ASME UV Stamped Valve on Regulator Station - Overpressure? 1

[Bourbon105]

If you have a B31.3 piping system (e.g. 100 psig nitrogen) and a UV stamped PSV immediately downstream of a pressure regulator used to limit the maximum downstream pressure in a failed open scenario, all downstream users/vessels have their own adequately sized and installed pressure relief devices, and your piping system is built to contain the maximum possible pressure (and therefore does not need relief device protection):

1. What overpressure do you use for sizing the orifice?
2. What overpressure do you use for sizing the inlet/outlet piping?

Since this is not a required valve for vessel or piping overpressure protection, are you still beholden to the 10% overpressure just because the PSV is ASME stamped even though there's no Code requirement (i.e. the PSV could be removed)? Section VIII, I believe the non-mandatory?, says the piping should be sized at stamped rated flow (10% overpressure) but where does it say that if the valve is not required by Code does it need to be sized with overpressure?

This all came about as we were debating if it should be conservatively sized for 0% overpressure if, say, you had some downstream API 650/620 tanks. And if you did, would then the piping be sized to the proportional rated flow at 0% overpressure?

Thanks again

 

[Latexman]

It seems if there was downstream API 620/650 tanks, there should be another letdown regulator with downstream PSV upstream of the tanks, with pressure settings less than those low pressure tanks, in the same spirit as those of the pressure vessels.

I still use 10% OP, maybe it’s habit.

We like and use the arrangement you described so the fail wide open regulator does not blow through the vessels and tanks. They may contain nasties, like Methyl Ethyl Death.

Good Luck,
Latexman

 

[don1980]

Codes require you to prevent equipment from exceeding the code-specified limits (i.e. accumulation limits), but there are no code limitations on overpressure. I think you may have meant to say accumulation rather than overpressure, but I'm not sure. Regardless, if the system is code-compliant with or without this PSV, and you choose to keep it in place anyway, then there's no design/sizing basis for this PSV. There's no required size, because there's no required flow rate. But even without a required flow rate you do have a rated capacity, because that's based on size of the orifice and the set pressure (independent of the required flow rate). You can arbitrarily choose to say that the PSV's relieving pressure is based on the max allowable pressure for the B31.3 piping, or based on the max allowable pressure for the vessels.

If the system is code-compliant without this PSV, it's technically OK to design for a relieving pressure of say 500 psi, even though the system is nominally designed for 150 psi. But doing that obviously doesn't make sense because it invites confusion and trouble from auditors who don't recognize that the PSV isn't serving any safety purpose or code-mandated purpose.

If the PSV is really unnecessary, I think it's best to just remove it.

 

[LittleInch]

Well do you want one device to buy / service test or several?

Most people would choose the one device and then rely on that rather than multiple devices all of the same size as they would need to be independent.

difficult to say without a diagram and a bit more explanation which will also define the pressures you refer to.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

I don’t know LI, I’ve never, ever had to witness at a Plant Manager’s Review Board a PSV that only blows nitrogen.

I’ve sweat thru a few when monomer, resin, solvent, caustic, or acid were involved. Accounting for every pound with the Environmental Manager for review with the local jurisdiction is never fun. While justified, I’ve heard the decisions being made that affects operators livelihoods, and saw the after effects.

So, if I can engineer one scenario away from blowing the relief on a reactor with relatively inexpensive components, I call that a good day.

Good Luck,
Latexman