US3401674A

US3401674A - Steam generator recirculating pump operation

Info

Publication number: US3401674A
Application number: US580765A
Authority: US
Inventors: Palchik David
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1966-09-20
Filing date: 1966-09-20
Publication date: 1968-09-17
Anticipated expiration: 1985-09-17
Also published as: ES344615A1; GB1168669A

Description

p 17, 1968 D. PALCHIK 3,401,674

STEAM GENERATOR RECIRCULATING PUMP OPERATION Filed Sept. 20, 1966 3 Sheets-Sheet l FIG-1 INVENTOR DAV/D PALCH/K AGENT United States Patent 3,401,674 STEAM GENERATOR RECIRCULATING PUMP QPERATION David Palcliik, Bloomfield, Conn., assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Sept. 20, 1966, Ser. No. 580,765 Claims. (Cl. 122-406) ABSTRACT OF THE DISCLOSURE A once-through supercritical pressure steam generator of the recirculating type, employing a centrifugal pump to effect recirculation. A measure of the density of the fluid passing through the pump is determined and the pump speed is regulated in response to this measurement. The measure of density may be the density itself, temperature, power consumption of the pump drive motor, or any other reliable parameter under the particular operating conditions.

This invention relates to recirculating type supercritical pressure steam generators and in particular to a method and apparatus for operating a recirculating pump so as to obtain the maximum recirculation consistent with pump drive power consumption limitations.

US. Patent 3,038,453 to W. H. Armacost illustrates a once-through steam generator with a recirculation system superimposed on the through-flow circuit. The recirculating pump is located in the recirculating line so that it withdraws fluid from a location downstream of the heating surface and reintroduces it upstream of the heating surface. This pump is controlled to obtain a constant pressure drop across the heating surface.

US. Patent 3,135,252 to W. W. Schroedter also describes a once-through boiler with a recirculation system superimposed on the through-flow circuit. The recirculating pump used to produce or induce recirculation is a free-floating centrifugal pump. The free-floating pump on the system produces a generally uniform velocity of the fluid entering the waterwall system over the operating range of the pump. This patent illustrates the pump as located not only in the recirculating line but alternately in the through-flow line in such a location that flow is induced from a location downstream of the heat absorbing location to a location upstream of the recirculating pump. In such a location the hot recirculated fluid is mixed with the incoming feedwater so that the temperature at the recirculating pump is lower than the temperature of the fluid naturally being recirculated.

The temperature of the fluid entering the Waterwall circuits varies substantially over the load range even though the velocity of the fluid is approximately constant. This difference in temperature results in a considerable change in density so that the mass flow of the water entering the waterwall circuits is considerably reduced at low loads where high temperatures exist at the pump. Since the resistance of the internal film inside the boiler tubes to transfer heat is a function basically of the mass flow rate rather than velocity, a relatively poor conductance results when high temperatures exist at the waterwall inlet. Even though relatively low average absorption rates exist at this time, local rates may be relatively high with resultant high metal temperatures. It is desirable to increase the mass flow rate at this time to increase the factor of safety in the design of the boiler tubing.

It is an object of my invention to provide a method and apparatus for operating a recirculating pump which is located intermediate the mixing vessel and the waterwall heating surface in such a manner as to obtain the maxirnum recirculation consistent with the circulating pump drive power consumption limitations.

Other objects and advantages of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIG. 1 illustrates a once-through recirculating type steam generator having a mixed flow pump with the pump speed being controlled by use of a motor generator set in a fluid coupling, and the control being responsive to the power consumption of the motor driving the recirculating p p;

FIG. 2 is a curve illustrating velocity conditions entering the waterwall with constant pump speed;

FIG. 3 is a curve illustrating the mass flow rates at the waterwall inlet for different pump speeds;

FIG. 4 illustrates the kilowatt input to the motor for various pump speeds;

FIG. 5 illustrates the motor revolutions per minute for a constant kilowatt input; and

FIG. 6 illustrates a steam generator wherein the variable speed motor driving the circulating pump is controlled in response to the temperature of the fluid passing through the circulating pump.

