GB2466473A - A liquid sealing system for inertial pistons - Google Patents

Abstract

The present invention relates specifically to sealing for inertial pistons, and discloses a sealing system for a piston 1 naming longitudinally in a cylinder 2 which divides the cylinder 2 into a high pressure volume 4 and a low pressure volume 5 and the space between the piston 1 and the cylinder 2 defines a seal volume 6. The piston 1 includes a dense internal plunger 7 which is free to slide relative to the piston in the direction of travel of the piston 1 within a volume of sealing liquid 9. The movement of internal plunger 7 serves to inject a sealing liquid into the seal volume 6 via the volume 9, and the pressure difference maintained between the two sides of the piston 1 acts to try to drive or pump the sealing liquid through the seal volume 6. The movement of the plunger 7 within the piston 1 is brought about by the difference between the actual acceleration of the piston and the natural acceleration in that direction due to gravity, the difference resulting mainly from the pressure difference between the two sides of the piston 1. The system may include a set of small wheels 3 which aids the free movement of the piston 1 in the cylinder 2.

Description

LIQUID SEALING SYSTEM FOR INERTIAL

PISTONS.

Field of the Invention:

This invention relates to sealing around a piston moving axially in a cylinder (possibly reciprocating) for the purpose of compressing a gas or, potentially, for the expansion of pressurised gas. Its main area of applicability lies in situations where the main force caused by the pressure difference between the gas on the two sides of the piston is inertial. That is to say, the force arising as (A.iXP) is reacted primarily by a combination of gravity and acceleration or deceleration of the piston.

Back2round.

Reciprocating machines for the compression / expansion of gas are very common. One central feature is common to the operation of all such machines -a pressure difference is caused to exist across a piston which is movable relative to a cylinder such that high pressure gas is present on one side and lower pressure gas on the other. In virtually all such machines (perhaps all) it is desirable to minimise the leakage of gas across the piston. Such leakage obviously dissipates energy. This invention describes a seal for minimising the gas leakage flow. There are many seal options which can reduce gas leakage flow to virtually zero (for example in gas-springs). However in these cases, the near-zero leakage flow has to be set against high friction losses and relatively low lifetimes in terms of the net linear distance travelled by the seal before its performance degrades significantly. The present invention is intended to achieve very large net linear distances with relatively low friction losses and still a very high sealing performance.

In most machines using a piston reciprocating relative to a cylinder, the force required to move the piston relative to the cylinder is derived from some mechanical or magnetic linkage. Most commonly, the piston is driven from a crankshaft. However, there are machines in which the movement of piston relative to cylinder is caused by purely inertial forces.

A trivial illustration of such a machine is a closed "cigar-tube" with a free-moving piston in the centre and two one-way valves at each end -one allowing air to enter freely from atmosphere and the other allowing air to exit into a high-pressure plenum whenever the pressure inside the cylinder at that end exceeds the pressure of air in the plenum. In the case of such a simple machine, the only forces (ignoring friction) allowing air to have a pressure difference across the piston are inertial forces. If the external tube of such a device is oscillated along the axis of the cigar-tube, inertial forces will cause the piston to move relative to the tube and air will be compressed and discharged into the plenum if the tube accelerations are sufficiently large and sufficiently long duration. A hand-held version of such a device might easily be made *.. The main motivation for this invention derives from machines which harvest renewable energy directly in : the form of compressed-air. There are numerous ways in which wind, wave or tidal power can be used to * cause an external cylinder to move such that resulting inertial forces induce an internal piston to move axially within the cylinder. One notable case is outlined in W02008003950-A2 where a (predominantly) *..: horizontal axis wind-turbine converts some or all of the power extracted from the wind by allowing free- *:. moving masses to travel internally within (blades of) the turbine rotor. The movement of these free-moving masses is driven primarily by gravity. When one blade is oriented vertically downwards relative to the rotation axis, the free-moving mass (or masses) in that blade will tend to pull towards the outermost * : radius. When the rotor has turned by more than a quarter of one revolution, the same mass (or masses) will have a tendency to return radially inwards. In some embodiments of the invention in W02008003950-A2, energy is extracted from the free-moving masses by allowing them to compress air to high pressures and discharge it.

