GB2125118A - Hydraulic seal - Google Patents

Abstract

The efficiency of an hydraulic seal is improved by the provision of a pump rotating with the part of the seal formed with the liquid-containing recess. The pump receives liquid either directly from the supply or as a recirculatory flow from the recess and directs it at maximum angular velocity into the radially outermost part of the recess. The pump enables the seal to be used between contra-rotating shafts, and gives the seal a self- priming capability. Referring to Fig. 1 of the drawings two contra-rotating shafts 1 and 2 of a gas turbine engine are shown with an hydraulic seal between them. The seal has a recess 3 formed in the radially outer shaft which rotates at a higher speed. The recess is defined between upstream and downstream weirs 4 and 5 respectively. The pump 9 consists of vanes which extend between the weir 5 and a further radially inwardly extending wall 10 on the shaft downstream of the weir. The pump receives oil from the supply 7 to the recess, and from the recess, and delivers it to the recess at the radially outermost part thereof via an annular passage 12. A second embodiment is disclosed in which, to ensure self-priming of the pump, all of the oil supplied to the recess passes through the pump and is delivered to a passage outside the outer shaft and is delivered to the recess through apertures in the shaft. <IMAGE>

Description

SPECIFICATION Hydraulic seal The present invention relates to hydraulic seals.

Hydraulic seals are known between relatively rotatable components and generally include a rotatable component in which is formed an inwardly facing recess, and a second component rotatable, in the same sense but at lesser speed and which has a sealing member which projects into the recess. The recess is defined between upstream and downstream weirs, and is supplied with a liquid. The liquid is held in place in operation by centrifugal action, and the sealing member is of sufficient radial extent to dip into the liquid in the recess to form a barrier against a flow of gas past the seal. The terms upstream and downstream are used in relation to the direction of flow of the gas.Provided that the downstream weir is of sufficient radial extent, the centrifugal force on the liquid between the sealing member and the downstream weir can be arranged to be greater than the pressure forces on the liquid on the upstream side of the sealing member, so that a complete barrier is formed to the flow of gas past the sealing member.

The driving force causing rotation of the liquid in the recess is friction between the liquid and the walls of the recess, and within the fluid itself.

There is, therefore, a slip factor which prevents all of the liquid from reaching its maximum angular momentum with a consequent loss of centrifugal pressure in the liquid.

Such seals have, therefore, been used and work adequately with small weir depths, against relatively low pressure gas flows, for example, 5 psi to 10 psi. Also their use has so far been limited to seals between relatively rotating components which are para-rotating.

When the gas pressure is much higher, for example, 60 to 80 psi much higher centrifugal forces are needed in the liquid which leads to much greater radial depth in the downstream weir.

Due to the space considerations it may not always be possible to use weirs of the required radial depth.

In the case of contra-rotating components, such seals have not found application and this is believed to be because the liquid, being dragged round in opposite directions by the surfaces of the weir and the seal member, attains an average rotational velocity which is less than that of the higher speed shaft by a considerable margin, and becomes very inefficient.

Another problem is that, if the seal should break down, due for some reason to a temporary loss of liquid, after the gas pressure has reached its steady maximum, the resulting flow of gas past the sealing member can prevent the re-establishment of the liquid layer in the recess when the liquid supply is on the downstream side of the seal.

It is an object of the present invention to provide a hydraulic seal which is of sufficiently increased efficiency that the above-mentioned problems may be at least partially overcome.

According to the present invention an hydraulic seal between two relatively rotatable components comprises a radially inwardly facing recess defined between upstream and downstream weirs on the component which rotates at the higher speed, means for supplying a liquid to the recess, a sealing member on the other one of the components and which is arranged to project into the recess to a sufficient extent to dip into the liquid which is held in the recess by centrifugal action, and a pump rotating with the higher speed component said pump being arranged to receive liquid from the supply or the recess, to positively drive the liquid at the speed of the component and to deliver it to the recess at a radially outermost part thereof.

