US3818699A

US3818699A - Feed and injection water control for steam generators

Info

Publication number: US3818699A
Application number: US00247279A
Authority: US
Inventors: E Pritchard
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-04-30
Filing date: 1972-04-25
Publication date: 1974-06-25
Anticipated expiration: 1991-06-25
Also published as: ES402265A1; GB1381908A; CA965657A

Abstract

A steam generator control system including a once through steam generator, a superheater thermostat sensing the temperature of superheater steam in the generator and controlling a fluid injection circuit connected in parallel to a portion of the steam generator coil to supply injection water to the coil and also, controlling the supply of feed water supplied to the steam generator, the feed water supply being controlled substantially proportional to the amount of exhaust steam issuing from a steam consuming apparatus together with extra water when called for by the thermostat, said control being provided by a positive displacement motor such as a rotary motor driven by the exhaust steam, and including auxilary governing means for the rotary motor to ensure more accurate proportionality between the speed of the rotary motor and the supply of feed water requirement to the steam generator so that full utilization of steam generator burner output is obtained for any given requirement.

Description

United States Patent [191 Pritchard 7 June 25, 1974 FEED AND INJECTION WATER CONTROL FOR STEAM GENERATORS [76] lnventor: Edward Pritchard, Unit 11, 176

Canterbury Rd., Bayswater, Victoria, Australia 3153 v 22 Filed: Apr. 25, 1972 211 Appl.No.:247,279

[30] Foreign Application Priority Data Primary ExaminerEdgar W. Geoghegan Assistant ExaminerH. Burks, Sr. Attorney, Agent, or FirmWenderoth, Lind & Ponack 5 7 ABSTRACT A steam generator control system including a once through steam generator, a superheater thermostat sensing the temperature of superheater steam in the generator and controlling a fluid injection circuit connected in parallel to a portion of the steam generator coil to supply injection water to the coil and also, controlling the supply of feed water supplied to the steam generator, the feed water supply being controlled substantially proportional to the amount of exhaust steam issuing from a steam consuming apparatus together with extra water when called for by the thermostat, said control being provided by a positive displacement motor such as a rotary motor driven by the exhaust steam, and including auxilary governing means for the rotary motor to ensure more accurate proportionality between the speed of the rotary motor and the supply of feed water requirement to the steam generator so 22 Claims, 6 Drawing Figures PATENTEUJUNZS 1974 I sum 1 OF 3 PAIENTEDJUNZEIBH V 3.818.699

SHEET 3 BF 3 CHA R66 cu 7- IN Q PUMPS (P, +1

021? area \i E Q IPUMP (P,

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FROM EXl/dl/ST MUTOQ IN TA HE 5705 71: o'urLer- 5/05 6 FEED AND INJECTION WATER CONTROL FOR STEAM GENERATORS This invention relates to the control of feed water and injection water flow into steam generators of the type known as flash boilers and once through steam generators having single or parallel coils. Fluids other than water can be used in similar vapour generators. It is pointed out, therefore, that fluid can be read for water and vapour for steam.

One of the main problems in the development of compact steam generators as used for automotive steam engine systems is in the control of the generator. A large percentage of experimental steam car projects have failed because of the inability of the designers to solve the control problem. The aim is to obtain reasonably constant steam pressure and temperature at the outlet of the steam generator. During normal operation over a wide range of loads, control should not be at the expense of a reduction in burner output which causes undesirable reduction in steam generator pressure, in order to maintain safe temperatures throughout the steam generator.

The principle of the once through steam generator appears deceptively simple. Water is pumped in at one end and superheated steam is led away from the other end. A survey of the rather voluminous patent literature on the subject of control systems shows, however, that a wide variety of control schemes are proposed. It is clear to the applicant from extensive experimental trials and an appraisal of prior proposals that the correct control of a steam generator is not obvious even to those supposedly skilled in the art. Some of the problems involved are:

CONTROL OF THE BURNER This is not a difficult problem. Quick response or feed back can be obtained with either pressure or temperature control.

CONTROL OF WATER SUPPLY This is a more difficult problem. Although, for exam ple, an increase in feed water supply will cause an almost immediate response on the feed water heating or economiser section of the steam generator in which the fluid is largely incompressible, there will be a delay in the feed back to a thermostat fitted to the steam generator in a steaming or superheater zone in which case there exists compressible fluid (steam) between the feed pumps and the thermostat.

SEVERAL PRIOR ART CONTROL SYSTEMS ARE REFERRED TO IN A GENERAL WAY:

a. Pressure Control of feed pumps (e.g., early White steam car). A disadvantage of this system was excessive blowing of the safety valve especially when delay in the pressure control caused too much water to be fed into the steam generator. The burner, under thermostat control, would endeavour to bring temperature back up, even at zero power output.

b. Temperature Control of feed pumps and water supply. Burner main control was usually a pressure type with an overriding high temperature cut-off. Some variations in temperature control systems as outlined in (b) are:

1. Final Thermostat type. Control thermostat is situated in the superheater zone. Disadvantage too much response delay.

II. Thermostat at end of evaporative zone. (e.g., British Patent 254,774, 1926, W. M. Cross.) Disadvantage Too much response delay. III. Thermostat(s) in evaporative zone. (e.g., US. Pat. Re 20045, 1936, J. Fletcher). Disadvantages Too much response delay. Also effected by inherent changes in boiling point temperatures with steam pressure changes. The latter applies in particular to automotive systems, where certain steam pressure changes take place in normal operation.

IV. Thermostat in feed water heating zone. Small response-delay with this system but thermostat is situated so far away from the final steam generator zone that poor control can result from secondary effects such as soot on generating coils modifying water-steam zone positions.

