Storage Tank API 650 - Design External Pressure

[msshah17]

Hello,

I have a SS-304 storage tank with 1" insulation (ID=14 feet X 20 feet High and self supporting cone roof). I am using Tank software for analysis purpose. In Tank software, under external pressure analysis - specify external pressure (Pe)in lb/ft^2.

I have a client with 40 Yrs old tank. The drawing did not have any design condition so what is the best guess?

Please, Can some one explain regarding design external pressure. What will be the value?

I am using -1.73 inch of water OR -8.999 lb/ft^2. Is it correct??

I appreciate it.

Thanks - Mayur

 

[bimr]

Why are you concerned about the external pressure?

 

[JStephen]

Under the current standard, the external pressure is taken as 1" of water column if not otherwise specified, or zero for tanks with circulation vents.
With the 1" of water column, no calculations are required for the vacuum condition.
In the current standard, the 1" vacuum is accounted for in the roof loading (with 40% factor) and in the shell stability calculations/ wind girder calculations.

For a 40-year old stainless tank, it was built prior to Appendix S, so it may not comply with Appendix S.

 

[LittleInch]

bimr, I don't believe you are correct. For tanks, pipes, rods, etc, strength in tension is far stronger than compression, hence worst case is nearly always compression / collapse or essentially vacuum conditions where inside pressure is > outside.

Think of your humble Coke can. It can handle high internal pressures, but with much much less than 1 bar across it it will crumple inwards very easily

For a tank this only relates to instances when the tank is empty usually, but on any negative pressure compared to atmospheric pressure, the strength of a tank is very low.

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

 

[bimr]

LittleInch, you are correct and I deleted it. The tank walls are much more flexible than an egg. In regards to that matter, I was looking at the Saturn 5 rocket last year and it is probably a better example. The designers had to pressurize the kerosene fuel tanks as fuel was spent, otherwise the thin rocket walls would collapse.

Don't understand why the OP is concerned about the external pressure unless he is planning to steam clean or make a boat.

 

[msshah17]

All,

I appreciate your input and time.