GB2343498A

GB2343498A - Safety valve with shape-recoverable sealing element

Info

Publication number: GB2343498A
Application number: GB9823911A
Authority: GB
Inventors: Clive Phillip Dingley; Raymond Charles Morton
Original assignee: Seetru Ltd
Current assignee: Seetru Ltd
Priority date: 1998-11-03
Filing date: 1998-11-03
Publication date: 2000-05-10
Also published as: GB9823911D0

Abstract

A sealing arrangement for a safety valve comprises a seat and a closure member. The seat or the closure member has a shape-recoverable sealing element which provides edge contact with a sealing surface, and is preferably made of P.T.F.E. The sealing element may present a right-angled (or otherwise) edge contact and the sealing surface may be angularly oriented, preferably oblique in relation to the sealing edge of the sealing element. The sealing surface may be internally or externally frusto-conical and the edge contact correspondingly outwardly or inwardly oriented. The shape-recoverable element may be located in a recess in the closure member and is preferably supported in the region of the edge. The sealing surface is preferably in the form of an upstanding inverted "V" shaped rim surrounding a fluid flow aperture.

Description

A DESIGN FOR A SAFETY VALVE SEALING SYSTEM EMPLOYING P. T. F. E. OR SIMILAR POLYMER MATERIAL The purpose of this specification is to describe a sealing arrangement wherein a metal surface comes in contact with a polymeric seal in the closed position of the valve and is able to provide a substantially leak proof seal by doing so and employing relatively light sealing loads. The polymeric material envisaged primarily is P. T. F. E. but there may be other materials whose properties are broadly similar and could be used as well. P. T. F. E. has long been used as a sealing material in valve design and also in safety valve design. It is characterised mainly by the fact that while not being in elastomer it is less hard than metal and has a degree of defomability which can make it useful for the design of sealing systems. Also P. T. F. E. has very good chemical resistance and also temperature resistance for which reason it is widely used in process engineering applications.

Nevertheless, the incorporation of P. T. F. E. into a sealing system for safety valves has proved difficult because of the propensity of the material to cold flow which causes some dimensional instability and the possibility that the required shape configuration might be lost with time and service use. These factors have made it difficult to produce adequately leak tight seals for many chemical and process application for which the material was nevertheless quite appropriate from the point of view of its general properties. The present design addresses these difficulties in P. T. F. E. design and describes a configuration in which the sealing material is used to best advantage and where it is able to provide efficient sealing and to maintain this during prolonged service.

The use of the P. T. F. E. seal according to this specification can be envisaged either in the plunger or disc of the safety valve or in the seat thereof. Although not limited to flat disc type safety valve configurations, the specification mainly refers to this type of configuration. In it a substantially flat disc moves perpendicular to a ring form seat along the seat axis. In the open position of the safety valve fluid flow will take place axially through the seat aperture and subsequently radially outwards past the disc or plunger into the discharge galeries of the safety valve construction. When the fluid flow is sealed off when the disc makes contact with the seat there will be in the general case a spring urging the disc or plunger into this contact against the fluid pressure applying in the seat aperture. The spring force will counterbalance the fluid pressure to a certain point which is defined the set pressure of the safety valve.

When this is exceeded the safety valve will begin to open.

In such a general safety valve construction, where seat and disc will be of metal, a ring form P. T. F. E. seal is set either into the seat or disc or plunger, but generally it will be into the plunger. The P. T. F. E. ring will have a cross section designed to offer a well defined edge, right angled or otherwise, which will make contact with an oblique face of the mating component, the seat in this case, and this contact will essentially be an edge contact which represents the essence of this specification.

The metallic face bearing on the edge of the P. T. F. E. seal configuration is able to secure a good seal because the entire closing force of the safety valve is concentrated around a circular edge of minimal area. This closing force will cause some deformation of the P. T. F. E. edge, but tests have shown that this recovers to its original shape when the valve opens under fluid pressure in opposition to the spring.