V. Thermostat plus water injector. In British Patent Specification 568,722, 1945, M. H. Lewis states that up to 5 percent only of total feed water capacity is fed to the water injector nozzle in the superheater zone. Otherwise there is a danger of a high temperature peak before the injector point. See later for argument showing that this amount would be insufficient to result in good control but that, in some systems, additional increments of base water can be fed equal to the quantity of water injected.

There have been tried and proposed various combinations of thermostatic and water injection systems. Estimates are made later which show that a thermostatic and injection system alone cannot provide sufficient basis for a correct control but, from certain considerations, can be used to control up to only approximately 65 percent of the total water. An additional form of control must therefore be provided.

Early systems using variable capacity vaporising burners proportioned water and fuel. (e.g., Serpollet, later White steam cars.) Note that, with the type of vaporising burners used, roughly proportional air-tofuel ratios were maintained. With modern pressure atomising burners, systems using variable fuel and air supplies are complex and relatively expensive, particularly as applied to small units.

There have been systems using auxiliary reciprocating engines or turbines driving feed water pumps (and other auxiliaries) in order to assist in matching water flow with burners demand. Some systems use water metering valves, sometimes dependent on hand adjustment. With some water injection systems, relatively large amounts of water, which are sometimes relatively cool, are injected into superheated steam causing thermal shock to the piping system. Thermal shock is a serious problem particularly under the difficult conditions encountered with the varying power requirements of an automotive steam power system where frequent operation of the injection control may be required. Thus with such systems it is undesirable to inject into the superheater zone.

Main engine-driven pump systems have the disadvantage that, at low speeds, particularly with a cold engine, the feed pumps pump insufficient water. Some prior systems are not fundamentally sound, in that they will not cope with a wide range of power demands. On some, to prevent local overheating, the burner is cut. This may reduce available power.

Applicants earlier Australian Patent No. 226,096, Improvements in Steam Plants for the Control of Plant Auxiliaries proportional to the Steam Consumed, stated that, preferably the drive arrangement according to the invention is operated in conjunction with conventional temperature actuated means (thermostat) controlling a secondary feed pump, which is cut-in to boost the primary pump, responsive to changes in steam temperature within the steam producing unit." In practice, such conventional means did not prove adequate. Very considerable experimental and theoretical work was carried out before the control system according to the present invention was evolved.

As described in Applicants earlier US. Pat. No. 226,096, water quantities bear a direct relation to exhaust steam quantities rather than to burner rates. This means that steam generator water control can be largely independent of burner operation. Thus, boosting of the burner will not directly effect the water control system.

It is a principal objective of the present invention to overcome the abovementioned problems and provide a steam generator control system in which the quantitative components affecting the operation of the system namely the feed water pump means, feed water injection system and burner are controlled.

It is a further objective of the invention to provide a steam generator control system in which the auxiliaries are driven by an improved proportional exhaust steam motor drive in combination with a water injection system in which feed water flow rates and injector flow rates are controlled within certain proportions calculated empirically.

It is a further objective of the present invention to provide a steam generator control system in which known definite quantities of water are automatically fed into the base of the steam generator coils by proportionally driven feed pumps or controlled metering means and known definite quantities of water are injected (as required) into a known desired evaporator zone point of the steam generator coil, thus resulting in a fast response and stable control with minimum thermal shock at the injection point, and enabling full utilisation, under normal operation, of the burner output for a given steam generator capacity.

There is provided according to the present invention a steam generator control system comprising a steam generator, a burner, a once through coil heated by said burner supplying superheated steam to a steam consuming apparatus, a superheater thermostat disposed on the coil at or near the outlet end of the coil in proximity to said burner arranged to sense the temperature of superheated steam, a water injection circuit connected in parallel to at least a section of said coil and arranged to carry feed fluid by-passing said coil section to inject said feed fluid into a zone of the coil carrying fluid of higher temperature, said injection circuit including valve and metering means for controlling flow of feed fluid therein, feed water supply means arranged to normally provide feed water at a rate below the requirements of the steam generator and to intermittently provide an increased flow of feed water when there is a flow of fluid in the injection circuit said increased flow resulting in a total feed water flow in excess of the requirements of the steam generator, a positive displacement motor operated by the exhaust steam from the steam consuming apparatus and arranged to control at least said feed water supply means at a rate substantially proportional to the volume of steam consumed by the consuming apparatus.

The superheater thermostat may be positioned anywhere in the superheater zone of the generator. In another aspect of the invention there is provided according to the present invention a steam generator control system comprising a steam generator, a burner, a once through coil heated by said burner supplying superheated steam to a steam consuming apparatus, a superheater thermostat disposed on the coil at or near the outlet end of the coil in proximity to said burner arranged to sense the temperature of superheated steam. a water injection circuit connected in parallel to at least a section of said coil and arranged to carry fluid from by-passing said coil section to inject said fluid into a zone of the coil carrying fluid of higher temperature, said injection circuit including valve and metering means for controlling flow of fluid therein, feed water supply means arranged to normally provide feed water supply means arranged to normally provide feed water at a rate in the range of 60 percent to percent of requirements of the steam generator and to intermittently provide an increased flow of feed water when there is a flow of fluid in the injection circuit, said increased flow resulting in a total feed water flow in the range of percent to percent of the requirements of the steam generator, the volume of fluid arranged to be injected by the injection circuit being in the range of 30 percent to 90 percent of the increase in feed water flow above that otherwise provided, a positive displacement motor operated by the exhaust steam from the steam consuming apparatus and arranged to control at least said feed water supply means at a rate substantially proportional to the volume of steam consumed by the consuming apparatus.

The feed water supply means may comprise a feed pump and associated metering means for providing feed water at the desired rate. Preferably the supply includes a feed water pump means driven by said positive displacement motor.

