GB2301168A

GB2301168A - Pressure relief valve

Info

Publication number: GB2301168A
Application number: GB9509798A
Authority: GB
Inventors: James Ian Mcwilliam
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-05-15
Filing date: 1995-05-15
Publication date: 1996-11-27
Anticipated expiration: 2015-05-15
Also published as: GB9509798D0; GB2301168B

Abstract

A pressure relief valve has two pistons 5, 6 and a shuttle plug 7 which operate within a cylindrical bore such that liquid may be relieved from an enclosed volume, particularly a gate valve chamber, via a connection to a primary port 1 through one of two auxiliary ports 2, 3, in a direction and at a pressure determined by whichever of the two auxiliary ports is at the higher pressure. Flow of liquid from one auxiliary port to another is automatically prevented, as is flow from either auxiliary port to the primary port. The pressure relief device operates on a pressure balance principle. The higher of the pressures in the ports 2, 3 moves the pistons 5, 6 and the intervening plug 7 towards the port at lower pressure. Excess pressure in the primary port 1 then lifts the piston adjacent the auxiliary port at the higher pressure from the plug 7, thus opening a flow path through the piston.

Description

- HYDRAULIC PRESSURE RELIEF DEVICE This invention relieves hydraulic pressure from an enclosed, or primary, volume to either of two other auxiliary volumes (or parts of a hydraulic circuit) also connected to the invention, at a pressure and in a direction determined by whichever of the two auxiliary volumes (or parts of a hydraulic circuit) is at the higher pressure.

Isolated volumes within hydraulic systems, or within individual components in hydraulic circuits, can be subjected to large increases in internal pressure. If the isolated volume is liquid solid and experiences either an input of energy to the liquid, or a reduction in physical volume, then potentially disabling or damaging hydrostatic pressures can be generated.

According to the present invention there is provided a pressure relief device comprising: a a cylindrical bore in a body with a central, primary, hydraulic connection to the isolated volume to be relieved; and 2 opposed pistons operating within the bore, hydraulically sealed to prevent leakage past the pistons, with integral orifices and a central, freely moveable, shuttle plug, located between the two pistons, which seals the orifices in the two pistons; and l 2 pressure retaining end caps, with a hydraulic connection from each end of the bore to other, separate, auxiliary volumes (or parts of a hydraulic circuit) to which the relieved liquid is delivered.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a cross section of the pressure relief device; Figure 2 shows a cross section of the pressure relief device and the position of pistons 5 and 6 and shuttle plug 7 when pressure 3 is greater than pressure 2 and pressure 1; Figure 3 shows a cross section of a parallel slide double disc gate valve.

Referring to Figure 1 the pressure relief device comprises: a body 4 which contains a cylindrical bore; a hydraulic connection, port 1, is provided from the centre of the bore to the volume from which pressure is to be relieved.

Pistons 5 and 6 are located within the bore and are hydraulically sealed to the bore by '0' ring seals 15 and 16 respectively; the nose of piston 5 and piston 6 each contains an orifice.

Shuttle plug 7 is positioned between pistons 5 and 6 within the bore of body 4; the shuttle plug seals the orifice in each piston 5 and 6 with '0' ring seals 8 and 9 respectively. The shuttle plug 7 is designed to allow pressure from port 1 to communicate freely with either end of the shuttle plug 7, up to seals 8 and 9. In this particular embodiment the shuttle plug 7 is shown with a central circular groove and a full length flat on a minor chord of its otherwise circular cross section.

Pressure retaining caps 10 and 11, and seals 12 and 13, are fitted to each end of body 4.

Caps 10 and 11 are retained by screws 14. Ports 2 and 3, in caps 10 and 11, hydraulically connect each end of the bore in body 4 to the two auxiliary volumes (or parts of a hydraulic circuit) to which the relieved liquid is delivered.

Springs 17 and 18 are strong enough to overcome the friction of the '0' ring seals 15 and 16 on pistons 5 and 6 and so pre-position pistons 5 and 6 against the shuttle plug 7. In this position the shuttle plug 7, in conjunction with '0' ring seals 8 and 9, seals the orifices in pistons 5 and 6. Otherwise, sealing forces and the forces to move pistons 5 and 6 and shuttle plug 7 are produced by hydraulic pressures.

The pressures at the three hydraulic connections, ports 1, 2 and 3, are identified as pressures 1, 2 and 3 respectively.

The combined length of the pistons 5 and 6 and the shuttle plug 7 is less than the distance between the fitted end caps 10 and 11. It is this clearance and the ability of a piston, either 5 or 6 (depending on whether pressure 2 or pressure 3 is the greater) to lift away from the shuttle plug 7 under the action of pressure 1 which gives the device its pressure relief capability.

