US3640251A

US3640251A - Control of fluid flow through centrifugal pumps

Info

Publication number: US3640251A
Application number: US67772A
Authority: US
Inventors: Jeremiah M Ferguson
Original assignee: Riley Power Inc
Current assignee: Riley Power Inc
Priority date: 1970-08-28
Filing date: 1970-08-28
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08

Abstract

The flow through a steam power plant recirculation circuit is increased and made variable by injection of a certain amount of boiler feed water into the suction side of a centrifugal pump in the recirculation circuit thereby increasing the density of the fluid being pumped and correspondingly increasing the effective pumping pressure in the recirculation circuit.

Description

O United States Patent [151 3,640,251

Ferguson Feb. 8, 197 2 [54] CONTROL OF FLUID FLOW THROUGH 2,277,100 3/1942 Hartmann ..55/201 CENTRIFUGAL PUMPS 2,495,525 1/1950 Karassik I 22/406 2,662,507 l2/l953 Loumiet et al ..l22/448 Inventor: Jeremiah Ferguson, Northboro, Mass- 2,679,83l 6/1954 Henkel 1 22/386 [73] Assignee. 113:; Stoker Corporation, Worcester, Prima'y Emminer xenneth wp g AttorneyWard, McElhannon, Brooks & Fitzpatrick [22] Filed: Aug. 28, [9'70 [21] Appl. No.: 67,772 [57] ABSTRACT The flow through a steam power plant recirculation circuit is increased and made variable by injection of a certain amount [52] "122/406 S of boiler feed water into the suction side of a centrifugal pump [51] Int. Cl ..F22b 35/12 in the recirculation circuit thereby increasing the density of [58] Field of Search 1 22/406 R, 406 S, 406 ST the fluid being pumped and correspondingly increasing the effective pumping pressure in the recirculation circuit. [56] References Cited 19 Claims, 4 Drawing Figures UNITED STATES PATENTS 1,908,265 5/1933 Lucke ..l22/406 CONTROL OF FLUID FLOW THROUGH CENTRIFUGAL PUMPS This invention relates to the control of fluid flow and more particularly it concerns novel arrangements for controlling the fluid delivery rate of constant speed centrifugal pumps.

A particularly advantageous application of the present invention occurs in the operation of steam generators, especially those which produce supercritical steam for use in power plants. In steam power plants a working fluid, such as water, is converted to steam by heating in a boiler or steam generator. The steam is then expanded and reduced in pressure in an engine or turbine and the exhaust steam from the turbine is thereafter cooled and converted to liquid form in a condenser. The liquid is then pumped at high pressure and is returned to the steam generator or boiler for reheating and reconversion to steam.

Supercritical boilers are those which heat the working fluid to a temperature at which the phase of the fluid is no longer responsive to pressure. That is, the fluid at the supercritical temperature cannot be liquefied no matter what pressure is applied to it. Generally, operation of steam power plants above supercritical temperatures will provide moreefficient and effective operation since the working fluid at such temperatures is capable of transferring substantially greater amounts of heat energy.. There are however a number of problems that must be accommodated in the design and construction of equipment capable of handling working fluids, such as steam, in the supercritical range.

' One requirement of steam generators which operate at supercritical steam temperatures is that the fluid flow through the interior conduit system of the boiler must be maintained at high velocity irrespective of the amount of steam being delivered to the turbine or engine. If this internal flow rate should decrease appreciably, the heat transfer characteristics of the various internal components would deteriorate and dangerous overheating would occur.

In general, a high fluid flow rate through steam generators is achieved by means of a recirculation system which bypasses the engine or turbine and the condenser. Thus the total flow through the steam generator is made up of two flows, namely: a through flow, which passes from the steam generator to the engine or turbine then through the condenser and back to the steam generator; and a recirculation flow, which also passes from the steam generator but which bypasses the engine or turbine and condenser and is recirculated back into the steam. generator with the through flow from the condenser. If the engine or turbine load should decrease thereby decreasing the through flow, the combined flow through the steam generator will also decrease. Accordingly the recirculation flow should be sufficient to maintain an adequate total or combined flow through the steam generator under such decreased load conditions.

The recirculation flow is maintained by means of centrifugal pumps driven by constant speed motors; and in many instances these pumps are located in the recirculation line so that they will not have to handle the combined flows.

