a given steam pressure ratio and of constant turbine efficiency. Therefore, an increase in inlet steam temperature to produce more power, correspond- ingly increases the exhaust steam temperature. This results in a higher superheat at the turbine exhaust flange, necessitating the addition of some desuperheating water

$ 240 m c 220 .- $ 200 L

180

?! 140 Y) ; 120 P

E 100 0

$ 80

60 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Theoret~cal steam rate In Ib per kwh

Fig. 2. Theoretical steam rate curves dramatically illustrate lower steam consumption at higher- pressures and temperatures.

to reduce the temperature to give zoo F superheat. Although this has the effec't of reducing the total steam flow through the backpressure turbine by the volumeof water added at the exhaust there is a net gain in the power generation. Fig. 2. Theoret- ical steam rate curves and temperature dramatically illustrates lower steam consuinp- tion at higher pressures. Steam turbine optimum efficiency is a function of rotor diame- ter, rotational speed and number of stages to suit the selected steam conditions. The ratio: steam velocity/blade velocity, (W/V) velocity ratio gives maximum efficiency for an impulse Rateau stage when i t is between 0.4 and 0.5 and a Curtiss stage of approximately 0.300. The heat drop through the stage fixes the steam speed, but the wheel of blade velocity depends on both the blading diameter and the rotor speed.

Although it is difficult to generalize due to the large variations of steam pres- sures, temperatures, and flow possible through a given turbine, a high speed gear machine is usually more efficient than a direct coupled machine for the smaller powers and exhaust volume flows. The gain in power more than offsets the gearing losses. At larger powers with larger exhaust volume flows the direct coupled set becomes more efficient over all. In terms of high speed geared machines economic considera- tions are taken into consideration and where possible the manufacturer offers stand- ard design turbines rather than going to speeds above, 5,500 RPM requiring solid forged rotors and special- rotor materials.

The following is important when applying steam turbines to recover power from process steam: a. Use as high as practical initial pressure and temperature. b. Use as low as practical exhaust process pressure. c. Never permit a large volume of steam to expand from one pressure to a lower pressure without having useful work from the expansion.

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2. Estimation of ~ i a i l a b l e Process Steam Power

When process steam pressure and flow are fixed and consideration has been given to boiler conditions the next step is to assess the power available from the process steam flow. If a straight back pressure turbine c a n ~ o t produce enough power to balance the power demand and no public utility is available for running in pafallel, then further consideration can be given to extraction, condensing type turbines. The steam rates given in the accompanying charts can be readily obtained with back-pressure or condensing multi-stage turbines of reasonable cost as produced by several builders. Still lower steam rates can be obtained if conditions justify the extra cost.

The added cost is justified when conditions require a very good economy in the use of fuel and steam such as the following examples : a. When power is required outside the mill for irrigation or other purposes. b. When bagasse can be profitably sold and the substitute fuel such as coal, oil or

natural gas must be bought. c. When excess bagasse can be used for making paper and furfural. d. And when power is generated and sold throughout the year and there is a need to

obtain low generating cost.

I I I I I I I I I I I I I

20 000 60000 100000 140000 180000 220000 Steam quant~ty -pounds per hour

Correction factor-multlply power or dlvlde steam quantity by: 0 7 12 124

,",,,, 300 400 500 600 700 800 900 1000

Temperature OF

Fig. 3. Steam Consulnption and Available Power Chart 240000 lbslh or 12000 1rW.

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The chart in Fig. 3 shows relationship betweensteam consumption and available power for a given set of steam conditions.

An extraction back pressure turbine can be calculated by considering the first extraction pressure ,as the exhaust pressure of one turbine with initial steam inlet and the second turbine with inlet pressure equivalent to the extraction pressure and exhausting to the existing pressure.

The steam rates are only intended to enable preliminary survey to be made in overall terms to decide which one of several possible scl~emes merits further detailed investigations. Reference should be made to the turbine designer on the part load performailce of the plant as it would be impossible to incorporate other design points than that obtained at the best efficiency point.

I 3. Quality of E~thaust Steam

With the interest and illcrease in better steam and fuel economy and subsequent use of high pressure superheated steam, especially large new mills, turbines and t ~ b o - generators are used exclusively.

equalivant of a horsepower is 2545 BTU. The heat removed from each pound of steam may therefore be expressed by the formula:

Heat removed =

I 2545 I The steam rate must be that of the turbine. Steam rate per brake horsepower hour

itself not including gear losses, etc. In the cases of turbines for driving Sugar Mills, cane cutters or cane shredders, pumps, extra large gears are used and the gear loss is about 3% if the steam rate includes the gear loss, deduct 3% to find the steam rate of the turbine itself.

TABLE I

APPROXIMATE FACTORS FOR CONVERTING FULL LOAD STEAM RATES O F GEARED TURBINE GENERA-

TORS IN lbs./lrWh TO STEAM RATE OF TURBINE ALONE IN lbs./RHP/h.

1cW rating Factor

2 0 0

300 0.687 400 0.690 500 0 . 6 9 2

1000 0.697 1500 0.698 2 5 0 0 0.700 3 500 0.705

For turbine generators the loss in both gear and generator must be considered also as the steam rate is usually given in pounds per kilowatt hour which must be converted in pounds per brake horsepower hour. Table I gives a factor by which the

S. VAN DER LINDEN* I577

steam rate per kilowatt hour may be multiplied to give the steam rate of the turbine itself in pounds per brake horsepower hour.

To find the condition of the exhaust steam from any noncondensing turbine use the following procedure : a. On curve Fig. 4 find the total heat contknt of the steam at initial pressure and

temperature.

Total heat content m

m + m a m o o mo ooo o 00% goo o N NO + LD(D m g 2- Q Wg

Steam pressure -pounds gage

Flg. 4. Curve showing total heat content of steam above water a t 32" F.

