GB2318403A

GB2318403A - Piston and cylinder device

Info

Publication number: GB2318403A
Application number: GB9722056A
Authority: GB
Inventors: James Brian Wilson
Original assignee: ABB Seatec Ltd
Current assignee: Baker Hughes International Treasury Services Ltd
Priority date: 1996-10-18
Filing date: 1997-10-17
Publication date: 1998-04-22
Also published as: BR9711938A; GB9722056D0; AU4712797A; NO991804D0; GB9621769D0; NO991804L; WO1998017930A1

Abstract

A metal piston (15, 16) has a skirt (17, 18) which engages a resilient seal (19, 20) mounted within a bore (13, 14) of a metal cylinder (11, 12). The skirt (17, 18) has a thin walled portion (34, 35) which is resiliently flexible and is dimensioned relative to the bore (13, 14) such that, whenever fluid pressure acting on the piston (15, 16) exceeds a predetermined value, the thin walled portion (34, 35) will be deformed to provide a secondary metal-to-metal seal (36, 37) protecting the resilient seal (19, 20) from the external ambient environment. In this manner the clearance space between the piston skirt (17, 18) and the cylinder (11, 12) is closed by the outward deformation of the thin walled portion (34, 35) due to the increasing fluid pressure. This inhibits corrosion of the engaged metal surfaces and denies space for the accretion of marine growth or sediment.

Description

PISTON AND CYLINDER DEVICE The invention relates to a piston and cylinder device for use in harsh environments which may cause deterioration of the conventional resilient seal between the skirt of the piston and the bore of the cylinder.

One particular harsh environment is the ocean floor where subsea oil field apparatus are typically exposed externally to high water pressures and intemally to high oil pressures. A piston and cylinder device used in this environment typically has high water pressure applied to one face of its resilient seal and a high oil pressure applied to the opposite face of the resilient seal. Such conditions can cause progressive deterioration of the material from which the resilient seal is formed. Furthermore the face of the resilient seal exposed to the sea water is positioned within the clearance space between the piston skirt and the cylinder bore which can be affected by corrosion and by the accretion of marine growth and can cause the piston to jam or may otherwise damage the resilient seal during piston movement.

Other harsh environment exist in many industrial processes such as chemical plants and foundries.

It is an object of the present invention to provide a piston and cylinder device in which the resilient piston seal can be protected from such a harsh environment.

According to the invention a device comprises a metal cylinder defining a bore, a metal piston having a skirt engaging a resilient seal mounted within the bore, a fluid connection to admit pressurised fluid between the piston and the cylinder to generate an axial movement of the piston, and the skirt is resiliently flexible and is dimensioned relative to the bore such that, whenever the pressure of the fluid exceeds a predetermined value, the skirt will be deformed to provide a metal-to-metal seal with the bore. In this manner, after a predetermined load has been applied to the piston, the clearance space between the skirt and the cylinder is closed by the outward deformation of the skirt due to the increasing fluid pressure. This prevents corrosion of the engaged metal surfaces, provided that they are the same material, and denies space for the accretion of marsh growth or sediment.

Preferably the skirt is dimensioned to form the metal-to-metal seal in a region lying between the resilient seal and a head of the piston. In this manner the metal-tometal seal forms a barrier protecting the resilient seal from the environment outside the cylinder and also protects that part of the skirt which will slide through the resilient seal when the piston is eventually relocated into the cylinder.

Preferably an outer surface of the skirt is formed with a thin walled portion which distorts into an external land to form the metal-to-metal seal with the bore. The use of this land defines the exact position of where the metal-to-metal seal will be formed and limits the amount of the skirt that need to be machined to a fine tolerance relative to the cylinder bore.

Desirably axial movement of the piston out of the bore is limited by a stop to prevent the piston being ejected from the cylinder. This is particularly useful where the piston only requires a slight travel and provides a reaction point for the axial piston load as the pressure of the fluid is increased to cause the skirt to expand and grip the cylinder bore. Preferably, the stop is provided by an external cylindrical flange of the piston and a retaining ring secured within the bore.

