Pressure Relief Valve Connection 2

[MelBe]

When installing a pressure relief valve, does the outlet pipe need to use the same flange pressure rating as the valve?
i.e. a PRV with DN50 PN40 flanges, can a DN50 PN10 flange be used for the blow off piping, seen as it is exposed directly to atmosphere and does not actually see any pressurised conditions?

 

[LittleInch]

Providing there are no valves or potential for any blockage (e.g. hydrates or solid material) then this pipe is often classified as "atmospheric" pressure and not hydrotested either, especially if it is short and free to air. If it is a connection into a long flare line then it can have a back pressure and have a pressure rating, but not usually above 5 bar max

See what your line list calls it.

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

 

[shvet]

Are you able to guarantee such non-standard connection:
- is proven tight?
- is able to withstand a load from downstream piping during relief?
Why do you believe discharge is proven non-pressurized? Why mechanical load to the fitting is ignored? What is calculated built-up backpressure of this PSV? What is sound pressure and velocity (or Mach number)? Describe a relief - is it toxic, explosive, or similar?

Note that a designer of this PSV thougth exactly the same as you do that PSV discharge to atm is a minor issue

https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/docs/El Paso-Mojave GT 2012-5-2 Redacted.pdf

 

[MelBe]

Thanks LittleInch.
The pipe is indeed just out of the PRV, with a short length to atmosphere.

@shvet, this PRV is installed on a saturated steam line, with the lift pressure set to 22 barg, and is piped straight-up approx. 8 meters.
I'm confident that there is no back pressure issue, and very little mechanical loading.

 

[LittleInch]

Commonly the relief valve will have different rated flanges on the inlet and discharge size.

PN10 may not be available so you might find its a PN16 flange. But depends on what you order I guess.
As shvet says, noise and vibration will be quite high here and you might even have sonic flow. You will probably here it in the next town....
You need to check the back pressure and also check the force loading on the elbow and the pipe as this has bent relief valves and their exit pipe before.

You could easily need a bigger vent pipe than DN50 to allow for the steam expansion.

Also you need a very small drip connection to remove rain and condensed water right before the valve.

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

 

[Snickster]

For compresible gas flow in a relief valve discharge to atmosphere, the pressure inside the discharge pipe is determined by both friction loss and critical flow pressure at pipe exit. Pressure inside the tip of the pipe at the exit may be significantily above atmospheric if the flow is sonic at the exit. For instance the pressure at the exit could be approaching 22 bar relief pressure theoretically depending on flowrate. A check of backpressure needs to be performed based on the flowrate to make sure sonic flow does not exist at the exit of the discharge. If it does then the actual critical flow pressure needs to be determined at the exit. Then a friction loss is done between the exit and relief valve discharge adding the friction loss to the critical flow pressure at the pipe exit to get the total backpressure at the relief valve exit.

 

[mk3223]

IMO, as PSV to be discharged to atmosphere, the outlet connection can be PN10. For a PSV PN40 flange connection, you may need a transient flanged spool from PN40 flange to the PN10 discharge piping.

 

[georgeverghese]

To keep exit backpressure low, exit line velocity should not exceed 0.5Mach. Standard design practice is to document calcs on this. Such calcs on safety relief valve selection and operation are usually HSE critical and may be subjected to external HSE auditors review when requested.

 

[shvet]

I'm confident that there is no back pressure issue, and very little mechanical loading.

If you are confident then why did you start this topic? I am confident you have not calculated backpressure and stress analysis. I am confident there IS a built-up backpressure caused by friction and a significant mech loading caused by combination of thermal expansion, flow induced forces and vibration. Have you ever seen how steam relief 22 bar is discharging?

As per my experience with such flanges non-standard PN10/40 connection poses a threat to a personnel (leaks of steam and sound) as it is located at a personnel service area. A spool PN40/10 described above is the best way to avoid this. Curious to test if bolts are able to be inserted in such PN10/40 fitting - we used thin bolts and large washers for such flanges.

You can provide more details to discuss whether this threat is credible or not.

 

[GD2]

OP,
Many Owners would need a PN 40 discharge flange for a PN 20 valve that discharge to atmosphere. The reason being to provide stiffness to the discharge piping by the reaction load.

GDD
Canada

 

[MortenA]

@shvet, i dont see any "non standard" in this arrangement? Its the most common. But as Littleinch says you shall only do this for single valve arrangements w.o. block valves downstream the PSV

--- Best regards, Morten Andersen

 

[The Obturator]

@MelBe To understand your concern better, can you advise both the Inlet and Outlet size and rating of the Pressure-relief Valve you are discussing.


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

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

 

[shvet]

@MortenA
OP considers to connect DN50 PN40 flange to DN50 PN10 flange. It does not seem 'standard' and 'common'. I had been practicing such in the past and my experience is negative.