Steam flow b. Find the steam rate of the turbine itself or

hp

c. Find the heat removed from each pound of steam (use th r formula). d. Subtract the heat removed (c) from the total heat content of the entering steam

(a) to determine the heat remaining in the exhaust steam. e'. From curve Fig. 5 find the point at which the vertical line for the exhaust pressure

intersects the horizontal line for the total heat content (d) of the exhaust steam '

and read the superheat or moisture content of the exhaust steam at that point. Example A. 125 pounds saturated steam. I5 pounds back pressure. Steam rate 40 pounds per brabe horsepower hour, not including gear loss. Exhaust steam less than 4%,wet. Exam9le B. 400 pounds zoo0 superheat. 20 pounds back pressure. Steam rate 22 pounds per brake horsepower hour including gear loss. Exhaust steam 92" F S.H.

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Example C. I500 kW turbo-generator. 600 pounds zoo0 superheat. 25 pounds back pressure. Steam rate 21.6 lb./kWh. Exhaust steam 10" F S.H.

Calculation for part load or low speed conditions will show that the exhaust steam is hotter than at full load and full speed. This is correct because the steam rate is higher under such conditions and less heat is removed from each pound of steam.

10

Direct acting oil pump (Nema A )

8

7

'I relay ( Nema C

Precision ( Nema

Steam pressure, %

90 95 100

40 60 80 100 Load, %

acting oil pump ( Nema A )

Fig. 5. A Typical Speed Regulation Curves of Three Types of Governors. B. Typical Curves showing Speed Change with Initial Steam Pressure Change for Three Types of Governors.

When the superheat and exhaust steam is great enough to be objectionable a desuper- heater must be used. Simple and reliable spray type desuperheaters with automatic temperature controls are available. The water used is not wasted as it is evaporated and becomes added steam available for process. The quantities of water required for desuperheating are not very large. Any convenient and reliable source of purewater at adequate pressure and at any temperature may be used. An example of the amount of boiler feed water at 180" F required to desuperheat 50000 lbs of steam per hour at I5 lbs. pressure from 100" F superheat to 10" F superheat will be equivalent to 4-113 GPM which can usually be supplied from the regular boiler feed pump.

S. VAN DER LINDEN

B. DESIGN CRITERIA

I. Impulse us. Reaction

The advantages of an impulse turbine are reliability, ruggedness, simplicity and ease of maintenance.

The distinguishing characteristic of the impulse turbine is that steam expansion and pressure drop occur in stationary parts only - the nozzles. The nozzles, arranged in a ring at one side to clear the blades feed steam at an angle to the travel of the rotating blades (buckets). The crescent shaped blades permit free entry and discharge and produce the change of speed and direction which gives the rotational force. No expansion takes place in the buckets; therefore, the pressure is the same on both sides of the rotating blades - i.e., both upstream and downstream, and consequently, the rotor is completely free from end thrust. Furthermore, ample clearances do not impair turbine efficiency.

In a reaction turbine expansion of steam actually takes place in the rotating bucket, with the resulting thrust moving the wheel. Furthermore, since the higher pressure steam upstream of the blade will tend to follow the path of least resistance and try to by-pass the blade, very close clearances are required if a good turbine efficiency is to be accomplished.

Impulse turbines with no end thrust and ample clearances are ideally suited for application in the sugar industry where reliability, simplicity and ease of main- tenance are desirable characteristics.

2. General Construction Details

The construction details of a turbine depend on so many variables such as pressure and temperature, horsepower, speed, etc. that i t becomes impractical to mention all possibilities.

A few general comments, however, applicable particularly to the sugar mill applications are well worth mentioning.

( a ) Centerline Support Centerline support minimizes steam and vertical expansion and allows for uniform expansion, thus, practically eliminating misalignment due to temperature changes.

(b ) Casing The horizontally divided casing permits easy removal of the turbine upper half for routine inspection and repair.

Casing supports are designed to allow for casing expansion while maintaining correct alignment under all conditions of operation within the range of the unit. k keyed support guards against casing movement toward the driven machine.

( c ) Dia$hragms In the cast diaphragm construction, usually found in sugar mill turbines, diaphragm nozzles are formed by casting the inner and outer diaphragm rings around extended sections of the diaphragm blading. Regardless of the type of diaphragm used, stainless steel' blading to resist corrosion and erosion in the steam path..is always used.

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( d ) Blades and Wheels For turbine efficiency and long life, blade design is important. All turbine bladingis usually made of steel to resist corrosion and erosion. The blades are made of 12%

chrome steel, an alloy tha has high resistance to erosion and excellent damping qualities. The turbine wheels are machined with extreme accuracy from heat-treated alloy steel.

Avoiding blade resonance is important in steam turbine design. I t is more of a problem in variable speed mill drives than in constant speed turbine generators or other mecl~anical drives, because of the greater probability of running at some reson- ant speed. Variable speed turbine blading is usually designed $or a high ratio of blade frequency to design running speed. This gives the prospective user a very rugged blading design.

( e ) Rotor The rotor is the power producing component of the turbine and one of the most im- portant. The shaft is constructed from a carefully heat-treated alloy steel forging. After it is completely assembled, the entire rotor is dynamically balanced.

In single stage turbines, the rotor consists of one two-row (Curtis) stage. In multi-stage turbines, the first stage is Curtis and is then followed by several Ratea (single row) stages.

Mill drive turbines will usually have 5 to 7 stages; one Curtis stage and 4 or 6 Rateau stages. In turbo-generators, however, there may be as many as I5 to 20 or more stages, depending on the size and desired efficiency of the machine.

While on the subject of rotor design, it is worthwhile mentioning something about critical speed and "stiff shaft" construction.

Critical speed is the speed at which the rotor experiences a self-excited vibra- tion and begins to vibrate excessively. This is an inherent characteristic of all flexible shaft turbines. The first critical speed will usually occur at around 55% of the turbine design speed.

