Discharge Piping Diameter Reduction
Discharge Piping Diameter Reduction
(OP)
I have always believed that we shouldn't reduce the diameter of the PSV discharge line. We now have a client questioning my belief (can you imagine that!) - now I can't seem to find any reference that specifically forbids me from reducing my 2.5" line down to a 2" line. Sure, it's not good practice and you're all going to say that - so "Show me the paper". Can anyone find this reference? B31.3? API ?





RE: Discharge Piping Diameter Reduction
What you need to do is demonstrate / calcualte the effect of the reduced diameter on the pressure at the relieving point.
In B 31.3 section K322.6.3 ( a simple search found this) the relief system which includes all the downstream piping needs to be sized such that at your reliving flowrate, the presusre in the thing you're protecting doesnt exceed 110% of the deisgn presusre (116% for multiple reliefs). for a long high flow rate system, 2 1/2" might not be big enough (you need to check) or a short low flow rate, 2" might be good enough. This is called design, not rule following and is why the codes normally don't get too prescriptive, but provide boundaries and limits on things that actually matter like pressure, temperature and flow, not line size.
As an aside, whilst I'm sure 2 1/2" piping exists, I've never come accross it and would prefer 3" if you're going to go for a bigger size.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Discharge Piping Diameter Reduction
RE: Discharge Piping Diameter Reduction
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Discharge Piping Diameter Reduction
I think we need some sort of a basics prep course on process equipment.
RE: Discharge Piping Diameter Reduction
RE: Discharge Piping Diameter Reduction
I'd have to check the latest version of API 520 to see if that's been changed, I thought I had seen something that mirrored the inlet piping, the outlet piping shall not be less than the outlet of the PSV but I'm not certain.
I'd echo the comment about everything not being in the codes. If that was the case, lawyers would do the engineering.
RE: Discharge Piping Diameter Reduction
RE: Discharge Piping Diameter Reduction
I agree it seems logical and a requirement to always increase the discharge pipework since as the gas expands it would need a larger pipe to accommodate/maintain the mass flowrate unless you allow the velocity to increase. However I remember years ago working on a project where the owner insisted on a decrease in pipe size local to the end of the discharge pipe. (i.e. the tailpipe was 3in and a 3in x 2in concenric reducer was welded on the end) The reason being to ensure high velocity at the tailpipe end to disperse the contents. Obviously the backpressure on the RV was checked for compliance.
RE: Discharge Piping Diameter Reduction
...
"Each pressure-relief device that vents directly to the atmosphere should normally have an individual vent pipe sized for a relatively high exit velocity; however, the outlet piping should not be smaller than the pressure-relief-device outlet."
...
From the language, it appears that this applies only to atmospheric relief, not to relief going to a flare, treatment, or collection system.
RE: Discharge Piping Diameter Reduction
RE: Discharge Piping Diameter Reduction
Discharge piping from a single safety valve must not be smaller than the safety valve nominal outlet flange size and must meet pressure drop constraints.
For valves discharging into a header system, the size of discharge piping must ensure that piping discharge pressure drop under any expected condition/scenario will not adversely affect set pressure or capacity of any safety valve discharging into the header system.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Discharge Piping Diameter Reduction
My experience has been (in Western Canada, specifically Alberta and Saskatchewan) that the Regulators indeed have not accepted installations in which the outlet piping from the valve to the header is a smaller size than the outlet connection on the valve, and that further, for several PSV outlet pipes tying into one header, they have enforced the interpretation that the summation of cross-sectional areas for the individual pipes shall be less than or equal to the cross sectional area of the header.
I think I came across a written Information Letter or interpretation about this a couple of years ago. If I can find it, I will post a link.