RELIEF ENERGY

[roker]

Hello all,

when a high pressure PSV at 150 barg set pressure is opened to a flare system which is operating at 0.5 barg to what type of energy (beside the friction at the outlet line)is this high pressure converted, where it can be detected and what is the exact location of this DP conversion.
beside a verbal reply a scheme will be helpful.

regards,
roker

 

[chemjock]

When a gas moves through a PSV there is a hugh conversion of pressure head to velocity head as the fluid accelerates to sonic velocity and then reconverts some velocity head to pressure head as it slows down in the the outlet reducer and finally reaches about 3/4 Mach in the tailpipe on a new design. Thermally, the process is adiabatic. Benoulli's therom applies and the friction through the nozzle can be caluculated using Crane.

 

[insult2injury]

Typically, relief valves are modeled as isentropic nozzles with the vent stacks modeled by the fanno flow equations.

I2I

 

[4Pipes]

chemjock

"as the fluid accelerates to sonic velocity and then reconverts some velocity head to pressure head as it slows down in the the outlet reducer".

This is a common process assumption but I wouldn't be so sure.

A reducer is like the exander of rocket motor. The velocity could increase. Even if the reducer is not very efficient as a "nozzle", you will get increases in velocity to supersonic through the shock waves that will be present. (M depends on how under expanded the nozzle is). A short distance further downsteam, more shock waves will reduce the flow back to subsonic from where your assumptions will hold true.