Max Flow Determination for PSV Protecting Pipeline

[mechanicalchad]

Hi All,

I am looking at having 3 low pressure natural gas wellsites being tied into a higher pressure natural gas pipeline at a compressor pod site (for those who don't know what that is: a site where a future compressor can be added). The low pressure pipelines are all plastic and have an MOP of 1100 kPa where as the MOP of the high pressure pipeline is a steel line rated at 3450 kPa. At this site I will have a spec change from 150 ANSI to 300 ANSI at a check valve which is located after the low pressure header, but before the high pressure pipeline riser. Immediately before the check valve, on the 150 ANSI side, I will have a PSV to protect the low pressure pipelines.

Although, the steel pipeline is rated at a higher pressure it is actually running at a fairly low pressure and hence the reason why we are planning on tying in a low pressure PL.

My question is this, to size the PSV should I use the pressure differential between the two MOPs and a valve flow equation with the ball valve to calculate the maximum flow the PSV could see? Or, should I use the flow rates that operations provides to size this PSV? My concern with the latter is that since it is a gathering system, more pipelines could be tied into this system and the pressure and flow could increase.

Sorry for the longwinded explanation. I am not that experienced with pressure protection of pipelines so if I have made a mistake please feel free to point it out.

Thanks in advance,

Chad

 

[BigInch]

It appears you are trying to design a relief station to protect the low pressure gathering system pipelines from high pressure backflow from the collection lateral, right? Proceeding on that assumption,

For sizing the PSV, you must use the lowest MAOP as the inlet pressure to calculate the flow capacity across the PSV, so put 1100+10% = 1210 kPa on the inlet side of the PSV and 100 kPa atmos pressure at its outlet to atmosphere.

The remaining problem is to determine the maximum possible flow going to the PSV in the hi pressure backflow scenario. How much flow can you get from your high pressure source? With those parameters identified, you can determine the orifice size of the PSV that is required to protect this installation.

I think you would want a block valve in front of your check, as check valves don't count as positive shutoff should you need to isolate the gathering lines to do some work in there or make a new tie-in to the station, etc.

If hi pressure backflow from the compressor station to the LP gathering lines is really possible here, I would put a 300# Emergency Shut Down block valve before the check you mention with a HI Press switch on its upstream side to close the ESD valve and also shut down the compressor. Then put the PSV 300# inlet x 150# outlet, the 300# check running to the compressor inlet flange.

If the compressor's discharge pressure can get higher than the 3450 kPa MAOP of the collection lateral pipeline, you will need a similar installation on the discharge side of the station. Put a high discharge pressure switch (set at 10% over MAOP 3750) running to a station discharge PSV and ESD valve coupled to a HI press compressor shutdown there.

Verify that the 10% overpressure setting is allowed by your code.

 

[pleckner]

Adding:

The pressure into the PSV is as given by "BigInch" in the post above. But the flow will be restricted by any fittings existing between the high pressure line and the PSV. It's hard to quantify what the flow would be through a failed check valve so the conventional wisdom is to just use the maximum flow that the system will ever see. But, if you have a valve between the high pressure line and the PSV, then by all means use the fully opened Cv of this valve with a differential pressure equal to the MAXIMUM obtainable pressure of the high pressure line minus the PSV relieving pressure (as given by "BigInch" above).

Note that you do not take credit for the instrumentation in determining relieving rates. You can take credit for the instruemntation in determining if you can develope a relieving scenairo.