External pressure (vessel)

[mech0346]

Can anyone please tell me what is basically meant by external pressure when we talk about pressure vessel , is it atmospheric or what .. please elaborate
thanking in advance

 

[chaulklate]

No, it is not atmospheric pressure. When we talk about external pressure, we are usually implying a "negative internal" pressure, and is used to describe the effect of the vessel contents experiencing a very instantaneous change of state (liquid to gas). The rapid forming and evacuation of the gas from the vessel creates a vacuum-like effect making the vessel want to implode or collapse inwards. As it is impossible to create a true vacuum condition, we view the loadings as a positive external load inversely equal to the actual negative internal load. In this case, the external load would not be greater than atmospheric conditions. Neglecting sea level elevation, it is widely accepted as 15 psig or 103 kPag.

Reading back over that, I am not convinced my explanation is going to click. Anyone want to elaborate or correct that??

Granted, it can also be applied if the vessel is buried underground or water; it will be subject to an external pressure equal to the loading caused by the water (hydrostatic head) or soil.

 

[SnTMan]

talha0346, usually meant to note a vacuum condition in a pressure vessel. Full Vac = atmospheric, half-vac = half atmospheric.

However, it can carry other meanings as well. For example, tubes in a shell & tube exchanger are subject to internal (tubeside) pressure and external (shellside) pressure which are not limited to atmospheric..

And as chaulklate noted, hydrostatic pressure on a submerged vessel is external pressure.

Depends on the physical situation.

Regards,

Mike

 

[mech0346]

thanx alot from side ... for your promt response

 

[C2it]

Adding to the examples .... jacketed pipe needs careful design with regard to the external pressure imposed on the core pipe by the jacket pipe pressure.

I saw the results of omitting this in a nitric acid plant. The entire core pipe, several hundred metres of expensive stainless was collapsed into two lobes when the jacket was hydrotested. One simple calc would have avoided this multi-million £ error.

 

[PVnew80]

Search "you tube pressure vessel external pressure" and you'll see an example of some failures due to "external pressure". Should give you an idea of what you're dealing with!