US2163281A

US2163281A - Means for controlling feed water pumps

Info

Publication number: US2163281A
Application number: US191138A
Authority: US
Inventors: Hillier Harold
Original assignee: G&J Weir Ltd
Current assignee: Weir Group PLC
Priority date: 1936-12-10
Filing date: 1938-02-18
Publication date: 1939-06-20
Anticipated expiration: 1956-06-20

Description

' June 20, 1939. H $163,281

MEANS FOR CONTROLL INGT FEED WATER PUMPS Original F iled Dec. 10, 1936 3 Sheets-Sheet 1 Jun 20, 1939.

' Original Filed Dec. 10 1936 5 Sheets-Sheet 2 June 20, 1939. I H. HILLIER 2,163,231

MEANS FOR CONTROLLING ,FEED WATER PUMPS Original Filed Dec. 1Q, 1936 3 Sheets-Sheet 3 Patented June 20, 1939 UNITED STATES MEANS FOR CONTROLLING FEED WATER PUIWPS Harold Hillier, Cathcart, Glasgow, Scotland, assignor to G. & J. Weir, Limited, Cathcart, Glasgow, Scotland, a corporation of Great Britain and Northern Ireland Original application December 10, 1936, Serial No.

115,213. Divided and this application February 18, 1938, Serial No. 191,138. In Great Britain January 2'7, 1936 2 Claims.

This invention relates to means for controlling pumps used for discharging feed water into boilers, and particularly pumps of the centrifugal type driven by steam-actuated turbines as deaascribed in my application No. 115,213, filed De,-

' cember 10, 1936, and later abandoned, of which the present application is a division.

For a centrifugal boiler feed pump to run satisfactorily, the pump impeller or impellers must 10-,be so designed that the discharge pressure of the pump, when operating at constant speed, falls continually from that developed by the pump when no water is being discharged to that developed by the pump when discharging water at its maximum capacity. The relation between the discharge pressure of the pump and the quantity of water discharged by the pump is known as the pressure capacity characteristic, and is termed a falling characteristic when it falls continuously m from no load to full load.

Centrifugal boiled feed pumps must develop such a falling characteristic when running at constant speed if they are to operate stably when running solo and stably when operating in parallel with other feed pumps which may be sharing the duty of supplying water to a boiler or boilers.

The resistance to be overcome in discharging water into a boiler system is represented by the a summation of the pressure in the boiler, the static lift from the feed pump to the water level in the boiler, the pressure drop across the boiler feed regulator, and the frictional resistance in the piping, valves, heaters and economisers through which the water flows fromthe feed pump to the boiler. These frictional resistances increase approximately as the square of the rateat which water is discharged through the feed system to the boiler. The static lift to the boiler is constant, and, to obtain the best results from the boiler feed regulator, the pressure drop across the feed regulator should be maintained as nearly constant as possible.

It will be seen that the characteristic of the resistances to be overcome rises from a minimum when an extremely small quantity of water is flowing into the boiler to a maximum when the maximum quantity of water is discharged into the boiler. The relation between the resistance to ,be overcome and the quantity of water discharged 'by the, pump may be termed the resistancecapacity characteristic of the feed system, and, since itrises from no load to maximum capacity, it is a rising characteristic.

55 The difference in pressure between the falling characteristic of the feed pump and the rising characteristic of the feed system resistance must be absorbed by the feed regulator, and, with the usual method of governing a feed pump, this entails a pressure drop across the feed regulator which is very much greater at low loads than at the maximum load. Hence are caused difficulties in the operation of boiler feed regulators at low loads, and a considerable expenditure of power in excess of the minimum necessary to feed the boiler.

The present invention consists of improved means of controlling turbine-driven centrifugal boiler feed pumps whereby such pumps can be operatedstably and satisfactorily when running solo or in parallel and with a minimum expenditure of power for the duty required.

The invention is illustrated in the accompanying drawings in which Fig. 1 is a diagrammatic view of an installation of feed pumps with control means according to the invention. Fig. 2 is a sectional detail View. Figs. 3 and 4 show a modification, Fig. 3 being a diagrammatic view, and Fig. 4 a sectional detail view.

In the arrangement shown in Figs. 1 and 2,

which relate to installations of two centrifugal pumps operating in parallel, there is disposed in the discharge line of each pump l a convergentdivergent tube 2, and the discharges of the two pumps are connected in parallel. A valved crossconnection 2 is provided between the throats of the two tubes 2'. The throat of each tube 2 is connected to the cover 6 of a cylinder 5 in which is fitted a movable piston 4', so that the throat pressure is applied to the upper face of the piston t. The cylinder 5' and the piston 4' each have two diameters, the upper diameter being larger than the lower diameter. The top face of the piston l is exposed to the pressure at the throat of the tube 2'.

