GB2243406A

GB2243406A - Centrifugal pump

Info

Publication number: GB2243406A
Application number: GB8820806A
Authority: GB
Inventors: Trevor Stanley Smith
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1987-10-07
Filing date: 1988-08-25
Publication date: 1991-10-30
Also published as: IT8947978A0; IT1233500B; FR2664663A1; GB8820806D0

Abstract

The impeller 13 of a pump lies in a chamber 12 into which liquid can flow through an inlet 11 which can be shut by a valve 14. A plurality of chambers 25-28 are defined within a sleeve 17 by discs 21-24. Circumferential clearance between the discs 21-24 and the sleeve 17 provide flow restrictions by means of which the chambers 25-28 communicate with each other and with the chamber 12. When the valve 14 is opened the chambers 25-28 fill successively to accommodate differences between flow rates through the inlet 11 and the pump delivery, and thereby to minimise pressure surges arising from these differences. <IMAGE>

Description

CENTRIFUGAL PUMP This invention relates to centrifugal pumps and in particular to such pumps when intended for supplying fuel to gas turbine engine reheat systems. Such pumps are commonly supplied with fuel by means of a boost pump and have an engine-driven impeller. The pumps are provided with a valve for shutting the pump inlet when the reheat system is not in use, in which condition the pump empties through a drain valve. When the reheat system is brought into use the pump is required to be primed in about 0.1 seconds. The inlet valve is opened and rotation of the impeller intitially creates a vapour core which exists until the pump is full, at which time the inlet flow has suddenly to reduce to a level equal to that of the pump delivery. This reduction results in a pressure surge which will travel back to the fuel tank by way of a supply passage and the boost pump.In order to avoid damage it is desirable that this pressure surge is prevented from becomming excessive. It is an object of the present invention to provide a centrifugal pump in which this requirement is met.

According to the invention there is provided a centrifugal pump comprising a housing having an inlet and a pumping chamber, an impeller in said pumping chamber, a valve for shutting off said inlet from said pumping chamber, a container and a first flow restriction interconnecting said container and said pumping chamber.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawing in which: Figure 1 is a longitudinal section through part of a centrifugal pump and Figures 2 to 5 are views on respective arrows in Figure 1.

As shown in Figure 1 a centrifugal pump has a housing 10 having an inlet 11 and defining a pumping chamber 12 in which a bladed impeller 13 is rotatable.

Sealingly slidable in the housing 10 is a valve control element 14 which is biased by a spring 15 towards engagement with a seat 16 in the housing 10. The spring 15 surrounds a cylindrical sleeve 17 which forms part of the housing 10 and which serves to guide the control element 14. The element 14 and sleeve 17 are as shown, axially aligned with the impeller 12. The element 14 is movable against the spring by a biassing fluid pressure HP applied through a port 18 and acting on a piston portion 19 of the element 14. A servo pressure is derived from the biassing pressure HP by means of a valve 31 and is applied to a port 32 to urge the element 14 to a shut position.

Rigidly mounted in the sleeve 17 is a stem 20 having four axially spaced discs 21, 22, 23, 24 which define four volumes 25, 26, 27, 28 within the sleeve 17. Each of the discs has between .152 and .252 mm diameteral clearance within the sleeve 17. The discs 21, 22, 23 are also provided with four equi-angularly spaced radial slots 30 all of which are 2.54mm in width. The slots in the disc 21 have depths of 5.2mm, those in the disc 22 depths of 3.43mm and those in the disc 23 depths 1.6mm. The radial clearances of the discs and, where appropriate the slots 30 provide flow restrictions between the volumes 25, 26, 27, 28 and the, or each, adjacent volume. The axial spacings of the discs 21, 22, 23, 24 are such that the volumes 25, 26, 27, 28 are 26.2, 19.7, 13.2 and 6.6ml respectively.

The dimensions of the volumes and flow restrictions given by way of example above are based on an assumed pipe length of 6 meters between the pump and the tank from which it draws its fuel, in which case a pressure surge will travel from the pump to the tank and return to the pump in approximately .01 seconds.