Water from the hot well 2 passes through feedwater line 4 which includes the condensate pump and low pressure feedwater heaters (not shown). Feedwater pump 6 raises the pressure of the water to about 4000 p.s.i. with the water then flowing to the steam generator through feedwater valve 8.

This high pressure water is passed through the economizer 10 to the mixing vessel 12 and continues through the circulating pump 14 and the furnace wall circuits 16. These furnace wall circuits are in the form of a plurality of parallel tubes which line the walls of a radiant furnace. Fuel burner 18 fires coal or oil into the furnace for combustion with the heat being radiantly transmitted to the furnace wall circuits 16.

The through-flow of water continues through the furnace wall outlet line 20 and the superheater 22 to the main steam line 24. Combustion gases formed by the combustion form burner 18 pass over the surface of superheater 22 heating the steam passing through that surface as well as over the surface of economizer 10 thereby heating the water passing through that surface.

Turbine control valve 26 controls the steam flow through steam line 24 to turbine 28. This turbine is directly connected to electric generator 30 for the generation of power. Steam is exhausted from the turbine 28 and condensed in the condenser 32 so that it returns to the hot well 2. The water is then again recycled as in the conventional power plant cycle.

Recirculating line 34 is connected to withdraw a portion of the through-flow from the furnace wall outlet line 20 and return this portion to the mixing vessel 12 which is located upstream of the furnace wall circuit 16. The circulating pump 14 adds pressure energy to the Water passing therethrough, and in conjunction with the recirculating line 34 operates to maintain a flow in the furnace wall circuits higher than the normal throughflovv as explained in US. Patent 3,135,252 to W. W. Schroedter. This recirculating line includes the stop check valve 36 which operates to prevent reverse flow in the recirculating line.

The circulating pump 14 is of the conventional centrifugal type having an impeller which is rotated at 1800 r.p.m. Since the pressure of the water at this'location is about 3900 p.s.i., it is difficult to properly seal a shaft passing to the pump from an external motor. Accordingly, a motor of the canned type is used to drive this pump with the rotor of the motor being contained within a thin metallic structure. This rotor is of the squirrel cage design With no electrical connections to the rotor.

With this motor driving the pump at constant speed, the velocity characteristic is as indicated by curve 38 in FIG. 2 and more fully described in US. Patent 3,135,252 to W. W. Schroedter. In this curve the ordinate indicates velocity in feet per second of the fluid entering the tubes of the furnace wall section 16. The abscissa generally indicates the quantity of through-flow through the steam generator as a percentage of design full load flow. Since these units are generally started up by initiating flow at percent While the steam generator is cold and then firing to warm up the unit before increasing flow, the 10 percent condition is shown at two locations along the abscissa. The first condition indicates 10 percent flow with cold water in the steam generator while the second 10 percent point indicates the conditions with the furnace wall circuit up to full temperature (in the order of 800 F. at the outlet). It can be seen that the velocity entering the waterwalls is essentially constant throughout the range in which the pump operates. Curve 40 indicates the velocity without recirculation. At 78 percent load the output of the pump is insufficient to induce recirculation and from this point on the through-flow alone causes the 'velocity to increase.

The solid line 44 in FIG. 3 indicates the mass flow characteristics at constant pump speed of 1800 r.p.m. throughout the same operating range. The ordinate in this case is the mass flow of the fluid entering the furnace wall tubes expressed in pounds per hour per square feet. It can be seen that at the 10 percent through-flow condition as the Water is heated up, while the velocity increases slightly, the mass flow rate decreases substantially. At increasing through-flows a mass flow rate generally increases again.

FIG. 4 using the same abscissa indicates the kilowatt input to the canned motor in curve 46 for the same 1800 r.p.m. impeller speed. Due to the natural characteristic of centrifugal pumps, the kilowatt input decreases as the water density decreases and subsequently increases in a manner similar to the mass flow.

