On the National Transmission System (NTS) there are two types of ball valves used for pipework isolation purposes, Cameron and Cort. The primary interest on this project is the large ball valves used to isolate pigtraps across the NTS, necessary for inline inspection of the pipelines. There are over 200 of these valves of various sizes of which 127 are Cameron type 31. Due to the age and condition of the asset, some of these valves are experiencing seal leakage. It has been identified that there is a mechanism on the Cameron type 31 valves by which if gas is leaking into the body cavity and the cavity becomes pressurized, the valve will selfrelieve into the downstream pipework. This is a serious safety consideration for National Grid as the valve would relieve at full line pressure (35-94 bar) into open pipework where an employee working on the pigtrap would be situated. Additionally, seal leakage and the subsequent release of natural gas to atmosphere is a contributing factor to National Grid’s overall carbon footprint. There is an opportunity to investigate both the self relieving mechanism on the Cameron type 31 valve and methods for reducing fugitive emissions as one such valve, 42” diameter and weighing over 13 tonnes, was removed from a National Grid site, due to it’s age and condition (see figure 1). This proposal looks to make full use of the opportunity; the removal of this asset facilitates a research project designed to fully simulate the performance of the valve in service in a way that would otherwise not be possible.
The project scope is to develop an additional safety control measure when work is being carried out downstream of a closed-off valve. The Cameron type 31 has a self-relief (single piston) system which is designed to avoid dangerous pressure build up in the body cavity. However, there is the potential for a hazardous situation to arise, whereby if a valve is closed, gas leakage past the upstream seal will start to pressurise the body cavity. This pressure should be relieved through the vent line(s), if the leakage rate is high enough then the pressure will rise and if this cavity pressure exceeds downstream line pressure by 200 psi, this will cause the downstream seat to lift off the ball, and allow the pressure to relieve into the pipeline. It would therefore be advantageous to understand this self-relief behavior and to consider how its occurrence might be foreseen. The project looks to investigate options for innovative alarm system solutions, by using the 42” Cameron self-relieving ball valve which has recently been taken out of service.
The second aspect of the work concerns methods to detect and quantify leaks on self-relieving (single piston) ball valves in operation on the gas transmission system. One method used by National Grid for the detection and quantification of leaks is the use of a portable acoustic emission device where a sensor is directly coupled to the surface of the valve. This device can be used on above ground assets but a significant number of National Grid Gas’ assets are below ground where access to the surface of the valve is not possible. Listening to the valve and/or vent pipe can also provide an indication of leakage. A 42” Cameron self-relieving ball valve has recently been taken out of service as it was leaking (see figure 1 in Appendix 1).
Benefits
The project will identify suitable alarm options for the self relieving single piston valve, and provide an assessment of the performance of the valve leakage detection equipment in quantifying leakage for both above and below ground process valves.