In turbo-generators, this presents no problem since it is a constant speed ma- chine. I t requires care during startup, but that is all.

In sugar mill drive turbines, however, the situation is quite different. The tur- bine is a variable speed drive with a governor that probably allows 3 to I speed range.

I t becomes obvious that a turbine with a first critical speed within the governor speed range is unacceptable. "Stiff Shaft" construction with considerable increase in shaft diameter is required with first critical speed 25% above turbine maximum design speed. "Stiff Shaft" construction is a must for all sugar mill drive turbines and must be supplied on all such variable speed applications.

(f) Thrust Beavings For sugar mill applications, the thrust bearing of a turbine can be the most important individual component. It can mean the difference between long trouble-free operation and disaster. Boiler swings often result in severe water slugs which impose terrific thrust shock loads on the turbine rotor. If the thrust bearing does not withstand this momentary - and sometimes prolonged - thrust loads, the rotor will eventually move in the direction of the thrust, the blades will rub against the diaphragms and

S. VAN DER LINDEN 1581

The thrust bearing that is generally employed on all turbo-generators and on most mill drive turbines is of the Kingsbury type.

This type of bearing offers the best chances of survival in cases where severe water slug conditions are frequent and inevitable. The bearing is also greatly oversized to further ascertain trouble-free operations.' '

Alignment of both thrust and journal bearings is made with the aid of a spheri- cal seat.

(g ) Shaft Packing Where the shaft passes through the casing diaphragms, carbon ring or metallic laby- rinth packing is used to control leakage. The type of paclciilg depends upon peripheral speed of the shaft and steam temperature - carbon ring being used for low speeds - labyrinth for high speed and temperature.

Carbon packing is practically always used in sugar applications. Carbon rings are made in three 120-degree segments, held together by a garter spring to a clearance of 0.001 inch between carbon body and shaft. A stop at the horizontal split keeps the carbon ring from turning with the shaft and serves as a spring anchor bolt. Solid stainless steel spacer rings separate the carbon rings, while stainless steel buttons maintain clearance to allow removal of leakage steam froin paclcing case.

(h) Lubrication Systervl Turbine parts which require oil for lubrication are the journal bearings, thrust bearing, worm and worm gear for governor drive, and other governor moving parts; in addition oil is usually supplied to the driven machine, be that a generator or a high speed gear. The oil must be cooled to remove the heat generated by friction and conducted to the bearings from other hot turbine parts.

Basic oil system components include a reservoir, pumps and piping, pressure control valves, oil coolers and filters.

The most important consideration in turbine oil system design is absolute con- tinuity of service, and in that connection, we deal with the starting time and accelera- tion rate of the auxiliary oil pump. If the main oil pump fails, a standby unit must quickly come into service to prevent an oil pressure drop great enough to actuate any of the safety devices; i.e., the trip and throttle valve or the low oilpressure trip mechanism. The lubrication system must also be designed to prevent tripping of shutdown devices when a sudden turbine load change demands large amounts of oil for control purposes, as is the case in full oil relay governors. Such design criteria are of paramount importance if such "accidental" shutdowns are to be avoided.

No matter how well designed and rugged a turbine is, it is impossible to have a succes- ful installation with improper, incorrect or defective governing system.

The types of governors that are used in sugar applications are (Figs. 5 and 5A) : (I) Mechanical Shaft Governor, (2) Oil Pump Governor, (3) Full Oil Relay Governor, (4) Automatic Nozzle Control, ( 5 ) Diaphragm Actuator or "Outside" Control.

i I

' Two types of special interest are the Direct Acting Oil Relay Governor for mill

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drives and the Full Oil Relay Governor as used in most turbo-generators above 1000

bW. Fig. gA shows typical Speed Regulation Curves of three types of Governors

whereas Fig. 5B shows speed change with initial steam pressure change for three types of governors.

The Direct Acting Oil Relay Governor is a simple and versatile governing ar- rangement consisting of three major elements: the governor head, the governor valve and linkage, and the mechanical trip and throttle valve.

The governor head, which contains built-in flyweights and a completely self- contained oil relay system, actuates the double-seated ba1an;ed-type governor valve directly through an adjustable connecting link and pivoted lever. Time lag is ex- tremely small. lkesult is highly stable, instantaneous governing.

The emergency or overspeed trip assembly is a mechanical trip and throttle valve which, although built into the turbine, is entirely independent of the main speed governor and functions separately.

The mechanical trip and throttle valve is actuated by a weight mounted on the turbine shaft. This trip can easily be reset with the hand wheel, without relieving steam pressure ahead of the valve.

Every governor head is equipped with a hand speed changer. In many inst an air or electric motor speed changer is also provided. When the air or electric is used, the turbine remains under control of the speed governor at all times. his arrangement, Fig. 11, provides isochronous (constant speed) governing at any speed for which the air motor is set regardless of changes in load or steam conditions. This is particularly desirable when a remote control panel is also installed.

The Full Oil Relay Governor follows the same principle as before, except that it no longer acts directly on the steam admission valve. Power amplification is used. This amplifier is called a servo-motor and can (a) actuate a single large steam admis- sion valve or (b) actuate a power piston which in turn, through a rack and gear assem- bly serves to rotate a camshaft and thereby a number of smaller automatic valves, thus eliminating the need for hand valves.

This latter governing system is also referred to as Automatic Nozzle Control Governor and it has three distinct advantages: (I) The automatic valves are arranged so as to feed steam to separate nozzle groups and are opened in sequence. Each begins opening when the nozzle ring pressure under the preceding valve is within a small percentage of the turbine's steam chest pressure. This reduces throttling to a minimum and results in a more efficient turbine opera- tion, particularly at part load conditions. (2) Governing stability is much better with a number of small valves than with one large valve. (3) No need of any manual adjustments.

