PRV Sizing for Use during Hydrotest of Piping Systems 1

[zengx]

I work as a client rep. and in one of the clients specs there is a requirement for sizing the PRV used during hydro testing any piping system per API 520.

I did some research on sizing the PRV and found that the relief capacity depends on the flow rate. We mainly use water for pressure testing, so I assume that there will be no much flow coming from the hydro test pump after reaching the required test pressure and before over-pressurization, specially for small piping systems.

Can you advise how can we size PRV for a hydrotest? and what would be the worst over-pressurization scenario?

Thanks.

Regards,
Mohammed

 

[The Obturator]

Hi. I think someone may have their wires crossed with that requirement. A SRV has its size (nozzle) determined by various factors. These include data such as the minimum flow rate to pass at a predetermined set pressure. This can be done by API-520 method. The HydroSTATIC test is to test the integrity of major cast parts such as the body and nozzle. This does not require a flow rate as such, but for the tested part to be filled with water and pressurised in place ("static") at the test pressure.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Latexman]

I have never seen or used a pressure test failure as a PSV sizing scenario. Has anyone ever done this? Why?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[mk3223]

for testing a piping system, a hand pump and a small PSV (3/4" or 1") are normally adequate for the work. The pressure is to go up with a small amount of water pumped with the hand pump, so a small PSV is adequate for the job.

 

[zdas04]

I have done this. When test pressure results in stresses that are over some percentage of SMYS (in the case I needed a PSV the test pressure resulted in 85% of SMYS), tiny changes in test fluid temperature can quickly put you over SMYS. I set a 3/4" PSV at a pressure corresponding to 90% of SMYS (so that if it lifted it wouldn't invalidate the test). In this case the operator was able to easily maintain pressure by bleeding off fluid, and the PSV didn't lift during the test, but if he had missed a temp change the PSV should have protected everything. Like mk3223 said, the relieving capacity is tiny (based on the flow rate of the injection pump).

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Latexman]

Basically, ambient thermal expansion relief.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

That is the only scenario that I've ever found credible.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Max flow is what the hydro test pump is capable of of no one turns it off. For a hand pump it's very low, but some pressurising pumps can be decent, especially if someone use a larger pump than the volume requires.

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

 

[zdas04]

Even a pretty big (for a pressurizing pump, not a fill pump) pump has a pretty small capacity. It is incumbent on the test designer (if they determine that a PSV is required) to properly size the PSV.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[zengx]

Thank you all for your replies.
We had some cases where the PRV did not function after exceeding the set pressure, but I believe this has nothing to do with the sizing. These are probably malfunctioned PRVs.

Regards,
Mohammed

 

[LittleInch]

Malfunctioned?

Spring relief valves are very reliable systems, but need to be set correctly and installed correctly ( top of pipe, no dirt etc). If they really did malfunction I would start to get very worried indeed. PRVs are relied on the world over to open when required to a very high level of reliability.

I strongly suspect / hope you mean incorrectly set. What the valve says on its name plate is only as good as the day it left the factory. Once people start messing around with them and re-setting pressures, changing springs etc, the name plate should be amended or ripped off but no one does in practice.

Insist on up to date calibration certs for the valves or get them bench tested before use if the paper work is poor or non existant.

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

 

[mk3223]

It could be a serious safety consequence for the worker or equipment if the PSV for the hydro test was not correctly calibrated or installed. It should be a lesson learned of the near missed event and make sure it won't be happened.

 

[ChemE08]

I have seen that program being adopted at a site. They have a set of PSVs that vary in set pressure depending on the pipe class. The driving force in the sizing is the capacity of the hydrotest machine (which is normally very small) and the idea that it continues pressuring up. Given the normal hydrotest machine capacities and pressures, you likely won't need any huge PSV. The more common occurrence is the leaking of the PSV during the process of pressuring up the pipe segment.

So the sizing wouldn't be any different than a normal blocked outlet case. The main driver here is making sure the desired set pressure is correct.

The pressure spikes cause the PSV to lift with every stroke. They tend to hold just fine after we have reached our desired test pressure. However, they will likely be repaired more frequently due to this.