GB2213541A

GB2213541A - Pump impeller seals

Info

Publication number: GB2213541A
Application number: GB8828711A
Authority: GB
Inventors: Duane Mosure
Original assignee: Sundstrand Corp
Current assignee: Sundstrand Corp
Priority date: 1987-12-10
Filing date: 1988-12-08
Publication date: 1989-08-16
Anticipated expiration: 2008-12-08
Also published as: GB8828711D0; GB2213541B

Abstract

A seal 52, 54 for a pump impeller 22 comprising an annular ring freely received within a cylindrical opening in the pump housing 12 and disposed around a cylindrical portion of the impeller. <IMAGE>

Description

MECHANICAL SHAFT SEAL Technical Field The present invention relates to pumps, and more particularly to a seal for an aircraft fuel pump.

Backaround Art Fuel pumps usable in aircraft engines are, obviously, crucial to the safe operation of the aircraft.

Such pumps are commonly centrifugal pumps with impellers operating at 25,000-35,000 rpm to raise fuel pressure from around 50 psi at the pump inlet to over 900 psi at the impeller outlet. This pressure differential between the inlet and the outlet results in significant back flow or leakage back around the impeller (between the impeller and the housing) from its high pressure outlet to its low pressure inlet.

one approach to sealing against such leaking has been to minimize the clearance between the impeller and either the housing or an intermediate sealing member fixed to the housing. Even with close manufacturing tolerances, however, it has not been possible to reduce this diametral clearance to less than about 6-10 thousandths of an inch, which minimal clearance is required in view of a number of factors, including centrifugal and thermal growth, and imprecision in establishing and maintaining the axis of rotation of the impeller in the housing. As a result of this required clearance, prior art pumps moving, for example, 200 gallons per minute have commonly leaked as much as 30-40 gallons per minute of that back to the inlet.

Still other seals which have been used in the prior art include labyrinth seals which define tortuous paths in order to attempt to minimize any leakage of fluids therethrough. Such seals are shown, for example, in Naffziger Patent No. 4,269,564 and Hartland Patent No.

3,238.534. Such pumps still can allow 15-20* leakage such as previously described.

Leakage as described above not only reduces the capacity of the pump, but also increases the horsepower required for a desired pump output. For example, during flight and ground idle when the engines require little fuel but the pump is still operating at design speeds, the action of the pump will increase the fuel temperature, which also results in a requirement of more power for the pump (e.a., r from SO horsepower to so horsepower). It is, of course, desirable to minimize the horsepower requirements of any aircraft component.

The present invention is directed to overcoming one or more of the problems as set forth above.

Summary of the Invention The invention peoviaes a pump having a housing, an impeller inside the housing for impelling a fluid from an inlet to an outlet, at least one cylindrical opening in the housing about an associated cylindrical portion of the impeller, and, for the or each housing cylindrical opening, an annular seal ring which is freely received within that cylindrical opening and which has an outside diameter less than the inside diameter of the associated housing cylindrical opening.

The invention could be utilised to minimise the leakage of fluid back around the impeller from its inlet to its outlet. The invention could also be utilised to minimise the size and weight requirements of a pump necessary to produce a desired output. The invention could be further utilised to minimise the horsepower requirements for a given pump output.

Brief Description of the Drawings Fig. 1 is a cross-sectional view of a portion of an aircraft fuel pump illustrating the impeller seals of the present invention: and Fig. 2 is an enlarged view of a portion of Fig. 1 showing the seals of the present invention.

DescriDtion of the Preferred Embodiment A fuel pump 10 embodying the present invention is shown in Fig. 1. The pump 10 includes a housing 12 which suitably supports a drive shaft 14. The drive shaft 14 has a spline connection 16 with an inducer 18 and also has a spline connection 20 with the pump impeller 22.

The pump 10 is a centrifugal pump which draws fuel through the input 26, axially along the inducer 18, and then (as indicated by arrow 28) through radial blades or vanes 30 in the impeller 22. The fuel is ejected from the impeller 22 into a dircumferential ring 32 around the impeller 22, and then from the ring 32 through tangential passages 34 to the pump outlet 36.* It should be understood that the illustrated pump lo is merely exemplary of the type of pumps with which the present invention could be used. Modifications to various parts of the illustrated pump 10 could thus be made within the scope of the present invention.