4. Safety Devices

The most important requirement is that a steam turbine must shut itself down when its rotating speed exceeds its maximum operating speed by 10 to 15 percent. This keeps the centrifugal stesses that are developed in rotating members of turbine (wheels and buckets) and driven machine within safe limits.

S. VAN DER LINDEN 1533

All turbines must be equipped with a separate emergency governor (overspeed trip) which operates completely independent of the normal controlling speed governor. I t must be possible to shut down the turbine manually in an emergency by means of a hand trip device located at the front of the turbine.

I Since most turbines are equipped with pressure lubrication to journal and thrust bearings, loss of lubricating oil pressure will cause damage to the bearings and force a costly shutdown. To prevent this, turbines must be equipped with low oil pressure trips (LOPT) .

If a turbine is equipped with a shaft driven main oil pump - which is usually the case - but without LOPT, then a separate auxiliary oil pump is a must; otherwise, in case of main pump failure, the turbine will be damaged. An audible alarm or visual light can be connected to the turbine panel to announce that the auxiliary oil pump has cut into the system, and warn the operator, that something is out of order.

I t is desirable to connect the low speed gear or driven machine lube system by means of a pressure switch to the turbine solenoid dump valve, this will cause the

I turbine low oil pressure tripping mechanism to shutdown the turbine if the low speed gear or driven machine oil pressure fails in spite of the fact that the two systems are completely separate.

Turbine control panel with alarm contact thermometers and gauges are recom-

r mended improving the overall system reliability and making for safe operation. Greater sophistication can be provided by the use of a multi-point indicator that gives

i continuous monitoring of temperatures.

1 C. APPLICATION AND SELECTION CRITERIA

I I . Mill Drives

(a) Turbine Horsepower The maximum output of the turbine is limited by the nozzle area and therefore, it has little overload capability unless specifically incorporated in the turbine in the form of extra nozzles.

The load on each individual mill varies in accordance with blanket thickness (Fig. 6), cane variety, fiber content, hydraulic pressure, and other factors of lesser importance. The amount of load variation is considerable, and the time between load changes quite short. Occasionally, high peak loads occur, probably due to a thick blanket of cane coming through. The turbine must be selected with sufficient nozzle area to take care of any probable peak condition.

. After careful consideration of all the data that we have gathered from our instal- lations, we have concluded that a turbine should be selected on the basis of 20 hp per short ton of fiber per hour per 3-roller mill. This should be the turbine nominal rating, especially if it drives the first mills. In the beginning of the tandem, for the first, second and third mills, we sometimes go as high as 24 hp per short ton of fiber per hour per 3-roll mill.

The power to drive a 2-roller crusher is approximately 60% of the power re- quired for the first 3-roller mill, or approximately 12 hp to 14.4 hp per short ton of fiber per hour.

The decision as to which of the two figures to use, or something in between, can

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be arrived at by examining what other equipment is installed ahead of the crusher, and also the hydraulic pressure of the mills. If, for example, two sets of Bnives, or a knife and shredder are installed, the preparation of the cane is much better and less load will be imposed upon the first two mills. Therefore, in a case like this, we can ~ safely use the figure of 20 hp per short ton of fiber per hour.

0 10 20 30 40 50 60 70 80 90

Short tons pressure 1 foot length of roll

Flg. 6. Relatlon between h p per short t o n fiber per hour and hydraulic pressure for one 3-roller mlll a t di f ferent rates o f blanket thickness.

As mentioned above, this should be the nominal rating of the turbine. In addi- tion, the turbine should be provided with enough nozzle area to enable the turbine to : (I) Talie care of excess starting torque in case the turbine has to be started with the

tandem fully loaded - such as is the case after a short stoppage. We recommend 100% starting torque capability at design steam conditions.

(2) Enable the turbine to carry its rated load with lower than normal throttle pressure, higher than normal back pressure, and fouled blading.

(3) Enable the turbine to carry froill 15% to 25% extra overload capacity at normal steam conditions.

We normally provide extra nozzle area for 100% starting torque, low steam pressure and a minimum o 15% overload capability. This extra nozzle area can be put under three or four hand valves so as not to impair turbine efficiency when not in use.

( b ) Gearing We have emphasized the necessity of rating the turbine for the peak conditions with ample overload capability, but do not think this necessary for the reduction gears. Gear teeth are designed on the basis of wear at a normal load and will withstand peak loads. Furthermore, according to AGMA standards (American Gear Manufacturers

S. VAN DER LINDEfJ 1 9 5

Association), high speed gears for mill drive applications have a service factor of 1.75 and low-speed gears a Service Factor of 1.50. This Service Factor is primarily for shock loads, but provides for overload capability as well.

We, therefore, judge that the gears can be selected and rated according to the turbine nominal rating - but not more th i6 20 hp per short ton of fiber per hour. The Service Factor automatically taltes care of overload capacity.

( c ) Speed In selecting turbines, another major factor is the r.p.m. We limit the maximum speed of a mill drive turbine to 5000 RPM. However, it would be an application error if the turbine, when first installed, is designed to operate at its inaximuin speed, it is advisable to fix the turbine speed corresponding to the present full load at some 10-

15% below the maximum allowable speed. In our case, it would be somewhere in the vicinity of 4000-4500 r.p.m. Future speeding up of the tandem for increased capacities can be done simply by raising the turbine speed on the governor, with the extra nozzle area, this increase can be achieved at no cost to the mill.

Constant flow 1000

800

600

... $ LOO 0 a 01 9 fl

200

0 0 1000 2000 3000 4000 5000

Turbine speed - r p. rn.

Fig. 7. Brake horsepower turbine 1-.p.m. at constant thro t t le flow.