The recirculation pumping requirements in steam generators of large capacity have been difficult to fulfill. This is because the centrifugal pump in the recirculation line must act upon a working fluid, namely supercritical fluid of rather low density; and as a result the pump is incapable of generating sufficient pressure to force fluid through the recirculation path at large mass flow rates. I

As steam power plants increase in size the requirements of the recirculation pumps also increase and in fact they may reach a point where the cost of a pump to handle the recirculation flow may become far out of proportion to the overall cost of the plant. One reason for this is that the recirculating fluid, being of relatively low density must be pumped at very large volume rates to achieve a given mass rate of flow. Pump sizes therefore have to be quite large. In addition, centrifugal pumps are only capable of producing a pressure difference directly proportional to the density of the fluid being pumped;

and since this density is low the effective pumping forces are also low.

The present invention overcomes the above described difficulties of the prior art. With the present invention it is possible to vary or to increase the pumping capability and capacity of a fixed speed centrifugal type pump. It is also possible, with the present invention to achieve satisfactory recirculation in large steam power plants with much smaller centrifugal pumps than has heretofore been necessary.

According to the present invention advantage is taken of the principle that the pressure difference generated by a centrifugal pump is proportional to the density of the fluid being pumped. Thus the present invention provides for an increase in the densityof the fluid passing through the pump. In this way the pumping pressure is increased and fluid is forced by the pump around the recirculation path at an increased mass rate of flow.

As illustratively embodied, the present invention obtains an increase in the density of the recirculation fluid in a steam generator by injection of a certain amount of the boiler feed or through flow into the recirculation flow at a location upstream of the recirculation pump but in the recirculation path. The boiler feed flow being of higher density than the recirculation flow, mixes with it to produce a combined flow of increased density at the recirculation pump input. The pump in turn produces a greater pressure differential or pumping force and thereby causes a greater mass flow rate of recirculating fluid.

Various further and more specific objects, features and ad-,

vantages of the invention will appear from the description given below, taken in connection with the accompanying drawings, illustrating by way of example a preferred form of the invention.

In the drawings:

FIG. 1 is a diagrammatic view of a steam power plant in which the present is embodied;

FIG. 2 is a graph illustrating typical pump head characteristics of a recirculation pump used in the power plant of FIG. 1;

FIG. 3 is a diagrammatic view of a centrifugal pump used for recirculation in the power plant of FIG. 1; and

FIG. 4 is a series of graphs useful in explaining the operation of the recirculation control obtained in the power plant of FIG. 1.

The steam power plant represented in FIG. 1 includes a steam generator unit 10 having a boiler section 12 and an economizer section 14. The steam generator unit supplies hot fluid, e.g., at supercritical temperatures through an outlet connection 16. The fluid passes from the connection 16 to a recirculation bypass 18 where the fluid is divided into a through flow and a recirculation flow.

The through flow passes along a through flow line 20 to a superheater and hence to a turbine 22 or other utilization device where the heat content of the steam is utilized as by expansion and conversion to mechanical energy. After passing through the turbine and any other steam utilization devices the exhaust steam passes through an exhaust line 24 to a condenser 26 where the steam is further cooled and condensed to liquid form. The liquid output or feedwater, from the condenser then passes to a feed pump 28 which forces the liquid at high pressure back into the economizer section 14 of the steam generator 10. The feedwater receives heat in the economizer from furnace exhaust gases, and is brought up to a temperature for injection into the boiler section 12. Upon exiting from the economizer section 14, the preheated feedwater passes into a mixer 30 where it is mixed with recirculation flow. The resulting combined flow passes through a downcomer 32 and back through the boiler section l2.

The recirculation flow, which is divided from the combined flow at the recirculation bypass 18, passes along a recirculation line 34 and through a recirculation pump 36. The output of the recirculation pump 36 is directed to the mixer 30 where it is combined with the preheated through flow for return to the boiler section 12.

Because of the recirculation flow the combined flow which passes through the boiler section 12 is increased. Moreover, by maintaining a high recirculation flow rate, it is possible to provide a substantial combined flow through the boiler section 12 even at low load conditions when only a relatively small through flow is called for.

It is important, especially in supercritical boilers, to maintain a high fluid flow rate through the boiler section even at low load conditions. This is because a high heat transfer is required internally of the boiler to prevent burning and other damage to its structure; and the heat transfer characteristics along most internal boiler surfaces are a function of the fluid flow rate along them.

