Process Piping Code Question 1

[vtme90]

Does B31.3 process piping code require a relief device if a pipe cannot withstand the max pressure from a positive displacement pump or can credit be taken for pressure interlocks on the pump controls. I've been out of piping for a while but thought it was similar to ASMEBPV code which was prescriptive for the requirement of relief when a max pressure source is above the design pressure capability. Hi viscosity incompressibles are often difficult to put rupture discs in and LOPA analysis has been used for the change to interlocks. I don't remember if this is adequate and don't have a recent copy of the code. Any help would be appreciated.

 

[The Obturator]

ASME B31.3 (2020) Paragraph 322.6.3 References overpressure protection to ASME VIII (Note. Pressure-relief Device requirements are now in new ASME XIII since 2021).

Paragraph 322.6.3 does also mention some set pressure setting variations from the norm, which may or may not answer your question.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[RVAmeche]

In my opinion, PD pump systems should have an external relief device to protect the system. Interlocks only get you so far (depending on the LOPA/system being used) and the pumps internal PSV shouldn't be relied on to protect the piping.

 

[LittleInch]

The key section may is actually at the start, my emphasis.

301.2.2 Required Pressure Containment or Relief
(a) Provision shall be made to safely contain or relieve
(see para. 322.6.3) any expected pressure to which the
piping may be subjected. Piping not protected by a pressure-
relieving device, or that can be isolated from a pressure-
relieving device, shall be designed for at least the
highest expected pressure.
(b) Sources of pressure to be considered include
ambient influences, pressure oscillations and surges,
improper operation, decomposition of unstable fluids,
static head, and failure of control devices.
(c) The allowances of para. 302.2.4(f) are permitted,
provided that the other requirements of para. 302.2.4
are also met.

302.2. f gives you potential for occasional overpressure of up to 33%.

So my interpretation of the above is that if you don't have a pressure relieving device, then you need to rate the system for the highest expected pressure. It's not immediately clear what is meant by a "pressure interlock". An interlock usually prevents something from happening, so if e.g. the pressure was too high, then the pump would not start.

What you are implying is a pressure trip though which is something different. The SIL of that trip I would have thought needed to be a SIL 2 or SIL 3. Then it might just qualify by preventing the pressure from reaching such a high pressure. Maybe.

So please explain what you mean a bit more before anyone can give any more comment.

But why does anyone want to do this? The releif can be back to the pump inlet side, not always the tank or a drain.

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

 

[georgeverghese]

It is for this reason that mechanical design pressure for the discharge piping and downstream system be preferably 30-40% higher than max normal operating discharge pressure, to account for the risk of poor pressure pulsation attenuation at the discharge pulsation dampener. Rupture discs can blow at pressures as low as 80% of set pressure. At the least, also select a triplex pump.

 

[TiCl4]

VTME90,

No, there is no requirement specific to PD pumps that I know of. However, if you are pumping a incompressible fluid, how do you expect to protect against deadhead? Process response times of interlocks are too slow to mitigate the pressure rise of a valve closed quickly against a running pump.

If rupture discs or relief valves are troublesome, you could explore mechanical torque-limiters on the pump driveshaft. These are essentially break-away couplings that will fail at a specific torque value. You’d need to run the numbers on your specific pump to determine correct torque value to select, but it can be done.

Typically one would run a LOPA and take the 0.01 credit of a relief device because it’s usually cheaper than other means of obtaining that reduced risk. If you can satisfy the LOPA to your internal standard, then effectively the case is sufficiently mitigated such that the overpressure is not “expected”, per LittleInch’s quote above.

 

[shvet]

can credit be taken for pressure interlocks on the pump controls.

See API 521 annex E