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Design Pressure vs Operating Pressure

Design Pressure vs Operating Pressure

(OP)
Dear All

I have a basic question regarding design pressure of pressure vessels designed as per ASME code.
What is the basis for defining design pressure for an equipment? Is there any sensible relation between operating pressure and design pressure?
I have some vessels to deal with, and their operating and design pressures are as follows:
1-operating pressure: 3.5 barg --- design pressure: 7.5 barg
2-operating pressure: 0 barg --- design pressure: 5.5 barg
3-operating pressure: 3.5 barg --- design pressure: 18.1 barg
4-operating pressure: 0 barg --- design pressure: 12 barg
5-operating pressure: 9.5 barg --- design pressure: 18.1 barg
There are other vessels with different operating and design pressures but I can not find any relation between them. I mean if operating pressure is the pressure that the equipment is going to have in service then why should we design it for a higher pressure. After all we have considered safety margins for allowable stresses and there seems to be no need for increase in design pressure for safety reasons.
One more question is that for fatigue calcs shall I consider design pressure or operating pressure? because that could make significant change in stresses.

Warm Regards

RE: Design Pressure vs Operating Pressure

The (normal) operating pressure and the design pressure are defined in 3-2 by the code (Div.1). The operating pressure has no practical influence on the design (minimum thickness). The design pressure (that doesn't include the static head) is normally determined by the purchaser based on some extra criterion, such as to impose a minimum wall thickness; it might include a safety margin required by the owner, but only against eventual pressure surges, not to complement code's safety margins.
And fatigue calculations are based onto operating pressure cycles.

prex
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RE: Design Pressure vs Operating Pressure

(OP)
Thank you prex
What do you mean by "eventual pressure surge"? Is it occasional pressure surge?
Warm Regards

RE: Design Pressure vs Operating Pressure

Nice philosophical question to start the week.

"What is the basis for defining design pressure for an equipment? "
"Is there any sensible relation between operating pressure and design pressure?"

Design pressure for a pressure vessel comes from a variety of places and overall philosophy

You start with "normal" operating pressure.
Then add any fluctuations
then add a margin for instrument alarm and trip levels for high pressure and then high high pressure
Then add margin for your pressure relief valve ( usually 10%)

Then you need to look at what is the MAXIMUM pressure the vessel can see due to blocked outlet. Sometimes designers go for the strength method whereby you set the design pressure of your vessel higher than the largest pressure that can come from upstream. So e.g. if your pressure vessel is fed by a pump which has say a normal operating pressure of 5 bar, but on no flow the inlet pressure to the pump increases and then the no flow differential pressure increases and so the max pressure from the pump is 10 bar.

So if you had set the design pressure of the vessel at say 7 bar, then you now need full flow pressure relief to ensure the pressure in the vessel doesn't exceed the design pressure or MAWP. This can be very onerous, so the alternative is to set the deign pressure to say 12 bar. Then the pressure relief can be for emergency thermal / fire events only as the design pressure of the vessel cannot be exceeded by any of the inputs.

Pressure vessel design is also complicated a bit as you set the design pressure, then design it, then back calculate the MAWP based on the components you have used and often this is a lot higher than the design pressure.

You also sometimes find for pressure vessels that companies have philosophies whereby some of the margins I referred to above have minimum amounts, so the high pressure alarm for example could be 20% of operation or 2 bar, whichever is higher. Therefore for relatively low pressure pressure vessels you can find these margins get big in relative terms.

So yes, there is a sensible relationship, but this might not be obvious based off "normal" operating pressure. The design case might be something which only happens occasionally or just that the margins of design are set in absolute terms.

BTW, a pressure vessel operating at 0barg is surely not actualy operating??

Hope this helps.

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

RE: Design Pressure vs Operating Pressure

(OP)
@LittleInch
Thank you very much for your detailed response.
The equipment that I mentioned with zero barg operating pressure were actually some storage drums with design pressure higher than 1 bar so I had no choice but to use pressure vessel code to design them.

As you have mentioned, the difference between operating and design pressures is mainly based on the occasional events that might or might not happen during the lifetime of the equipment. Therefore, the equipment has to be designed based on these possible loading in the future. So I think for designing the pressure vessel against fatigue failure these loads and other occasional loads like earthquake cannot be included since they are not some sort of cyclic events happening to the vessel. And therefore, as prex has mentioned for the fatigue calculations it is the operating pressure cycles that matters not the design.

Warm Regards

RE: Design Pressure vs Operating Pressure

LittleInch,

When you are calculating MAWP, do you need take into account nozzle loads applied to your pressure vessel's nozzles? I think, you need to apply all other loading conditions, when you are calculating MAWP. Any thoughts?

Thanks,
Curtis

RE: Design Pressure vs Operating Pressure

curtis2004, you do not take nozzle loads into account for calculating MAWP. Instead, you ask nozzle designed to be good for MAWP+ external loads.

There is a certain margin to set the design pressure based on operating condition. It is purely preference by each engineering company. However, upset condition shall also be taken into account and a process engineer who supervises P&ID and HAZOP review will be able to set the final deign pressure for each equipment such that the design pressure may be higher than the margin criteria. A pure mechanical/vessel engineer who has no process background shall avoid setting the design pressure.

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