Discharge flow rate of natural gas

[mamed]

Typically, how much is the discharge flow rate of a PSV in a gas pipe line with the two approximate operating conditions as follows:
Diameter: 12"
pressure: 1000 psia and 400 psia (two cases)
flow rate: 2 MMSCMD

I have used API 520&521 but I do not know the value of discharge rate to be considered. I have considered 10,000 m3/day. Is it okay? Or not? Why? Is there any standard in this regard?

Thank you for your Time.

 

[LittleInch]

The discharge rate nerds to be the flowrate that is required to keep the pressure below your design pressure. So if 2mmscmd is coming in and nothing going out then your relief rate is 2mmscmd.

I think we're missing something here...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mamed]

In my thoughts, practically the PSV acts when the input flow rate is zero (or negligible) due to shut off via any problem or failure.

 

[LittleInch]

A PSV acts to restrict the pressure in a vessel or pipe to a certain value. The amount of flow it needs to handle is driven by the circuamstances that can feasibly occur to cause the pressure to rise abouve the PSV set point ( usually the design pressure,but can be lower). For a gas pipe, there is nomrally only the blocked outlet flow case to consider, when you need to consider what is the incoming maximum pressure and flow rate, without considering other valves acting correctly.

This is a fairly vague question so it only gets a fairly vague response. If you provide more details of our system you may get more of a respponse back - e.g. if your system is designed for 1000 psi, but only run at 400, then there does not appear to be any requirment for a PSV, unless the thing supplying the 400 psi can deleiver more than 1000 if the control mechanism fails.

PSVs represent a safety fall back if other control functions or valves fail so need to be sized for the worst possible case which could occur.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

What??????????? When a PSV opens to atmosphere with the vessel at 400 psig or 1000 psig (note, you always do PSV calcs in gauge pressure because you are actually doing a dP calculation and P_{upstream gauge}+P_atm-P_atm=P_{upstream gauge}) you have sonic velocity through a known orifice size. Perfectly well defined parameter, simply a function of gas composition and temperature. Same speed of sound for all upstream pressures greater then critical pressure (which is about P_atm / 0.6). The mass flow rate is a function of velocity and density upstream of the PSV so you would have a different mass flow rate for every upstream pressure.

The API 521 calculation assumes that you know the required mass flow rate to keep your vessel from exceeding MAWP plus allowable build up pressure (for other than the fire case allowable build up is to 110% of MAWP). If you already have a PSV in place and you are trying to find the flow rate, it is a trivial bit of Algebra to solve the API 521 equation for mass flow rate and you have all of the unknowns in the new equation.

If you are asking about the required flow rate, read API 521 a bit more carefully. It says that you need to investigate the credible scenarios and determine the inflow rate from your system for each credible scenario. Then pick the scenario with the largest inflow rate as the required PSV flow rate. Plug that into the API 521 equation and get the required orifice size.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[mamed]

Dear zdas04,
Thank you for replay.
You are absolutely true. Exactly my problem is the selection of the discharge flow rate you mentioned. Are there any rules of thumbs.