7 replies to this topic

[Victor 8790]

How to estimate relief load for block discharge case for natural gas line to KO drum.
Max. Op. Pressure is 430 pig and temp is 130 deg.F, max inlet flow is 100 mmscfd. Normal flow 60 to 90 mmscf.
PSV set at 473 psig.

Query:
1. Should I size the psv for max inlet flow of 100 mmscfd (releif rate is so high requires bigger psv outlet line, but existing line is small).
2. Can I calculate flowrate to be relieved for the pressure rise between 430 pig to 473 pig for blocked discharge case.
3. Any other way to reduce the blocked discharge flowrate by taking some credit for outlet lines (two gas outlet lines are there from KO drum, therefore can I assume one outlet will always be open and subtracting 100-x flow)?

[Bobby Strain]

You must size for the flow at relieving pressure, in this case (1.1 x 473) psig. Nobody can answer 1 and 3 without a lot more detail.

 

Bobby

Note change to 1.1 x 473

Edited by Bobby Strain, 02 December 2016 - 11:00 PM.

[Victor 8790]

Thanks for your reply sir.. but can u explain how to calculate the gas flow corresponds to 473 psig ( or relief load)..
The hp natural gas comes from export facility, the design pressure of the export line is 500 psig. But from battery limit of export line to our facility tie in design pressure is 473 psig only.. Normal operating pressure is 425 psig at tie in point and KO drum to which psv is available operates at 350 to 400 psig. Therefore our facility psv will pop up first during blocked discharge case before the export line psv relieves.

Only flow range as I mentioned before (60-90 mmscfd, max 100 mmscfd) was given. Based on consumption in downstream units the flow will vary.. Normal operating pressure is 350 psig & 120 F in downstream units.. Assume mol wt of natural gas is 17.14.

[Saml]

Sorry for changing the topic a little. May be it grants a new thread but...

 

I was answering to Victor and found myself in doubt.... so would like your opinion.

 

- The feed line is protected at 500 psig by an upstream valve.

- I assume that there is a pressure reduction station (and possibly a metering station) between the "export line" and the KO drum.

- The metering station does not have a PSV downstream of it. If it has, then it should suffice to protect the vessel if set at or below KO drum MAWP (there still may be a fire case, but  that would be a much smaller valve)

- Then there is a KO drum with an MAWP of 473 psig

 

Now, on blocked outlet or valve failure it can go above 473 psig. However, below 10% allowable accumulation (520 psig) the flow is zero since feed line goes up to 500. Does it means that any valve would do it and still meet the 10% accumulation? 

 

Regards

Edited by Saml, 01 December 2016 - 05:14 PM.

[sanderson231]

Victor, here is how I would look at your situation.

 

1)  I am assuming that there is a PSV on the incoming line upstream of your pressure letdown station set at 500 psig.  If it is a conventional PSV and not a pilot PSV, the operating pressure needs to stay below 90% of 500 psig or 450 psig to keep the PSV from simmering. A pilot PSV can go to 95% of set pressure or 475 psig.  Going forward I assume that the supplier is holding a normal operating pressure upstream of your pressure controller of no more than 473 psig.  I also am assuming that your pressure controller is holding the downstream pressure i.e. your K.O drum pressure.

 

2) If the K.O. drum outlets are blocked and your pressure controller works normally, there is no overpressure event.

 

3) If the K.O. drum outlets are blocked and your pressure fails to respond there is no overpressure as the upstream normally operating pressure is no more than the K.O. drum MAWP

 

4)  Even if we assume that the incoming line pressure is at its max pressure (500 psig plus 10% accumulation or 550 psig).  The maximum overpressure is 550/473 or 116%.  While it is more than the 110% overpressure allowed for a single PSV it is well within the hydro-test pressure of the K.O. drum of 130% (current) or 150% (past).  Add a second small relief valve and 116% overpressure meets code.  This scenario takes three failures - blocked discharge, failure of your pressure controller and failure of the supplier's pressure controller.  Coupled with modest overpressure potential, loss of containment is not a credible event.

