Introduction to Pressure Vessel Safety Awareness Tutorial
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Pressures in Pressure Vessels
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 inchesH_{2}O;
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 14.7psia psi describes a relative pressure
that starts from atmospheric pressure. Therefore if you are on a mountain
and the pressure vessel gage reads 1000psi and relocate the pressure vessel to
sea level the gage will read 1000psi. We can further say that at sea
level there is 0psi. 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 H_{2}O and inches of Hg?
There is little difference except
they have to be converted. Inches of H_{2}O
is more sensitive than inches of Hg because mercury is heavier than water. At
sea level there is 407.1 inch H_{2}O 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 [0100psi gage]. For instance a
100psi vessel should have a gage graduated no higher than a 0300psi gage or
no lower than a 0150psi gage.
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