The best approach is to design the turbine for future maximum speed and horsepower - if known - and, for initial operation, use the governor for down speed adjustment. Since at the reduced speed operation, the hp will also be less, the speed- 11p cl~aracteristics of the turbine must be designed carefully to match exact Inill requirements. Fig. 7 shows the relationship between BHP/Turbine r.p.m. at constant throttle flow.

( d ) Single Stage ov Multistage: Cost vs. Efficiency Turbines will operate well on the low pressure saturated steam that is used in so many mills. These pressures were dictated by the limitations of the reciprocating engines and steain pumps when the mill was first put into operation some 40 or 50 years ago.

Turbines, however, will operate equally well and give considerably better

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economy on steam pressures and temperatures much beyond the range of the recipro- cating engines. The stainless steel, so freely used in the blading, valves, and other parts of a modern turbine is very resistant to wear from either saturated or highly superheated steam. Therefore, the choice of steam pressure and temperature may be based solely on economic and heat balance considerations.

Either will give good mechanical performance and require little maintenance. Single stage turbines require up to 30% or more, more steam than a Corliss engine, at -

the same low pressure of approximately IOO to 125 psig. Multistage turbine steam consumption is as good as the Corliss even at throttle pressures as low as 85 or go psig. At around 125 psig, the turbine does better than the engine. At 250 psig, the turbines consume approximately half the steam quantity required by a Corliss operating at a low pressure. Steam consumption of reciprocating engines when given in pounds of steam per indicated horsepower must be increased by 11% to 15% for direct comparison with steam rates per brake horsepower hour.

The question of steam balance may very well rule out the single stage turbine. Generally speaking, single stage machines are used to drive knives or shredders, ind occasionally, mills and crushers if the horsepower is not above 500 hp. Above 500 hp, the steam balance of the house will usually dictate a multistage turbine.

Therefore, when considering a new boiler, it is worth while to look into possibility of going to a higher pressure. The trend in modern sugar mills is for hig pressures, up to 450 psig or even 850 psig. With such pressures turbine efficiencies as: dramatically improved resulting in considerably less steam requirements and boiler capacities and the turbines may be cheaper.

(e) Individual or Combined Drives Both types of installations have been made with complete satisfaction. Individual drives, whenever possible, are preferable because all the mills may have their speeds adjusted differently with respect to each other, which has proven to result in better milling. Table 2 illustrates a tandem with an average grinding rate of 222.4 metric tons per hour 11.76% fiber and 95.5% extraction rate.

TABLE 2

ILLUSTRATES A MILLING TANDEM WITH AN AVERAGE GRINDING RATE OF 222.4 METRIC TONS PER

HOUR, 11.76% F I B R E AND 95.5% EXTRACTION RATE -

Mill Size of mill Openings in inches Total Surface in inches Feed Discharge pressure speed

tons ft/min

In replacing a Corliss, this is usually not feasible because of the existing gearing which is retained. If, however, new mills are added to the tandem, or the existing

1; S. VAN DER LINDEN

(f) Governors In mill drive turbines, fast response governors are of paramount importance if a constant speed is to be maintained at the rollers. The load fluctuation is so dramatic that a slower response governor will produce severe turbine hunt, detrimental to efficient milling and turbine performance. "-*

This type governor is a NEMA Class D usually with a 65% speed range (I.e., a 3 to I speed ratio) ideally suited for the wide range of speeds at which mill drives are so frequently required to operate, and equipped for remote control.

CONTROLS - Centralized Remote Contvol Panel I

When several mill drive turbines are installed in one tandem, it may be desireable to have a centralized control panel which enables four variations of control: ( i ) Individual Speed Control - The panel mounted individual speed controls may be adjusted to operate each turbine at the speed which best fits the operating conditions desired. ( i i ) Master Speed Control - Grinding capacity can be adjusted instantaneously by changing the speeds of all turbines simultaneously. ( i i i ) Remote Stop and Restart Control - Each turbine is arranged so that it can be stopped and restarted from the panel. ( i v ) Emergency Stop - All the turbine emergency trip valves can be closed simul- taneously from the control panel by the use of the emergency stop button.

The entire Remote Control System is pneumatic. Our experience has proved that air operation provides the simplest and most reliable system available.

If a mill is contemplating the replacement of one engine now, a second engine in the future and possibly the addition of an extra mill, it is well worthwl~ile to plan the control system of the first turbine in such a manner that a centralized control system can be installed in the future, if desired, with only minor modifications.

(g) Special Controls ( i ) A i r Clutch - In modern sugar mill installations, it is getting increasingly popular to provide an air clutch between the high speed and low speed gear, in place of a flexible coupling. The slight extra cost of the air clutch is easily justified by the many advantages it offers.

Operated by air pressure, the clutch can be engaged and disengaged from the control panel, providing the following benefits: I. Ease of disconnecting the turbine from the mill. This feature is useful for emergen-

cy stopping, warming up the turbine and checking overspeed trip. 2. Adjustable slip joint permitting adjustment of torque to prevent breakage if the

mill becomes jammed. 3. Torsional vibration damping is inherent in the design of the clutch, protecting the . high speed components from mill vibration. 4., Flexibility equal to gear type flexible coupling.

, 5. Positive drive in case of air failure. The clutch controller can be mounted on the remote panel and not only permits

the engaging and disengaging operation of the clutch, but also controls the air pressure to the clutch. There is a straight line correlation between air pressure input and torque trahsmitted.

1588 ENGINEERING

( i i ) Remote Halzd Nozzle Colztrol - When steam is at a premium it is essential to operate the turbine at best efficiency at various 11p loadings. The most economical way of accomplislling this would be to open or close the hand nozzle valves provided in the steam chest to give optimum nozzle area.

Remotely operated nozzle valves with switches on the central control panel will permit the operator to open or close one or two nozzle valves and so pernlit opti- mum nozzle area to meet HP requirements of the mill.