Secondary applications for the present invention exist in cases where an inertial piston is associated with the rapid one-off expansion of a gas such as in some satellite-launch schemes or artillery/small-anns rounds.

Some features of reciprocating seals.

Most, if not all, seals presently used in reciprocating machines are contacting seals where a sealing element fills the radial space between piston and cylinder completely in some axial locations. In reciprocating internal combustion engines, piston-rings form these sealing elements and these are usually split rings which fit snugly into grooves in the piston and allow very little leakage past them. Elasticity within each ring keeps its outside diameter pressed against the inside wall of the cylinder. A second class of contacting seals uses elastomeric elements fixed to the piston to provide a light force between a piston member and the cylinder inside wall. The softness of the elastomer ensures that a good seal can still be achieved despite relatively light normal forces. The geometry of the elastomer ring(s) is usually configured such that as the pressure difference across the piston increases, the contact force between the elastomeric ring(s) and the cylinder inside wall is increased also.

All contacting seals have the two undesirable features of substantial wear and relatively high friction losses. Non-contacting seals are possible. Whilst these have effectively zero wear and very low friction losses they obviously allow much higher leakage flows across the piston and consequently there may be substantial power losses associated with these leakage flows.

In turbo-machinery, it is common to use so-called labyrinth seals to try to minimise flow between different internal volumes in the machine defined partly by rotor geometry and partly by stator geometry.

Labyrinth seals are non-contacting seals and they comprise a number of discrete small gaps between which there is some space. As fluid passes through each small gap, it acquires a high velocity and when the fluid exits the gap to enter the space at the end, it diffuses and loses its kinetic energy (turning it into heat). Then as the fluid enters the next gap, it must be accelerated again and so forth. In their applications in rotating machines, labyrinth seals often comprise a plain cylindrical inner-surface on the stationary side and a finned structure on the rotating side where the fins run in the circumferential direction. Although the rotor spins relative to the stator, the geometry of the space comprising the seal does not change substantially with time.

Labyrinth seals of similar general geometries can also be used in the context of reciprocating machines. In this context, if they are to be non-contacting, the piston must have some means of being guided relative to the cylinder which does not rely on rubbing between the labyrinth fins and the cylinder inside wall. For a non-contacting seal, the tips of the labyrinth fins must remain clear of the cylinder wall and for practical reasons associated with thermal gradients and elastic deformations of components, there will be a minimum average gap over the tip of each fin. Because these average gaps must be a minimum size, the * . leakage flow through such seals is regarded as being unacceptable for most practical purposes.

*S..SI * For the discussion of non-contacting seals, it makes sense to divide the entire volume of the cylinder into three main volumes: the volume of fluid in front of the piston (i.e. at the higher pressure side), the volume of fluid behind the piston (i.e. at the lower pressure side) and the volume of fluid occupying the space * : * around the piston walls. This latter volume is of most interest in this document and we will refer to it henceforth as the seal volume. S. * * . S * .. * S * * *.

Employing water or another liquid as the sealing fluid.

The pressure-drop in a fluid as it passes through a non-contacting seal comprises some combination of inertial and viscous effects. Like almost every liquid, water has substantially higher density and viscosity than air (or any other common gas) -even at high pressures. For any given geometry of non-contacting seal and any given pressure difference, we may reasonably expect that the power losses associated with the flow of water through this geometry will be significantly lower than the power losses associated with the flow of air through the same geometry. The viscous drag between the piston and the cylinder will be higher but this is likely to account for a very small net power loss compared with leakage. In the context of most reciprocating pistons, there are strong practical objections to introducing a liquid as a sealing fluid. It will be seen that for reciprocating pistons where the net gas forces across a piston are reacted by inertial forces produced by the piston, there is an elegant natural way to introduce liquid into the seal volume automatically and to main this volume of fluid in the vicinity of the piston.

