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Design Pressure Selection

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ChemEngSquirrel

Chemical
Jun 10, 2010
72
What is the minimum recommended design pressure for a hydrocarbon vessel which is atmospherically vented?

The vessel operates at < 0.1 barg so I am considering setting design pressure at 1 barg. The vent is to a safe location and has a flame arrestor on it.

Thanks.
 
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No chance of a vacuum being created I presume!
 
1 barg will make it a pressure vessel, or nearly so. "Nearly so" is no problem- but a design pressure above a certain point implies a RELIEF pressure above that point, which puts it into the scope of a pressure vessel code. Don't tread across that line arbitrarily as it has a steep cost associated just with inspection and registration.

If your tank is relieved to the atmosphere by way of a flare or other similar device, a little additional design pressure will help to reduce the size of relief device you require, or to make the relief device more reliable.

There are plenty of atmospheric tanks which contain hydrocarbons and which are designed for hydrostatic head plus a very small positive pressure. There are applicable API standards, which although intended for much larger tanks than what I assume you need, may be used for guidance in selecting a design pressure.
 
At 1barg, your flat floor has to be anchored and reinforced. Keep the design pressure sufficiently low, such that the total pressure uplift of the roof is offset by the weight of the roof, shell, and appurtenances [like ladders & catwalks]. No anchoring required, but enough design and operating pressure to allow a 'Conservation Vent' to work well.
 
If you have a "small" vessel with formed heads, then small pressures have little effect on the design and construction and it doesn't much matter what you use, as long as you don't get too carried away. It may be advantageous to keep in the range of pressures where it is exempt from pressure vessel laws.

If you have a "large" vessel, or more critically, a flat-bottom tank, then fairly small pressures or pressure increases can have major effects on design and pricing, and it would be helpful to have input from the vessel designer as well as looking at vent sizing.
 
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