Air head oil relay governors are very flexible and can react to any signal elec- trical or mechanical which in turn is converted to a 3 to I5 psig control signal. Through the use of additional computing relays, the whole milling tan4em including the cane carrier system can be automated to meet exacting milling requirements for maxikum extraction.

2. Turbine Gelzerators

Cane sugar factories throughout the tropical parts of the world have widely varying local conditions and there is no common pattern for steam/power ratio.

(a) Turbilze Generator Ty$e Most sugar factories use back pressure turbine generators, where there is steam and '

Horsepower load

Steam turb~ne performance curve- controlled extrachon

Fig. 8. Typical steam turblnes performance curve with controlled extraction.

power balance. Some have large irrigation pumping load in the off season when steam is not used in the factory. In other factories electrical demand is in excess of the power generated by process steam. In these cases back-pressure units alone will not be sufficient. This problem is overcome in some cases by running diesel alternator sets in parallel with the turbo-generator or operating in parallel with outside power

S. VAN DER LINDEN 1 9 9

supply. Others install an extraction condensing turbine. The extraction pressure is the process pressure, and additional steam passes to the condensing section to gener- ate the additional power required. Where no public electricity supply is available this type of turbine provides maximum flexibility to balance varying steam and power demands. During the off-season the set operates as a full condensing turbine with efficiency close to that expected from a turbine built strictly for condensing purposes. A typical performance would be as per Fig. 8.

The process pressure is kept constant by automatically controlled extraction valves located adjacent to, but following the extraction point. These valves similar to inlet valves are also under control of the governor and actually serve as inlet valves to the low pressure end.

The flexibility of this machine further permits the installation of a high pressure boiler with extraction pressure to suit old low pressure system especially where low pressure turbines of 125 to 150 psig are already installed.

Increasing electrical demands in sugar factories, and installation of 450 psig 600" F or higher boilers will require serious consideration of the extractioncondensing turbogenerators.

SIZES

Most turbo-generators presently being installed are multi-stage with outputs between 500 1cW and 5000 1cW and as large as 12500 1cW. Direct connected machines are pre- ferred for sizes 2500 1cW and up, however, geared units are being supplied for 3500 1cW with rotational specs below 5000 r.p.m., some manufacturers have speeds as high as 8000 r.p.m.

CONTROLS

Units up to 2000 kW can be provided with hand nozzle control, however, it is usually preferred that units form I500 kW to 7500 1cW be provided with automatic nozzle control. Where automatic extraction is required, the turbo-generators always have automatic nozzle control.

Turbo-generators are provided with oil relay governors within the limits of Nema Class "D". These are isochronous governors (zero droop) with dial adjustment to 6% or more. This droop can be changed while the machine is operating, for case of synchronizing and operating in parallel with other sytems or for load distribution between units, Figs. g and 10 illustrate. Assume now that unit A is set for no load speed 100% and 4% droop, while B is set for no load speed 100% but 8% droop (Fig. 9). In this case, when unit A is carrying 100% load and both units are therefore operating at 96% speed, unit B is carrying only half load. Unless the speed setting of 33 js changed, therefore, only three fourths of the capacity of the combined units (150%) can be used without overloading A. Suppose now the speed setting of B is raiked 4% (Fig. 10) with the load remaining fixed at 150% of the capacity of a single unit. The new load division will be 67% to A, 83% to B and the speed of the system will increase to 97 113%. Now, however, the load can be increased to a full zoo%, each unit then carrying 100% load at 96% speed.

0 50 100

Load, %

0 67 83 100

Load, %

Fig. 9. Units 100% no load speed; A 4% droop, B 8% droop a t 96% speed; A carries loo%, B 50%, maximum total load 150%.

Fig. 10. Unit B speed setting 104%. New load will be A 67%, B 83% system speed 97 113% at 96% speed, both units carry 100% load.

BACK PRESSURE CONTROL I I Acting through the speed governor pilot valve, the back pressure regulator controls the nozzle valves to pass steam flow according to process requirements allowing paral- lel operation with a Public Utility. The power generated depends on the steam fiow and the factory imports or exports power from Utility to meet steam/power balance. The regular speed governor acts only as overspeed governor. The back pressure can be easily disengaged for regular use of standard governor.

Mechanical or electrical load limit devices for control of amount of power exported or control of maximum turbine output are also provided.

GENERATORS

Generators are supplied in low voltage 480 V up to 2000 kW and high voltage 2400 and 4160 from 1500 kW and up. Geared generators running at speeds below I500 r.p.m. are generally open air cooled units, but 1500 r.p.m. and up are enclosed because of windage noise. All 2 pole generators are closed water to air cooled machines.

3. K n i f e and Shredder Drives

Steam Turbines are ideally suited for Cane Knife and Shredder Drives and may be of single or multistage construction. Fig. 11 shows Torque vs. Speed ax-motors and Steam turbines. As the speeds required for this application are normally 400 r.p.m. to 1200 r.p.m., the turbine is always provided with a gear and mounted on a single baseplate with integral lubrication system. Gears should have rating factors of 1.5 to 1.7 depending on ratio and hp.

H P REQUIREMENTS

Normal requirements can be determined from the chart Fig. 11 for knife sets running at 600 r.p.m. Approximately I+ to 2 hp per ton of cane per hour.

S. VAN DER LINDEF

0 10 20 30 LO 50 60 70 80 90 100

Synchronous speed, %

Fig. 11. Torque vs. speed a.c.-motors and steam turbine.A. Normal torque, low starting current, squirrel cage motor; B. Normal torque, normal start current, squlrrel cage motor; C. High torque, low starting current, squirrel cage motor; D. High torque, high slip, squirrel cage motor; E. Steam turbine with H P rating equal to above motors; F. Steam turbine with H P rating normal for cane cutter drive.

150 50 100 150 200 250 300 330 Short tons 1 h

1200 2400 480060007200n20 Short tons /day

Kn~fe horsepower

Fig. 12. Turbine speed us torque %.

