GB2182113A - Valved pig - Google Patents

Abstract

An emergency shutdown valved pig for a pipeline comprises a stator R1 and armature R2 and a venturi-like flexible membrane F which is kept open by a spring S but which closes under differential pressure caused by a leak (Figure 3 not shown). The pig may be clamped in place where desired and may be part of a train of articulated units. <IMAGE>

Description

SPECIFICATION Fluid control valve This invention relates to a shutdown valve for gas and oil supply pipelines.

In the vicinity of fixed platforms, for example gas and condensate pipelines,thefracture of such a pipeline could cause a very significant hazard where large volumes of inflammable material are lost into the sea or into the atmosphere in the vicinity ofthe platform.

It has been proposed to install safety barriers in such pipelines so that in the event of a large leakage causedforexamplebyfractureofthepipeline,the safety barrier would close down the pipeline to thus prevent a significant leak.

Although shutdown safety valves are known and can be installed when a pipeline is being constructed, there remains the problem of existing pipelines. It would of course be possibletofitshutdownvalves into existing pipelines by cutting into the existing pipeline and fitting such valves. Howeverthiswould be a difficult and expensive task and would lose val- uablerevenuewhilethepipelinesareshutdownfor the valves to be fitted owing to the time it would take.

It is an object of the present invention to provide a shutdown valve which can be installed into an existing pipeline by "pigging" it along the pipelinefrom one end to thus avoid the disadvantages mentioned above.

According to the present invention a shutdown valve comprises a circular valve stator which can be pigged along a pipeline and which has means for sealing the statorto the pipeline at a predetermined position, and a valve armature which is responsive to a predetermined minimum flow rate to move under the influence oftheflowing fluid in the pipelineto close the pipeline down.

In orderthatthe invention can be clearly understood reference will now be made to the accompanying drawings in which: Figure 1 shows in cross section and somewhat schematically an emergency shutdown valve according to an embodiment ofthe present invention; Figure 2 shows the valve of Figure 1 beginning to close, and Figure 3shows diagrammatically the valve of Figure 1 closed.

In the embodimentto be described it is assumed that it is required to close down under emergency conditions a pipeline P having an internal diameter of 14"(102 mm) at a predetermined location. The valve to be described is capable of being pigged down the pipeline and to pass around bends in the pipeline and to negotiate other minor obstructions such asjunctions and minor distortions of the pipeline. It is proposed that the shutdown valve forms part of a total package accommodated within a "train" of short modules of about 300 mm in length which are interconnected by a series of articulated links. The modules are shaped to ease their passage down the pipeline during installation and, if necessary, recovery.

The valve is designed so that in operation the bar rierthat it forms closes automatically, should the leak in the pipeline exceed a predetermined minimum flow rate which would be a mass flow rate of 1 kg per second in the case of a gas. Normally such pipelines would be continuously welded so that there are no flanged joints.

Since failure of the pipeline would be "catastrophic" there will be an immediate and significant drop in line pressure on the side of the shutdown valve where the failure has occurred. A differential pressure will be set up across the valve, the higher pressure being "outboard" of the valve regardless of whetherthe pipeline is importing or exporting gas.

Where gas is being exported the pumping pressure will be relieved by the rupture which has occurred and there will be a back-pressure from the gas downstream ofthevalve. This differential pressure is utilised to assist in the closure of the valve and in the designs to be described forms the basis of an automatic mode of operation which would be independent of any remote operating signal.

It is also proposed to protect the barrier provided by the valve against the rupture which propogates across the valve, by installing two such valves separated by a distance which exceeds the crack propogation length (which can be calculated given the pipeline specification). Alternatively it is proposed to install crack arresters on either side of the barrierwhich in some circumstances can be integrated within the design ofthe mounting for the remote control tele metry module which will be external to the pipeline.

Referring to Figure 1 of the drawings a pipeline P houses an emergency shutdown valve which comprises a stator ring R1 sealed to the pipeline wall by means of a clamp and seal S. A relatively streamlined body C1 is attached to the stator ring R1 by means of webs Wand fluid indicated by the arrows can flow relatively freely past this body Cl.

Coupled to the body C1 is a second relatively streamlined body C2 supported on an armature ring R2 by means of webs W2.

Atubular guide G is fixed to the body C1 butthe body C2 can slide on the guide G via a bore B in the body C2. A helical spring SP maintains the ring R2 spaced apart from the ring R1 and a flexible circular membrane F of Venturi-like axial section and made of e.g. nitride rubber is secured to the rings R1 and R2 at its opposite ends. The spring SP maintains the membrane F in the configuration shown in Figure 1.

Under normal operation when there are no leaks from the pipeline Pthe pressure at regions I and ll is substantially equal due to a connecting hole H in the stator ring Rl The pressure at region III is slightly higherthan at I and II due to friction pressure drop through the valve. However the spring SP keeps the rings R1 and R2 apart and thus keeps the valve open under these normal operating conditions.

If howeverthere is a sudden downstream leak i.e.

in region I the pressure there drops and the flow rate increases considerably. Thus the pressure dropped through the valve increases and the spring begins to compress. However meanwhile the pressure in the region II remains at a pressure above that in there gion I because leakage of gas or liquid from there gion II is controlled by the bleed hole H. Pressure in region I has dropped pressure in region II is also dropping by leakage through bleed hole B but con- trolled to be higherthan the pressure at region Thus the flexible wall F distorts in the manner shown in Figure 2. Meanwhile pressure in region Ill remains at working level and assists in thefurther movement of the armature body C2 towards body C1 which is acting as a valve seating.

Turning now to Figure 3 which shows the valve in its completely shutdown condition, the valve has now sealed on to surfaces at 1 and 2 and pressures in regionsland II are nowequal and lowerthanthatin region lil. Pressure differential from region Ill to region I maintains the valve in the closed position.

When the pipeline is repressurised, when the fracture or leakage has been repaired or prevented, the spring will restore the valve to the open position as shown in Figure 1.

It is envisaged thatthe valve described would be applicable to pipelines in the range 14" to 40"diameter, and also to both gas and liquids.

Stainless steel parts would normally be used but the bodies C1, C2 could be of plastics material.

Claims (1)

1. A shutdown valve comprises a circular valve statorwhich can be pigged along a pipeline and which has means for sealing the statorto the pipeline at a predetermined position, and a valve armature which is responsive to a predetermined minimum flow rate to move underthe influence of the flowing fluid in the pipeline to close the pipeline down.