API 650 TANK: DIFFERENCE BETWEEN VAPOR PRESSURE AND INTERNAL DESIGN PRESSURE

[inammanj123]

HI,

What is difference between vapor pressure and design pressure. As per my knowledge design pressure will still be zero, even if there is vapor pressure given on data sheet.
I am working on a utility water tank. vapor pressure of 0.09 bar(abs) is mentioned, but there is no information regarding design pressure. API 650 has addressed product vapour pressure and tank design pressure differently.
Kindly eleborate

thanks,
Inam

 

[DSB123]

I think you need to do some basic reading regarding "Static Head" and Tank design pressures.

 

[IFRs]

The design pressure for an API 650 tank is the pressure exceeding atmospheric that you are designing for.

For product stored in API 650 tanks, vapor pressure is typically a partial vapor pressure. The total pressure is still atmospheric. If the vapor pressure is over atmospheric, the liquid is boiling and would not be in an API 650 tank.

Vapor pressure - Wikipedia

https://en.wikipedia.org/wiki/Vapor_pressure

What is vapor pressure? definition and meaning - BusinessDictionary ...

http://www.businessdictionary.com/definition/vapor-pressure.html

ChemTeam: Vapor Pressure - Chemteam.info

http://www.chemteam.info/GasLaw/VaporPressure.html

 

[LittleInch]

You need to use the correct units.

The design pressure of API 650 tanks is often stated as "atmospheric". At sea level this equates to 1 bara.

Vapour pressure is an indication of volatility of a liquid and varies with temperature. If the the vapour pressure exceeds the gas pressure above the liquid then vapour bubbles appear in the liquid and the liquid "boils".

Hence for your situation, the design pressure is 1 bara, but vapour pressure is 0.09 bara. Therefore all is good.

Even if the liquid boiled, so long as the venting system was big enough, the pressure in the tank would still be very low. max design pressure in an API 650 tank is 2.5 psig, but this is quite rare.

Remember - More details = better answers
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[inammanj123]

thx...so it means that if vapor pressure mentioned is more than 1 bar(a), but client does not specify design pressure, than atlesat we should inform him??
yes i am aware of difference between static head and design pressure

 

[LittleInch]

If VP is more than 1 bara in an atmospheric tank then pretty soon you won't have any liquid left!

You would though have a LOT of vapour to deal with.

As said VP is variable by temperature so it's always important to say VP @ X C.



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

 

[JStephen]

For petroleum tanks in the US, the vapor pressure is one determining factor in whether a floating roof is required per air pollution regulations, etc, so it is commonly furnished in inquiry documents. It really doesn't enter into the tank design itself, though, and a water tank would normally be atmospheric as indicated above.