Question about set pressure/location about PSV 1

[billbusy]

Hello everyone,

I am a mechanical guy and want to ask some basic questions about PSV.

1. IS there any set pressure requirement for PSV by codes, like for a pressure vessel, should the PSV pressure set as 1.0* MOP or 1.0* Design pressure? OR is there any other general rules for process guy to determine the PSV set pressure?

2. In a system, where should PSVs be used/added? Is there any general rules?

Thanks.

3 years industry experience;
Oil & Gas industry in Canada.

 

[billbusy]

found a very helpful post

http://www.eng-tips.com/viewthread.cfm?qid=258862

Summarized:

• The Safety-Relief Valve shall be set at or below MAWP (normally same as Design Pressure), Ref. ASME Sec. VIII, Div. 1, Para. UG-134(a).

Engineers try to be "safer" by having a relief device lift earlier than MAWP but it typically results in events that should have never happened.

• The allowable accumulation is 3 psi or 110% of MAWP, whichever is greater, Ref. ASME Sec. VIII, Div. 1, Para. UG-125(c).

(Accumulation pressure can be greater than MAWP. Just because a valve is set to pop at a given pressure doesn't mean that it is fully open at a set pressure. If a valve is set to open at 100 psig it is not fully open and flowing until it reaches over pressure, which in most cases is 10% over the set pressure the difference in over pressure and set pressure, is called accumulation pressure.
Snap acting pilot valves will go full open on initial pop for some high pressure, high performance service. These designs are more expensive. The majority of Sec. VIII, direct spring PRVs will not go fully open on initial pop. More likely, they go 50 to 70% open and then gradually reach full lift at or before 110% of Set Pressure (Accumulation Pressure). To set a valve at or below MAWP does not mean the system will not exceed MAWP when it is relieving.
For a conventional type PRV, the maximum operating pressure may needs to be approx. 10% lower than the set pressure to avoid unnecessary premature opening. Whilst for pilot operated PRV, it may be approx. 3 %.)

• The Set Pressure Tolerance for Safety-Relief Valves is +/- 2 psi up to 70 psi and 3% of Set Pressure above 70 psi, Ref. ASME Sec. VIII, Div. 1, Para. UG-126(d). (Manufacture/installation tolerance)

• When deciding on the PRV set pressure, at least two points you may have to look at.

first point where set pressure can same as or lower than MAWP.
Second point you have to looks at is premature opening of PRV by considering
- maximum operating pressure
- type of PRV
- superimpose back pressure
- blowdown
- net spring setting
- tolerance,
- etc

• If you have a string of equipment with the same MAWP (no rotating equipment or block valves in between them) and a single relief valve protecting all of them after the last piece of equipment (as is the case in some gas plants), you may want to set the relief valve at a pressure that takes into consideration the pressure drop through the system.

Say you have a pressure drop at normal operation of 200 psi through your system, and the MAWP for all equipment is 600 psig, your relief valve may be set at 400 psig to protect the equipment at the front end.

However I would still be very hesitant to place a new relief valve at the end of a chain of equipment like that. Pressure drop is not always a reliable value in gas plants due to the multiple variables that go into it and there are also lots of potential complications (methane hydrate formation, liquid slugs, etc) that could make reliving difficult. Almost without exception I would recommend placing the relief valve at the first equipment item. It makes more sense to place the relief valve at the front end, closer to the source of the overpressure in the example I gave.


3 years industry experience;
Oil & Gas industry in Canada.

 

[billbusy]

Plus, I am still looking for answers for Q.2.

If anyone can give me some tips, I will appreciate it.

3 years industry experience;
Oil & Gas industry in Canada.

 

[DSB123]

Answer to question 2. PSV's or some form of overpressure protection is needed in any sections of the process plant which are not protected from possible overpressure caused by process upset events or any other causes (e.g. fire).

 

[RaRo]

I don't know what is the exact meanaing in Q2 for you. But I experienced having to take a litteral/geographical approach for some mechanical engineers that just began in chemical industry.

