US2372087A

US2372087A - Boiler feed pump control

Info

Publication number: US2372087A
Application number: US460135A
Authority: US
Inventors: Igor J Karassik
Original assignee: Worthington Pump and Machinery Corp
Current assignee: Worthington Pump and Machinery Corp
Priority date: 1942-09-29
Filing date: 1942-09-29
Publication date: 1945-03-20
Anticipated expiration: 1962-03-20

Description

Fiq.

I. J; KARA'SSIK' BOILER FEED PUMP CONTROL Filed Sept. 29, 1942 3 Sheets-Sheet l m fiunnu nl 1 I I IIIAIIIIIII lqnr J. Karafisik ENTOR March 20, 1945; J. KARASSIK BOILER FEED PUMP CONTROL 3 Sheets-Sheet 2 5 PurnP' Filed Sept. 29, 1942 a l ,E FuE EmLEMF 4 m 2 F E E w a m m w M W W 2 mm M W E D-LH a J V E r arm n 1+ P z D W M H T 2 .m m 5 F H mm MD.

Closed Heater Ficp t Dir: :t Ennfaci Heater lqur J. Karassik ENTOR ATTORN Y March 20, 1945, l. J. KARASSIK BOILER FEED PUMP CONTROL Filed Sept. 29, 1942 3 Sheets-Sheet s arca m mmzmuznu lEnn nmm L m ummnm m m m LE: mm L u L m umm m m mzmu L mm ummEu Lm m ummu m 3 Lmmcmucnu z m mE L Ll. Karassik NTOR ltlnr ATTORNE Patented Mar. 20, 1945 2,372,087

UNITED STATES PATENT OFFICE BOILER FEED PUMP CONTROL Igor J. Karassik, South Orange, N. J., assignor to Worthington Pump and Machinery Corporation, Harrison, N. J., a corporation of Delawar Application September 29, 1942, Serial No. 460,135 14 Claims. (Cl. 122-451) This invention relates to the automatic control the features forming the invention will be speof boiler feed systems, and of centrifugal boiler cifically pointed out in the claims,

water entering the boiler feed pump and the vafeedwater, and consequently the vapor pressure feed pumps. More specifically, the invention In the drawings: contemplates the control of the temperature of Figure 1 is a diagrammatic, drawing showing a the boiler feedwater entering a boiler feed pump, Control mechanism employing h f t es o of the pressure of the feedwater at its point of p e in t n. entry into a boiler feed pump, or of both of the re 2 i dia ram at c d w gsh me foregoing, for the purpose of maintaining aminie pp c of the present inventlon o a mum difference between the pressure and the tfeed v vapor pressure corresponding to the temperature Flgure 3 is a dlaglammatlc drawlng ShOWmg of said entering feedwater for preventing flashanother form of employing the features of ing of the feedwater into steam with the resultant the Present invention h damage to the pumping system and interruption Flgure 4 is dla'gramma drawmg s Owmg of the pumping action a thepresent invention as applied to another type An object of the present invention is the proof boflerrfeed system vision of a mechanism and method of control r 5 e t i m fi i which will provide an impulse when the margin the present invention hasi app ed am close d or differential between the pressure of the feedboner Bed System em 0 ymg vana e spee boiler feed pump.

' Referring more specifically to the drawings, in

Figure 1 is shown an electrical type control mechanism A embodying the principles of the present invention. In this type of control, the instrument is actuated by the relative positions of two 225" indicators, one of which converts temperature measurements into corresponding vapor pressure, while the other reads the actual pressure either at the point where the temperature measurement is taken or at the critical point where the differential between the vapor pressure and the absolute pressure of the feedwater is desired for preventing flashing.

Temperature or pressure changes which would reduce the margin between the absolute pressure 5 of the feedwater and the vapor pressure below the predetermined minimum actuate the control mechanism. and open or close contacts or relays in the electrical control circuit. This control is made up of two separate pieces of mechanism, the

.10 first of which is used to measure pressure differpor pressure corresponding to the temperature of such entering feedwater reaches a predetermined minimum at which flashing will successfully be avoided.