If pressure 3 is greater than pressure 2 and pressure 1, then piston 6 will press on shuttle plug 7 and its orifice will be sealed by '0' ring 9; shuttle plug 7 will press on piston 5 and the orifice in piston 5 will be sealed by 'O' ring 8; piston 5 will compress spring 17 and sit on end cap 10. In this condition liquid from port 3 cannot leak to port 1 and liquid from port 1 cannot leak to port 2. (As the invention is symmetrical about the axis of port 1, then if pressure 2 is greater than pressures 3 and 1, leakage cannot occur from port 2 to port 1, nor from port 1 to port 3).

If pressure 3 is greater than pressure 2, then pistons 5 and 6 and shuttle plug 7 will take up the position as described above and shown in Figure 2. Should pressure 1 increase such that it exceeds pressure 3 then piston 6 will be lifted away from shuttle plug 7 and liquid will be relieved from port 1 to port 3. The shuttle plug 7 will continue to press against piston 5 and hence port 2 will remain sealed from pressure 1.

Referring to Figure 3: the principal elements of a typical parallel slide double disc gate valve are: a body 21, seats 24 and 25, a slide gate 20 and stem 26. Typically, the gate 20 is constructed in two pieces, pre-loaded by a spring so that each disc is allowed to float independently and mate with its seat, 24 and 25.

With this type of valve the double discs of the gate 20 can trap liquid within the body 21.

When the valve closes, an isolated volume is created within the body 21 at the instant both discs of gate 20 cover the seats 24 and 25 and, as the gate continues to move to its central position, stem 26 continues to enter the trapped volume so generating potentially damaging hydraulic pressures. In addition thermal transients can cause the trapped liquid within the valve body 21 to expand, so increasing the internal pressure within the body 21 and preventing valve operation by jamming the discs of gate 20 against seats 24 and 25. This phenomenon is known as pressure locking.

According to this invention, potentially damaging or disabling pressures within parallel slide double disc gate valves can be prevented by connecting the volume within valve body 21 to port 1 of the invention, by connecting nozzle 22 to port 2 of the invention, and by connecting nozzle 23 to port 3 of the invention. These identified parts of the gate valve, if so connected to the invention as indicated, correspond to the previously defined primary and auxiliary volumes.

The liquid relieved from the valve body 21 is returned to either nozzle 22 or nozzle 23 (which, when the valve is closed, are connected to separate parts of the hydraulic circuit).

Thus the liquid that is relieved from valve body 21, which may be noxious or hazardous to health, is retained within the hydraulic circuit.

As the invention operates on a pressure balance principle, the pressure within valve body 21 at any time (i.e. pressure 1) can be no greater than the higher of the pressures in nozzle 22 (i.e. pressure 2) or nozzle 23 (i.e. pressure 3).

Claims

1. A pressure relief device comprising: a cylindrical bore in a body with a central, primary, hydraulic connection to the isolated volume to be relieved; and v 2 opposed pistons operating within the bore, hydraulically sealed to prevent leakage past the pistons, with integral orifices and a central, freely moveable, shuttle plug, located between the two pistons, which seals the orifices in the two pistons; and 2 pressure retaining end caps, with a hydraulic connection from each end of the bore to other, separate, auxiliary volumes (or parts of a hydraulic circuit) to which the relieved liquid is delivered.

2. A pressure relief device as claimed in Claim 1, wherein the isolated primary volume to which it is connected relieves to either of the two auxiliary volumes (or parts of a hydraulic circuit), at a pressure and in a direction determined by whichever one of the 2 auxiliary volumes (or parts of a hydraulic circuit) is at the higher pressure.

3. A pressure relief device as claimed in Claim 1 or Claim 2, which hydraulically isolates the auxiliary volumes (or parts of a hydraulic circuit) from each other.

4. A pressure relief device as claimed in Claim 1 or Claim 2 or Claim 3, which prevents leakage to whichever one of the 3 volumes (or parts of a hydraulic circuit) is at the lowest pressure, from either or both of whichever two of the 3 volumes (or parts of a hydraulic circuit) are at higher pressures.

5. A pressure relief device as claimed in Claim 1 or Claim 2 in which the liquid relieved from the primary volume, which may be noxious or hazardous to health, is retained within the auxiliary volumes or returned to the hydraulic circuit.

6. A pressure relief device as claimed in Claim 1 or Claim 2, which ensures that the pressure in the primary volume can at no time exceed the higher of the pressures in the two auxiliary volumes (or parts of a hydraulic circuit).