US2819701A - Boiler feed pump control - Google Patents

Description

Jan. 14, 1958 l. J. KARASSIK I 2,819,701

BOILER FEED PUMP CONTROL Filed Jan. 26, 1956 2 Sheets-Sheet l DIRECT CONTACT o HEATER IGOR J. KARAgl gw FIG. I WWW-ML l. J. KARASSIK 2,819,701

BOILER FEED PUMP CONTROL Jan. 14, 1958 2 Sheets-Sheet 2 Filed Jan. 26, 1956 STEAM mnem- CONTACT HEATER STAND 50 STER Boner:

PUMP v FEED Pu M P IGOR KARAEJfi gyg'luii/o w y BOILER FEED PUMP CONTROL Igor J. Karassilr, Maplewood, N. J.

Application January 26, 1956, Serial No. 561,414

2 Claims. (Cl. 122-451) The present invention relates to a boiler feedwater system of the open type, that is, one which embodies a direct contact feedwater heater.

More specifically, the invention contemplates the protection of the main boiler feed pump against flashing in a boiler feedwater system wherein a separate booster feed pump is interposed between the direct contact heater and the main feed pump to provide additional pressure at the main feed pump suction. The booster pump is used to maintain a sufficient pressure differential between the suction pressure at the main feed pump and the vapor pressure corresponding to the temperature of the entering feedwater to prevent flashing of the feedwater into steam in the first stage of the main feed pump to avoid damage to the pump and interruption of its pumping action.

The pressure generated by-the booster pump is a reasonable criterion of the pressure difierential between the main feed pump suction and the vapor pressure corresponding to the feedwater temperature at any given load, as can be seen from the following relation:

Boiler feed pump suction pressure=heater pressure-I-static head-l-net pressure of booster pump-friction losses in the piping Since the vapor pressure of the feedwater in a feedwater system embodying a direct contact heater is equivalent to the heater pressure:

The available net positive suction head (NPSH), or the pressure differential between the pressure of the feedwater entering the boiler feed pump and the vapor pressure corresponding to the temperature of the entering feedwater at the suction of the main feed pump=the static head-l-the pressure of the booster pump-the friction losses in the piping.

Thus, by selecting the net pressure of the booster feed pump in a proper manner, the net positive suction head at the main feed pump suction can be made to exceed the NPSH required to avoid flashing of the feedwater into steam in the first stage of the main feed pump. This, of course, is based upon the assumption that other control means are available during transient conditions to prevent the reduction of the available NPSH by an excessive decrease in the direct contact heater pressure. Therefore, since the pressure generated by the booster pump is a reasonable criterion of the pressure differential between the main feed pump suction and the vapor pressure corresponding to the feedwater temperature at any given load, then the only other cause for a decrease in this pressure differential would be a decrease in the pressure generated by the booster pump, through failure of thepump, or of the power supply to the pump driving means.

The failure of the booster pump may occur independently of the prevailing conditions at the main feed pump since the booster pump is commonly driven by an electric motor while the main feed pump may be turbine driven. Thus, should the power supply to the electric motor driving the booster pump fail, the pump would atent ice automatically stop. Even when both the booster and main feed pumps are driven by electric motors, the motors may operate on different voltage circuits because of a wide difference in motor size. Here too, a failure in the power supply to the motor of the booster pump would cause failure of the pump. In addition, the booster pump may break down independently of conditions prevailing at the main feed pump through a mechanical defect in the pump end thereof.

It the pressure at the suction of the booster feed pump were to remain constant at all loads, the failure of the pump would not be important since the use of pressure switches set at specific pressure settings could transmit a pressure responsive signal to start a standby booster pump, to provide the predetermined pressure diflerential, or to provide an alternate source of feedwater for the main feed pump. However, in modern steam plants the direct contact heater at the suction of the booster pump may operate over a wide range of pressures and temperatures and the use of pressure switches would not be.

satisfactory since a given suction pressure at the main feed pump may be satisfactory for one set of load and temperature conditions, while unsatisfactory for another set of conditions.

In accordance with the present invention, a method and apparatus for controlling the pressure in a boiler feedwater system is provided to maintain a predetermined pressure diiferential between the suction pressure at the main feed pump and the vapor pressure corresponding to the temperature of the entering feedwater to prevent flashing of the feedwater into steam upon failure of operation of the booster feed pump. The present invention provides a pressure responsive signal indicating the pressure differential across the suction and discharge side of the booster pump or the net pressure of said booster pump, which signal is transmitted to control means operatively connected to a standby booster pump in parallel with the main booster pump to start the standby pump when this pressure signal decreases below a predetermined value. The control means are also operatively connected to valve means adapted to admit feedwater under pressure to the suction of the main feed pump from a. different source when the pressure signal received decreases below a second predetermined value, be1ow the first predetermined value.