The impulse provided by the mechanism and method of control of the present invention may be utilized in several different manners to provide the result sought, depending upon the type of boiler feed system to which the invention is applied.

In systems wherein the pressure at the point of entry of the feedwater into the boiler feed pump is fixed or not subject to control, the present invention may be applied to provide an impulse which will effect the control of the temperature of the thereof, for maintaining the necessary differential between such vapor pressure and; the pressure of the feedwater at its point of entry into the boiler feed pump for preventing flashing.

In so-called closed boiler feed cycles, the i ences', and the other to translate these differences pulse provided by the application of the presen into the motions required to open or. close certain mventlon may be used to control the pressure of control valves or otherwise effect changes in the the boiler a f e thereby maintaining e boiler feed system for controlling the absolute necessary mmlmllm margln between Su p pressure or the temperature of the feedwater, as

e and e vapor pressure corresponding to the will be hereinafter described in connection with temperature of the feedwater and preventing the application of the mechanism flashmg of the feedwater- In Figure 1, a temperature sensitive bulb is In some systems, a combination of both of the used t measure the temperature t t point 1 above methods of control effected by the present all ired, transmitting variations of internal pressure invention may be employed, bo e ods being due to temperature changes through the capiloperatcd y e Same Impulse mechanism, as lary tubing 2 and a specially constructed and caliheremafter descrlbed. brated bellows 3 to a movable indicating pointer With these and oth r o je s in w, as lrn r 4. This is accomplished by filling the bulb I ppea from the accompanylng Specification, e with a liquid or gas so arranged that the temperainvention consists of various features of constructure changes at the bulb cause changes in the tion and combination of parts, which will be first internal fluid corresponding to changes in the described in connection with the accompanying pressure at which the feedwater would vaporize drawings, showing a boiler feed pump control of corresponding to its temperature. The bellows the preferred form embodying the invention, and 3 *is "so calibrated that the attached pointer 4 moves along an indicating scale 5'which reads the vapor pressure corresponding to the temperature directly-mp, s. i;. A, further feature of the pointer travel is that the amount oftravel per unit of pressure change is independent of the absolute value of the actual vapor pressure measured.

A similar pointer 6 is'connected to the absolute pressure measuring device 5 and. IQQSSQsses the. same features with respect to, travel per unit. of pressure change. This device-.lfincludes-a bellows 8 connected by tubing 9 to the point where the pressure measurement is talgemor tothecritical. point where the margin of difference is required.

The distance along the scale 5 between the two point r 4 d. 6. represents, at. all temperatur s. and p ssu s the dif ereneeb weeh ther por pressure corresponding to thewtehlperature of he eedwater at h ri ical point and t e. abr iutepressu e atsu h point- An extension arm 4a is attached; to. the vapQr press re, ndicating p nter 4,, the. e gth i. wh p s nt he min mumpermissible.pressure iie ferential between the absolute pressure available and the, apor pr ssure... electri a rcuit. in is established, withint r d al;connections H, and. I2 to each of the two pointersi land .5,.. .espec,-. tively. ele e as the ava la le. p ss re. d flernce remainsih excess f thepredeterminedmim imum diiferential, the electrical circuit will. re, ma n. open, but up n l wer hsorthe pr hre i rehce to, th minimum. ermissible. aspre e: termip d; b he en th. o the or extension. 4a, this extension will contact the. pointer Sand, cl se the. lectr cal circuit. ll. In iieet, h echanism ould. pert he. tunetip o a Wit actuated, b thev difie i hce between. ahse:

.lulie pr ssure ndvap r ressure.

