Seal for a joint or juncture – Seal between relatively movable parts – Piston ring or piston ring expander or seat therefor
Reexamination Certificate
1999-11-29
2002-10-01
Sandy, Robert J. (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Piston ring or piston ring expander or seat therefor
C277S546000
Reexamination Certificate
active
06457722
ABSTRACT:
The invention relates to a sealing element for dry running systems, in particular for dry running piston compressors, in accordance with the preamble of the independent claim 1 and to its use for dry running piston compressors.
Sealing elements for dry running systems, such as for example dry running piston compressors for the oil-free compression of gases, serve to seal off a pressure difference between a high pressure and a low pressure side of the system, with a foreign lubrication, e.g. by means of oil, being dispensed with. Since sealing of this kind is based on dry friction, sealing elements in dry running systems are usually subject to a natural wear which limits their lifetime—also called the working life.
Sealing elements for dry running systems are nowadays mainly manufactured of plastic, for example of filled polymers. A frequently used polymer material is for example polytetrafluoroethylene (PTFE). Fillers such as amorphous carbon, graphite, glass fibres, metals, ceramics or solid lubricants are introduced into the PTFE matrix.
In a dry running piston compressor, sealing elements are used for the sealing of the piston rod and/or as piston rings for sealing the piston. The counter-running partner of the sealing element, which means the surface which is moved relative to the sealing element, is the piston rod in the first case and the inner wall of the cylinder, the so-called cylinder running surface, in the second case.
The performance of a sealing element in a dry running system is frequently characterised by the so-called pv value. This value is the product of the contact pressure (p) with which the sealing element is pressed against the counter-running partner and the frictional velocity (v) at which the sealing element moves with respect to its counter-running partner. For a dry running piston compressor, to which reference is made in the following as a representative example of a dry running system, the pv value is to be understood as the product of the average pressure difference (p) to be sealed off and the average piston velocity (v).
In practice it is desirable for the operation of dry running piston compressors to have a high performance sealing element which can withstand pv values which are as high as possible for as long as possible. In this, the leakage rates along the sealing element should be very low. Furthermore, the sealing elements should have the longest possible working life, which means that their sealing properties should remain substantially of unaltered high quality over the longest possible working life.
It is usual, to increase the working life in particular, to arrange a plurality of sealing elements one after the other in the axial direction, both for the sealing of the piston rod and for the sealing of the piston, and form a sealing element set. For the sealing of the piston rod, a component which contains a sealing element set of this kind is called a packing.
It is also known to provide special sectional shapes of the sealing element to increase the working life. In CH 439 897 a packing for the sealing of a piston rod is for example disclosed having a plurality of sealing rings each of which is segmented, with the section lines between the individual segments of a sealing ring extending approximately tangentially to the inner jacket surface of the sealing ring. A hose spring encircling the sealing ring provides a prestress and presses the individual segments against the piston rod. The wear caused by abrasion of material is compensated in that the individual segments are pushed inwards by the pressure exerted on them in the direction of the piston rod.
However, for the maximum pv value which a packing of this kind can withstand in continuous operation, it is not the number of sealing elements in a packing which is primarily decisive but the performance of an individual sealing element. It is known, namely, that the distribution of the pressure difference to be sealed off is usually not uniform over the different sealing elements of the sealing element set. In practice the realistic extreme case can even be such that the entire pressure difference is substantially sealed off by only one of the sealing elements.
Sealing rings for dry running piston compressors are usually manufactured nowadays with an axial ring height of at least 7 mm. This minimum height is based, on the one hand, on the consideration that the friction surface of the sealing ring becomes too small at smaller ring heights and thus that its sealing action becomes too poor so that at typical pressure differences of, for example, 40 bar, the leakage rate would take on unjustifiably large values. On the other hand this minimum thickness is motivated by the fact that the sealing ring must also be sufficiently stable mechanically in order to withstand the stresses without a significant deformation, that is, one which considerably influences the sealing action. In particular PTFE, which is frequently used as sealing ring material, is known to have a pronounced tendency to creep (high cold flow) so that an additional support ring is necessary to avoid deformation of the sealing ring at pressure differences of for example 40 bar.
Most of the sealing rings which are segmented for compensation of the wear admittedly have a segment profile which is gas-tight in the radial direction; the usual direction of cutting however produces gaps and joints which are open in the axial direction and must be sealed off by an additional cover ring. This cover ring is frequently of three piece radially cut design and arranged together with the actual sealing ring to form a sealing ring pair in such a manner that through-going gaps are no longer present. A fixing pin between the two rings is intended to prevent the alignment of the ring gaps to form through-going gaps during operation. The cover ring usually has the same axial height as the sealing ring. Such known sealing ring pairs typically achieve a total axial height of 18 mm and more. These axial heights limit the number of sealing ring pairs which can be arranged in a packing with a predetermined constructional length.
One of the main problems which limit the maximum pv value which a sealing ring can withstand over longer periods of time is the frictional heat arising in dry running sealing elements. With increasing pv values, such high temperatures rapidly arise in the frictional surfaces that the sealing elements, which frequently consist of plastic nowadays, are thermally destroyed. Practice has shown that currently known sealing elements for dry running systems are immediately destroyed or have unusably short working lives at pv values of more than about 140 bar·m/s. For an average piston velocity in a dry running piston compressor of 3.5 m/s, there thus results a pressure difference maximum of 40 bar which can be sealed off by a sealing element.
With this state of the art as a starting point it is an object of the invention to provide a sealing element with a better performance for dry running systems, in particular for dry running piston compressors. The sealing element should be able to withstand pv values significantly greater than 140 bar-m/s and also have a long working life during which the sealing action substantially remains of the same quality. In addition the sealing element should also enable high pressure differences to be sealed off in a dry running system with very low leakage rates and have only a minimum wear.
The sealing element for dry running systems, in particular for dry running piston compressors, satisfying this object is characterised by the features of the independent patent claim
1
. The sealing element in accordance with the invention has a segmented sealing ring which comprises at least two ring segments which are adjacently arranged in its peripheral direction, with in each case two adjacent ring segments contacting one another in the assembled state along a cutting line which forms an angle of intersection with a tangent at the inner jacket surface of the sealing ring which is different from 90°. The se
Maschinenfabrik Sulzer-Burckhardt
Sandy Robert J.
Schwing Karlena D
Townsend and Townsend / and Crew LLP
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