As will be understood by those skilled in the art, the fluid pressure at the input of the impeller blades 30 is substantially less than the fluid pressure at the impeller blade output to the circumferential ring 32 (for example, 130 psi versus over 900 psi). Thus, there is a tendency for the fluid at the output to leak back between the housing 12 and the outside of the impeller 22.

The gaps 40, 42 through which the above-described leakage can occur can, with the present invention, be greater than would be provided by close machining of the parts, to thereby minimize the power lose from fluid friction during pump operation, The shaft seal structure of the present invention is best illustrated in Fig. 2, in which the spacing of the components is exaggerated for purposes of illustration. The seal structure includes a front seal or annular ring 50 and a back seal or annular ring 52 which seal against leakage around both sides of the impeller 22, and also aid in balancing the thrust loading of the impeller 22.

The front seal 50 comprises a ring of very hard material such. as CPW1OV (vanadium carbide) or silicon carbide within a support ring 54 made of hard steel or the like and interference fit within the housing 12 (which is commonly made of aluminum). The cylindrical section 56 defined by the support ring 54 could also be formed by a suitably hard integral portion of the housing as well.

The front seal 50 has a diametral clearance between its outer surface and the support ring 54 (of, e.rr,, about twenty thousandths) so that the seal floats in the cylindrical section56. The seal 50 has a radial flange 58 at its input end, which flange 58 seats on the radial shoulder 60 of the support ring 54 during operation. An offset portion 62 is also provided at the outlet end of the seal 50, which portion 62 both minimizes the mass of the seal 50 and provides a spacing from the impeller 22 to minimize fluid friction and resulting power loss.

The seal 50 includes a cylindrical portion 64 which is disposed around the cylindrical portion 66 of the impeller 22. Inasmuch as the seal 50 floats, it will track the outer diameter of the impeller cylindrical portion 66 and can thus be provided with a minimal diametral clearance from the impeller cylindrical portion 66 tea. five thousandths of an inch and less), and therefore substantially minimize leakage therethrough (e.a., to substantially less than ten gallons per minute versus 30-40 gallons per minute leakage commonly found in prior art pumps pumping 200 gallons per minute).

The back seal 52 floats within a support ring 70, and functions in substantially the same manner as the above-described front seal 50 to both minimize leakage, and balance the thrust loading.

A pump having the seal structure of the present invention will thus have minimal leakage around the impeller from its outlet to its inlet. Accordingly, such a pump will have minimal horsepower requirements for desired outputs, which in turn allows for the size and weight of the pump to be minimized for partIpular design outputs.

Other aspects, objects and advantages of the present invention can be obtained from a study of the drawings, the specification and the appended claims.

Claims

1. A pump comprising a housing, an impeller inside the housing for impelling a fluid from an inlet to an outlet, at least one cylindrical opening in the housing about an associated cylindrical portion of the impeller, and, for the or each housing cylindrical opening, an annular seal ring which 'is freely received within that cylindrical opening and which has an outside diameter less than the inside diameter of the associated housing cylindrical opening.

2. A pump according to claim 1, wherein there are two annular seal rings and two associated cylindrical openings in the housing, axially spaced one on each side of a transverse plane containing the impeller outlet.

3. A pump according to claim 1 or claim 2, wherein the diametral clearance between the or each housing cylindrical opening and the or each impeller cylindrical portion is less than or equal to five thousandths of an inch (0.127mm).

4. A pump according to claim 3, wherein only the diametral clearance between the or each housing cylindrical opening and the or each impeller cylindrical portion is less than or equal to five thousandths of an inch (0.127mm), and the diametral clearance between the impeller and the or each annular seal ring at other locations is greater than five thousandths of an inch (0.127mm).

5. A pump according to any preceding claim. wherein the or each housing cylindrical opening is defined by a cylindrical steel collar secured within the housing by an interference fit.

6. A pump according to any preceding claim, wherein the or each annular seal ring is made of silicon carbide.

7. A pump according to any of claims 1 to , wherein the or each annular seal ring is made of vanadium carbide.

8. A pump according to any preceding claim. further comprising a radially extending shoulder at an end of the or each housing cylindrical opening, and a radially extending flange on the or each annular seal ring which seats on the shoulder during operation of the pump.

9. A pump* substantially as herein described with reference to the drawings.