I received a call from a customer asking about a safety shutdown application. He read that a double block and bleed arrangement could be used in a safety shutdown valve application if a mechanical hold close system was used. His question is if someone can explain what this mechanical hold close system would be.
He'd appreciate the community's thoughts on this... thanks!
I did hear back that it is not uncommon to use a double block and bleed configuration in a safety instrumented system. The person with whom I connected was not familiar with the standard referring to a “mechanical hold close system” as being required (unless the person asking the question is referring to an actuator or something to fail the block valves to their safe state.) The question may be best answered by the gate valve manufacturer who may be familiar with other requirements.