It ist he.unde stoodthat heah re s nly one. f. several. po sible app cation of. the principl f t e resehtipvehtion vthat.otherv construe: tions a e. possib e, to. provi e the des r d. r sult w thout departing ro. i renti.o

The openin andlclosipe. at he eleetiiea l th wit. I 0.. maybe employed. for ct ing asolenoid operated or motor driven modulating valye of; any. approv d. types which. may. be purchased. upon the. market. and whic may. e, used as a pilot. alye o aprepo ticnihevalve o e ntrolth eedater system. n F ure t e, valv structure 31is sho n as. as leh i operat d va rapt an desired type which ma be. ur hased. upon ehesp ritp the present.

valve 28 is interposed in this bypass line 26 for controlling the flow of feedwater through this line 26.

This Valve 28 is shown in Figure 2 as a solenoid operated valve of any approved construction, controlled by a control mechanism A such as is illustrated, inFigure 1. It is to be understood however that the mechanism illustrated in Figure 3 couldalso be utilized for the purpose in this system.

If Q1 is the flow through the main suction line 24, Q2 the flow through the auxiliary line or bypass 26, and Q3 the sum of Q1 and Q2 (or the total flow to the boiler feed pump), the temperature T3 of the mixture is represented approximatelr he i l i e ati n;

. and Pv the vapor pressure of the feedwater at tionately controlled but when employing the boiler feed system wherein th e boiler f eed akes. s suction from. a. dire t. ontac edater. hea er 22 hrou h. the ma n feed. uc ion. e 2 This inel2 i .a e.s..the. dwater. ate. t mp r tur Itwhichhasa crres -clin a or. ess re v l he p ssur at heisu t phififef. h o l r feed; ump, s 133- An pxi iar eed. lineor. bypass fi isipro ded,leading directlx r t e. dis har e. Qt. hev condensate. pum into, the main eedlipe, .4 beyond the-, e water heater 2,. p ov din a b pass. arquhclthe fe dwat r. heat.- r. 2: T eemperature. of the feedw ter this. auxi iary li e 6. is 12, and, lower than. that. the line 2. beyond the; eclw terhea er 2.2,- A

structure shown in Figure 3, the control is proportionate.

The prime requisite of the arrangement is to maintain a given minimum excess of pressure at the boiler feed pump suction over the vapor pressure PV3. In other words, to maintain the following relation:

.3%P l pl a constant er comes generally from an extraction sta e of the main turbine and when the electrical load undergoes a sudden drop, this latter is accompaniedby a sudden drop in steam pressure at theheaterwithout a corresponding temperature drop atthe pump suction. A safety factor must therefore be provided to take care of the time lag which exists between a sudden reduction of pressure in the heater and the ultimate reduction, oftemperature at the pump suction.

Iii-would bepossible to install both a thermometercalibrated to read directly the vapor pressure eOrresponding; to. the temperature T3 and a pressure gauge; at the pump suction for reading the pressure P3. The position of the valve could then. be varied manually in such manner as to fulfill the requirements of the equation (2) above. This however would be bothtedious and impracticaland the, object of the present invention isto provide such a result automatically. In other words, when the difference between the vapor pressure at the; critical, point 25 and the absolute pressureat the critical point 25 falls below the desirable minimum, the extension contactarm 4a of the. control mechanism A will engage the pointer 6 and close the electrical circuit Ill through the solenoid valve structure 13. opening the valve 28 to permit a predetermined quantity of feedwater to flowv from the. condensate pump 2'! directly to. the. boiler feed pump 21,,bypassing the feedwater heater 22 and con: sequently lowering the. temperature of the feedwater at. the critical point, or at the suction of the, boiler feed pump 21., raising the pressure dif- I the boiler feed pump 3|.

ierential between the vapor pressure and the absolute pressure and preventing flashing of the boiler feedwater in the boiler feed pump. I

'In Figure 3 of the drawings a. spring-loaded diaphragm type valve is employed for obtaining the same control as provided by the mechanism shown in Figure 1. In this structure the stem l5 of the valve I4 is connected at its upper end to a diaphragm [6. The chamber lBa on one side of the diaphragm it is subjected to the absolute pressure of the feedwater at the critical point by means of the tubing 11, while the chamber l6b on the other side of the diaphragm I6 is subjected to a pressure corresponding to the vapor pressure at the temperature being measured by means of a temperature sensitive bulb [9 located at such critical point and connected to the diaphragm casing by the capillary tubing l8. This bulb I9 is similar to the bulb I employed in the structure shown in Figure 1 of the drawmgs.