l4 surrounding the small diameter portion of the piston l within the cylinder 5 is connected to the inlet side of the tube 2', so that the annular area of the piston is subjected to the pressure at the inlet side of the tube 2. The smaller diameter portion of the piston i passes through the smaller bore of the cylinder 5' and is centred in an upper spring cap 8', the lower extremity of the piston i being subjected .to atmospheric pressure. The piston rests on the steam valve spindle l0 which also passes through the upper spring cap 8' and through a screw-threaded sleeve l I which is screwed into the lower spring cap 9 and which is provided with a collar resting on the bottom of a spring cage securely attached to the top of the The annular space steam valve body. The sleeve H is rotatable for the purpose of raising or lowering the lower spring cap 9 and adjusting the stress of the spring 1' held between the upper and lower spring 5 caps 8', 9'. Upward movements of the piston 4' are effected by the spring 1' in combination with the pressure at the inlet of the tube 2 applied to the annular area of the differential piston and atmospheric pressure on the lower end of the 10 piston. Downward movements of the piston 4 are the steam valve l3, whereby more steam is ad' mitted to the turbine. Downward movement of the piston 4 causes the steam valve I3 to close and reduce the flow of steam to the turbine. The difference in pressure between the inlet to' the g convergent-divergent tube 2 and the throat of the tube increases as' the square of the rate of flow through the tube, and is negligible at light flows.

It will be seen that, since the pressure differencc acting upon the differential area of the governor piston 4 is negligible at small rates of flow, the governor is controlled during. flow at such rates by the tube throat pressure acting on an. area equivalent to the smaller diameter of the piston in opposition to the datum pressure and the spring load.

Throughout the range of flow the arrangement gives a falling pressure capacity characteristic measured at the throat of the convergentdivergent tube 2, and maintains a rising pressurev capacity characteristic for the greater part of the flow measured at the inlet to the convergent-divergent tube.

Within the range of small capacities Where the s pressure difference between the inlet and throat of the tube is negligible'the discharge pressure characteristic of the pump is a falling characteristic corresponding tothe slope determined by the spring 7'. As the rate of fiow increases, the

';clischarge pressure characteristic changes to a rising characteristic when the rising slope due to the increasing pressure difference between the inlet and the throat of the convergent-divergent tube is greater than the falling slope determined by the characteristic of the spring.

It will be seen that the pumps are constrained to operate stably in parallel at all loads, even at very small loads, and at the same time are constrained to operate on a rising discharge pressure capacity characteristic, which can be determined'to suit the corresponding feed system resistance characteristic by suitably proportioning the convergent-divergent tube 2.

By a suitable choice of the spring and of the form of convergent-divergent tube, the amount of pressure drop through the tube 2 can be limited to that necessary to obtain stability in operation even though the pressure difference across the convergent-divergent tube is considerably less than the frictional resistance in the feed system.

The pressure losses due to the flow of water through the tube 2 are thereby reduced to a minimum with a view to obtaining the maximum possible economy associated with control to give -2. risingpressure capacity characteristic.

It will be seen'that, with the inter-coupling of the dischargesof the two pumps, and the interaction of the two governors the feed pumps are constrained to share the duty of supplying water to the boilers and are further constrained to operate stably in parallel.

A boiler installation usually comprises a number of boilers with several feed; heaters in series. The numberof heaters in operation and the number of boilers in operation may vary, with the result that the system resistance characteristic varies with the different arrangements of heaters in use and the different arrangements of boilers in use. It is desirable, therefore, to be able to modify the discharge pressure capacity charac teristic developed by the feed pump. For this purpose, as shown in Figs. 3 and 4, alternative connections 3 3 may be provided to different points of the throat of the tube 2 and coupled up to the top of the governor piston 4 such points being chosen asnecessary to give desired pump discharge characteristics, a selected control point being put in operation to suit the particular arrangement of the installation in service at a given time.

l. Ina boiler feed system, the combination'with a centrifugal feed pump having a discharge branch anda turbine for driving said pump, of a convergent-divergent tube directly connected to said discharge-branch; a valve controlling the supply of motive fl'ui'dto said turbine, a valvegov'erni-ng device-including a cylinder and a piston having diiferential surface areas movable in said cylinder, whereby" an annular space is formed in-said cylinder behind" the portion of the piston having thesm'aller surface area, the end of the piston remote from-t-he portion having the larger surface area being subjected to atmospheric pressure, aconnection from't'he throat of said tube tothat'pa-rt of the cylinder adjacent the surface of the piston having the larger surface area; whereby the piston surface of larger area is subjected to the pressurea't said'throah'a'connect'ion between the inlet end of-said tube and said annular space; a spring acting on-said piston in supplement to'the opposition presented by at mospheric pressure'to movement of said piston, andan;operativeccnnection between said piston and saidvalve effective'to control said valve in dependence on the position of said piston.

2. 'Ir'r'a boiler feed system; the combination with a" centrifugal feed pump having a discharge branchand aturbinefor'driving. said pump, of a convergent' divergent tube" directly connected to said discharge branch, a valve" controlling the supplyof" motive" fluid" to said turbine, a valvegoverning device" including. a cylinder and a pistonhaving"diifeientiarsurfac'e areas movable in said cylinder, whereby an annular space'is l formed in said cylinder behind th'e'portion of the piston haviligthe' smaller surface area, the end' of the piston remotefroni the portion having the'larger surface area being subjected to atmospheric pressure," selectable valved connections from about tliethroa'tbf'said' tube'to the part of said'cylinder adjacent the surface of the piston having the largerarea, whereby the piston surface oflargr area is subjected to the pressure at said throat, a'connectiori between the'inlet' end of said tube and said annular space, a springacting on said" piston in supplement" to the opposition presented by atmospheric" pressure to movement of said