The output of the feed water pump means may be increased by increasing pump speed, increasing the stroke of the pump or by providing a stand-by pump.

Conveniently the feed water pump means includes a primary feed water pump arranged to continuously supply feed water to the steam generator whilst in operation and a secondary feed water pump arranged to intermittently supply feed water to the generator under control of said superheater thermostat.

The superheater thermostat is arranged to actuate said injection circuit valve means to allow fluid flow therein and to simultaneously actuate said secondary feed water pump to supply additional feed water to the steam generator coil. The injection circuit is arranged to by-pass a section of the coil, and preferably the inlet of the circuit is connected into the feed water heating zone of the coil and the outlet of the circuit is connected into a fast moving steam zone. The feed pumps are driven by a proportional drive so that good control can be obtained with the injection point located as far back as at a point in the evaporator zone of the steam generator, despite the difficult conditions encountered in an automotive steam power system having widely varying power requirements. It is preferable that the amount of feed water in the injection circuit is limited so as not to deplete the amount of water upstream of the injection outlet, thereby assisting in preventing the production of superheated steam upstream of the injection outlet.

The present invention allows close control over:

i. the amount of fluid injected by the injection circuit, and

ii. the amount of additional feed water administered by the secondary feed water pump thereby giving a rapid response to shortage of water signalled by the outlet thermostat in the steam generator. Furthermore, the additional feed water acts as a follow up to the rapid response provided by the injector.

It has been found that this injector-outlet thermostat system does give a rapid response to shortage of feed water, however fluctuations in the final steam temperature may still occur especially where the coil is composed of lightweight tubing having little heat reserve. To reduce these fluctuations even further, there is also provided by the present invention means for more accurately controlling the proportioning drive motor over its speed range by compensating for the effects of steam leakage and the effects of back pressures in the exhaust steam line to the drive motor at low and high motor speeds respectively. Said means includes a bypass valve or an electric/motor/generator controlling the speed of the motor over the middle or middle and high speed range. Conveniently the drive motor is a simple rotary motor.

The invention will now be described in greater detail having reference to the accompanying drawings.

FIG. 1 is a semi-schematic view of an overall steam plant showing various auxiliaries arranged in accordance with the present invention.

FIG. 2 shows a steam generator temperature curve of steam temperature against heat added to the steam generator.

FIG. 3 shows a feed water pumps performance curve of water pumped Q as a percentage of total weight of steam required R against power.

FIG. 4 also shows a similar curve of water pumped Q as a percentage of total weight of steam required R against power showing the effect of bypass correction by a meteringvalve of the feed pump drive motor.

FIG. 5 shows a curve similar to that in FIGS. 3 and 4 in which speed correction in the middle and high speed range of the feed pump drive is obtained by an electrical generator.

FIG. 6 shows a sectional view of a metering valve calibrated to relieve high back pressures in the exhaust steam line and also bypass some exhaust steam to provide the correction shown in the curve depicted by FIG. 4.

Referring to FIG. 1 a steam generator 10, is adapted to supply steam to an engine 11, by means of pipe 12 and throttle valve 13. A feed water system comprising a tank 19, a positive displacement pump 16, and feed water'pipe line 17, is adapted to feed lower tubes 18 of the steam generator through pre-heaters 20 and 20a disposed in the exhaust conduit 21 leading from the engine 11 and engine 24 respectively.

The feed water pump arrangement 16 shown in FIG. 1 includes an auxiliary or secondary feed water pump in parallel with a primary pump 16a, both pumps being preferably driven by the common drive 26. In this arrangement the secondary pump will run free while solenoid 43 is energized. Solenoid 43 is arranged to actuate an armature 42 constituting a valve controlling inflow of water to the pump 40 from tank 19. The current to the solenoid 43 is controlled by superheater thermostat 14.

Thermostats

14 and 27 are connected to pivotable arms 15 and 28 respectively arranged to actuate contactors 14b and 2712 between two way contact points 14a and 27a. The solenoid 43 is connected to the power supply through contactor 14b and is energised whilst the superheater thermostat 14 is sensing a temperature lower than a preset maximum.

If the preset maximum temperature is exceeded the arm 15 moves a sufficient distance to open the solenoid circuit and practically simultaneously close the injector valve circuit through the other contact point 140. The injector valve, 52 which is also preferably solenoid actuated as shown at 50, is energized, provided the safety thermostat contactor and contact 27a and b are in the normal position as shown. Safety thermostat 27 is arranged to sense overheating in that part of the steam generator coil which is connected in parallel with the injection line. Alternatively, instead of closing the injector line the water injection point may be temporarily varied to a position upstream of the normal point (not shown). Alternatively the safety thermostat 27 may be arranged to reduce or cut off (not shown) the output of the burner 31. Thus, if the safety thermostat 27 senses a temperature above a predetermined maximum the solenoid circuit 50 is opened by movement of contactor 27b away from contact 27a thereby opening the circuit to the injector solenoid 50 and causing the injector valve 52 to close injector line 51 and thus restore full feed to the by-passed section of the coil.

A manually controlled switch 58 is provided to control operation of the system. An outlet steam pressure switch 53 is provided which is arranged to open and close switch 54 and disconnect and reconnect the burner motor with the power supply, when steam pressure exceeds a predetermined maximum, or falls below a predetermined minimum respectively.

As a precaution the superheater themostat 14 is also arranged to control operation of the burner motor. This control is shown in FIG. 1 comprising a burner switch 55 controlled by arm 15 connected to superheater thermostat 14. The contacts of the burner switch are opened upon the superheater thermostat 14 sensing a steam temperature in excess of (by a predetermined amount) the temperature of the steam which causes the superheater thermostat 14 to open contact 14a. Thus, the opening of the burner switch 55 is a second stage operation which shuts off the burner 31 as a back up to the pressure switch control 53 and the introduction of injector water and additional feed water if (despite the introduction of additional water) the steam temperature continues to rise to an undesirable level.