A loading spring is provided which exerts a constant pressure upon the diaphragm [E in opposition to the absolute pressure exerted thereon. The tension or compression of this spring 20 is-determined in accordance with the desired minimum pressure differential or margin. When the vapor pressure rises to a point where it, plus the pressure exerted by the spring 26, is greater than the opposed absolute pressure, the valve H5 will open. When the sum of the vapor pressure plus the constant pressure as exerted by the spring 20 is less than the absolute pressure, the valve will close. In fact, as the sum of the vapor pressure and the constant pressure of the spring approaches the absolute pressure, through the gradual rising of the vapor pressure due to the increase in temperature, the valve will begin to open, and a proportioning effect'is, thus provided.

It will be seen that the valve will be opened only i when the difference between the vapor pressure and the absolute pressure ,is less than the predetermined minimum, as represented by the constant pressure of the spring 20. This type of mechanism is especially suitable for use as ,a proportioning valve to be employed. in systems where such type of control is adaptable.

In Figure 4 is illustrated a boiler feed system wherein the feedwater is drawn from a direct contact feedwater heater 3'! through a suitable line a by the suction of the multi-stage boiler feed pump 3|, and is then withdrawn from an intermediate stage. of the boiler feed pump through the line 32, heated in a closed feedwater heater 33, and returned to a following stage of the pump 3| through the line 34.

In this'type of system there is always available feedwater at a temperature lower than that prevailing at the given point of reentry of the feedwater into the boiler feed pump. The impulse provided by the control of the present invention is thus used to control the admission of such colder feedwater through an auxiliary feed line, the final temperature of the resulting mixture being reduced sufficiently to permit satisfactory operation of the boiler feed pump.

In this system shown in.Figure 4 an auxiliary pipe or bypass is provided between the discharge pipe 32 from an intermediate stage .of the boiler feed pump 3| to the feedwater heater 33 and the return pipe 34 to a succeeding stage of A diaphragm operated valve I4 of the type shown in Figure 2 of the drawings is located in this pipe or bypass 35.

Tubing l8 connects the chamber lBb on one side.

of the diaphragm IE to a temperature sensitive bulblocated' in the return pipe 34, while tubing i1 connects the chamber 16a on the other side h of the diaphragm I6 to the boiler feed pump 3! at a lower stage.

Generally the pressure at the intermediate stage of the boiler feed pump from which the feedwater is withdrawn for heating is suificiently greater than the vapor pressure corresponding to its return temperature, and the application of the control mechanism here is not for the purpose of establishing a relationship between these two pressures. However, through the design of the boiler feed pump 3|, some of the feedwater returning may be permitted to leak into a lower stage of the pump, the pressure of which may fall below the vapor pressure corresponding to the temperature of such returning feedwater. The control is employed to prevent this possibility.

I the event that the pressure in the lower stage of the pump 3! to which the tubing 1! is connected falls below the vapor pressure corresponding to the temperature of the feedwater being returned to the pump 3| through the line or pipe 34, the decrease in such pressure will cause movement of the diaphragm It to open the valve l4 and allow bypassing of the feedwater around the feedwater heater 33 directly to the stage of the boiler feed pump 3! to which the feedwater returns from the feedwater heater 33.

thus lowering the temperature in this stage of the pump to prevent flashing, and reestablishing the desired pressure difi'erential, which will in turn cause operation of the diaphragm I8 to close the valve I4. I

In both of the systems just described and illustrated in Figures 2 and 4 the principle of the present invention is employed for controlling the temperature of the feedwater for maintaining the required differential. In Figure 5 of the drawingsthe principle-of the present invention is applied for controlling the absolute pressure of the feedwater, but for accomplishing the same result, i. e. the maintenance of the minimum differential between the absolute pressure and the vapor pressure corresponding to the temperature of the feedwater to prevent flashing in the boiler feed pump.

