2 replies to this topic

[stu]

Dear Sir,
I came across OISD-106 in that the following statement was found
" spare safety valves should no be installed unless absolutely necessary or are required by a statutory authority. A detailed examination of the service conditions of the system should be done and if the conditions are very critical, only then spare safety valves should be installed. If spare safety valves are absolutely necessary, three way isolation valves should be installed at the inlet and outlet with proper manifolding."
under section 6.3 Spare Safety Valve.
Please explain how we provide three way isolation for PSV? normally we provide isolation valve on Upstream & Downstream of PSV,how we provide three way isolation please explain
Thanking You.
Regards,
Stu.

[Art Montemayor]

Stu:

I don't know what "OISD-106" is, but I suspect it is one of your local codes or regulatory laws on PSV employment and design.

A 3-way valve is a common method used to install two PSVs on a common protected vessel nozzle. The valve communicates to either one of the two mounted PSVs – relieving one at a time, and never two at the same time. You simply turn the 3-way valve in one direction or the other. These are special-built valves and I recommend you contact your OISD-106 regulator agent and ask them what specific valves they approve of. You then obtain installation and operating instructions from the valve manufacturer.

What your regulation obviously doesn't want is individual block valves on the inlet and outlet of each PSV – even though these may be controlled by "Locked Open" or "Car Seal Open" techniques. Your regulators probably feel "safer" with 3-way valves.

[JoeWong]

stu,

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