Design Pressure vs Operating Pressure (Too much difference) 1

[Dome Design]

Hello all, I hope everybody is safe!

I have a question that maybe can be very simple but I want to hear your comments also, thank you. The question is that, I see some datasheets says design pressure is 100 mbarg and operating pressure is 20 mbarg for storage tanks. How can be a difference between them that much?
They want to create N2 blanketing system inside of tank, it is okey but as a operating pressure it says "ATM". I think there is a misswriting or what do you think? Thank you.

 

[LittleInch]

Depends on your design code.

If you're talking API 650, then for atmospheric tanks the definition of "ATM" is 1kpA (10 mbar) or 4 inches water.

100 mbar is actually quite high as design and sometimes put there by mistake because someone wants to insert a figure instead of "ATM"

otherwise not easy to decide as we have few other details.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

It depends.
In a lot of cases, somebody has picked an arbitrary number without any consideration of the effects.
In other cases, they are properly considering API 650 or 620 provisions for vent set points, overpressure to achieve flow, etc.
In some cases, they may be anticipating future changes, or may be designing all tanks on a site for the same pressure to avoid confusion later.

 

[don1980]

Note that tanks operate within a pressure range rather than a fixed "operating pressure". For a padded tank, that's the pressure range between the pad gas set pressure and the set pressure of the tank's breather device. For a non-padded tank it's the range between the vacuum vent set pressure and the breather vent set pressure. This is the "operating pressure range" and it should be optimized by weighing the cost for a higher design pressure against the benefit of minimizing tank vapor losses. Each application is different, and needs to be evaluated on a case-by-case basis.

If the tank contains a liquid that is volatile, and/or expensive, and/or harmful (personnel or environment), then one has the incentive to maximize the operating pressure range, which means an incentive to maximize the tank's design pressure.

For the tank we're discussing, we don't know the contents of this tank or the size of the tank. If this tank is large in diameter, or contains a inexpensive & nonvolatile liquid like mineral oil, then a design pressure of 10 mbarg is excessive. In other cases it may be very appropriate.