Curves

48 and 50 in FIG. 3 show the mass flow entering the furnace wall circuits for impeller speeds of 2250 and 2700 r.p.m. respectively. Similar curves are illustrated in FIG. 4 where curve 52 shows the kilowatt input for 2250 r.p.m. and curve 54 the kilowatt input at 2700 r.p.m. While the curves are shown for only a few of the impeller speeds, obviously there are an infinite number of speeds which could be used and a corresponding infinite number of curves. An inspection of FIGS. 3 and 4 will show that when the mass flow is low, the kilowatt input for a given impeller speed is also low. Therefore in order to improve the mass flow conditions, the impeller speed is increased where the mass flow increases. It, however, is desirable to limit the kilowatt input to the motor. This kilowatt input affects not only the cooling requirements of the motor but also the power plant switch gear and transformer capacity which must be installed to supply the energy requirements for the pump. In FIG. 4 line 56 represents a constant kilowatt input to the motor driving the circulating pump. By interpolating between

curves

44, 48 and 50 the appropriate r.p.m. can be obtained to maintain this constant kilowatt input. This motor speed is shown by curve 57 in FIG. 5. Its effect on the mass flow is illustrated by line 58 shown on FIG. 3. It can be seen that by operating the circulating pump in such a manner the particularly low mass flow operating condition is avoided with no increase in the power requirements of the pump. At all Conditions the maximum mass flow 4. is obtained compatible withpump motor power limitations.

In order to obtain this characteristiathe pump speed must be changed. Throttling of the flow through the pump or throttling of the recirculating flow will'not suflice, since this will not effectively change the power consumption of the motor driving the pump.

While the pump speed may be controlled manually,

FIG. 1 illustrates an .automatic control for maintaining the maximum mass flow consistent with power consump-' tion requirements. The circulating pump 14 is driven by directly connected canned motor 60. The constant speed alternating current motor 62 is connected to the line and operates to rotate alternating current generator 64 which is connected through the fluid coupling 66. The output from the generator 64 passes to the motor 60 through Watt meter 68. By changing the frequency ofthecurrent' generated by generator 64, the speed of rotation of "the pump 14 is changed. Watt meter 68 senses the kilowatt consumption of the motor 60 and passes acontrol signal representing this power consumption through control line 70. At set point 72 this signal is compared to a signal representing the desired kilowatt consumption with the error signal passing through control line 74 to controller 76. This controller then operates on the fluid coupling 66 changing the speed of the generator 64 and therefore the speed of motor 60. This is automatically readjusted until pump 14 is operating at such a speed that the kilowatt consumption remains at the desired value. This control circuit could be arranged in such a manner that the kilowatt speed of the motor is changed in steps, allowing some deviation from the desired kilowatt consumption rather than using the exact approach illustrated here.

FIG. 6 illustrates a power plant which is identical with that illustrated in FIG. 1 except for the method of controlling the speed of the circulating pump 14. The density of the fluid flowing through the pump is affected by temperature at a constant pressure. In most circumstances nominal variations of pressure do not significantly affect the density. Therefore when operating in the usual pressure range, temperature alone may be used as an indication of the density. The kilowatt consumption of the motor driving a circulating pump is directly proportional to the density of the fluid being pumped. Therefore, this temperature may be used as an indication of the kilowatt consumption of the motor. A variable speed motor which drives the circulating pump 14 is of the two-speed two-winding type. This motor will operate at either 1800 r.p.m. or 2700 r.p.m. This motor is connected through controller 82 to the line power supply. This controller is operative to activate the motor 80 selectively on the 1800 or 2700 r.p.m. windings.

Temperature transmitter 84 senses the temperature of the fluid passing through the conduit 13 and the circulating pump 14 and sends a signal indicative of this temperature through control line 86 to the controller 82. As indicated before, this temperature signal is a function of the density of the fluid being pumped. When the temperature of this fluid drops below 540 F., the controller 82 operates to activate the 1800 r.p.m. windings of pump 80. When the temperature rises to about 550 F., the controller operates to increase the speed of the motor 80 to 2700 r.p.m. With various multiple winding motors, this operation may be used to select any desired steps in motor speed. Where pressure varies to such. an extent and in such a range that it significantly affects the density of the fluid being pumped, the pressure transmitter must also be included so that the pressure and temperature signals are both used to determine a signal representative of the density of the fluidpassing through the motor.