I ' The load on cane knives is very erratic due to clumps of cane and provision

must be made in the turbine design horsepower to take care for peak loads. Generally a figure of 25% to 33% is sufficient and is provided by means of additional nozzle area under hand nozzle control. Turbines with two or three hand valves would have on; valve shut off to provide further additional nozzle area when low steam condi-

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1 throttle the steam flow and will increase the m number of hand valves open as per Fig. 12.

e oil relay governors resulting in faster response (isochronous ttle speed variation and less chance of clloking. Higher initial

nozzle ring pressure.

4, Boiler Feed Pum9 D~ives

Both the pump and the turbine driving the pump must be designed and built ,for reliable and continuous service as serious injury may result to the boiler due to failure of the pumping unit.

n \ 7

Feed water regulator Boller

L

Drop thru feed water regulator at full load

B

Bo~ler rl Pressure drop across feed water regulator constant at all loads

Fig. 13. Turbine equipped with constant speed governor only. A. Feedwater regulator control only. Here the feedwater regulator throttles all the difference between pump discharge and the required pressure. B. Feedwater regulator and excess pressure control. In order to avoid too wide a variation in the pressure drop across the feedwater regulator which will result in poor level control, an excess pressure regulator is frequently installed in series with the feedwater regulator. The excess pressure regulator may be arranged to maintain a constant pressure drop across the feedwater regulator.

S . V A N DER LINDEN I593

The turbine may run continuously, may be used as a standby or may be part of a dual driver unit.

Standby units start up when a quick opening valve opens ahead of the turbine, in the event of an electrical failure.

Dual drivers use turbine and motor or"&ither thru adjustment of a setting of the turbine governor by means of a speed changer.

Normally mechanical shaft governors are provided and may employ any of the following three types of controls.

C O N T R O L S

(I) Turbine equipped with constant speed governor only. Fig. 13 A and B. (2) Turbine equipped with constant pressure governor to maintain a constant dis-

charge pressure at all loads. Fig. 14.

Feed water regulator 'drop at full load

Flg. 14. Turblne equ~pped wlth constant pressure control. The feedwater regulator throttles a variable amount equivalent to the difference between the constant pressure and the requlred system curve less feedwater regulator drop.

Feed water regulator drop at full load

Excess pressure constant

kig. 15. Turblne equlpped with constant differentla1 pressure governor. The most colnmon form of excess pressure control on turbine dr~ven pumps. The feedwater regulator pressure drop 1s not constant, but rather malres up the difference between the total system pressure drop (less feed- water regulator) and pump discharge pressure a t variable speed.

(3) Turbine equipped with constant differential (excess pressure governor to maintain the pump discharge pressure at a pressure higher than the boiler pressure by a predetermined amount). Fig. 15.

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(4) Constant pressure drop across feedwater regulator control. Fig. 16.

5. Fan Drives

Forced draft fans ordinarily operate at relatively high speeds 1150 to 2200 r.p.m., which permits direct connection to turbine in most cases.

Turbine

Steam supply

Pressure drop across feed water regulator constant at al l loads

I Volume, %

Fig. 16. Constant pressure drop across feedwater regulator control. Similar to Fig. 13. B exc,ept that instead of throttling pump discharge, the governor controls the pump speed to establish a constant pressure drop across the feedwater regulator. Fig. 17. Methods of volume control. Forced draft fan; I, outlet damper control; 2, inlet valve control; 3, slipping motor step control; 4, variable speed control, turbine driven.

0 20 40 60 80 100 120 140 160 Boiler output % rated capacity

0 20 40 60 80 100 120 140 Brake horsepower, % +overload

Fig. 18. Variable speed operation of forced draft fan turbine driven. Fig. 19. Turbine design horsepower for induced and forced fan drives.

For normal boiler operation fans are designed for 75% to 85% of maximum fan volume to allow for reasonable factor of safetty for emergency conditions calling for extreme boiler output. I t is therefore advisable to design the turbine for best economy

--

S. VAN DER LINDEN I595

at normal rating point, using hand valve nozzle control for power loads above this point.

Induced draft fans operate at relatively low speeds, 400 to 1200 r.p.m. which require geared turbine drivers.

Induced draft fans will generally not come up to speed with cold air. Volume control of gases follow in general the accepted methods fo forced draft air volume control.

Same turbine design or selection applies here as for forced fan drives (Fig. 19).

CONTROLS

Constant Speed O$eration - Mechanical shaft constant speed governor with air control by manually automatically adjusted damper or inlet control hand speed changer can be used for plus or minus 10% adjustment in combination with damper setting.

VARIABLE SPEED OPERATION (MANUAL)

(I) Mecl~anical shaft constant speed governor, with speed varied by throttle valve in steam line. (2) Direct acting oil pump (DAOP) governor 3 to I speed range Nema A with speed varied by needle valve adjustment. (3) Remote Control - DAOP governor and motor operated needle valve from central push button station.

AUTOMATIC SPEED CONTROL

(I) ' Constant Speed Governor - combustion control equipment operates a chronome- ter valve or steam control valve in steam line. (2) DAOP variable speed governor, 3 to I range, with speed varied by combustion control and adjustment of needle valve in governor control system.

6 . Other P u m p and Auxi l iary Drives

Steam turbines are also ideally suited and employed for injection water pump drives - compressor or other auxiliary pump drives. Multistage turbines are applied for larger horsepowers and low steam consumption depending on steam conditions available. Constant speed governors are adequate of Nema A regulation, speed variation up to 20% is available.

D. INSTALLATION

After the proper horsepower and type of turbine are selected, it is equally important to insure that the turbines are properly installed and supplied with dry clean steam.

I. Feedwater

Sugar factories must have suitable feed and boiler water control, or else, large and costly shutdowns are inevitable.