 

5) If you are concerned about leak-by of your pressure control valve in the closed system it is no worse than 4).

 

6)  The K.O. a pressure vessel so code requires a PSV if the vessel can go over the 473 psig MAWP.  Since this is a K.O. drum I'd look at the maximum amount of liquid that normally could be in the vessel i.e. at what point does the BPCS or operator initiate draining.  From this get the wetted surface area and calculate the fire case relieving rate.  Assume both the inlet and outlet are blocked in.  Check your PSV for this case.  PSV needs to be set at 473 psig but K.O drum over-pressure is allowed to go to 121% of MAWP for fire case.  This may have been the sizing basis for the PSV.  Check the PSV data sheet.  It should say what the sizing basis was.

 

7)  In summary you may need a PSV set a 473 psig for a fire case (can the vessel be exposed to an external fire?) but I believe you are adequately protected for both blocked outlet and wide open inlet.

 

8)  All the blocked outlet and wide open inlet discussion goes away if the K.O. design MAWP had been set at 500 psig.  This would have cost few dollars if done during original design.  This is especially true for heat exchangers where the tube rupture case goes away if the low side hydro-test pressure is equal to the high side MAWP.  Again cheap if done during original design.

 

Hope this helps.

[Victor 8790]

Thanks for the answers all:
@sanderson231
1. U are right about upstream line psv. Actually its op.press is 425 to 500 psig and psv set pressure is 550 psig, but not sure about pressure reducing station(it's a huge distribution network to lot of consumers through 52",42" lines, 16" lines with about 900 mmscfd capacity coming out from LPG unit). we take tie in from that network from 12" line and after 3.8 km line of 12" line the Natural gas is sent to KO drum and outlet to our distribution network (boilers etc..). The k.O. drum in our unit pressure is controlled by pcv in KOD downstream gas outlet line @ 350 psig (i.e in downstream distribution network). We are planning on providing pcv in the kod inlet line in future to control the flow.

2. I don't know pressure controller in distribution network, but maybe you are right, it could be in LPG plant.

3. Agreed.

4. If the incomming line is pressurized more than 500 psig then the psv in ko drum set at 473 psig will pop up and relieve the gas. API states if adminstrative procedures are in place and if overpressure due to blocked discharge (BD) doesn't reach corrected hydrotest pressure then no need to consider the BD case. However u asked to add second small relief valve to meet 116% overpressure (already 4L6 psv is available on the drum).  

5. Agreed

6. I checked for Fire case both wetted and unwetted. Relief load is less than 15 mmscfd (I dont have existing dataheet to see th sizing case). My only concern is if psv has to be sized for BD case for total inlet flow of 100 mmscfd the psv outlet line of 6" is not adequate also 4P6 psv is required. Do i need to consider maximum inlet flow?

I studied that if incoming pressure at Tie in point is 500 psig and for the pressure to reach 473 psig  in KO drum, only 68 mmscfd can be pushed (considering 27 psig pressure drop across 3.8 km 12" line). So should i size PSV only for 68 mmscfd?

7.psv is sized for Fire case. But BD flow I don't know how much to take.

8. Agreed but already designed for lower pressure of 473 psig.

 

Please conclude if i should consider blocked discharge or not? if i consider how much flow i should take.

Edited by Victor 8790, 04 December 2016 - 12:03 PM.

[sanderson231]

Victor, in order to provide further assistance, I need to be sure how your system is configured.

You said you have a pressure control valve downstream of the KO drum with set point of 350 psig. I presume this is regulating downstream pressure to your users. If this is the case, then the KO drum pressure is set by the pressure your supplier is sending. Is this correct?

[sanderson231]

Victor, in order to provide further assistance, I need to be sure how your system is configured.

You said you have a pressure control valve downstream of the KO drum with set point of 350 psig. I presume this is regulating downstream pressure to your users. If this is the case, then the KO drum pressure is set by the pressure your supplier is sending. Is this correct?