By this means, it is ensured, particularly in a seal between contra-rotating components, that more of the liquid will be driven at the speed of the high speed component and will have the maximum centrifugal force thereon offsetting the opposing effect of the contra-rotating components. Also by arranging for the liquid supply means to deliver the liquid to the inlet of the pump, after any temporary loss of liquid in the seal, the pump will re-prime the recess by delivering liquid to the recess with maximum centrifugal pressure, which will be able to counter the established air flow over the sealing member.

The pump is preferably in the form of vanes or a porous structure which provides a positive drive to the liquid.

In one embodiment of the invention, the two relatively rotatable components are contrarotating shafts of a gas turbine engine and oil is used as the liquid to seal an air flow between the shafts.

The invention will now be more particularly described by way of example only and with reference to the accompanying drawings in which: Fig. 1 is a sectional elevation of a pair of shafts including one form of seal of the present invention, and, Fig. 2 is a sectional elevation of a pair of shafts including an alternative form of seal of the present invention.

Referring now to Figure 1 there is illustrated a pair of shafts 1 and 2 which are contra-rotating, the radially outer one having the higher speed of rotation. A recess 3 is formed in the radially inner surface of the outer shaft, and is defined between upstream and downstream weirs 4 and 5 respectively. The words upstream and downstream are used in relation to the direction, shown by arrow A, of a gas flow between the shafts.

The inner shaft, which is the lower speed shaft, is provided with an annular fin 6 which projects radially outwardly from the surface and is of sufficient radial extent to dip into liquid trapped in the recess by the weirs. The liquid is supplied to the recess by a tube 7 which directs the liquid into the recess from the downstream end.

Air flowing down the annular space between the shafts in the direction of arrow A has to pass over the tip of the sealing fin 6 in order to get past the seal. Thus, provided that the downstream weir 5 is deep enough, the centrifugal force on the depth of oil between the weir 5 and the fin 6 will create sufficient pressure to overcome the pressure of the air on the oil on the other side of the fin, and create a total blockage to the air.

Because of the contra-rotation of the shafts, the sealing fin will exert a viscous drag on the oil in opposition to the driving force exerted by the surfaces of the recess. This tends to lower the average velocity of the oil in the recess by a "slip factor" which can be quite high. In order to counter the slip, a pump 9 is introduced, which takes the form of vanes provided between the downstream side of the weir 5 and a further radially inwardly extending wall 10 on the shaft 2.

The liquid supply tube, which in this example supplies liquid at the downstream end of the recess, is arranged to deliver the liquid to the radially inner ends of the vanes which constitute the inlet of the pump 9. The outlet from the pump delivers liquid at the radially outermost diameter of the recess via an annular passage 12 terminating adjacent the tip of the fin 6. By arranging that the radially inner ends of the vanes collect any liquid passing over the weir 5, a recirculatory flow can be established through the recess, with high velocity liquid from the annular passage 1 2 passing radially inwardly through the spaces 13 between the fin 6 and the wall 14 of the passage 12. The recirculatory flow may form an additional or an alternative supply for the pump 9.Thus it can be seen that the pump 9 positively drives the liquid at the speed of rotation of the shaft 2. This ensures that at least some of the oil achieves the maximum centrifugal pressure. The depth of the annular passage 12 is equal to the clearance between the fin 6 and the base of the recess 3, so that the maximum centrifugal pressure is available in the oil over the whole of the clearance.

The pump, therefore, increases the efficiency of the seal by eliminating the slip factor in the liquid which is present in all seals where friction between the liquid and the surfaces of the recess is the only driving force on the liquid. Hence a greater pressure can be generated in the liquid for a lesser radial depth of liquid.