While I have illustrated and described a preferred ;embodiment of my invention is Y to be understood that such is merely illustrative and not restrictiVeJand that variations and modifications may be made therein without departing from the spirit and scope of the invention.

I therefore do not wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is:

1. A supercritical pressure steam generator comprising: a heat exchange section; means for establishing a throughflow of water through said heat exchange section; a mixing zone upstream of said heat exchange section and conduit means for conveying fluid from said mixing zone to said heat exchange section; means for withdrawing a portion of the throughflow from a location downstream of said heat exchange section and reintroducing said portion in said mixing zone; a centrifugal pump having an impeller located in said conduit means; driving means for rotating the impeller of said centrifugal pump; sensing means for sensing a measure of the density of the fluid as it passes through said conduit means and centrifugal pump; means for varying the speed of rotation of said impeller in response to said sensing means.

2. An apparatus as in claim 1 wherein said sensing means comprises means for sensing the power input to: said driving means.

3. An apparatus as in claim 1 having a furnace, and means for burning fuel within said furnace; said heat exchange section comprising tubes lining the Walls of said furnace.

4. An apparatus as in claim 3 wherein said driving means comprises a canned squirrel cage motor; the means for varying the speed of rotation of said pump comprising a constant speed electric motor, an alternating current electric generator, a fluid coupling connecting said alternating current motor and said alternating current generator, and electrical connections for conveying the output of said alternating current generator to said canned motor.

5. An apparatus as in claim 1 wherein said sensing means comprises means for sensing the temperature of the fluid passing through said conduit means.

6. A method of operating a supercritical pressure recirculating type steam generator comprising: establishing a through-flow of Water; heating said through-flow to a temperature level by passing the water in heat exchange relationship with a heat source; forming a mixed flow portion of water flow by returning a portion of the through-flow at said temperature level to a location in said through-flow path upstream of said heat exchange relation, and mixing the returned portion with the through-flow; increasing the pressure of said mixed flow portion by mechanically adding pressure energy to the mixed flow portion of water flow; determining a measure of the density of said mixed flow portion of the water flow before heating the water flow; and regulating the increase in pressure in response to said measure of density.

7. A method as in claim 6 wherein the measure of density of the mixed flow portion is determined by deter mining the energy required to mechanically add pressure energy to the mixed flow portion of Water flow.

8. A method as in claim 7 wherein the increase in pressure is regulated in response to the energy required, in such a manner as to maintain at a constant value of energy required to mechanically add pressure energy.

9. An apparatus as in claim 6 wherein the measure of density of the mixed flow portion is determined by measuring the temperature of the mixed flow portion.

10. A supercritical pressure steam generator comprising: a heat exchange section; means for establishing a through-flow of Water through said heat exchange section; a mixing zone upstream of said heat exchange section and conduit means for conveying fluid from said mixing zone to said heat exchange section; recirculating pipe means for withdrawing a portion of the through-flow from a location downstream of said heat exchange section and reintroducing said portion in said mixing zone; said recirculating pipe means and said conduit means comprising a first portion of a recirculating loop; a centrifugal pump having an impeller located in said first portion; driving means for rotating the impeller of said centrifugal pump; sensing means for sensing a measure of the density of the fluid in the condition in which it passes through said centrifugal pump; means for varying the speed of rotation of said impeller in response to said sensing means.

References Cited UNITED STATES PATENTS 2,255,612 9/1941 Dickey 122451 XR 2,324,513 7/1953 Junkins 12l2-451 XR 3,135,252 6/1964 Schroedter 122406 3,194,219 7/1965 Henzalek 122--406 KENNETH W. SPRAGUE, Primary Examiner.