The exhaust conduit 21 carries exhaust steam from the engine 11 to rotary motor 24 through heat exchanger 200 and thence to a condenser 23. Water from the condenser 23 is returned to feed water tank 19. The rotary motor 24 drives a shaft 25 which in turn drives feed water pump 16 through belt 26. The rotary motor is arranged to drive other auxiliaries such as condenser fan 33, motor generator 56 and the like. The condenser fan drive may come from either side of the one way clutch 57.

The motor generator 56 operates as a motor at starting primarily for driving the feed water pump 16. It may operate as a generator for, charging the battery power supply during normal running of the system and also may be used for a further useful purpose in governing the speed of the rotary motor. This latter purpose will be described in greater detail later. The one way clutch 57 is provided to transmit drive from the rotary motor to the motor/generator 56 and feed water pump 16 and condenser fan 33 when the rotary motor 24 is producing power but will not transmit drive from the motor/- generator 56 when operating as a motor, as at start thus avoiding unnecessary load on the motor/generator 56. The one way clutch 57a is arranged to free wheel and thus prevent the burner motor 36 from driving the auxilaries on the other side of the clutch 57a.

The present invention has analysed the operation of the various components of the above described system in providing a steam generator control system consisting of:

l. a burner preferably of the ON/OFF type primarily controlled by a device responsive to generator steam pressure, and also an overriding temperature controller responsive to steam temperature in the superheater zone.

2. an exhaust steam rotary motor system preferably driving two feed pumps.

Operation of the feed pump has already been described in which one pump 16a is operable to pump water whenever it is rotated whilst operation of the second, auxiliary, pump 40 is under the control of the superheater thermostat 14 in the generator coil 18. The arrangement is such that the superheater thermostat 14 also controls the flow of injection water through the parallel injector circuit 51 on the generator coil at the same time as the second feed water pump 40 is operating dependant upon normal temperature conditions in the bypassed section of the generator coil.

The analysis of the variable components controlled by the invention is best shown by reference to various equations as hereinafter described in which the following symbols will be used.

P, rate of water by weight pumped by first pump.

P rate of water by weight pumped by second pump.

EW (Extra Water) rate of water by weight injected through injector nozzle when water is flowing through injector circuit.

R (Requirements) rate of steam by weight passing out of the steam generator.

I. The first consideration to be outlined here is the amount of extra water EW to be injected as compared with the water pumped by the second pump P That is the ratio of EW to P It has already been mentioned above that one reason for the long delay in response of the superheater thermostat situated in the superheater zone to the change in feed water quantity into the base of the steam generator is due to compressibility of the fluid between these two points.

To illustrate a point, it could be said that the effect ofa change in base water feed is similar (in that part of the steam generator containing compressible fluid, i.e., steam) to that of a wave front carrying a higher level (high tide) or a lower level (low tide) of the density of the fluid behind the wave front.

The wave can be considered to be traveling at the speed of the actual fluid through the steam generator. In the evaporative steam zone where the dryness fraction of the steam is low, the velocity will also be low.

The response of a thermostat situated in the superheater zone to a change in feed from a water injector located at a point after which the steam is of a dryness fraction of 50 percent or more, or superheated, is rapid. In this case, the steam speed is relatively high and a short time period only is required before mixture is carried from the injector point to the thermostat. With such a rapid response, the thermostat control may turn water injection on and off rapidly enough such that there will be no resultant large fluctuations in the final steam temperature.

It has been found that an additional quantity of water, only up to a rate approximately equal to that fed by the water injector can be fed into the base of the steam generator in step with water injection fed directly from a feed water supply, by dividing the additional water fed into the base. Alternatively all additional water is fed into the base and injection water is obtained from a feed water heating economiser zone as shown in FIGS. 1 and 2. In either case the following discussion generally follows although it particularly applies to the first case in which the additional base water is divided.

The superheater thermostat l4 acts to regulate the quantity of injection water required and could be said to act as an early warning regulator on the amount of water entering the base of the steam generator. If, on the other hand, the increase in the amount of water fed into the base of the steam generator is greater than the injection water quantity, there is the likelihood that, when this increased flow comes throug to the superheater zone thermostat, it will be too much and it will be too late to turn it off soon enough to prevent an excessive down swing in final steam temperature followmg.

Thus the water control system operates as follows: P is always less than R and, from the above, the additional water feed into the base of the steam generator when the second pump is pumping, i.e. (P EW) must be equal or less than EW i.e.: P EW EW and EW 2 .50 P (I) II. Of major importance in the control of a steam generator is the ability of the system to control events following a change from the pumping of a smaller quantity of water (such as P to a greater quantity of water (such as P, plus P Consider the case where temperature is rising at the superheater outlet thermostat and the latter has caused the second pump and the injection water to be switched on. The flow of steam after the injection point in the steam generator must match R without waiting for additional feed from the base of the steam generator. The worst case would be where the flow in the steam generator just before the water injection point may have fallen to P, (low tide).

To satisfy the above, P EW 2 R. If this requirement is not met, in the above case, temperatures will continue to rise and the thermostat override control will shutoff the burner. This will lead to a loss of available power if the steam generator pressure has fallen into the range where burner operation is otherwise required.

In order to allow for such factors as steam generator thermal delay, P EW should have some margin over R, especially if a more rapidly fluctuating injection water control is required in order to assist in smoothing out fluctuations of water feed through the base of the steam generator. With 10 percent margin, P, EW 2 1.10 R.