Summary of the Invention.

The present invention is based upon the idea of pumping water or another liquid into the seal volume towards its leading edge at a pressure to match or exceed the pressure of the gas in front of the piston. In this way, little or no gas can leak into the seal volume. In most embodiments, the sealing liquid would be water and so for brevity in the remainder of this document, "water" will be used to represent the sealing liquid although other liquids can be considered. Because the entire purpose of the seal is to maintain a pressure difference between the two ends of the piston with minimum gas leakage, the water will tend to flow through the seal volume from the high-pressure end to the lower pressure end and thus there will be some power loss associated with this flow. The flow will be induced naturally by exploiting the fact that the pressure difference across the piston is automatically matched to the net inertial force on the piston.

Within the piston itself, water will be contained and provision will be in place to drive this water in the desired direction. Usually, this provision will involve another element movable along the axis of the piston relative to the piston itself. However, in some circumstances, the inertia of the water itself can be sufficient to achieve this.

Novel Features.

The present invention uses an intrinsic feature of inertial pistons to considerable natural advantage. The same acceleration/gravitational force which causes the pressure difference to occur across the piston also serves to pump a sealing fluid into the sealing gap. No prior art is to be found on sealing technologies specifically for inertial pistons which exploit the fact that the pressure-difference across the piston is proportional to its acceleration. *S.* * *

Specific Embodiment A. * ** *** * Figure 1 shows a free piston body (1) inside a cylinder (2). In this figure, small wheels (3) are indicated * : as the guide system ensuring that the piston can move freely relative to the cylinder without contact between the piston body and the cylinder wall. A volume of gas at high pressure is indicated as (4) and the * end of the piston facing this high-pressure gas volume will be termed the front of the piston. A volume of gas at lower pressure is indicated as (5) and the end of the piston facing this lower-pressure gas-volume : will be termed the rear of the piston. Note that the termsfront and rear are defined here in terms of which * * end of the piston faces the higher pressure and not by the instantaneous direction of motion of the piston within the cylinder. The space (6) between the piston and the cylinder wall is termed the seal volume here.

The magnitude of the seal-volume is exaggerated for clarity. In a typical application, the piston body (I) might have a mean radius of 250mm while the maximum radial distance through the seal volume Within the piston itself, a dense internal plunger (7) is free to slide relative to the piston in a direction collinear with the main direction of motion of the piston relative to the cylinder. One or more light centering springs (8) cause the plunger (7) to have a preferred position towards the centre of its travel when the gas pressure difference across the piston is very small but even when the plunger is close to one end of its travel, the net restoring spring force is relatively small compared with the inertial force on the plunger when the piston experiences full rated pressure-difference across it. The plunger is in contact with a volume of water (9) on either side. When it moves relative to the piston (caused by inertial forces associated with a pressure difference across the piston), the plunger tends to expel water into the seal-volume (6) at the front edge and it tends to induct water at the leading edge.

On the side-walls of the piston, there are raised annular fins (10) which impede the flow of water from the front of the seal volume towards the rear. (liven the rated pressure-difference between front volume (4) and rear volume (5) together with the properties of the water and the geometry of the seal-volume, some volume flow-rate Q of water is expected to occur. This quantity can be calculated or measured. For obvious reasons, this flow-rate will depend at least slightly on the velocity of the piston relative to the cylinder wall and the maximum expected flow rate is chosen as Q. Note that Q will be a function of the pressure difference AP.

The cross-sectional area of the cylindrical portions of the water volumes (9) on either side of the plunger is matched to the mass, M, of the plunger such that the inertial force caused to exist on Mby the deceleration of the piston body (I) produces the appropriate pressure in the water being expelled into the seal volume.

The mean axial length of the cylindrical portions of the water volumes (9) on either side of the plunger is determined by the maximum volume of water expected to be circulated through the seal-volume in any one "rated stroke" of the piston. This volume is computed as the integral of Q(AP) over the time taken for a single (one-directional) stroke.