I 1596 ENGINEERING I

The whole turbine installation can be a failure if piping is installed to impose an ex- cessive strain on the turbine casing. This applies to both steam and exhaust piping.

To avoid such a strain on the casing, certain rigid rules must be followed. The

limit the movement of a line to prevent excessive deflection at any point. A rigid support is not satisfactory where thermal expansion may cause the pipe to move away from the support.

Place corrugated expansion joint at the exllasut flange of the turbine then come down and out and connect to the exhaust header and anchor the exhaust header rigidly at this point. Then place an expansion joint in the header near this anchor point to permit the header to expand and contract.

expansion joints on either side of the anchor point. This should be done for each fur- bine connected to the header.

The use of expansion joints on the inlet are preferred and are the ideal method of preventing stresses and strains transmitted to the steam end. Where pipe sizes are small these could be deleted. However, i t is recommended that the turbine steam connection should be taken from the top of the supply header, and led to the turbine in such a way that there will be at least three right angle bends in the line. In this way,

the movement of the engine frame may be communicated to the turbine casing. It is advisable to always provide the turbine manufacturer with drawings

showing installed or proposed inlet and exhaust piping. There are limits to allowable forces and moments that can be exerted on steam turbines, and recommendations made by the manufacturer should be followed for proper operation of the turbine.

Pipework should be straight and short as possible, with adequate provision for thermal expansion, and minimum number of bends and fittings.

3. Condensate

It is very important to provide drains for the steam line above and also below the throttle valve, as vell as a drain for the turbine exhaust casing at its lowest point. These connections should drain automatically by using a colnbination of traps and hand controlled valves.

Normally, steam carries the condensate along the periphery of the pipe and it's removed by a steam separator or collected in water legs and trapped out. This works

If proper control is used, long and trouble free operation can be confidently expected from the turbines.

I

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Tro~~ble is, many lines are designed for a maximum but are used at reduced flow. Condensate will collect in the bottom of such lines because steam velocity is too low to carry it along. The writer has seen the finest grade of stainless steel blading erode badly in several seasons due to water in the steam.

a w

4. Slugs of Water

Slugs of water have damaged or destroyed a lot of 'steamdriven machinery. And a good many more units have shut down for inspection after catching a slug. Water slugsalways result in downtime, which costs money in any plant.

200

175

150

125 Excess or overload nozzle area ( handvalve )

100 Governor nozzle area

75 Kn~fe requlren~eni

25 0 o 0 25 50 75 100

Speed, %

Fig. 20.

If a water slug replaces the steam entering the nozzles at 150 fps, its velocity isn't increased by expansion so it leaves the nozzle at the same speed. Assuming the blading is moving at 300 fps the water can't enter and is carried around in the space between the blading and nozzle ring. Due to the blade's shape, the water slug becomes a number of small wedges that must be brought up to blade speed. As they are, the wedge shape plus friction of the water produces a heavy force that tends to push the blades away from the nozzle ring. This force causes a bending moment at the blade roots that may break them, and an excessive force on the thrust bearing that may damage it.

Due to lower velocity ratios and re-heat effect between stages, Multi-Stage turbines can tolerate certain amounts of moisture and water better than single stage units.

Separators are available for removing water from steam lines. Their effectiveness is limited by their size and capacity, plus good maintenance, of the traps you used.

We would strongly recommend that a centrifugal type of steam separator be used. In this type, the steam is made to whirl in a circular obit as it enters the separator and'most of the water is removed. I t is recommended that an atmospheric relief valve

I 1 i

1598 ENGINEERING

be placed between the turbine exhaust flange and the exhaust valve. I t is permitted that the pressure in the turbine casing build up to the safe limits of the strength of the casing so that this valve may be of small size.

6. Alignment

In erecting the turbines, extreme care should be taken in aligning the turbine to the high speed gear. I t is recommended theat they be lined up carefully when cold, but that the final aligning be done after steam is passed through it and the casing and piping is at operating temperature. After the high speed coupling is checked when all elements are at operating temperatures, the turbine should be aowelled, and not until then.

The turbine manufacturers always recommend a good grade of turbine oil of given characteristics, and their recommendations should be carried out to the letter, else bearing failures will result.

Mechanical drive turbines are extremely reliable and yet, once in a while, an obscure vibration sets up in the rotor. The cause is sometimes difficult to locate. The common causes are (I) pipe strain, (2) mis-alignment, (3) worn bearings, (4) water in casing, (5 ) sticking carbon rings, (6) bent shaft. Most of the usual causes of vibration may be corrected by the local forces possibly with the exception of a bent shaft

We would recommend that all turbines be furnished with steam gauges, (preferably recording) which show respectively initial pressure, ring pressure, first stage pressure, and exhaust pressure. Gauges are invaluable to tell what the turbine is doing, and also to locate trouble.

The use of individual flow recorders checked against first stage pressure of each unit will give an exact indication of turbine horsepower and a constant check of the turbine performance.

SUMMARY

This paper is divided into four main parts, the first discusses briefly thermodyynainic considera- tions that must be talren into account and be properly planned by the final user and the manufac- turer so that the equipment will perform a t conditions for most efficient and economical power generation. The second part discusses some design criteria that should serve as a guide to Sugar Mill Engineers in their effort to select reliable, rugged, and efficient turbine. The third part dis- cusses the applications of turbines for the various duties and services as found in Sugar Mills. The fourth part discusses installation.

REFERENCES

CITURCH, M. D. (1953) Steam Turbine Drives in the Sugar Mi11 Industry, Sugar. DODDS, R. B. (1956) You Can Keep Water Slugs Out of Machinery, Power. HOPPE, P. (1959) Fundamentals of Steam Turbine Control Systems, Power and Fluzds. HUGOT, E. (1960) Handboek of Cane Sugar Engzneering Elsevier, Amsterdam. STONE, E. F. (1959) A Problem of Steam Balance, Power alzd Fluids.