5 replies to this topic

[erwinfs]

Dear All,

 

Can we calculate the pressure of a reliefing gas at the outlet of PSV? Will the gas' pressure be the same as the critical pressure?

 

I want to know that since I was asked by the Piping engineer to provide operating pressure of the PSV tail pipe.

 

Thanks..

Edited by erwinfs, 24 April 2018 - 01:23 AM.

[fallah]

erwinfs,

 

Please specify if the tail pipe is going to atmosphere or ids connected to flare network...

[Amit J]

I understand that you want to calculate PSV discharge pressure at outlet flange.

 

Whether the psv relieving to atmosphere or to flare header, following general guidelines are common for calculating PSV discharge pressure :

 

1. There will be 2 contributors for this discharge pressure: superimposed backpressure & built up back pressure. one is due to geometric configuration of downstream network and another is due to effect of actual flow through given path.

2. you need isometric drawings, PSV sizing data, PSV discharge flow & properties, flaresim software, possible relieving scenarios to calculate same.

3. Build model in flaresim and insert actual data. you will get actual pressure at PSV discharge.

4. If you use flaresim, you can get corresponding mach no. as well. This mach no. is useful to piping engineer to identify stress critical lines and further do support calculations for same.

 

If approximate data required then consider flare tip pressure drop ....plus.... calculate possible pressure drop across piping by line hydraulics check. you may consider this value with some margin of safety.

[fallah]

I understand that you want to calculate PSV discharge pressure at outlet flange.

 

Whether the psv relieving to atmosphere or to flare header, following general guidelines are common for calculating PSV discharge pressure :

 

1. There will be 2 contributors for this discharge pressure: superimposed backpressure & built up back pressure. one is due to geometric configuration of downstream network and another is due to effect of actual flow through given path.

2. you need isometric drawings, PSV sizing data, PSV discharge flow & properties, flaresim software, possible relieving scenarios to calculate same.

3. Build model in flaresim and insert actual data. you will get actual pressure at PSV discharge.

4. If you use flaresim, you can get corresponding mach no. as well. This mach no. is useful to piping engineer to identify stress critical lines and further do support calculations for same.

 

If approximate data required then consider flare tip pressure drop ....plus.... calculate possible pressure drop across piping by line hydraulics check. you may consider this value with some margin of safety.

 

Amit J,

 

By "Flaresim", do you actually mean "Flarenet"? 

[Amit J]

Thanks Fallah for correcting me....

I meant to say 'Flarenet'....

[serra]

erwinfs,
the maximum pressure limit at the outlet of PSV (not considering the influence on set point) is determined by PSV critical flow condition, for gas you can estimate this value at about 0.5 * Pin
since max pout (pressure at the outlet of PSV) must be below that value (see also the limits for conventional vs. balanced-bellows types) you should determine max pout adding dynamic contributes (pressure drop in piping etc.) and static contributes (for example if you discharge to atm),
you can solve these calc's manually (see API etc. formulations), with Excel (as I do) or some other software,
the usual procedure is to start from discharging point (for example atm) and then calculate all dynamic contributes testing for a mach number < 0.7 (or some other safe value),
as suggested by fallah you may start evaluating tailpipe connections etc.
btw, there are many threads at cheresources discussing these topics...

Edited by serra, 26 April 2018 - 09:01 AM.