In the system shown in Figure 5 the use of the impulse of the present invention to control the pressure at the point of entry of the feedwater into the boiler feed pump is shown in a so-called closed feed cycle, wherein several feed pumps are operated in series with closed feedwater heaters interposed between successive pumps, one or more of such pumps being operated at variable speed. Normally the variable speed operation is restricted to the low pressure unit in such a system, the controlling factor being the total flow to the boiler and the controlling mechanism either an excess pressure or a constant pressure regulator, which is used to vary the speed of the centrifugal pump, which may be driven by a steam turbine, a variable speed electric motor, or by a constant speed prime mover through a variable speed coupling.

Such means of control is obviously unaffected by the relationship between the temperature and the pressure at the suction of any one of the pumps, and conditions might arise where the speed regulation lowers the suction pressure of any given pump dangerously close to or even below the vapor pressure at the suction of such The system shown in Figure 5 includes a constant: p d ondensate nmn 44 ollow d by a. closed feedwater heater; 4!, variable speed; primary boiler feed pump 42, another closed feedwater heater 4.3, and finally a constant speed secondary boiler feed, pump; 44, whose discharge leads to the boiler 45- through the piping 48. I

It-is obvious that the margin between the. pressure at the suction of the secondary. pump 44 and the vapor pressure corresponding to the temperature of the feedwater entering the pump 44 must never be allowed to fall below a certain predetermined value, which depends on the pumpv design, else flashing would occur, disrupting the operation of the pump and of the, system. Thus, while normally the speed of the primary boiler feed pump 42 may; be-left under the control of the excess pressure or constant pressureregulator 46, the electrical. type control. mechanism shown in Figure 1 and indicated at A should be employed to set a definite limit to the speed reduction so occasioned. This limit, as'provided by the control mechanism A, will of course be variable, depending entirely upon the interrelation between the pressure of the feedwater at the suc-. tion of the pump 44- and the vapor pressure cor responding to the temperature of' the. feedwater at that point, asmeas ured' by and acted upon by the control mechanism A.

The excess pressure regulator 46' may be of any suitable type such as-may be purchased upon the open market-for providing regulation of thespeedof the electric motor 49which drives thepump 42, as for example the Copes electric pump control sold by the Northern Equipment Co. of Erie, Pa. Theregulatorindicated at 46 embodies a rheostat, theoperation of which is controlled by a mechanism connected to and controlled by the pressure in the boiler feedwater line 48 and the steam pressure in the line 56 to the main turbine for maintaining an excess of boiler feed pressure over the steam pressure. Upon an increase in available excess feedwater pressure over boiler pressure due to operating conditions or load, the regulator 46 would act to reduce the speed'of the pump motor 49' and the boiler feed pump 42, reducing the pressure in the line 48- and boiler-'45. This would likewise reduce the pressure at the suction of the constant speed boiler feed pump 44. Such a reduction would not necessarily be accompanied by a companion drop in temperature at that point, so that the differential between the vapor pressure and the absolute pressure might become so small as to cause flashing.

To prevent this, a control mechanism A such as shown in Figure 1 is introduced for limiting the reduction in speed of the pump 42 by the regulator 45. The control mechanism A of the present invention may be adapted to provide such a result in any of a number of manners, only one of which is shown, by way of illustration. The application of the control in any definite installation will depend upon the apparatus employed and other factors.

In Figure 5 the control mechanism A is employed for operation of a three-way solenoid operated pilot valve 5| of any approved form. The electrical circuit It, opened and closed by the operation of the control mechanism A under the influence of the differences in the absolute and vapor pressure at the suction of the pump 44, is connected to the solenoid 54 for energizing and deenergizing it. A drop in the absolute pressure to a point sufficiently near the vapor pressure, depending upon the setting of the control mechthe right,- increasing the speed of the motor 49 and pump 42, or in other words, limiting the reduction in motor speed. The result is an increasein pressure of the boiler feedwater at the suction of the pump 44, thus avoiding the danger of flash ing. The increase in absolute pressure, reacting onthe control mechanism A, actuates the control and opens the circuit It, deenergizing the solenoid 54and cutting on the flow of pressure fluid through the valve 5| to the cylinder 56. The pressure fluid in the cylinder 56 will then bleed back through the three-way valve 5| to atmosphere, releasing the pressure of the rod 58 against the arm 59 of the rheostat and allowing the excess pressure regulator 46 to resume control over the speed of the motor and pump 42.

