Equipment Design and Gauge Pressure vs. Absolute Pressure

Vessels are designed for subjected pressure and

subjected pressure for a vessel= (Absolute pressure - ATM Pressure)= Gauge pressure

If you want to design a vessel for space or for vacuum then you need to consider absolute pressure.

In design of pressure vessel, the pressure taken into consideration is the difference in pressure between the pressure inside the vessel/chamber and the pressure outside the vessel/chamber. So the difference in between any two pressures shall be subtracting which will demolish the components of atmospheric pressures.

You can fellow the following example: Suppose you have a pressure vessel which have an inside pressure, Pi and subjected to an outside pressure, Po:

- Pressure inside the vessel, Pi = 150 psia

- Pressure outside the vessel, Po = 30 psia

- Design pressure, P = Pi - Po = 150-30 = 120 psig (not psia) ..................... (1)

• The same result shall be attained if you considered the gauge pressures:

- Pressure inside the vessel, Pi = 150 - 14.7 = 135.3 psig

- Pressure outside the vessel, Po = 30 -14.7 = 15.3 psig

- Design pressure, P = Pi - Po = 135.3-15.3 = 120 psig ............................. (2)

• Also, in case of a vessel subjected to internal pressure and located at atmospheric, the design pressure, P = Internal atmospheric pressure - Atmospheric pressure = Gauge pressure.

Please read the following definition of design pressure quoted from ASME Section VIII, Division 1:

UG-21 DESIGN PRESSURE

Each element of a pressure vessel shall be designed for at least the most severe condition of coincident pressure and temperature expected in normal operation. For this condition and for test conditions, the maximum difference in pressure between the inside and outside of a vessel, or between any two chambers of a combination unit, shall be considered [see UG-98, UG-99(e), and 3-2].

In What Way Shal We Design For Atmospheric water reservoir since It Is At Atmospheric pressure, Design Pressure Will Be Zero Or Very Small Value?

There will be a relatively small pressure proportional to the depth of the water at whatever level of interest; 1 psi ≈ 2.31 feet of water depth.

Depending on circumstances, you may need also to consider seismic or wind loads. In other words, pressure isn't the only concern.

If you state what the shape of the tank/reservoir is intended to be, then better advice will be possible.

If you start designing anything based on absolute pressure,

1. You will have to consider inside pressure as absolute design pressure and external pressure as atmospheric pressure ! Why to generate more complexity?

2. You will have to design even the empty vessel without any inside pressure.

So the gage reads zero when the tank is equalized...