One final feature provides for ensuring that the net water body travelling with the piston remains reasonably constant. Soft elastomeric lips (11) are located at the extreme ends of the piston to help to define the seal-volume (6). These serve the joint purposes of scooping water back off the inside walls of the cylinder as the piston moves along within it and providing a natural barrier between the air and the water to prevent extensive dissolution of air into the water at high pressures. Note that these lips are not performing a sealing function as such. The pressure difference across each one will normally be very small.

To maintain a supply of sealing water in the piston, water sprays in the walls and ends of the cylinder can * . provide a constant wetting of the cylinder walls and an air-trap (not shown in Figure 1) can be integrated into the piston such that undesired air inside the internal piston volumes can be purged automatically in * * : order that the water volumes (9) remain largely full.

* : It is obvious that seals of some description are also needed between the plunger and the internal cylinders of the piston. These are obviously liquid seals and they need not be very effective. The net linear distances travelled by these seals will generally be much smaller than those travelled by the main seal and so contacting seals may often be a logical choice here. * * * * S * S.

S S S* S *e

Specific Embodiment B. Figure 2 shows part of a free piston body (1) inside a cylinder (2). In this figure, small wheels (3) are indicated as the guide system ensuring that the piston can move freely relative to the cylinder without contact between the piston body and the cylinder wall. Note that the piston depiction is not complete here.

The piston is in direct contact with a continuation (12) which must experience the same inertial forces along the axis of the cylinder as the depicted portion.

A volume of gas at high pressure is indicated as (4) and the end of the piston facing this high-pressure gas volume is termed the front of the piston as before. The space (6) between the piston and the cylinder wall is again termed the seal volume. The magnitude of the seal-volume is exaggerated again for clarity. In a typical application, the piston body (1) might have a mean radius of 250mm while the maximum radial distance through the seal volume Within the piston itself, a dense internal plunger (7) is free to slide relative to the piston in a direction collinear with the main direction of motion of the piston relative to the cylinder. One or more light centering springs (8) cause the plunger (7) to return to a positively-determined mechanically-stopped home position when the net gas pressure difference across the piston is very small. As before, the spring constants are chosen deliberately to be light such that even when the plunger is close to the downward extreme of its possible travel relative to the piston, the net restoring spring force is relatively small compared with the inertial force on the plunger when the piston experiences full rated pressure-difference across it.

In this embodiment, the plunger is in contact with a volume of water (9) on only one side side. When the plunger moves relative to the piston (caused by inertial forces associated with a pressure difference across the piston), it tends to expel water into the seal-volume (6) at the front edge and it tends to induct water at the leading edge.

On the side-walls of the piston, there are raised annular fins (10) which impede the flow of water from the front of the seal volume towards the rear, Some volume flow-rate Q of water is expected to occur and this quantity can be measured (or calculated if the characteristics of further sealing components in the piston further along from the high-pressure end are known). As is also the case with specific embodiment A above, the flow rate Q will tend to be larger while the high-pressure gas is expanding and lower while the high-pressure gas is being compressed.

The cross-sectional area of the cylindrical portions of the water volume (9) in front of the plunger is matched to the mass, M, of the plunger such that the inertial force caused to exist on M by the deceleration of the piston body (1) produces the appropriate pressure in the water being expelled into the seal volume.

* **., Generalisations. **e*

* : * * The invention described here affords several obvious generalisations over what has been described in the specific embodiment above. Some of these are enumerated here.

*..: Firstly, the "bearing" system used to allow the piston to move freely relative to the cylinder might not *. comprise the rollers indicated in Figures 1 and 2 which have axles mounted in the piston. The bearing system might have no moving parts and might rely on hydrodynamic or hydrostatic pressure forces. S. * * . . * S. *. . S* S.

The term cylinder has been applied throughout to represent the prismatic hollow space within which the piston runs. It is not at all necessary that the cross-section of this prismatic space should be circular. It could take any general 2D form with certain polygons such as square and equilateral triangle being obvious choices.

