[djack77494]

MTV,
I think there is a misunderstanding of the situation. PSV's are often spared to allow testing of one PSV while a second remains active. In this way, the safety of the system is not compromised by testing and/or maintenance of the valves. However, it is imperative that both PSV's are never simultaneously out-of-service. Since it is easy to defeat the PSV by closing a block valve on its inlet or outlet line, measures are required to see that this is not inadvertently done. The most common safeguard is to car seal open all of the block valves around all of the PSV's. When a particular valve is to be tested or maintained, it is necessary to cut the car seal (normally a plastic close once/never open flat device) and this typically requires the permission of a supervisor or someone aware of the consequences of this action. Immediately after the testing or maintenance, the block valves are fully opened and car sealed in that position. Note: The car seals are on the BLOCK valves and not on the PSV's. Sorry but I don't have any pictures of car seals.
Hope that helps,
Doug

[MTV]

Dear Doug,
Thank you very much for your anwser. However I don't know why block valves are open/closed, we call Car seal open/closed, I mean what is Car!!!.
Best regards,
MTV

[gvdlans]

From http://www.eng-tips....ad.cfm?qid=8440

Car seal comes from the rail car and freight industry. Once rail cars were filled, an inspector placed a metal wire on the loading valves. If the wire seal was broken, then the shipment could be refused. Each seal had a serial number or an imprint of the inspectors company. In the chemical industry it is an administrative control system to assure valves are not used inappropriately. For example, valves under relief valves are sealed open. This is better than chains and locks that used to be used in plants. If the valve is accessable by the public, like pipelines, then chains and locks are still used.

See also: http://www.americanc...mical_seals.asp