While the general principe of edge contact with P. T. F. E. seals can be realised in a variety of ways, particular preferred execution consists in placing a P. T. F. E. ring of suitable cross section in to a valve plunger which offers a generally horizontal face for the valve seat, if the axis of seat and plunger are taken to be vertical. The seat itself is provided with an upstanding metal rim broadly in the form of an inverted'V'surrounding the aperture the seat through which fluid pressure is applied. In this execution an inner conical flank of the seat rim is designed to make contact with the circular edge of the P. T. F. E. seal which is mechanically retained in the valve plunger. The P. T. F. E. profile is suitably recessed to accommodate the crest of the seat rim without making any other contact with the seal which contact is restricted to the inner flank as described.

The seal edge in this case is at the apex of the right angle with one side horizontal and the other side vertical. The width of the ring portion of the P. T. F. E. seal which takes the sealing pressure is relatively narrow and this contributes to limiting the deformation to which the P. T. F. E. profile will be subject, because of the close support which is given to the P. T. F. E. material by the retaining metal element of the plunger design.

This arrangement is illustrated on the attached drawing.

As mentioned, the execution of the edge contact principe can be realised in a variety of ways, such as e. g. employing the outer flank of the seat rim to make contact with a radially inward pointing edge of the P. T. F. E. seat in the plunger, as distinct from the radially outward pointing arrangement as described above.

In all cases the P. T. F. E. profile must be closely supported by metallic surfaces so as to limit its deformation in order it may recover its original accurate shape when the valve load is removed.

Altogether this arrangement offers an efficient and economic method for safety valves for a wide range of duties ranging from cryogenic cold applications through wide use in process plant to service on steam up to temperatures which can be satisfactorily withstood by P. T. F. E. material or similar polymers.

Claims

CLAIMS: 1. A sealing arrangement for a valve including a valve seat and a valve member cooperable therewith to effect closure of the valve, the sealing arrangement being associated with the valve seat and valve member and comprising a shape-recoverable sealing element and a sealing surface in combination providing a substantially edge contact for effecting in use sealing closure of the valve.
2. An arrangement according to any one of the preceding claim in which the sealing element is made of P. T. F. E.
3. An arrangement according to any one of the preceding claims in which the sealing element presents a right-angled edge contact.
4. An arrangement according to any one of the preceding claims in which the sealing element presents an acute-angled edge contact.
5. An arrangement according to any one of the preceding claims in which the sealing surface is angularly orientated in relation to the edge contact of the sealing element.
6. An arrangement according to Claim 5 in which the sealing surface is oblique in relation to the edge contact of the sealing element.
7. An arrangement according to any one of the preceding claims in which the sealing surface is frusto-conically formed.
8. An arrangement according to Claim 7 in which the frusto-conical surface is formed internally and the edge contact of the sealing element is outwardly orientated.
9. An arrangement according to Claim 7 in which the frusto-conical surface is formed externally and the edge contact of the sealing element is inwardly orientated.
10. An arrangement according to any one of the preceding claims in which the shape-recoverable element is formed as part of the valve member and the sealing surface is formed on the valve seat.
11. An arrangement according to Claim 10 in which the shape recoverable element is secured to the valve member and sits in a recess formed in the member and the sealing surface is formed around the aperture of the valve through which in use flow occurs.
12. An arrangement according to Claim 11 in which the sealing surface is formed of an upstanding inverted'V'shaped rim surrounding the said aperture.
13. An arrangement according to any one of the preceding claims in which the sealing element is supported in the region of the edge contact thereby to prevent permanent deformation upon release from the sealing abutment with the sealing surface.
14. An arrangement according to any one of the preceding claims in which the edge contact between the sealing element and the sealing surface is of narrow annular dimension.
15. A sealing arrangement according to any one of the preceding claims for a safety valve.
16. A safety valve incorporating a sealing arrangement according to any one of the preceding claims.
17. A sealing arrangement substantially as hereinbefore described with reference to the accompanying drawing.