Definition of shutoff valve:  A valve designed for and capable of positive closure to prevent flow within a piping system. Typical shutoff valves include, but are not limited to, manually-actuated, power-actuated, or spring-actuated fail-safe shutoff valves. Usually excluded are self-actuated valves, such as check valves, pressure regulators, flow controllers, and other devices that are not intended to provide positive shutoff for safety isolationForged Steel Valves.

I am working with our plant operators to ensure our LOTO program meets OSHA standards, and I keep getting asked the question why certain types of valves can't be used, specifically check valves.  I have been able to find plenty of statements to not use them, but no engineering reference is provided.  The closest document I found is "The Safe Isolation of Plant and Equipment" which I learned about in a post on this website.  However, it doesn't reference an engineering spec either.

Is the SEMI F1-90 definition as close as I am going to get to an engineering reference?Needle Valves

From common practice all over the world a 'positive shutoff' is a shutoff where the closing element is mecanically secured against accidental opening, eg. valves closing directly with mechanical devices and mechanical force and securly locked in the closed position, before mechanically operated to open position again.

FilterAnd again by one sided closing pressure from springs or helping one-sided closing pressure from fluid, a positive closure can not be said to be given.

Exception on above list of constructions is globe-check valve with spindle (and handwheel) for shut off, and regulating valves constructed with (often protected or retarded, away from cavitation exposed areas) positive sealings with spindle and construction for drop-thigt closure.

Even those, especially the regulating valves, will in addition normally have mounted revision valves (positive closure valves) upstream.

If you work in a nuclear plant in the US, you can look at the Nuclear Regulatory Commission's Information Notices.  There are more than a few that discuss check valves leaking.  I think the worst case detailed was a plant where they had something like 9 different check valves in series leaked by (and caused problems).  Look under the subject of "air entrainment in emergency core cooling piping."

There is another thread of similar content but a diferent question. If I am not mistaken JMichler you are looking for positive and verifiable isolation of the energy source. LOTO is Lock Out Tag Out? and would be for man entry or other work to access a service.

This is common in procesing industry and is generally a Double Block and Bleed isolation point. Simply this is two valves in series that provide a positive shut off. The cavity created between the two valves has a small bleed valve installed into itAutomatic Control Valves. The two main valves provide two points of isolation between the energy source and the LOTO point. The bleed valve is opened to vent/drain the cavity between the isolatino valves. This "bleed" valve provides a visual check that the primary isolation valve is holding and a relief point to ensure the secondary isolation valve is not over pressurised.

The style of valves generally used are ball, gate, or globe. Butterfly valves can be used but are not as secure (reliant on disc to seat pressure for seal), diaphragm valves are not ideal for the same reason. Check valves or line pressure activated valves are not at all suitable.

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