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Set Pressures Of Releif Valves On Api 650 Tank


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#1 rxnarang

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Posted 20 February 2007 - 10:28 AM

I have the following situation:

a) New tank design -API 650 tank - design pressure is 2.5 psig, 70"WC ( so Appendix F of API 650 and no accumulation)
cool.gif The product is VAM
c) The tank was originally designed as API 620
d) Tank is nitrogen blanketed
e) The present design shows 3 valves as follows

1) A big 20" - presumabaly for fire. Could be weight or pallet loaded. We understand that the overpressure for such valves is 100%, so we have set this at 35"WC. Is this correct?

2) A valve for liquid overfill. Is this a normal thing to have in a API 650 tank? If so, we need advice on the set pressure of this valve. Obviously it has to less than 35"WC. 25"WC? What is the typical overpressure of such valves? Preferably this valve should completely open and relieve before the valve at 1) opens

3) A conservation vent ( pressure/vacuum). The set pressure shoul be less that 2) above. 10" WC? Again will we get a valve with appropriate overpressure?

Would be much obliged for your advice.

Please let me know if any further clarifications are needed.

Regds

#2 proinwv

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Posted 20 February 2007 - 10:50 AM

Hello,

Well let me make some comments on your post. But first, I do not know what VAM is so I am speaking generally, and as a ME not a ChE.

1) Regarding the 20" vent: Find the manufacturers name, model and setting and contact them as to the set pressure, the maximum pressure at maximum rated flow and the flow curve. Next determine from the codes what your required flow rate for the application is. If applicable, see API 2000.

2) I do not have a copy of API 650 so I am asking others to comment on that factor. But you are on the right track in that the vent must operate outside of the operating range of the first mentioned vent. Again, find the manufacturers name, model and setting and contact them as to the set pressure, the maximum pressure at maximum rated flow and the flow curve.GENERALLY, if you had a 35in.wc set point on the first then 20 in.wc on the second would make sense as long as it's pressure at maximum flow would still be below the 35in.wc.

Have you considered overfill warning devices and overfill shut offs?

3) The conservation vent should again be sized appropriately as mentioned in (1) and also have a setpoint appropriate to the vent above it, or GENERALLY we would consider a 10in.wc setpoint reasonable here as long as its full open flow pressure were below the 20in.wc setting. And again, find the manufacturers name, model and setting and contact them as to the set pressure, the maximum pressure at maximum rated flow and the flow curve.

Also keep in mind that once any vent is actuated, the reseating pressure is below the set pressure so that should always be in the back of your mind also. This number should be available from the manufacturer and keep in mind also that the manufacturer is limited to test data under controlled lab conditions which are always on new, clean product. In time the values will likely degrade .
Regards

#3 djack77494

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Posted 20 February 2007 - 06:03 PM

rxnarang,
I don't normally like to "chime in" and just endorse what has already been said in a posting, but I think Paul did an excellent job in responding to your interesting question. I agree with what Paul has said pretty much completely. Remember that the idea of allowing for 100% above setpoint is a fairly conservative assumption and the manufacturer's flow curve will provide a better number, if it is available. Like Paul, I question the strategy of using a PSV for overfill protection. I think of it as providing an outlet for vapors trapped in the top of the tank when filling, but would hope that the tank never completely fills with liquid due to provided instrumentation. Overall, I like your approach and like even more some of the numbers you are using. It's rare to have a situation where you have the range of values to work with that you seem to have. Happy designing.
Doug

#4 proinwv

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Posted 21 February 2007 - 06:16 PM

I'll add onto what djack says.

A conservation vent is not used as a liquid overflow protection device. It is for passing vapors.

They are not tested for liquid flow nor rated as such and without that information could not be relied upon to provide a safe pressure build up for liquid flow.




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