Pressure

Pressure Vessel Safety

Awareness Tutorial

We express pressure in different ways [English units].

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pounds per square inch -or- psi
```

pounds per square inch absolute -or- psia

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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.

Question.

Should a pressure vessel have a pressure gage?

A.) Yes B.) No

Answer

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.