In addition to the above a further factor must be considered in the case where an ON/OFF burner is used. Consider the case where the system is running at light load, the burner is operating and the second pump and injection circuit have been switched on by rising temperatures in the superheater thermostat 14. In the evaporative zone, a temperature change of from, say, 544F to 587F, i.e. 43F, is required to raise boiling point pressures from 1000 psi. to 1400 psi. at which later pressure it is assumed the burner would be switched off. The above temperature rise may be achieved with a corresponding temperature rise at the superheater thermostat of twice this amount i.e. 86F. (depending on steam generator tubing layout etc.). Now it is not desirable to have to set the temperature for operation of the burner over-ride control at a large amount above that temperature at which the control operates the second pump and water injector, in order that the burner over-ride will not operate under normal conditions.

Sufficient pressure rise throughout the steam generator can be obtained with a more moderate temperature rise at the superheater thermostat 14, if the quantity of water and steam in the steam generator is increased. Thus, if P, EW is increased to be greater than R by an additional margin, (i.e. feed will tend somewhat to better match the momentary burner rate rather than the steam output rate) satisfactory results may be achieved with closer temperature settings for the pump/injector control and the burner over-ride control.

Thus allowing the further margin for the ON/OFF burner system,

P, EW 2 1.20 R

Considering the above case but with a modulating burner which matches the load more closely, temperatures would not be expected to rise significantly with the two pumps and EW feeding with the relation P, EW 2 1.10 R. Thus it would be expected that satisfactory results would be achieved with the quantity P, EW less than for the case with an ON/OFF burner system. For an ON/OFF burner system at full load, in which water and burner rates are more closely matched, a relation similar to that applying to the modulating burner system would see applicable.

It should be noted that there are many factors which have some effect in connection with the above relation (2). The applicant has found, however, that experimental results do tend to support the above reasoning.

lll. To avoid internal steam generator temperature peaks, control should be exercised over the proportion of injection water provided.

Thefollowing method calculates the maximum rate of injection water EW injected so that the dryness fraction q of steam just before the injection circuit outlet is 100 percent i.e. just not superheated.

Having reference to FIG. 2 the full line I in the graph indicates water and steam conditions throughout the steam generator heating surface under steady conditions when all feed water is delivered into the bottom of the steam generator, and is equivalent to the burner evaporation capacity at the particular load. Note that a burner controlled on an ON/OFF basis can give roughly similar results in matching the load as a modulating burner. The dotted line II shows the variation from the above when total feed water pumped equals the burner capacity as before but part of the water EW" is taken from a feed water heating zone (as is good practice for injection systems) at point W" and injected into a point Z immediately after which the dryness fraction is 1 60 percent. Steady conditions are again assumed.

The percentage of heat received by water-steam following line I between points W where temp. 450F and Z l 1.2 28.8 40 percent, producing steam at q 60 percent at Z.

Considering unit weight of water/steam, the percentage of heat to produce steam at q percent from water at 450F l 1.2 48 59.2 percent of total heat added.

It can be seen that, if heat supplied to the steam generator section between W and Z remains constant, and quantity of water passing along this section drops in the ratio of 40 to 59.2 i.e., drops to 40 59.2 67.6 percent of its former value, steam of 100 percent will be formed just before A i.e., EW 100 67.6 32.4 percent of total water pumped. If EW 32.4 percent, steam will superheat just before Z.

It is possible to use an earlier injection point to enable EW to be greater. However, greater delay in response to the thermostat would occur. Conversely, with a later injection point, EW would have to be less but response delay would be less. It may be desirable to reduce EW to conform with the considerations discussed in paragraph I above. It is considered that the injection point shown is approximately at the optimum position.

It could be argued that a small superheat before Z could be tolerated. Care is needed if this is assumed for the design based on an ideal graph. The above examples assumes steady conditions and, in practice conditions are not steady. Variations can occur such as load changes which because of factors such as inertia in flow response to change of load, can lead to effects causing steam before Z to become wetter or drier (superheated) than estimated for steady conditions. A margin of safety is required over the ideal graphs shown for steady conditions. Thus, from the above considerations, it appears that the water injector control could control 2 X 32.4 64.8 percent only of the total feed water. An additional control system is therefore needed.

From the above calculations, it can be seen that, to avoid internal temperature peaks, base water f a .676 R. Since base water feed may, at times, approach P, (low tide) thus P, a .676 R.

Under some conditions, P, EW may be approximately equal to R, then P, z .676 (P, EW) from which EW s .48P,

Under conditions such as may occur immediately after start-up, the flow reaching 2" on the curve shown in FIG. 2 from the base of the steam generator, may temporarily be P,. The temperature before Z would be expected then to rise and the safety thermostat 27 (FIG. 1) would possibly operate.

IV. Considerations involving reductions in steam temperature fluctuations Some causes of temperature fluctuations in the steam leaving the steam generator are: (a) Response-delay in the superheater thermostat 14 in sensing the correctness of the mixture at Z, and (b). The magnitude of the error in the mixture reaching the superheater thermostat 14.

Assuming a fixed response-delay time, reductions in temperature fluctuations can be achieved by bringing P, closer to R and minimising EW and P Thus there is argument for P to be less than P, i.e. Pumps of different capacities, referred to in more detail later.

V. Consideration of the quantities and relationships between P, P and R as effected by Rotary Motor Characteristics The graph (FIG. 3) shows the effects of leakage and back pressure on the rotary/motor/feed pump/condenser-fan drive system. The effect of leakage is large at the low powers thus leading to low rotary motor speeds. The high back pressure of the fan, rising as the square of the speed, causes a rapid increase in back pressure required to operate the rotary motor at high powers again leading to reduced rotary motor speeds.

