Vacuum on Pressure Vessels

[RJB32482]

Hi,
I am new to the comncept of vacuum relief. What is exactly the definition of "vacuum" on a vessel (or full vacuum). What codes show how to calculate if a vessel can handle vacuum conditions or if a vacuum relief valve is needed.

Thank You

 

[elguero]

Full vacuum is the greatest amount of vacuum that can be experienced in normal operation on planet earth. If there is any chance that an operation will operate at full vacuum (even by accident) then design for full vacuum should be done. Design for full vacuum is design for an external pressure of 14.7 psi.

To design for vacuum relief, decide what external pressure should be the MAXIMUM for design purposes. Then provide a measure such that said maximum is not exceeded.

 

[RJB32482]

Any codes that I could look at (ASME or API) to help me understand the concept more? Maybe the scenarios for vacuum and sizing?

Thanks for the reply.

 

[TD2K]

Codes won't help you.

If you can find a copy of the Ingersoll Rand pump book (any any pump book for that matter), it will discuss suction pressure/vacuum with some sketches which should help you understand it vaccum relative to atmospheric pressure (vacuum isn't an absolute number. Rather, it's used to refer to any pressure less than atmospheric).

As Elquero put it, since 14.7 psia is atmospheric pressure at sea level and the minimum pressure that could exist in the vessel is 0 psia the maximum vacuum rating is 14.7 psi.

 

[BenThayer]

most vessels that we have purchased recently actually state that they are designed for 15 psig external pressure.

 

[jproj]

RJB32482:

"Vacuum" is probably easier to understand in terms of external pressure instead a lack of internal pressure. A vessel that is open to the atmosphere (in equilibruim) is under no vacuum (no external pressure). If you close the vessel and evacuate enough fluid to reduce the internal pressure to 10 psia, the vessel would be under a 5 psi vacuum (total external pressure (assumed 15 psia) - 10 psia internal pressure = 5 psi vacuum). Obviously in this situation, the maximum vacuum (full vacuum condition) would occur when there is no internal pressure (0 psia).

There are equations used to calculate the required thickness for a given external pressure. Check the Pressure Vessel Hamdbook by Eugene Megyesy.

Hope this helps.

Jproj

 

[elguero]

To RJB32482 and others: ASME Section 8 has info on design for vacuum operation.

 

[JStephen]

When a round tank is pressurized, the pressure develops hoop tension in the shell, and the tank is very strong in that case. When subjected to external pressure (IE, pressure outside greater than pressure inside), the walls are loaded in compression. When so loaded, they can buckle, instead of failing by stretching, and the allowable vacuum may be a lot lower than the allowable pressure. For an example, take a plastic 20 oz soda pop bottle- you can probably buckle the sides in by sucking on it, but can't pop it by blowing in it.