It is also assumed that a boost pump mounted at the tank will deliver fuel at approximately 1380 kN/m2.

In use the pump impeller 13 rotates continuously. As the valve 14 opens fuel flows through the inlet 11 at an increasing rate as the pumping chamber 12 fills. Immediately prior to the chamber 12 becoming full fuel flows through the inlet 11 at about 5 litres/second, which is approximately twice the pump delivery flow of 2.5 litres/second, determined by a fuel metering device downstream of the pump. At this point fuel flows into the volume 25 through the restriction provided by the disc 21 at 2 litres/second.

The volume 25 thus fills in .013 seconds, which is greater than the time of .01 seconds for travel and return of the pressure surge arising from the excess of pump inlet flow over delivery flow. As a result of flow into the volume 25 the flow through the inlet 11 reduces by 5 - (2.5 + 2) = 0.5 litres/second instead of reducing by 2.5 litres per second as would be the case if the volume 25 were not provided. The magnitude of the surge pressure, over the boost pump delivery pressure, is correspondingly reduced to 420 kN/m2, which is a fifth of that which would otherwise be experienced.

When the volume 25 has filled fuel enters the volume 26 at 1.5 litres/second, filling that volume in .013 seconds. The inlet flow reduces to 4.0 litres/second and another 420 kN/m2 pressure surge passes to and from the fuel tank. The process is repeated as the volumes 27, 28 fill successively in .013 seconds each, accompanied by the aforesaid reduced pressure surge in each case, after which the inlet flow has reduced to 3.0 litres/second. A final pressure surge of 420 kN/m2, unaccompanied by filling of any additional volume, reduces the inlet flow to 2.5 litres/second, which corresponds to the delivery flow.

The pump thus fills well within the required 0.1 seconds and a succession of reduced pressure surges occurs instead of the 2100 kN/m2 which would otherwise result.

Each pressure surge is generated as a positive wave at the centrifugal pump and is reflected back from the tank as a negative surge. Return of the negative surge to the inlet 11 thus causes the inlet pressure to fall to the delivery pressure of the boost pump.

Selection of dimensions so that each successive volume 25, 26, 27, 28 fills in .013 seconds allows return of the negative pressure surge to the inlet 11 on each occasion before the next surge starts. The surges are thus not cumulative and the magnitude of each surge is restricted to 420 kN/m2 above boost pump delivery pressure at each occurrence.

The number of the volumes to be filled successively is selected so as to reduce each surge to an acceptable level commensurate with an acceptable time to fill the pump and having regard to the rate of pump delivery.

Claims

CLAIMS.

1. A centrifugal pump comprising a housing having an inlet and a pumping chamber, an impeller in said pumping chamber, and a valve for shutting of said inlet from said pumping chamber, said pump further including a container and a first flow restriction interconnecting said container and said pumping chamber.

2. A pump as claimed in claim 1 in which said container comprises a first volume communicating with said pumping chamber through said first flow restriction and a second volume communicating with said first volume through a further flow restriction.

3. A pump as claimed in claim 2 which includes a plurality of further volumes interconnected in series by respective flow restrictions, one of said further volumes communicating with said first volume.

4. A pump as claimed in claim 2 or claim 3 in which said housing includes a sleeve whose interior communicates with said pumping chamber, said volumes being defined by a plurality of partition elements within said sleeve.

5. A pump as claimed in claim 4 in which said sleeve is cylindrical and said partition elements comprise axially spaced discs.

6. A pump as claimed in claim 5 in which said flow restrictors comprise circumferential clearances between said discs and said sleeve.

7. A pump as claimed in claim 6 in which said discs are located on a central stem secured to said housing.

8. A pump as claimed in any of claims 5 to 7 in which said sleeve is axially aligned with said impellers.

9. A pump as claimed in any preceding claim in which said valve comprises a control element axially slidable in said housing to engage a seat, said container lying within said control element.