The present invention provides amethod and apparatus for detecting failure of the booster pump in a boiler feedwater system.

The present invention further provides a method and apparatus for controlling the suction pressure at the suction of a main feed pump in a feed-water system in response to pressure variations in the booster pump therein, rather than in response to variations in the temperature of the feedwater as in applicants Patent No. 2,372,087.

The invention will be better understood from the fol lowing description when considered in connection with the accompanying drawings forming a part thereof and in which:

Figure 1 is a diagrammatic view of a boiler feedwater system embodying the control means of the present invention.

Figure 2 is an enlarged view of the control means shown in Figure 1, and v Figure 3 is another embodiment of the invention shown in Figure 1.

Referring to Figure 1, the reference numeral 10 designates a direct contact heater embodied in a boiler feedwater system having a discharge line 11 therein in communication with a main feed pump 12 for flowing feedwater thereto. The main feed pump communicates by a conduit 13 with a boiler (not shown). A booster pump 14 is disposed in feedwater line 11 to generate additional pressure at the main feed pump suction to tached' thereto.

maintaina. predetermined. pressure. differential. between. the suction pressure at the main feed pump and the vapor pressure corresponding to the temperature of the feedwater-forz preventing fiashingiof: the feedwater into steam within: the first 1 stage of the main feed pump. A check valves. is'disposed. in line 11 on the discharge'side of' booster pump 14 to prevent: back flow of feedwater" therein;

A stand-by booster pump 16- is provided in a'feedwater" conduit 17' in. communication withconduit Iii on the suction and'discharge sides of booster pump 14 for gen-- erating pressure to maintain apredetermined pressure difierential: atthe main feedpump'suction in excess of thewapor'pressure corresponding to the temperature of' therentering feedwaterto prevent= flashing of the feed waterzinto: steam should booster pump 14 fail to providethe normalpredetermined pressure. differentialor margin;

check valve 18 is disposed in'conduit- 17 on the dis-= charge side of pump 16 therein.

A condensate pump'19 in communication with a condenser (notshown)-by line 19a is provided forsupplying feedwaterto the directcontact heater Idtbroughconduit- 20- in communication therewith. A- bypass line 21- is provided in communication with the condensate pump and conduit 11 atthe suctionof main teed pump 12to supplyfeedwater' under pressure directly to the main feed pump should stand-by booster pump 16- fail to operate, and the predetermined pressure differential between'the suction-pressureat the main feed pump and thevapor' p'ressurecorresponding to the temperature ofthe'entering feedwater decrease below a predetermined value; The flow of feedwater from the-condensate pump to themain feedpump through bypass line 21 is controlled by asolenoid or motorized valve-22 disposed in'line 21.

Referring to Figure 2, the reference numeral 24 designates. the control means embodied in the boiler feedwater' system: of the present invention to maintain a predetermined-pressure diiferential at the suction'of the main feed pump above the vapor pressure corresponding'tothe temperature of the feedwater' to prevent fiashingofthe feedwater into steam should'the boosterpump' pressure decrease or fail to operate, and to maintain a predetermined pressure differential above said corresponding vapor pressure when the stand-by booster pump pressure decreasesor fails to operate; prises a capillary tube 25connected at oneend to the suction of booster pump 14'and'at the other end to an expansible bellows 26 having a movable'pointer'27 at Bellows26 is socalibrated that the at: tached pointer27 moves along an indicating'scale 28 which reads the suction pressure at the booster pump'14' in'p. s. i; A similar'pointer29 is movably attachedto a bellows 30 similar to bellows 26; Bellows" 30 is connected by capillary tube 31 to'conduit 11 on the discharge side'of'booster pump 14 beyond line 17 'entering'therein. Bellows 30'is so calibrated that the attached pointer 29 moves along the indicating scale 28 to indicate the discharge pressure of the booster pump in p'. s. i. The distance along scale zsbetween'pointers 27 and 29represents at all times the net pressure generated by the booster feed pump 14.

