8 replies to this topic

[kpavan]

Hi guys,
I am designing a PSV to protect piping and a Jacketed pipe exchanger that heats gas. The PSV will be located on the cold outlet piping.
Gas being in the pipe (cold side) and is high pressure than jacketed side, the most probable case for this is Thermal expansion of gas.

I have calculated relief temperature using an expression of ideal gas law:
Tr = Pr * (T/P)
where,
P = Operating Pressure
T = Temperature
Pr = Relief Pressure
Tr = Relief Temperature
And calculated the relief flow using Q = m x Cp x (Tr-T)
where
Q= Normal exchanger heat duty
m=Relief flow
Cp=specific heat at relieving conditions.

Main thing is I need confirmation whether i can use this or is there any other method to calculate relief rate.

Thanks in advance.

[kpavan]

Dear all,
I request you to please give the comments on this topic.

Thanks in advance,
Pavan.

[Lowflo]

Perhaps you've done this already, but if not, determine if this will even cause the PSV to lift. Calculate the cold side pressure when it reaches the hot side temperature. P2 = (P1*T2)/T1. You'll often find that this P2 is lower than the MAWP. If it's higher than MAWP, then proceed with calculating the relief flowrate based on a corrected Q valve. Using the design Q is OK if the resulting relief valve is very small, but if it's not you can easily calculate a more accurate relieving Q by multiplying the design Q by the ratio of the relieving dT divided by the design dT.

The actual heat input will decrease from the moment the scenario first occurs until the cold side temperature reaches that of the hot side. The PSV will start to relieve when the cold side temperature has increased sufficiently to cause the pressure to reach the set point of the PSV. From that point, the temperature will continue to rise as the cold side T approaches the hot side T, and the resulting rate of heat transfer (and required relief flow) will decrease accordingly.

You could use the ideal gas law to calculate the number of moles in the cold side initially (P1,V1,T1) and then do calculate the amount at the final condition (the point where the cold side reaches the temp of the hot side). The rate of heat transfer tell you how long it takes to get from the first state to the second, therfore that allows you to calculate a required relief flow.

[fallah]

If gas is being trapped between e.g. two isolation valves and Jacketed pipe exchanger is still activated, you can use the model similar to what mentioned in API 521 for gas filled vessel in fire case.

[JoeWong]

I am designing a PSV to protect piping and a Jacketed pipe exchanger that heats gas. The PSV will be located on the cold outlet piping.
Gas being in the pipe (cold side) and is high pressure than jacketed side, the most probable case for this is Thermal expansion of gas.

kpavan,
I understood your configuration as

- Jacketed pipe exchanger.
- Gas is cold fluid in pipe
- Heating medium (i.e. hot water, hot gas, hot oil...) in jacket.

Gas in pipe being trapped while heating medium in jacket flowing continuously. Heat from heating medium transfers to gas in pipe, heated gas caused pressure increase upto PSV set pressure, PSV open and subsequently PSV relief. This is typical blocked-in-heat-on.
For BIHO, higher the heat input or heat flux, higher the relief load. Thus, relief load is subject to heat input or heat flux.

The equation that you mentioned is typical applicable to gas (or supercritical fluid) filled vessel expose to pool fire (as discussed in API Std 521 section 5.15.2.2.2). This equation may only applicable to heat input due to pool fire (with specific heat flux).

What you need to do is to find the gas expansion rate base on heat flux, find PSV relief load from gas expansion rate by keeping gas pressure in pipe constant at 110% of set pressure (if 10% overpressure is allowed by design code).

[kpavan]

Dear all,
Thanks for reply.
As described by Joe Wong, my calculation are almost like that of fire case in gas filled vessel. Because in fire case for gas, it is thermal expansion with some exceptions while taking relieving temperature and heat flux calculations. But here i have taken the design heat flux of exchanger as constant and used it further for calculating relieving capacity based on the relieving temperature as posted in the question i have earlier asked.

The equation that you mentioned is typical applicable to gas (or supercritical fluid) filled vessel expose to pool fire (as discussed in API Std 521 section 5.15.2.2.2). This equation may only applicable to heat input due to pool fire (with specific heat flux).

Joe Wong,
May i know which edition of API 521 you are describing, Because i am having 1999 edition in it fire case is described in 3.15.2.1.2 and i didnot find any section you have described, if there is anything different in that section please let me know.

Fallah,
My model is similar to that of Gas filled vessel in fire case.

Lowflo,
Initially I tried the way you described, but calculations seemed to be a little confusing. So, I switched over to this kind of calculation to make it simple.

Pls. give me some more suggestions/comments.

Regards,
Pavan.

[JoeWong]

Dear all,
Thanks for reply.
As described by Joe Wong, my calculation are almost like that of fire case in gas filled vessel. Because in fire case for gas, it is thermal expansion with some exceptions while taking relieving temperature and heat flux calculations. But here i have taken the design heat flux of exchanger as constant and used it further for calculating relieving capacity based on the relieving temperature as posted in the question i have earlier asked.

You may have different heat flux... Stick to constant UA could be easier for you.

Joe Wong,
May i know which edition of API 521 you are describing, Because i am having 1999 edition in it fire case is described in 3.15.2.1.2 and i didnot find any section you have described, if there is anything different in that section please let me know.

Latest release : API Std 521, 2007 edition with addendum May 2008.

Edited by JoeWong, 19 August 2009 - 09:38 AM.

[thakur]

Latest release : API Std 521, 2007 edition with addendum May 2008.
[/quote]

dear mr.wong,

from where one can get this latest edition of API Std 521?

[JoeWong]

Check this LINK...

Some update HERE...