Design & Relieving Temperatures

[Canoman]

I had originally specified a small vessel (filter shell) with a design pressure of 150psig and a design temperature of 350°F, but after looking more closely at the specified pump and the PSV loads, I realized that the design temperature and pressure might need to be changed. I changed the design pressure to 200psig, and revised the PSV set pressure to 200psig. The vessel is full of water, so the relieving temperature would be approximately 403°F in a fire case (assuming 21% overpressure).

The vessel was specified with a 150# flange rating. At 400°F, 150# flanges are good up to 200 psig. So this means that I would have to bump up the flange rating on the vessel if I change the design temperature to coincide with the relieving temperature.

ASME says that the design temperature needs to be at least the mean metal temperature under operating conditions but says nothing about upset or relieving conditions. API 521 says, "Relieving temperatures are often above the design temperature of the equipment being protected. If, however, the elevated temperature is likely to cause vessel rupture, additional protective measures should be considered (see 3.15.4)."

I guess my question is whether the scenario described above is "likely to cause vessel rupture," or how I can determine if it's likely.

 

[erpeterson]

Canoman,

You have very little to worry about. Here is why.

1) Typically, vessels designed properly will burst at 300% or more of design pressure at design temperature.

2) Relief valves are designed to protect the vessel and are very conservative, but if you wish, you can lower your relief pressure slightly to drop your relief temperature below 400F.

3) Realize that when relief devices are installed, they have a margin of error on the setting. For example, a 5% difference in pressure setting vs. rated setting is not uncommon. 3F out of 400F is less than 1%.

4) You can specify a valve that will be fully open at a lower pressure, thereby never reaching the 21% overpressure allowed by API 521.

5) Your corporate policies may force you in one direction or another on this question, so before doing anything, consult an internal expert first.

6) There are a wide number of other considerations, but don't forget materials of construction of your valve, such as seats. Many ordinary seat materials can't take 400F

 

[MortenA]

i second erpetersons comments.

remember that a fire PSV is "last line of defence". Its only supposed to protect in case a failure of deluge. The deluge will keep the temperature much lower. Should the deluge fail the PSV will protect the vessel against rupture but the vessel will be considered "lost" as such after the fire.

Most vessels where the psv is sized for fire that i have seen as a process engineer and consult in oil & gas for 8 years has had a design temperature much below the temperature require for creating a high enough "vapour pressure" for the PSV to lift.

Best regards

Morten