The limiting aspect of a boiler recirculation system is the pump which drives the recirculation flow. These pumps, for reasons of cost, operating life, maintenance characteristics and other reasons, are of the fixed speed, centrifugal type.

While in some boiler recirculation systems the pump providing the recirculation flow drive is not exclusively in the recirculation flow path, the present description will be directed to situations where, as illustrated in FIG. 1, the recirculation pump 36 is arranged exclusively in the recirculation flow path.

The recirculation pump 36 must handle high temperature compressible fluids of low specific density. Because of this it becomes difficult to achieve a large mass rate of flow for a given size pump. FIG. 2 shows a typical characteristic operating curve (a) for a fixed speed centrifugal pump where head (in feet) is plotted against delivery (in gallons per minute). As can be seen the curve rises in the low delivery rate regions and then falls off gradually as delivery rate increases. The shape of this curve is a function, primarily, of the pump blade design; and it indicates that as the pump blades (moving at constant speed) impinge on an incoming fluid they produce different kinetic effects on the fluid where the fluid enters the pump at different speeds.

The head produced by a centrifugal pump corresponds to the height to which the pump will lift an element of the fluid being pumped. As shown in diagrammatic fashion in FIG. 3 an element (e) of some fluid is impinged upon by a vane 50 mounted to spin with a rotor 52 within a pump casing 54. This impingement throws the element upwardly and out through an outlet 56 to a height H. Assuming that at the point of impingement the blade velocity is fully transmitted to the element, then the element should rise to the same height H irrespective of its density. Thus the head produced by a centrifugal pump is independent of the density of the fluid which it pumps.

The head produced by a centrifugal pump can be converted to pressure and in such conversion the density of the fluid being pumped becomes a factor. Thus where a fixed speed centrifugal pump produces the same head for two fluids of different densities, each fluid, because of its particular density, will have a different pressure producing momentum.

FIG. 4 shows at (b) and (c) the pressure curves (in pounds per square inch) corresponding to the head curve (a) of FIG. 2, for fluids of different density. As can be seen, where a greater density fluid is pumped to the same head the pressure corresponding to its momentum is greater than that of the lesser density fluid.

Also shown in FIG. 4 is a stylized curve (d) representative of the overall pressure losses throughout a fluid flow system (such as the recirculation portion of the system of FIG. 1). In most fluid flow systems pressure losses are due to the viscous drag produced by the mechanical elements of the system, such as piping etc., on the flowing fluid; and since viscous drag increases with increased relative velocity, the pressure losses are greater at high flow rates. Accordingly, as shown in FIG. 4, the pressure loss curve (d) rises at higher flow rates.

As indicated by the vertical dashed lines (f) and (g) in FIG. 4, the resulting flow through the system is at the intersection of the pump pressure curve (b) or (c) with the pressure loss curve (d). It can be seen from this that in a given fluid flow system where the fluid is driven by a fixed speed centrifugal pump, the flow rate through the system can be increased by increasing the density of the fluid which passes through the pump.

Reverting now to FIG. 1 it will be seen that a bypass 38 is interposed between the outlet from the economizer l4 and the mixer 30; and this bypass directs a portion of the preheated feedwater through a bypass line 40, through a valve 42 to a junction 44 where it combines with the recirculation flow in the recirculation line 34 near the inlet or section side of the recirculation pump 36.

By providing this bypass, a certain amount of relatively cool high density boiler feedwater is mixed with the hot recirculation fluid in the recirculation line 34. This mixing action serves to change the characteristics of the fluid so that for a given volume, the weight of the fluid is greatly increased. Because the pump 36 now operates on a higher density fluid its pressure characteristic changes, as from curve (c) to curve (d) in FIG. 4 and the resulting recirculation flow rate is correspondingly increased. The valve 42 can be adjusted to control the amount of feedwater admitted to the recirculation flow and in this manner the delivery characteristics of the pump 36 can be adjusted. Thus even though the pump 36 operates at constant speed, an effective adjustability of the recirculation flow produced by this pump may be obtained by a simple valve control. This adjustment can be controlled in accordance with load variations so that depending on the amount of steam demanded in the through flow circuit, including the turbine and condenser, the rate of recirculation through the recirculation circuit including the recirculation flow line 34 and the pump 36 can be correspondingly adjusted to maintain a proper combined flow through the boiler 12 for protection thereof.