Both the piston and the cylinder are formed preferably from the same metal which would normally be chosen to be highly resistant to corrosion by the ambient environment.

The invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a transverse section through a two-part connector for pressurised fluid illustrating their disengaged position; Figure 2 illustrates the engaged position of the two-part connector shown in Figure 1; Figures 1 and 2 illustrate a two part connector 10 as described in our co-pending British Applications GB 9621770.8A and GB 9621768.2A, the whole contents of each Application being incorporated herein by reference.

Referring to Figure 1, as described in our co-pending Applications, each part of the connector 10 comprises a metal cylinder 11, 12 which defines a bore 13, 14 and a metal piston 15, 16 having a skirt 17, 18 engaging a resilient seal 19, 20 mounted within the bore 13, 14. Each cylinder 11, 12 has a mating surface 21, 22 located at a head of the piston 15, 16 which is arranged to engage the mating surface 21, 22 of the other part of the connector 10.

Movement of each piston 15, 16 axially out of its bore 13, 14 is limited by a stop comprising an extemal cylindrical flange 23, 24 arranged to abut the inner end of a retaining ring 25, 26 that is secured within the bore 13, 14 by a threaded connection 27, 28. A pre-loaded compression coil spring 29, 30 reacts between each piston 15, 16 and cylinder 11, 12 to bias the position of the piston 15, 16 towards its retaining ring 25, 26 such that the flange 23, 24 is held against its respective retaining ring 25, 26.

The mating surfaces 21, 22 when in a engaged position, as illustrated in Figure 2, allow fluid pressure to be communicated between respective lines 31, 32 defined within the cylinders 11, 12 and pistons 15, 16. The fluid pressure acts on the effective areas of each piston 15, 16 to press their mating surfaces 21, 22 firmly together to provide metal-to-metal seal 33.

The resilient seals 19 and 20 provide secondary seals which are superseded by secondary metal-to-metal seals in the following manner. Each piston skirt 17 and 18 is formed with a respective thin walled portion 34 and 35 which is resiliently flexible and dimensioned relative to the respective bore 13 and 14 such that whenever the fluid pressure exceeds a predetermined value, the thin walled portions 34 and 35 of the skirts 17 and 18 will be deformed to provide secondary metal-to-metal seals 36, 37 protecting the resilient seals 19 and 20 from the external ambient environment. The dimensions of each thin walled portion 34 and 35 are preferably controlled by the position of external cylindrical lands 38 and 39, thereby limiting the fine tolerances to a very small portion of each piston 15, 16 and determining precisely where each secondary metal-tometal seal 36, 37 is positioned. In this manner the two-part connector 10 is completely sealed against the external environment by metal-to-metal seals 33, 36 and 37.

Claims

1. A device comprising a metal cylinder defining a bore, a metal piston having a skirt engaging a resilient seal mounted within the bore, a fluid connection to admit pressurised fluid between the piston and the cylinder to generate an axial movement of the piston, and the skirt is resiliently flexible and is dimensioned relative to the bore such that, whenever the pressure of the fluid exceeds a predetermined value, the skirt will be deformed to provide a metal-to-metal seal with the bore.

2. A device according to Claim 1, in which the skirt is dimensioned to form the metal-to-metal seal in a region lying between the resilient seal and a head of the piston.

3. A device, according to Claim 1 or 2, in which an outer surface of the skirt is formed with a thin walled portion which distorts into an external cylindrical land to form the metal-to-metal seal with the bore.

4. A device, according to any of Claims 1 to 3, in which axial movement of the piston out of the bore is limited by a stop.

5. A device, according to Claim 4, in which the stop is provided by an external cylindrical flange of the piston and a retaining ring secured within the bore.

6. A device, substantially as described with reference to the accompanying drawings.