Turndown in Relief Valves

[hianbotech]

In these moments we are designing a relief valve (for blocked outlet) of a natural gas gathering system from wells, considering the maximum flow of gas that hopes to be had in 25 years of production. However We would like to know that it will happen in those years that the gas gathering system have flows many minors to that of design and nonetheless the line wants to be protected by overpressure.
Will The relief valve be adapted for these low flows??.

Which is the turndown waited for conventional relief valves?.

Is it necessary to give this kind of information to the relief valve vendor?

Best Regars, Hianbo

 

[Montemayor]

hianbo:

You are installing a PRV on a natural gas gathering system from wells; where exactly is the PRV going to be? Wells inherently require no "over-pressure" to protect them, so you must be supplying the energy to raise the pressure of the well's gas. Are you using compressors? If so, what type of compressors? State the equipment that you are protecting.

There is no such thing as a "turndown" for a PRV.

You should calculate the magnitude of the worse, credible over-pressure scenario and supply that capacity, the type of PRV, and the set pressure to the PRV vendor.

 

[StoneCold]

Montemayor
Maybe he is worried about the valve chattering?

StoneCold

 

[btrueblood]

In the design of a relief valve, the spring can be selected so that the cracking pressure (pressure at which flow past valve first starts) is some fraction of the pressure for maximum or rated flow. Specify both pressures, with appropriate definitions, for the manufacturer of your relief valve.

 

[JimCasey]

He could protect the system primarily with a back-pressure regulator, which would throttle and offer impressive rangeability. At least in the US, piping codes would additionally dictate a safety-relief valve capable of discharging the entire system flow. THe setpoint of the SRV should be 10% higher than the pressure in the backpressure regulator at its wide-open position. If the SRVs need rangeability to prevent chattering, it is permissible to install multiple SRVs with staged (6%)setpoints. Consult ASME Sec VIII.

SRVs are designed to POP open to full flow and blow-down an overpressured system. If the SRVs open, it is to prevent the formation of a crater and avoid litigation, not to regulate the pressure in a system.

 

[hianbotech]

Montemayor the PRF will be installed in a rating 150 existing header that will be used as test header in some of our production clusters, where only one well will be alignment at the same time. The maximum pressure that the system could be reached is in the limit of rating 300, (maximum natural pressure of the wells without compressor). In the case that someone closes a valve (blocked outlet) in the system we must have a Pressure relief valve that does not permit exceed the pressure in the system of rating 150. As I said, this is the protection for a test header and the maximum flow that I considered in the design was for the maximum gas flow for the most producers well (this is the worst credible scenary).

The concern is if we used the test header with another well that produce a very low flow, the PRF will protect in the same way the test header, and also that valve will not suffer damage???. As StoneCold said a chattering effect could happened at low flows???

 

[EGT01]

Hianbo,

Going back to your original post, YES, you should always provide your equipment vendors with as much information as possible in order to get the equipment you need.

I don't really think of a relief valve as having turndown but I believe there is a range that you can expect them to operate without problems. In API RP-521 (4th edition, 1997) you will find a discussion (Section 4.3.2.2 Vapor Emissions) that suggests that spring loaded relief valves will stay open until the flow is reduced to about 25% of the valves capacity. I have also seen this comment in an older Farris relief valve catalog. The best thing to do is tell your relief valve vendor you needs and let them tell you if there is any problem with meeting your requirements.

If you know that your flows will be less than 25% of the relief valve's capacity that you select for your maximum relief condition, then you may want to consider installing a smaller relief valve to handle your low flow condition and set to prevent the larger valve from opening.

As another option, you could consider a pilot operated, modulating type valve. Again, this is something to discuss with your relief valve vendor.

 

[Guidoo]

It is not uncommon to have a HIPPS (High Integrity Pressure Protective System) i.s.o. mechanical protection for such a system. See Norsok P-001, section 4.4 for details.

Having a PSV in such an application can be unpractical because of the high relief loads.

Norsok P-001 can be downloaded from

http://www.standard.no/imaker.exe?id=1409

 

[hianbotech]

thanks for your useful advices.

Best Regards, Hianbo