Thus it will be seen that the limitation upon the reduction in speed caused by the excess or constant pressure regulator 46 is a variable one, created by the reaction of the control mechanism of the present invention, which provides an impulse at any time that the vapor pressure and the absolute pressure at the critical point approach within a predetermined distance of each other.

It is to be understood that the system as illustrated in Figure 5 could be controlled in either ofthe two manners illustrated in Figures 3 and 4. as by placing a bypass about the heater 43 in which a valve controlled by either of the mechanisms illustrated in Figures 1 and 2 is inserted.

Such construction could be employed in lieu of that just described, or even in conjunction therewith, as an added safety factor.

It will be understood that the invention is not to be limited to the specific construction or arrangement of parts shown, but that they may be widely modified within the invention defined by the claims.

What is claimed is:

1. The method of controlling the relationship between the absolute pressure and the vapor pressure corresponding to the temperature of the feedwater in a boiler feed system including a boiler feed pump and afeedwater heater to prevent flashing of feedwater into vapor in the boiler feed pump which includes controlling the temperature of the feedwater in advance of the suction of the boiler feed pump to insure at least a predetermined diflerential between the absolute pressure in the boiler feed system and the vapor pressure corresponding to the temperature of the feedwater by bypassingthe feed water around the feedwater heater in the boiler feed system and into the suction of the boiler feed pump when the vapor pressure corresponding to the temperature of the feedwater reaches a predetermined point approaching the existing .absolute pressure of the feedwater at a predetermined point in the boiler feed system.

2. In a boiler feed system embodying a boiler. a boiler feed pump and a feedwater heater, means for maintaining a minimum difierential between the absolute pressure at a predetermined point'in the system in advance of the suction of the boiler feed pump and the vapor pressure corresponding to the feedwater temperature at a predetermined point in the system in advance the flashing of feedwater into vapor in the boiler -feed pump, including means actuated by the temperature of the feedwater at said temperature predetermined point, means actuated by the pressureat said pressure predetermined point, and means actuated upo a. predetermineddegree of actuation of said temperature actuated means and said pressure actuated means for maintaining said minimum differential between said pressures.

3. In a boiler feed system embodying a boiler,

a boiler feed pump and a feedwater heater, means formaintaining a minimum difierential between the absolute pressure at a predetermined point in the system in advance of the suction of the boiler feed pump and the vapor pressure corresponding to the feedwater temperature at a predetermined point in the system in advance of the suction of the boiler teed pump to prevent the flashing of feedwater into vapor in. the boiler feed pump, including means actuated by the temperature of the feedwater at said temperature predetermined point, means actuated by the pressure at said pressure predetermined point, and means actuated upon a predetermined degree of actuation of said temperature actuated means and said pressure actuated means for limiting the temperature of the feedwater in the system to maintain said minimum differential between said absolute pressure and the vapor pressure corresponding to said temperature.

4. In a boiler feed system embodying a. boiler feed pump, a feedwater heater and a boiler, a

control for maintaining a minimum diiierential between the absolute pressure at a predetermined point in the boiler feed system in advance of the suction of the boiler feed pump and the vapor pressure corresponding to the temperature of the feedwater at a predetermined point in the system in advance of the suction of the boiler feed pump, embodying means actuated by the temperature at said temperature predetermined point for indicating the vapor pressure corresponding to said temperature, means for indicating the pressure at said pressure predetermined point, means operated by said pressure indicating means and said temperature actuated vapor pressure indicating 'means for creating an impulse when the dinerjence between the two indicated pressures reaches a predetermined minimum, and means actuated by said created impulse to vary the temperature or said feedwater and increase the differential between the two pressures.

