Introduction to Pressure Vessel Safety Awareness Tutorial

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Pressures in Pressure VesselsPressure Gauge

We express pressure in different ways [English units].
  • pounds per square inch -or- psi
  • pounds per square inch absolute -or- psia
  • inches of water or mercury -or- inches-H2O;inches-Hg

What is pressure?

Pressure is force divided by area -or P = force divided by area

What is the difference between psi and psia?

Psia describes an absolute pressure per square inch that starts from a perfect vacuum. 

Psia is influenced by weather and elevation. 

A good frame of reference is at sea level there is 14.7-psia psi describes a relative pressure that starts from atmospheric pressure.  Therefore if you are on a mountain and the pressure vessel gage reads 1000-psi and relocate the pressure vessel to sea level the gage will read 1000-psi.  We can further say that at sea level there is 0-psi.  Example, when purchasing a new gage the pressure gage reads zero.We can now say psia is an absolute pressure reading, and psi is a relative pressure reading.  In most cases, psi is used on pressure vessels.

What is the difference between inches H2O and inches of Hg?

There is little difference except they have to be converted.  Inches of  H2O is more sensitive than inches of Hg because mercury is heavier than water. At sea level there is 407.1 inch H2O  and 29.9 inches Hg at sea level.    

Should a pressure vessel have a pressure gage?

    A.) Yes     B.) No


     A) Yes. 

In fact, the gage needs a siphon to keep hot vapors from damaging the bellows in the gage; otherwise, the gage becomes damaged and unreliable. If there are pulsation's are experienced in the vessel, then a snubber is needed, for no gage can handle that kind of abuse. The gage needs to have the proper graduated range [0-100-psi gage]. For instance a 100-psi vessel should have a gage graduated no higher than a 0-300-psi gage or no lower than a 0-150-psi gage.


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