Extension arms32 and33are connected'to the suction pressure indicating'pointer'27. The length ofthe exten-. sion arms are such that contact will be made by them with pointer 29 before the predetermined value of pressure differential to be developed by the booster pumpl labove the vapor pressure corresponding tothe feedwaterterrn. perature at the suction of the main feedpump to prevent flashing'of the feedwater'into steam" is reached;

An electrical circuit 34 is'provided' for actuatingthe stand-by booster pump 16. Circuit. 34 comprises an electrical cable 35 counected'to pointer 27 atone end thereof and asouroe of power 36' at the other end thereof, and a'nxelectrical cable 37 connected to the source of. power and pointer" 29" at theotherend'thereofi" A starting switch" Control means 24 commined by the length of the extension arm 32, the arm will contact'the; pointer 29. to close theelectrical circuit 34 and energize. it thereby. closing starter switch 38 to energize the starting motor circuit and start the stand-by booster: pump in operation. Thus, control means. 24 performs the function of a switch actuated by the value of the net pressure generated by the booster pump 14. Thus, the failure of operation of booster pump 14 decreases the net pressure generated by the booster pump 14 so that electrical circuit 34 is energized and the stand-by booster pump, 16.- providesiadditional pressure at the maimfeed pump suction tov maintain .a suificient pressure diflerenuall between the suction pressureat main feed pump: 12. and; the. vapor pressure correspondingto the temperatureuof the entering feedwater to prevent flashing of the-feed;- Water into steam.

The subsequent openingof the electrical circuit 34d0$x not stop'the operationof the stand-by booster pump; but the. pumpmust be stopped manually and the mecha: nism reset; Thisactionzis desirable to prevent hunting. of thesystem.

An electrical circuit 42 comprising individual; cables" 35- and 43=-is provided for; energizing solenoid motor valve; 22.toopeu:the valvecandpermit deedwater under pressure to flow directly from condensate pump 19 through'byepass; line.21 to the suction of the main feed pump 12 bypassing thebooster. pumpianddirect contact beaten. Cable;431 is; connectedat one end' topointer 29 and at the other end; to. the solenoidtmotor valve 22. The other terminal. of: the solenoid valves22isconnected. through. cable 37 to the source. of power 36 to complete the circuit.

Upon failure of the starting of the standby booster pump 16 to restore the desired condition, pointer 29 indicating the discharge pressure of the booster pump 14*Wouldc0ntinue to move to the left looking at Figure 2, toward pointer 27 and arm 33 thereon.

Upon lowering of the pressure to that determinedby the lengthof the extension arm 33, the arm contacts pointer 29'and closes electrical circuit 42. The. closing ofthis circuit. energizes the solenoid motor valve-22 to open the valve and permit the flow of feedwater directly from thecondensatepump tothe main feed pump,.thus maintaining the pressure of the feedwater entering the main feed pump 12: sufliciently above the vapor pressure corresponding to the feedwater temperature to prevent flashingof the feedwater into steam. Thus, the present invention provides control means for maintaining a pre-" determined value of pressure differential on the discharge side of the booster pump in communication with the suction of the main feed pump above the vapor pressure corresponding to the feedwater temperature, should the booster pump, or standby booster pump, fail to operate due tofailure of either pump or the driving meansfor. said pumps, or should they fail independently of the conditionsprevailing'at the main feed pump.

Referring to Figure 3. of the drawings, a modified, form: ofthe invention. is illustrated wherein a spring.

' loaded diaphragm valve 45 is employed for obtaining thesame control as provided by the embodiment of the invention shown in Figure 1. In this structure, valve 45 is providedwith a.valve stem 46 connected at its upper end to a diaphragm 47. A chamber 48 on one side of the diaphragm 47 communicates with pressure line 31 soas to transmit thereto the pressure at the dischargeofi the booster feed pump 14; A chamber 49 on the other side of..diaphragm.47 communicates with the suctionof. theboostertfeedpumpltt by pressure line 25v so as. to. receive a pressure signal therefrom. A compression.

spring 50 is provided on the valve to exert a constant pressure on diaphragm 47 in opposition to the pressure exerted thereon from the discharge pressure of the booster pump. The tension or compression of this spring 50 is determined in accordance with the desired minimum net pressure to be developed by the booster pump. The degree of opening will bear a certain proportion to the amount by which the net pressure generated by booster pump 14 falls below its designed value. Valve 45 is disposed in a control air supply line 51 in communication with a source of air under pressure (not shown) and a relay mechanism 52. The valve 45 acts as an air pilot valve and sends a variable pressure signal or air impulse to the relay mechanism 52 readily purchased on the open market.