5. In a boiler reed system embodying a boiler feed pump, a feedwater" heater and a boiler, a control for maintaining a minimum differential between the absolute pressure at a predetermined point in the boiler feed system in advance of the suction of the boiler feed pump and the vapor actuatedby said created impulse to increase the difierential between the two pressures.

1 6, In a boilenieed system embodying a boiler feed pump and a feedwater heater, means for maintaining a predetermined minimum differential between the absolute pressure at a predeterlilil'le d point in the boiler feed system in advance ofthe suction of the boiler feed pump and the vapor pressure corresponding to the temperature of the ieedwater at said predetermined point to prevent flashing of the feedwater into vapor in the boiler feed pump, comprising means for determiningthe pressure at said predetermined point, means for determining the vapor pressure corresponding to the temperature at said predetermined point, an operating member subject to said absolute pressure at said predetermined point and to said vapor pressure at said predetermined point in opposed relation to each other, said member; operable when the said pressures vary to -with in the predetermined minimum, differential to increase the difierentialbetween said pressure beyond said predetermined minimum.

7.-;In a boiler feed system embodying a boiler feed pump and a'i'eedwater heater, means for maintaining a predetermined minimum diii'eren tial between the absolute pressure at a predetermined point in the boiler feed system in advance of the suction of the boiler feed pump and the .vapor pressurecorrespondingv to the temperature of the feedwater at said predetermined point to prevent flashing of thefeedwater into vapor in theboiler feedpump, comprising means for determining the pressure at said predetermined point, means for determining the vapor pressure corresponding to the temperature at said predeterminedpoininan operating member subject to said absolute pressure at said predetermined point and to said vapor pressure at said point in opposed relation to each other, said member operable when said pressures vary to within the predetermined minimum difierential to vary the temperature of the feedwater for increasing the difrerential between said pressures beyond said predetermined minimum. I

8. Ina boiler feed system embodying a boiler, a boiler, feed pump and a feedwater heater, means for maintaining a minimum diiierential between the absolute pressure at a predetermined point in the system in advance of the suction of the boiler feed pump and the vapor pressure corresponding to the feedwater temperature at a predetermined point in the system in advance of the suction of the boiler feed pump to prevent the flashing of feedwater into vapor in the boiler feed pump comprising, a temperature sensitive bulb inserted in the feedwater flow at said temperature predetermined point and having an internal pressure proportional to the vapor pressure corresponding to the feedwater temperature, a pressure actuated device responsive to variation of pressure within said bulooccasioned by variation in temperature of the feedwater, a pressure actuated device responsive to variations in absolute pressure in the system at said pressure predetermined point, an electric circuit, means carried by said pressure actuated devices for opening and closing said electric circuit when their relative movement responsive to the variations in absolute pressure and temperature is within certain predetermined limits, a solenoid operated valve in the system, said electrical circuit energizing and deenergizing said solenoid operated valve for bypassing feedwater around said feedwater heater and thereby varying the temperature bf the feedwater to increase the difierential 'be'twe'enthe absolute pressuraand the vapor pressure corresponding 'to the temperature of -the feedwater beyond the predetermined minimum. r

9. In a boiler feed system embodying a boiler, a Variable speed feed pump; a constant speed feed pump, and a feedwater heater; means for maintaining a minimum differential bet-ween the absolute pressureat-a predetermined point in the system in advance-of the suctionof the constant speed boiler 'feed pump and the vapor pressure cor-responding to the feedwater temperature at a predetermined point in the system in advance of the suction *of the constant speed boiler feed pump to prevent the flashing'of -feedwater -into vaporin the constant speed boiler ieed pump comprising, a temperature sensitive bulb inserted in the --feedwater newer said temperature predetermined point and having "an internal pressure proportional to the vapor pressure corresponding to the -feedwater temperature, a pressure actuated device responsive to variation of pressure within saidbulb occasioned by variation in temperature of the feed-water, a-pressure actuated device responsive to variations in absolute pressure in the system at said pressure predetermined point, an electric circuit, means carried by said pressure actuated devices for opening and closing said electric circuit when their relative movement caused by the variations in absolute pressure and temperature is within certain predetermined limits, and means actuated by closing of said electric circuit for-increasing the speed of the variable capacity feed pump to 3 vary the absolute pressure in the system for increasing the differential between the absolute pressure and the vapor pressure corresponding to the temperature of the feedwater at the predetermined point beyond the predetermined minimum.

