Southwark
Mechanical
- Feb 3, 2003
- 4
Operating a natural gas pipeline class 900# with MAOP 15MPa.
We have a 3 plug valve configuration bypass around a mainline valve. The valves connected to the mainline are weld/flange, while the middle valve is flange/flange. These valves have been found to be leaking and we are unable to seal them with grease for more than four hours(in some instances they do not seal). Connected between the plug valves is a vent stack. The plug valves would be used to throttle during venting of a long section of pipeline. The vent cap is required to be serviced annually, however, since we cannot guarantee the duration of the seal of the plug valves, we believe the design requires modification.
A suggestion has been made to install ball valves above the plug valves by flange connection thereby allowing a confident isolation. However as the plug valve will continue to leak, the flange will be in continual service. We will be unable to change out the gasket, if it fails, without blowing down the pipeline.
We have also considered using stopple and plug to isolate below the plug valves. Then cut out the plug valves and replace with weld/flange ball valves. This is considered to be high risk.
My questions:
Is there another solution that meets AS2885 that someone can recommend for rectifying this problem?
Is there a class 900# gasket that would be reliable for this service?
We have a 3 plug valve configuration bypass around a mainline valve. The valves connected to the mainline are weld/flange, while the middle valve is flange/flange. These valves have been found to be leaking and we are unable to seal them with grease for more than four hours(in some instances they do not seal). Connected between the plug valves is a vent stack. The plug valves would be used to throttle during venting of a long section of pipeline. The vent cap is required to be serviced annually, however, since we cannot guarantee the duration of the seal of the plug valves, we believe the design requires modification.
A suggestion has been made to install ball valves above the plug valves by flange connection thereby allowing a confident isolation. However as the plug valve will continue to leak, the flange will be in continual service. We will be unable to change out the gasket, if it fails, without blowing down the pipeline.
We have also considered using stopple and plug to isolate below the plug valves. Then cut out the plug valves and replace with weld/flange ball valves. This is considered to be high risk.
My questions:
Is there another solution that meets AS2885 that someone can recommend for rectifying this problem?
Is there a class 900# gasket that would be reliable for this service?
