1 reply to this topic

[Anil Panchal]

Hello to everyone,

Design Data:
Vessel side design pressure = Full vacuum
Jacket side design pressure = 5.0 kg/cm^2

My vessel is subjected to full vacuum while jacket side pressure is 5 kg/cm^2. so jacket side pressure will also act as external pressure on vessel.

Can anyone help me to find the final external design pressure for vessel of following.

final external design pressure for vessel = FV + 5.0 kg/cm^2
or
final external design pressure for vessel = FV

refer attached figure also.

Thanks,
Anil Panchal

Attached Files

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[ankur2061]

Anil,

I think you haven't picked up the concept of vessel design. Even if your normal operating conditions are vacuum in the vessel, you must realize that when the vessel is not operating and exposed to atmosphere it is subject to atmospheric pressure. Thus no vessel can be said to have a design pressure as just F.V.
Vessels are always specified for the atmospheric or above atmospheric pressure with a special case of vacuum (either partial or full) if by any reason (normal operation / process upset / startup / shutdown) they can be subjected to a vacuum condition. An example for a startup/shutdown condition can be steaming out of the vessel just after shutdown or prior to startup which would lead to steam condensation and formation of vacuum. A process upset could be the sudden condensation of saturated vapors due to sudden cooling by uncontrolled cooling media flow to the vessel causing a vacuum.

However, if any process has been evaluated to have no chance of vacuum formation then vacuum need not be specified for the design pressure. If a careful evaluation of your process concludes that under no circumstances your vessel can see a vacuum of more than a given value (e.g. 500 mm HgA) then you can describe a partial vacuum value in the DP column of your vessel data sheet. If you consider a margin of 20% then your design vacuum value based on 500 mm HgA would be:

500 -20%*500 = 400 mm HgA

Nevertheless, you still have to specify the DP in terms of atmospheric or above atmospheric pressure.

For your specific query the answer would be:

DP = 5 kg/cm^2g / F.V

BTW. make it a point to give the reference state for pressure (absolute or atmospheric). It is very annoying when the person trying to help you out has to guess that the pressure is gage or atmospheric.

Hope this helps you understand the concept of DP.

Regards,
Ankur.