New PSV with different body size

[Zoobie]

I am replacing a conventional PSV with a pilot operated valve with the same specified capacity and set pressures. This valve vents to atmosphere. The original valve was a 2.5"x4". The new valve is a 2x3 with the same orifice size. This means I will need an inlet swage and a new short tailpipe.

Do I need to account for this in the valve sizing? Is there a section of API 520/521 that would help me address this?

thx

 

[Montemayor]

Zoobie:

I haven't seen anything in API 520/521 regarding the changeout of an existing PSV with one that is physically different. What the API and ASME insist (& OSHA, in the case of hazardous chemicals) is that the PSV be sized to safely relieve ALL credible & possible over-pressure scenarios. Also, the vessel nozzle to the PSV (when the PSV is on a pressure vessel) must be the same size as the PSV's inlet. You are not allowed to use an expander; you are allowed to reduce the vessel nozzle size to a smaller, correct PSV inlet size. I believe this is related to the 3% of set pressure ruling on the inlet pressure loss to the PSV (as discussed in API 521, Section 5.4.1.1.

 

[TD2K]

I agree with Art that the only question here is whether the inlet line losses are less than 3% which simply means including the effect of the additional reducer on the losses.

 

[Safone]

Zoobie,

You didn't give full details on the pilot-op, so beware of these issues:

If the nonrecoverable losses on the inlet side of the pilot-op exceed 3% and you are using a remote sense line location, then capacity should be derated. The simplest way is to reduce the relieving pressure in the sizing equation by the amount of inlet line loss.

Accepted practice is not to derate the capacity of any PRV when the inlet losses are 3% or less. When the pilot-op sense line is at the valve inlet, it is treated like a spring-loaded valve on this issue.

 

[Zoobie]

Safone,

Thanks for your comments. These valves will not be remote sensing.

Montemayor,

Your comments about using expanders on piping attached to a pressure vessel is helpful. I presume this does not apply to pressure piping. In the application that I have, the PSV protecting a separator is attached just downstream on the vapor piping.