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#1 prajot

prajot

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Posted 19 June 2007 - 03:25 AM

Hi,
I am designing a nitrogen Pressure Control valve which is normally maintaining pressure inside a condensate storage tank (operating pressure 1.037 kg/cm2 a).
In order to design the control valve I have taken following flows
1) Normal N2 flow as difference between Normal liquid flow out and Normal liquid flow in to the tank.
2) Maximum flow as Maximum liquid flow out plus inbreathing due to thermal effects (As per API 2000)
3) Minimum flow as half the normal flow which is general practice in this project.
By taking into consideration all above condition I came up with normal flow as 386 Nm3/hr and maximum flow as 5786 Nm3/hr. As you know the difference is quite huge and our control valve vendor is suggesting to use two control valve one will take care of normal and other will take care for maximum.
My question is that Is this method is right? If not then could you suggest me the other consideration that I should use to reduce the maximum flowrate as API 2000 does not give pure guideline regarding the flowrate of this PCVs.

#2 proinwv

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Posted 19 June 2007 - 08:11 AM

Hi Prajot,

You said,

By taking into consideration all above condition I came up with normal flow as 386 Nm3/hr and maximum flow as 5786 Nm3/hr. As you know the difference is quite huge and our control valve vendor is suggesting to use two control valve one will take care of normal and other will take care for maximum.

This is API2000. I suggest you review the Appendices A and B of API 2000. You will better understand why this happens. Often, users reduce this amount based on their operating conditions and plant practices.

#3 prajot

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Posted 21 June 2007 - 06:13 AM

Thanks Paul for replying
But still I m not sure whether I have used right basis for my calculation.
You are absolutely right that basis varies with the operator and plant practices, but I just want to confirm that my basis is correct.
I have gone through API 2000 and your tank blanketing guide which gives me some confidence regarding the correctness of my basis.
But, If the basis is right then is it a normal problem for valve selection as I already mentioned that our control valve vendor is suggesting two control valve for this n2 blanketing valve.

#4 proinwv

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Posted 21 June 2007 - 08:54 AM

Prajot,

First I do not recommend a control valve for blanketing applications. They respond too slow and when the operating band is narrow, the tank pressure may go out of range. The best, and least expensive also, are direct operating regulators.

If you confirm that you require the larger capacity for thermal and if the regulator you choose will not be able to maintain the pressure over the total range of flows, then I would agree to adding a second regulator, in parallel with the first one. Set the larger one at a slightly lower pressure than the first so that it will come on before your process pressure goes out of range.




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