Having described the invention with particularity with reference to the preferred embodiment of the same, and having referred to some of the possible modifications thereof, it will be obvious to those skilled in the art, after understanding the invention, that other changes and modifications may be made therein without departing from the spirit and scope of the invention; and the appended claims are intended to cover such changes and modifications as are within the scope of the invention.

What is claimed is:

1. A method for controlling the delivery ofa given fluid by a centrifugal pump, said method comprising the steps of operating said pump, said method comprising the steps of operating said pump to deliver said compressible fluid, and during such operation mixing with said fluid, upstream of said pump, a second fluid of a different density than said given fluid thereby to provide a fluid mixture of a different density than said given fluid, which mixture passes through said pump to produce a changed pressure difference across said pump and a corresponding change in delivery of said pump.

2. A method according to claim 1 where said second fluid is of a greater density than said first fluid.

3. A method according to claim 1 wherein the amount of said second fluid is varied to change the delivery of said pump.

4. A method according to claim 1 wherein said first fluid is steam from a steam generator and wherein said second fluid is feedwater for said generator.

5. A method for controlling the recirculation flow in a steam power plant of the type having a steam recirculation circuit including a centrifugal recirculation pump to maintain high internal flow velocities at low load demands, said method comprising the step of injecting into the inlet side of said recirculation pump feedwater of higher density than the recirculation steam in said recirculation circuit.

6. A method according to claim 5 wherein said recirculation circuit is connected from the steam output of a steam generator to a mixing device for mixing the recirculation steam with boiler feedwater which is directed back into said steam generator.

7. A method according to claim 6 wherein said feedwater is obtained from upstream of said mixing device.

8. A method according to claim 7 wherein the amount of feedwater injected into said inlet side of said pump is controlled by a valve.

9. A method according to claim 8 wherein said valve is controlled to vary the amount of recirculation in accordance with the load demand on said steam power plant to maintain flow through the steam generator portion thereof.

10. Apparatus for controlling the delivery of a centrifugal pump, said apparatus comprising pump input conduit means connected to the input side of said pump, pump output conduit means connected to the output side of said pump, a first fluid supply means connected to supply fluid of one density to said pump, input conduit means and fluid injection means connected to inject fluid of a different density into the inlet side of said pump.

11. Apparatus according to claim 10 further including a recirculation circuit interconnecting the output and input sides of said pump.

12. Apparatus according to claim 11 wherein at least a portion of said recirculation circuit is shared with a second fluid circuit which supplies to said recirculation circuit said fluid of a different density.

13. Apparatus according to claim 12 wherein a further conduit is connected to direct a portion of said fluid of a difierent density from said second fluid circuit into the inlet side of said pump.

14. Apparatus according to claim 13 wherein said further conduit includes a valve.

15. In combination with a steam generator, steam outlet means from said generator, feedwater inlet means to said generator, a recirculation circuit including a bypass connected to said steam outlet means, a mixing device connected to said feedwater inlet means and recirculation conduit means including a centrifugal recirculation pump connected between said bypass and said mixing device and feedwater injection means connected to receive boiler feedwater from upstream of said mixing device and to direct same into the inlet side of said centrifugal recirculation pump.

16. A combination according to claim 15 wherein said steam generator is of the supercritical type.

17. A combination according to claim 16 wherein said mixing device is arranged to receive preheated feedwater from an economizer portion of said steam generator.

18. A combination according to claim 17 wherein said feedwater injection means comprises a conduit interconnecting the feedwater output from said economizer portion and said inlet side of said pump.

19. A combination according to claim 18 wherein said feedwater injection means includes a valve to control the ratio of feedwater and recirculation steam supplied to said pump.

Claims

1. A method for controlling the delivery of a given fluid by a centrifugal pump, said method comprising the steps of operating said pump to deliver said compressible fluid, and during such operation mixing with said fluid, upstream of said pump, a second fluid of a different density than said given fluid thereby to provide a fluid mixture of a different density than said given fluid, which mixture passes through said pump to produce a changed pressure difference across said pump and a corresponding change in delivery of said pump.

13. Apparatus according to claim 12 wherein a further conduit is connected to direct a portion of said fluid of a different density from said second fluid circuit into the inlet side of said pump.