Description of Incident
A crew narrowly avoided potentially serious problems with a high pressure gas supply by stopping and thinking things through. The near miss occurred whilst a crew were setting up for pigging of high pressure gas mains lines. A change to the flow configuration on two high-pressure gas mains to test flow rate was required; on the second to last step of the procedure, they closed a valve at the valve set.
Since one employee was unfamiliar with the location because it was not in their work area, they questioned the operator at the location to clarify what closing the valve actually did to the system flow. After a brief conversationthey realised that closing the valve would actually isolate the pipeline and prevent the flow of gas to a downstream valve set that they were intending to flow to.
Work was stopped immediately and the valve returned to its normal position.
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