3 replies to this topic

[ulka_19]

Hello Everyone,

 

I wanted to know how to decide the design pressure for LPG storage Vessel. LPG composition is 20% Propane and 80% butane. Design Temperature is 40 degree C. The vapor pressure of this mixture at 40 C is 76.3 psig.

In API 2510 it is mentioned "The design pressure of LPG vessels shall not be less than the vapor pressure of the stored product at the maximum product design temperature. The additional pressure resulting from the partial pressure of noncondensable gases in the vapor space and the hydrostatic head of the product at maximum fill shall be considered. Ordinarily, the latter considerations and the performance specifications of the relief valve require a differential between design pressure and maximum product vapor pressure that is adequate to allow blow down of the pressure relief valve (see API RP 520)."

 

But however this does not give me a clear picture as I am not able to decide by what amount I  should increase the design pressure from vapor pressure.

 

OISD 150 gives a guideline to have Design Temperature of -27 to +55 C and Design Pressure of 14.5 kg/cm2 g. Is this applicable for all compositions of LPG?

 

Please let me know if additional data is required.

 

Thanks

[ankur2061]

Akanksha,

 

There are no set guidelines for design pressure. Different engineering and / or operating companies have different criteria of fixing the Design Pressure (DP) based on the Maximum Operating Pressure (MOP) where MOP in your case becomes the highest vapor pressure at the design temperature (DT) with an LPG composition having the maximum content of propane or other light hydrocarbons. If the client has his own criteria for DP based on a certain margin over the MOP just follow that and make it black-and-white in your design basis document. If not, provide a margin of say 15 or 20% over the MOP and fix this as your DP. Make sure whatever DP you choose is documented in your design basis and approved by the client as well as any regulatory or statutory authority.

 

One thing I was not able to understand is how did you arrive at a DT of 40°C. Normally outdoor installations like LPG bullets and spheres which are exposed to the sun use a design temperature value of 55-60°C. In the middle-east , for equipment exposed to the direct harsh desert sun and with high incident solar radiation values, often a DT of 82°C is selected. In my opinion, you should not be selecting a DT of 40°C. The minimum DT selected should be 55°C.

 

Hope this helps.

 

Regards,

Ankur.

[fallah]

ulka_19,

 

As API 2510 well mentioned, you should set the design pressure for LPG storage vessel such that the difference between the MOP (vapor pressure at the design temperature) and design pressure (as PSV set pressure) wouldn't be less than the blowdown of the PSV...

[ulka_19]

 

Ankur,

 

Thank you sir for your valuable input. This Project is for a country where maximum temperature does not exceed 30°C that is why design temperature of 40°C is considered.