The sealing liquid may be other than water. Fluids with high surface tension but low viscosity are especially attractive.

In some cases, the plunger itself is not needed. If the ratio obtained by dividing the total mass of piston by the total volume subtended beneath the sealing volume is equal to or less than the density of the sealing liquid, then no plunger is required. In many cases, this relatively low mean piston-density is not attractive because it leads to a requirement for very long pistons but this may sometimes be acceptable.

The raised fins may have quite general optimised profiles to accentuate the pressure-drop across them as fluid is pumped past them.

The sealing system may be applicable in piston systems which are designed primarily for "single-stroke" use. Possible examples include pneumatic accelerators for satellite launching where one or more very large masses may be accelerated along a track to a high velocity before entering a compression cylinder where they are decelerated (as a piston) by their action of compressing air. The deceleration would automatically provide the flow of sealing fluid. The high pressure air created could drive another inertial piston in a communicating tube and that inertial piston could be connected to a payload of some description such as a satellite. Examples might also include large artillery where, in effect, a gas is being expanded. In the case of artillery and other single-stroke applications, the fluid need not be recovered into the piston and the upper elastomeric fins (11) shown in Figure 2 are not required.

In the case where a long inertial piston is fitted within a cylinder, a one-sided arrangement such as is described by Figure 2 and by specific embodiment B could be installed at each extreme of the piston or at positions which are not axial extremes.

In applications where the piston should overate over many strokes, the liquid in the piston will have to be constantly replenished. This can be achieved simply by wetting the side-walls of the cylinder and allowing the piston elastomeric lips to gather in top-up water.

In some applications -especially where the piston length is such that it is appropriate to implement it as a number of discrete components coupled via articulating joints, pressurised gas-filled interspaces between the main liquid seals could be implemented by implementing passively-controlled pressure-difference splitting and very small leakage paths through the entire multi-component piston.

An illustration of effectiveness. * 0.' *

* In a specific possible instance of the wind-turbine with integral compression (such as described in *: * . W02008003950-A2), pistons of total mass 55,000kg are present within compression tubes within each of four blades. The tip-diameter of the blades is 200m and the turbine has a rated power of 18MW with : rotational speed of 0.54 radls. Each individual piston has a total travel of 65m within a compression tube of diameter 0.82m and pressures of 7Obar are generated. A total outward mass-flow rate of air of over *. 27kg/s is required to absorb the 18MW of power if there were no internal losses and if compression were adiabatic. * . * 0S * *. * . * S S * .*

This pressure difference exists across a single piston for roughly one-sixth of the time. The piston has a total length of I Om in order to accommodate the requisite mass inside the available cross-sectional area (an effective mean piston density of 10,000 kg/m3 is required just to achieve this length).

If 500 individual fms are provided on the outside of the piston with 1mm radial clearance all around between fin tip and cylinder wall, then the mass-flow-rate achieved through a single piston at full pressure-difference (of 69 bar) is 1.678 kg/s. With each piston accounting for a leakage of this magnitude for one-sixth of the time, the net overall mass-flow leakage across the pistons is around 1.12 kg/s -around 4% of the total expected output. The equivalent Continuous power loss is 745 kW due to leakage alone.

It is desirable to reduce that leakage. Allowing for the same 500 fins to be present but water-filled, the volume-flow rate of the water (when the piston is stationary relative to the cylinder wall) is around 13.5 litres/second in the presence of the full pressure difference across the piston. This flow rate is reduced when (if) the high-pressure gas is expanding and increased when the high-pressure gas is being compressed-which accounts for most of the duty. On average, the volume-flow rate is lower than this and the average power-loss per piston associated with water flow is less than 10kW (hence less than 40kW for the set of four pistons). The volume of water required to be pumped in one direction through the seal-gap during a single stroke is less than 50 litres so a reservoir of 50 litres would be provided in each cylinder swept by the plunger. * ..* * * * S4S * *

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