The double block and bleed method of valve isolation has become almost the default method of isolation in the process industry. However, there are limitations and misunderstandings in the methods of proving integrity. This paper highlights several ways in which double block and bleed isolations can fail, resulting in hazards with major accident potential. Key learning points include:
• Implementing an isolation involves more than simply closing some valves;
• Multiple failures can and do occur — and because valves are often of the same type and in the same service, common cause failures are an issue;
• Valve integrity must be proven and this requires pressure. There will be times when no pressure is available from the process, or it is only available from the wrong direction;
• Reducing the risk to personnel carrying out maintenance will often be transferred to those implementing the isolation.
Published in the Loss Prevention Bulletin August 2016