API 650 Tank pressure

You need to know the difference between operating pressure, set pressure, overpressure, design pressure and accumulation.....Just read the code...this would be for an atmospheric pressure tank...

"Design pressure for a vessel is taken by adding a suitable margin to the maximum possible pressure during normal operation.

The maximum allowable working pressure is the highest pressure that the vessel can withstand based on actual nominal vessel thickness excluding the allowances provided for corrosion and loadings other than pressure.

Set pressure of a relieving device on a pressure vessel is the pressure when the relieving device just starts to open."....

https://www.enggcyclopedia.com/2011/06/key-terminology-relief-devices-vessels/

PSIG means Gauge pressure,as opposed to PSIA,which is Absolute pressure,which is referenced to a perfect vacuum.

Atmospheric pressure surrounds the outside of the vessel,as well as inside,so for all intents it can be ignored.

The tank, if designed for liquid, would need to be designed for the load of the liquid, vented or not. The forces at the bottom wall would be the weight of the liquid plus the absolute pressure on the top. The tank pressure above the liquid would be added to the liquid head to arrive at the bottom wall stress, so some 17.2 psia, in your case.

If vented to atmosphere,the head pressure is the only factor to consider,not absolute pressure.Atmospheric pressure is the same in all directions.

I don't think that's right. The forces at the bottom wall would be determined by the height of the liquid (and its density) plus the gauge pressure in the gas space. The outside of the tank also sees atmospheric pressure.

It's not unusual to have thicker plates at the bottom, getting thinner towards the top, especially with bolted sectional tanks.

Then it isn't a <...tank...>; it's a "pressure vessel" instead, and a different design code may be needed.

It is, however, impossible to see what's going on without access to a P&ID.

2.5 psig is the maximum pressure in the gas space allowed by API 650 (assuming you've quoted it correctly, I haven't used it recently and don't have it to hand). You need to add that to the liquid pressure to determine the plate thickness. I'm fairly sure 2.5 psig is the maximum allowed, if the pressure is lower you can use the actual figure. Also I believe API 650 only applies to circular tanks.

Agree with Codemaster .... API-650 addresses flat bottomed tanks and states that if you use the design methods and acceptable materials and construction methods outlined in this code, you can use these rules for a maximum pressure in the gas space of 2.5 psig.....

Contrast this with API-620, which has slightly different design rules, can be used for flat bottomed tanks and permits design to a slightly higher maximum gas space pressure.

The phrase <...we consider...> suggests that there is a Mentor available at that location. Good design practice is to get a second-person-check on designs before obtaining commercial approval. Discuss the project with that individual.

Perhaps the OP will confirm, but I doubt if the "we" in because "if we consider design pressure including liquid head....." refers to a specific person, it's just another way of saying "if design pressure including liquid head is considered....".

He just wanted clarification whether the 2.5 psig is the gas pressure alone or includes liquid head, and that's been done.

Quite.

That's the problem with undefined plural pronouns. What hasn't been done here, though, is a second-person-check as part of a design procedure. After all, no reader here is liable if construction comes out wrong and it has to be redone.

I think he just wanted to clarify a technical point. There's no need for CR4 to get involved in the process aspects which produce those conditions, or any contractual issues he might have.