Exact meaning of Design Pressure Expression

[Leclerc]

I have some vessel design pressure data on P&IDs in the form,
"DESIGN:600 PSIG @ 150 deg.F. External 14.7 PSIG @ 150 deg.F."

The vessel stands in the open air and can only be subjected to atmospheric pressure externally.

I assume the "External" refers to some vacuum pressure, but I would have thought the word "vacuum" would have been mentioned. How should I interpret the External part of the data?

Any clues?

 

[jproj]

Leclerc:

External design pressure is the vacuum design pressure. In this case, the vessel should be designed to withstand 14.7 psig of vacuum at 150ºF.

Since (like you stated), the vessel is located outside in the open air, 14.7 psia is the max. external pressure possible. To be technically correct, the P&ID should say -14.7 psig (or 0.0 psia... full vacuum)

Regards,

jproj

 

[abeltio]

When designing for atmospheric conditions if vacuum can be obtained inside the vessel the customary design pressure is full vacuum.
I've seen this indicated as: e.g. 150psig/FV meaning that the vessel can withstand 150psig internal press and Full Vacuum (atmospheric pressure outside).
HTH
Saludos
a.

 

[Leclerc]

I am also used to the expression, eg 600 psig/ FV. But I interpret this as the vessel has been designed to withstand an internal pressure of 600psig(0 psig external pressure understood); also the vessel design can withstand an internal pressure of minus 14.7 psig( 0 psig external pressure understood. Is this different from from your interpretation, Abeltio?

 

[abeltio]

There is no difference in the interpretation.
[14.7psia = 0 psig] external pressure = [-14.7 psig =0 psia]internal pressure.
FV (full vacuum) is the expression preferred for 0psia internal pressure which assumes atmospheric pressure outside.
To give another example consider a jacketed vessel (very common in the food processing industry).
The internal pressure of the vessel= 150psig
The internal pressure of the jacket= 350psig
BUT the internal pressure of the jacket is EXTERNAL pressure for the vessel.
So, the vessel must be designed for:
1. 150psig internal pressure
2. 350psig external pressure, if it is impossible to have vacuum in the vessel otherwise the external pressure should be considered as 364.7psi --> external pressure + full vacuum inside.
HTH
Saludos.
a.