It can be seen from the graph, and using the simplifled considerations the useful range is that in which P, R and P, P R, it can be seen that difficulties in obtaining a useful wide range increase as P becomes small in proportion to P,. (See later for rotary speed correction devices which assist in overcoming this factor).

The above considerations, 1 to V are in themselves narrow ones. Account is not taken of such factors as failure of one pump, thermal storage in the steam generator tubes, changes of steam zone positions with changes of load, inertia of the steam generator contents in following load changes. Because of the changes in rotary motor system performance with load, P, will not bear a fixed relation with R, for example.

The steam generator system described in this specification, however, is protected by the action of a safety" thermostat and the superheater thermostat as well as a steam pressure switch as previously described. Rapid accommodation to load changes is made with the rapid action of the water injection control system.

Summarising the relations evolved above:

EW 2 .50 P

P,+EW 1.2 R

EW .48 P,

Using a system with the position of the water injection point "2 as shown in FIG. 2, (i.e. so that the dryness fraction after Z is 0.60 with feed of water matching output for steady conditions.) and using twin feedpumps so that P, P with EW .5 P relation (1) will be satisfied and relation 3 will be approximately satisfied. From relation 2 1.5 P, 21.2 Rand P, z .80 R

Note that, if EW increased, relation 3" is not satislied. This means that there is a possibility, under abnormal conditions, of a temperature peak before Z. The safety thermostat would operate if necessary but this may cause a more serious loss of good control than if EW was not increased. In the latter case, the superheater thermostat may reduce burner output if required under abnormal conditions.

Some more latitude can be allowed for EW in a system using pumps of different sizes. With P, 1.15 P from relation 3,

EW s .48 P,

.552 P Thus EW, may be from 0.50 to 0.552 P For EW 0.50 P from relation 2, P, a 0.837 R,

For EW 0.552 P,., from relation 2," P, 2 0.81 R.

EARLIER INJECTION, AND MULTIPLE INJECTION With water injection earlier than shown, (FIG. 2) EW can be safely increased and a larger margin of operation of P, as a function of R can be achieved. Response delay can be reduced by injecting through more than one injection point.

EXAMPLE First Injection Point such that, under steady conditions, with no water injection, dryness fraction of steam 0.50. Using a method similar to that used for finding relation 3, total EW s .66 P,. Half of EW can be injected through a second injection point after which, under steady conditions, dryness fraction of the steam would be, say 0.75.

METERING AND PROPORTIONING OF INJECTION WATER The injection water line 51 in FIG. I incorporates a metering jet which, in the preferred arrangement is the orifice of the solenoid control valve 52 see FIG. 1. This jet is designed to allow the passage of quantities of water equal to approximately 0.50 P or as calculated by the use of the above relations.

The method of estimation of the jet size may be as follows:

A percentage load is assumed and the corresponding pressure drop of the water and steam passing through the steam generator proper, between W and Z, FIG. 2. is calculated. The jet size is then calculated so as to pass the correct amount of water at the estimated pressure drop.

EFFECT OF LOAD CHANGE ON EW The pressure drop from W to Z will vary approximately as the square of the load."l'he weights of water and steam passing through the steam generator proper between W and Z and also through the water injector will vary but will remain approximately in the same proportions.

, EFFECT OF PRESSURE DROP ON EW At low steam generator pressures, such as may occur immediately after start-up, pressure-drops through the steam generator will be higher (for the same load) due to the lower density of the steam and the higher steam speeds. The proportion of water through the water injector will thus tend to rise. However, the action of the safety thermostat will protect the steam generator if there is any significant upward surge of temperature because of the above.

Referring to FIGS. 4 to 6, FIGS. 4 and 5 show curves indicating the effect of speed correction of the rotary motor 24 (see FIG. 1) in the middle of the'range where the speed of the rotary motor tends to be higher than required for proportional control of the feed water pump 16 compared with the steam requirement of the generator. FIG. 4 shows by the dotted line, correction by a bypass or leak valve which has the effect of causing the speed of the rotary motor 24 to remain more closely proportional to the steam requirement, substantially over the useful load range of the power unit.

FIG. 5 shows speed correction by the connection of motor/generator 56 (see HO. 1) into the rotary motor drive circuit. The motor/generator 56 when operating as a motor is controlled automatically so as to cause feed water to be pumped into the steam generator at a rate approximately equal to percent of the full load rate at such times as the steam generator pressure is substantially below normal and the steamtemperatures are above normal. These conditions may occur just after initial start up.

The generator of the motor/generator is operative to impose a torque load on the rotary motor in the middle speed range which is inherently reduced because of the lower torque demand of the generator at higher rotary motor speeds.

The generator may be of the third brush or constant current type and cut in of the generator at low speeds may be suitably delayed to reduce torque load on the rotary motor.

FIG. 6 shows a metering valve for positioning in the exhaust steam circuit in parallel to the rotary motor. The valve includes a chamber 60 having a piston 61 therein, the piston 61 is movable between two positions under the controlling influence of biasing

springs

62, 63 and steam pressure. The chamber is ported at 64 to allow leakage of steam past the piston 61 at a predetermined pressure in the exhaust steam circuit representing the middle speed range of the rotary motor, thereby bypassing the rotary motor with some of the exhaust steam. The position shown in FIG. 6 is an intermediate position.

With back pressure higher than those normally encountered at full load, such as short term exhaust pressure surges, the piston may take up an extreme position thereby by-passing a considerable amount of steam and relieving the pressure surge.

