4 replies to this topic

[ankur2061]

Guruprasad,

28 inches Hg corresponds to 711 mm Hg. So tell us which is the exact value, 28 inches or 50 mm.

Normally the practice I have been following for specifying vacuum in my vessel process data sheet is Full Vacuum (F.V) simply because it gives me a inherently safe design and specifying a lower vacuum limit does not make an appreciable difference in the plate thickness (read cost) of the vessel specially if the vessel also has to withstand a significant positive pressure. This is all the more relevant to vessels normally operating at pressures significantly above atmospheric but are also specified for steam-out conditions during shut-down or maintenance turn-overs.

However, if cost is the overriding factor for you, you can can give a 30% margin on the maximum operating vacuum of the vessel e.g. if 700 mm HgA is your maximum operating vacuum you can specify a design vacuum of 500 mm HgA.

Hope this helps.

Regards,
Ankur.

I would like to know what is the equivalent Design Pressure to be considered while designing a Pressure Vessel for 28Inches or 50mm of Hg Vacuum Pressure?

[ankur2061]

Guruprasad,

There is nothing like equivalent positive pressure to vacuum. Pressure Vessels are designed with a suitable margin over and above the operating preesure envelope. If your operating envelope includes both above atmospheric pressure and sub-atmospheric pressure, then your design pressure would include both the margin above the above atmospheric pressure and sub-atmospheric pressure.

As an example, if my maximum operating pressure above atmospheric is say 3 bar(g) and maximum operating vacuum is 700 mm HgA, then my vessel data sheet would most likely show as follows:

Design Pressure: 4 bar(g) / 500 mm HgA (F.V, my personal preference)

500 mmHgA can also be written as -0.346 bar(g)

This implies a 30% margin on both the pressure and the vacuum side.

However, as mentioned in my earlier post I prefer to give full vacuum (F.V) for vacuum side specially where the vessel could be subject to conditions such as Steam-out and the vacuum operating conditions are some what vague and difficult to pin-point.

Hope this clarifies the matter.

Regards,
Ankur.

Thank You.
What I meant by 50mm of Hg was 50mm below 1 Atm or as you rightlu mentioned 710 mm of Hg.

What my exact question is that I'm used to designing pressure vessels using positive pressure calculations, so what equivalent positive pressure should I use to do the same?