In a seal for a gas turbine having contrarotating shafts, the liquid supplied is engine oil, the churning of the oil by the contra-rotating fin heats the oil, so that it is important to maintain a supply of cool oil to the recess to avoid boiling or frothing of the oil which would enable the air to blow past the seal. Apertures 8 at the upstream side of the upstream weir allow oil spilling over the upstream weir to escape back to the drains upstream of the engine.

During normal operation of the engine the pressure of the air being sealed is always related to the speed of the engine so that at low speeds, when the centrifugal pressure of the oil is low, the pressure of air which has to be resisted is low. If for any reason, however, the oil supply is interrupted after the full air pressure has been established, and the seal fails, there will be a high velocity jet of air passing over the tip of the fin 6 which could prevent the oil, when the supply is resumed, from reaching the recess and establishing its full centrifugal pressure. With the pump-assisted seal of the present invention, however, the pump will positively drive the oil up to its maximum rotational speed, and direct it into the recess with maximum centrifugal pressure.

This will ensure re-priming of the seal.

An alternative embodiment of the seal for dealing more specifically with this problem is shown in Fig. 2. Features of the seal which are identical to those in Fig. 1 are given the same reference numerals. In this embodiment the radial depth of the downstream weir is greater than the radial depth of the most downstream wall 10 of the pump 9, and all of the oil supplied to the seal is supplied to the inlet of the pump. The annular passage 12 is formed between the shaft 2 and a cylindrical shroud 1 5 which extends upstream beyond the fin 6, and the passage 12 delivers its oil supply directly into the recess on the upstream side of the fin via apertures 1 6. Additional pumping vanes 1 7 are provided on the downstream weir.

This arrangement ensures, after any breakdown in the seal due to a temporary loss of oil, that when the oil supply is resumed it is directed with maximum centrifugal pressure to the upstream side of the fin to re-establish the seal.

Claims (10)

1. An hydraulic seal between two relatively rotatable components comprising a radially inwardly facing recess defined between upstream and downstream weirs on the component which rotates at the higher speed, means for supplying a liquid to the recess, a sealing member on the other one of the components and which is arranged to project into the recess to a sufficient extent to dip into the liquid which is held in the recess by centrifugal action, and a pump rotating with the higher speed component, said pump being arranged to receive liquid from the supply or the recess, to positively drive the liquid at the speed of the component and to deliver it to the recess at a radially outermost part thereof.

2. An hydraulic seal as claimed in Claim 1 and in which the pump comprises vanes on a radially inwardly extending wall of said higher speed component.

3. An hydraulic seal as claimed in Claim 2 and in which the higher speed component has a radially inwardly extending wall on the downstream side of the downstream weir and spaced therefrom, the vanes extending between said wall and the weir.

4. An hydraulic seal as claimed in Claim 3 and in which an annular passage is formed between the radially outer end of the downstream weir and the radially inner surface of said component, the discharge from the pump being arranged to flow into said passage.

5. An hydraulic seal as claimed in Claim 4 and in which the radially inwardly extending wail extends inwardly to a greater extent than the downstream weir.

6. An hydraulic seal as claimed in Claim 3 and in which there is provided a cylindrical shroud surrounding and radially spaced from the higher speed component and terminating at its downstream end in said radially inwardly extending wall, said radially inwardly extending wall extending inwardly to a lesser extent than the downstream weir, the means for supplying liquid to the recess being arranged to direct the liquid towards the radially inner ends of the vanes, whereby the vanes direct the liquid from the supply means into the space between the shroud and the component, one or more apertures being provided through the component and through which the liquid passes, in operation, into the recess at the outermost radius thereof.

7. An hydraulic seal as claimed in Claim 6 and in which the passages through the component are disposed on the upstream side of the sealing member.

8. An hydraulic seal as claimed in any preceding claim and in which the relatively rotating components are two relatively rotating shafts of a gas turbine engine.

9. An hydraulic seal as claimed in Claim 8 and in which the two shafts are contra-rotating.

10. An hydraulic seal substantially as hereinbefore more particularly described with reference to the accompanying drawings.