10. In a boiler feed system embodying-a boiler feed pump and a 'feedwater' heater, means for maintaining a predetermined minimum differential between the absolute pressure at-a predetermined point in the boiler -feed system in ad- Vance of the suction of the boi-le'r feed pump a-nd the vapor pressure corresponding tothe temperatureof the feedwater at said predetermined point to prevent flashing of the feedwater into vapor in the boiler feed pump, comprising means for determining the pressure at said predetermined point, means for determining the vapor pressure corresponding to the temperature of the feedwater at said predetermined point, an operating member subject to the absolute pressure at said predetermined point and to the vapor pressure at said point in opposed relation to each other, a. bypass for bypassing water around said feedwater heater to said'boiler feed pump, a valve in said bypass controlled by said operating member, said member operable when the difference between said absolute pressure and said vapor pressure is less than the predetermined minimum to open said valve and bypass feedwater about said feedwater heater-for varying the temperature of the feedwater and there'- by the corresponding vapor pressure at said .predetermined point and increasing the differential between said pressures at said point.

11. In a boiler feed'system embodying a constant speed feed pump .and a variable speed boiler feed pump and'a feedwater heater, means ferential between the absolute pressure at a predetermined point-in the boiler feed system in advance of the suction of the boiler feed pump and the vapor pressure corresponding to the temperature of the -feedwater at said point to prevent flashing of the feedwater into vapor in the constant speed boiler feed pump, comprising, means for determining the pressure at said predetermined point, means for determining the vapor pressure corresponding to the temperature at said predetermined point, an operating member subject to said absolute pressure at said predeterminedpoint and to said vapor pressure at said point in opposed relation to each other, a speed regulator for controlling the speed of said variable speed boiler feed pump, said operating member operable when the difference between the absolute pressure and the vapor pressure becomes less than said predetermined minimum to actuate, said regulator for increasing the speed-of said variable speed boiler feed pump and thereby increase the differential between said pressures beyond said predetermined minimum.

in the boiler feed pump which includes limiting th temperature of the feedwater in advance of the boiler feed pump toinsure at least a predetermined differential between the absolute pressure in the boiler feed system and the vapor pressure corresponding to the temperature of the feedwater.

l3, The method of controlling the relationship between the absolute pressure and the vapor pressure corresponding to the temperature of the water in a boiler feed system wherein the feedwater is heated and which includes a boiler feed pump to prevent flashing of the feedwater into vapor in "the boiler feed pump which includes limiting the absolute pressure in the boiler feed system "in advance of the boiler feed pump to insure at least a predetermined differential between the absolute pressure in the boiler feed system anutnewm pressure corresponding to "the'temperature of the feedwater in. the system.

14. The fneth'odof controlling the relationship between "the absolute pressure and the vapor 'fr'ire'ssure corresponding to the temperature of the "feedwa'te'r 'in 'a boiler feed system including av'ariable speedfeed pum-p and a constant speed feed pump together with 'a feedwater heater to prevent flashing of 'feedwater into vapor in the constant speed pump,'which includes controlling the absolute pressure in the boiler feed system in advance of the suction of the constant speed boiler feed pumpyfor maintaining a predetermined 'differential between the absolute pressure in theboiler feed system and the vapor pressure corresponding to the temperature of the feed- Water-in the *system by increasing the speed of the variable speed 'fee'd pump to increase the absolute pressure when such absolute pressure fallsto-a predetermined degree above the vapor pressure cor-responding to the temperature of the feedwater at a predetermined point in the boiler feed system.

IGOR J. KARASSIK