I claim:

1. A steam generator control system comprising a steam generator, a burner, a once through coil heated by said burner supplying superheated steam to a steam consuming apparatus, a superheater thermostat disposed on the coil at or near the outlet end of the coil in proximity to said burner arranged'to'sense the temperature of superheated steam, a water injection circuit connected in parallel to at least a section of said coil and arranged to carry feed fluid by-passing said coil section to inject said feed fluid into a zone of the coil carrying fluid of higher temperature, said injection circuit including valve and metering means for controlling flow of feed fluid therein, feed water supply means arranged to normally provide feed water at a rate below the requirements of the steam generator and to provide an increased flow of feed water when there is a flow of fluid in the injection circuit said increased flow resulting in a total feed water flow in excess of the requirements of the steam generator, a positive displacement motor operated by the exhaust steam from the steam consuming apparatus and arranged to control at least said feed water supply means at a rate substantially proportional to the volume of steam consumed by the consuming apparatus.

2. A system as claimed in claim 1 in which the feed water supply means includes feed water pump means driven by said positive displacement motor.

3. A system as claimed in claim 2 in which the feed water pump means includes a primary feed water pump adapted to continuously supply feed water to the steam generator coil in operation and a secondary feed water pump arranged to intermittently supply feed water to the generator as required.

4. A system as claimed in claim 1 in which feed fluid is taken from the feed water heating zone of the coil and injected into an evaporation zone of the coil by said injection circuit.

5. A steam generator control system comprising a steam generator, a burner located within said steam generator, a once through coil heated by said burner supplying superheated steam to a steam consuming apparatus, a superheater thermostat disposed on the coil adjacent the outlet end of the coil in proximity to said burner arranged to sense the temperature of superheated steam, a water injection circuit connected in parallel to at least a section of said coil and arranged to carry feed fluid into a zone of the coil, carrying fluid of higher temperature, said injection circuit including valve and metering means for controlling flow of feed fluid therein, feed water supply means arranged to normally provide feed water at a rate in the range of percent to percent of the requirements of the steam generator and to intermittently provide an increased flow of feed water when there is a flow of fluid in the injection circuit said increased flow resulting in a total feed water flow in the range of 1 16 percent to percent of the requirements of the steam generator, the volume of fluid arranged to be injected by the injection circuit being in the range of 30 percent to 90 percent of the increase in feed water flow above that otherwise provided, and a positive displacement rotary motor operated by the exhaust steam from the steam consuming apparatus and arranged to control at least said feed water supply means at a rate substantially proportional to the volume of steam consumed by the consuming apparatus.

6. A system as claimed in claim 5 in which the feed water supply means includes feed water pump means driven by said positive displacement rotary motor.

7. A system as claimed in claim 6 in which the feed water pumped means includes a primary feed water pump adapted to continuously supply feed water to the steam generator coil in operation and a secondary feed water pump arranged to intermittently supply feed water to the generator as required.

8. A system as claimed in claim 5 in which feed fluid is taken from the feed water heating zone of the coil and injected into an evaporation zone of the coil by said injector circuit.

9. A system as claimed in claim 3 wherein said superheater thermostat is adapted to actuate said injection circuit valve means to allow fluid flow therein and to substantially simultaneously actuate said secondary feed water pump to supply additional water to the steam generator coil.

10. A system as claimed in claim 7 wherein said superheater thermostat is adapted to actuate said injection circuit valve means to allow fluid flow therein and to substantially simultaneously actuate said secondary feed water pump to supply additional water to the steam generator coil.

11. A system as claimed in claim 1 in which the inlet of the injection circuit is connected to a feed fluid supply and the outlet of the coil is connected into a fast moving steam zone of the coil, the amount of feed water flowing the injection circuit being controlled to prevent depletion of water upstream of said injection outlet in the coil.

12. A system as claimed in claim 11 in which the inlet of the injection circuit is connected into the feed water heating zone of the coil.

13. A system as claimed in claim 5 in which the inlet of the injection circuit is connected to a feed fluid supply and the outlet of the coil is connected into a fast moving steam zone of the coil, the amount of feed water flowing in the injection circuit being controlled to prevent depletion of water upstream of said injection outlet in the coil.

14. A system as claimed in claim 13 in which the inlet of the injection circuit is connected into the feed water heating zone of the coil.

15. A steam generator control system comprising a steam generator, a burner located within said steam generator, at once through coil heated by said burner supplying superheated steam to a steam consuming apparatus, a superheater thermostat disposed on the coil adjacent the outlet end of the coil in proximity to said burner arranged to sense the temperature of superheated steam, a water injection circuit connected in parallel to at least a section of said coil and arranged to carry feed fluid bypassing said coil section to inject said feed fluid into a zone of the coil, carrying fluid of higher temperature, said injection circuit including valve and metering means for controlling flow of feed fluid therein, feed water supply means arranged to normally provide feed water at a rate in the range of 60 percent to 90 percent of the requirements of the steam generator and to intermittently provide an increased flow of feed water when there is a flow of fluid in the injection circuit said increased flow resulting in a total feed water flow in the range of l 16 percent to 180 percent of the requirements of the steam generator, the volume of fluid arranged to be injected by the injection circuit being in the range of 30 percent to 90 percent of the increase in feed water flow above that otherwise provided, a positive displacement rotary motor operated by the exhaust steam from the steam consuming apparatus and arranged to control at least said feed water supply means at a rate substantially proportional to the volume of steam consumed by the consuming apparatus and rotary motor speed control means compensating for the effects of steam leakage at low exhaust steam pressure and controlling excessive motor speed at high steam pressures.

16. A system as claimed in claim 15 comprising auxiliary drive means including an electric motor for driving said rotary motor during periods of exhaust steam shortage to compensate for the effects of steam leakage thereby assisting to maintain a feed water supply.