Aside from thermal expansion (of liquids), the typical spots where PSVs are installed are:

At pump discharge to protect downstream equipments/lines

On the top of equipments (gas head) for vessels, tanks, hoppers..., preferrably mounted directly on the equipement
On the vapor outlet line for equipments where direct mounting is not possible (no room or no nozzle)

After condensers, on the vapor line to whatever (vacuum pump for example)

Hope this helps :-D

A less literral answer would be: where you need them, i.e. where there is the potential to exceed the MAWP/design pressure of one or several equipments. The study should consider what are the pressure sources (internal and external) and divide the equipment sets according to the product flows and the isolation possibilities between them.

 

[billbusy]

Thanks DSB123 and RaRo.
You answered what I asked.

As what I understand now, it seems there is no specific minimum requirement from codes/standards for PSVs, a psv must be used after a pump outlet or every 100m in a piping system or pressure higher than a number, etc.. Thus the Process engineers add PSVs based on their understanding of equipment/piping safety and clients' request.

Cheers



3 years industry experience;
Oil & Gas industry in Canada.

 

[billbusy]

, LIKE a psv must be used after a pump outlet or every 100m in a piping system or pressure higher than a number, etc..

Sorry for missing a LIKE there

3 years industry experience;
Oil & Gas industry in Canada.

 

[zdas04]

I don't know where you got the 100m of piping as a requirement. I can point to systems that are thousands of km long without a single PSV. You don't even need one on every single pressure vessel as long as the vessels that share the safety device are: (1) all rated at a pressure that is consistent with the set pressure of the device; and (2) separated by piping without integral block or control valves. The code says that the vessels must be protected as you showed in your second post in this thread, but it really doesn't say "how". That comes back to Engineering Judgement.

David

 

[chemebabak]

More important than all, API 521 offers a section titled, "Causes of overpressure". There are several scenarios that should be analyzed to determine if a PSV is required.

PSVs are required on all ASME stamped pressure vessels that can be blocked in. A pressure vessel requires an ASME stamp if the design pressure is 15 psig or greater. Refer to ASME Sec. VIII, Div. 1, Para. U-1(h) and UG-125(c).

If the process is designed with 2 or more pressure vessels in series without block valves in between, then only one PSV is required. If each pressure vessel could be individually blocked in, then each requires its own relieving device.

In many systems, such as in tank farms, piping is protected by PSVs commonly called TRVs (temperature or thermal relief valves). These open during thermal expansion. In a tank farm the piping is usually blocked at the tank and at the source. The sun could heat the pipe enough that the fluid will expand and overpressure the pipes. A small PSV jumps from the pipe to the tank inlet over the block valve. Thermal expansion will open up the TRV and just enough fluid to lower the pipe pressure will pass to the tank.

Also, PSVs are required on heat exchangers where the 10/13 rule is not applied. In simple terms, if the low pressure side is designed for less than 10/13 the design pressure of the high pressure side (or if the design pressure of the high pressure side is greater than 130% of the design pressure of the low pressure side) a relief device is required. Reference API 521, 5th edition, section 5.19.2.

 

[Duwe6]

Distance means nothing. ". . . every 100m in a piping system . . . "
Just need one PRV anywhere liquid can be blocked in [between isolation valves]. Also if your pump discharge pressure can exceed the MAWP of the pipeline.

Just one, if the discharge rate will be adequate.

 

[insproh]

Hello Everyone,

I am a mechanical engineer.
I just wanted to know if there was any set pressure requirements while testing an equipment.
For Ex. Suppose we are hydro testing an Exchanger or vessel. we install a PSV on the test circuit. what should the set pressure of the PSV be? is there any set recommnedation or can we assume 5~10% more than the test pressure.

 

[Duwe6]

I like 5-10% over Hydro Pressure. Back when Hydros were 1.5 x MAWP, I was taught 1.33 x Hydro pressure, or less. This was for Sect III work - Nuclear. 1.33 x current Hydro gets too close to Yield Strength for me.

 

[insproh]

Thanks Duwe6,
Good to see a reaction on this post. but from what i understand from your post. there is no set rules from any of the standards ASTM or API for the PSV set pressure for such cases. and interestingly 1.33 x hydro pressure was also practiced, which makes the case that upto 33% was used.
we carry out the hydro for the in-service equipments at design pressure itself.I understand that the PSV is for safety measure only. however i was keen on knowing if there was a standard to judge the over rating or set pressure for the PSV.

 

[Latexman]

Please don't add a post to an existing post. Now it is unlikely to be searched properly in the future since the title is for the original post. Start a new post and give it a meaningful title.

Good luck,
Latexman