The relay mechanism 52 is operatively connected to the starting switch 38 of the stand-by booster pump 16 so as to energize the electrical circuit of the stand-by booster pump 16 in a manner well known in the art when the net pressure developed by booster pump 14 decreases below a predetermined value. Thus, the standby booster pump 16 is started in operation to provide additional pressure at the suction of the main feed pump 12 to maintain a pressure differential between the feedwater entering the main feed pump and the vapor pressure corresponding to the feedwater temperature to prevent flashing of the feedwater into steam.

Relay mechanism 52 is also operatively connected to a switch 53 adapted to close the electrical circuit to solenoid valve 22 in bypass line 21. Switch 53 is set to close after a predetermined time delay when the net pressure developed by booster pump 14 falls below a predetermined value, above the pressure differential desired to be maintained between the feedwater pressure entering the main feed pump 12 and the vapor pressure corresponding to the feedwater temperature, but less than the predetermined value required to start the stand-by booster pump 16 in operation. Thus, the embodiment of the invention shown in Figure 3 provides control means for maintaining a predetermined pressure difierential between the feedwater pressure entering the main feed pump and the vapor pressure corresponding to the feedwater temperature, by starting the stand-by booster pump when the net pressure of the booster feed pump 14 decreased below a first predetermined value, and for opening solenoid valve 22 to flow feedwater directly to the main feed pump 12 when the net pressure of the booster pump 14 decreases below a second predetermined value to prevent flashing of the feedwater into steam.

What is claimed is:

1. In a boiler feedwater system, a direct contact heater, a main feed pump, a feedwater conduit connected between said heater and main feed pump, a booster pump disposed in said conduit to deliver feedwater under pressure from said heater to said main feed pump, a second conduit communicating at one end with the direct contact heater and at the other end in communication with the suction side of the main feed pump a stand-by booster pump disposed in said second conduit to increase the pressure of feedwater delivered from the heater to said main feed pump, a condensate pump, a discharge conduit in communication with said condensate pump and said heater for passing condensate therethrough to said heater, a bypass conduit in communication with said condensate pump to said main feed pump for passing condensate therethrough to said main feed pump, a solenoid valve disposed in said bypass conduit to control the flow of condensate therethrough and control means for maintaining a predetermined pressure differential between the feedwater entering the main feed pump and the vapor pressure corresponding to the feedwater temperature at the suction side of the main feed pump to prevent flashing of the feedwater into steam, said control means being operatively connected to said stand-by booster pump and solenoid valve to start the stand-by booster pump in operation when the net pressure of said booster pump falls below a first predetermined value, and to open said solenoid valve when the net pressure of said booster pump falls below a second predetermined value.

2. in a boiler feedwater system, a direct contact heater, a main feed pump, a feedwater conduit connected between said heater and main feed pump, a booster pump disposed in said conduit to deliver feedwater under pressure from said heater to said main feed pump, a second conduit communicating at one end with the direct contact heater and at the other end in communication with the suction side of the main feed pump a stand-by booster pump disposed in said second conduit to increase the pressure of feedwater delivered from the heater to said main feed pump, a condensate pump, a discharge conduit in communication with said condensate pump and said heater for passing condensate therethrough to said heater, a bypass conduit in communication with said condensate pump to said main feed pump for possing condensate therethrough to said main feed pump, a solenoid valve disposed in said bypass conduit to control the flow of condensate therethrough and control means for maintaining a predetermined presurre differential between the feedwater entering the main feed pump and the vapor pressure corresponding to the feedwater temperature at the suction side of the main feed pump to prevent flashing of the ieedwater into steam, said control means comprising an electrical circuit connected to said stand-by booster pump for starting the pump in operation, an electrical circuit connected to said solenoid valve for opening said valve, a relay mechanism connected to said electrical circuits to energize said pump circuit when the not pressure of said booster pump decreases below a first predetermined valve, and to energize said solenoid valve and open said valve when the net pressure of said booster pump decreases below a second predetermined value, and an operated relay actuating means operatively connected to said relay mechanism for actuating said mechanism and responsive to the net pressure of said booster pump.

References Cited in the file of this patent UNITED STATES PATENTS 2,372,087 Karassik Mar. 20, 1945 2,395,657 Dinsmore Feb. 26, 1946 FOREIGN PATENTS 193,971 Great Britain Mar. 2, 1923

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