17. A system as claimed in claim 15 in which said speed control means includes an electric generator driven by said rotary motor operative to impose a torque load on the rotary motor in the middle speed range, said torque load being reduced as an inherent characteristic of the generator at higher rotary motor speeds, said varying torque load being operative to maintain a feed water supply substantially proportional to the feed water requirements of the steam generator.

18. A system as claimed in claim 15 including a bypass governing valve positioned in an exhaust steam circuit parallel to said displacement motor operative to maintain a predetermined level of steam pressure flowing to said motor in instances of excessive exhaust steam pressures by by-passing excess steam thereby assisting to maintain a feed water supply substantially proportional to the feed water requirements of said steam generator.

19. A system as claimed in claim 15 in which said displacement motor is a rotary motor and said by-pass governing valve includes a chamber and a piston slidably mounted therein movable under the controlling influence of mechanical biasing means and exhaust steam pressure, said chamber including at least one steam port controlled by movement of said piston, the arrangement being such that during periods of excessive exhaust steam pressure and excessive rotary motor speeds said piston overcomes the mechanical biasing means to uncover said port and by-pass steam from said rotary motor thereby maintaining the speed of the rotary substantially proportional to the feed water pump requirement.

20. A system as claimed in claim 1 in which the feed water supply means includes a primary feed water supply means and a secondary feed water supply means wherein said primary feed water supply means is adapted to provide less than the normal feed water requirement of the steam generator and said secondary feed water supply means provides additional water to a maximum amounting to substantially twice the amount of water injected through said injection circuit.

21. A system as claimed in claim 1 in which the feed water supply means includes a primary feed water supply means and a secondary feed water supply means wherein at any given time the feed water mass/second supplied by the primary feed water supply means plus the feed fluid mass/second supplied through the injection circuit to the steam generator is equal to the product of the mass output/second of the steam generator at that time and a factor of at least about 1.1.

22. A system as claimed in claim 1 wherein said feed water supply means includes a primary and secondary feed water supply means and in which the extra water supplied through the injection circuit amounts to a maximum of about 0.5 of the amount of feed water supplied by the primary feed water supply means.

Claims

1. A steam generator control system comprising a steam generator, a burner, a once through coil heated by said burner supplying superheated steam to a steam consuming apparatus, a superheater thermostat disposed on the coil at or near the outlet end of the coil in proximity to said burner arranged to sense the temperature of superheated steam, a water injection circuit connected in parallel to at least a section of said coil and arranged to carry feed fluid by-passing said coil section to inject said feed fluid into a zone of the coil carrying fluid of higher temperature, said injection circuit including valve and metering means for controlling flow of feed fluid therein, feed water supply means arranged to normally provide feed water at a rate below the requirements of the steam generator and to provide an increased flow of feed water when there is a flow of fluid in the injection circuit said increased flow resulting in a total feed water flow in excess of the requirements of the steam generator, a positive displacement motor operated by the exhaust steam from the steam consuming apparatus and arranged to control at least said feed water supply means at a rate substantially proportional to the volume of steam consumed by the consuming apparatus.

5. A steam generator control system comprising a steam generator, a burner located within said steam generator, a once through coil heated by said burner supplying superheated steam to a steam consuming apparatus, a superheater thermostat disposed on the coil adjacent the outlet end of the coil in proximity to said burner arranged to sense the temperature of superheated steam, a water injection circuit connected in parallel to at least a section of said coil and arranged to carry feed fluid into a zone of the coil, carrying fluid of higher temperature, said injection circuit including valve and metering means for controlling flow of feed fluid therein, feed water supply means arranged to normally provide feed water at a rate in the range of 60 percent to 90 percent of the requirements of the steam generator and to intermittently provide an increased flow of feed water when there is a flow of fluid in the injection circuit said increased flow resulting in a total feed water flow in the range of 116 percent to 180 percent of the requirements of the steam generator, the volume of fluid arranged to be injected by the injection circuit being in the range of 30 percent to 90 percent of the increase in feed water flow above that otherwise provided, and a positive displacement rotary motor operated by the exhaust steam from the steam consuming apparatus and arranged to control at least said feed water supply means at a rate substantially proportional to the volume of steam consumed by the consuming apparatus.

15. A steam generator control system comprising a steam generator, a burner located within said steam generator, a once through coil heated by said burner supplying superheated steam to a steam consuming apparatus, a superheater thermostat disposed on the coil adjacent the outlet end of the coil in proximity to said burner arranged to sense the temperature of superheated steam, a water injection circuit connected in parallel to at least a section of said coil and arranged to carry feed fluid bypassing said coil section to inject said feed fluid into a zone of the coil, carrying fluid of higher temperature, said injection circuit including valve and metering means for controlling flow of feed fluid therein, feed water supply means arranged to normally provide feed water at a rate in the range of 60 percent to 90 percent of the requirements of the steam generator and to intermittently provide an increased flow of feed water when there is a flow of fluid in the injection circuit said increased flow resulting in a total feed water flow in the range of 116 percent to 180 percent of the requirements of the steam generator, the volume of fluid arranged to be injected by the injection circuit being in the range of 30 percent to 90 percent of the increase in feed water flow above that otherwise provided, a positive displacement rotary motor operated by the exhaust steam from the steam consuming apparatus and arranged to control at least said feed water supply means at a rate substantially proportional to the volume of steam consumed by the consuming apparatus and rotary motor speed control means compensating for the effects of steam leakage at low exhaust steam pressure and controlling excessive motor speed at high steam prEssures.

18. A system as claimed in claim 15 including a by-pass governing valve positioned in an exhaust steam circuit parallel to said displacement motor operative to maintain a predetermined level of steam pressure flowing to said motor in instances of excessive exhaust steam pressures by by-passing excess steam thereby assisting to maintain a feed water supply substantially proportional to the feed water requirements of said steam generator.