API 521 - Trapped Liquid expansion

[svi]

Can anyone please explain from first principals the API 521 equation used for the thermal expansion and the consequent confined volume pressure increase. I want to know whether the same equation may used for a ball valve cavity pressure estimation.

 

[BigInch]

It seems to be correct, but I think it is easier to calculate the expanded fluid pressure using the bulk modulus of the fluid.

Independent events are seldomly independent.

 

[BigInch]

maybe the bulk modulus is the isothermal compressibility coefficient.

Independent events are seldomly independent.

 

[svi]

Yes the compressibility coeffecient is the inverse of the bulk modulus. It is either dv/v/dp or dp/dv/v. Kpa^-1 or Kpa. For liquids the inverse of the isothermal values are near equal to the bulk modulus.

I can see that the equation takes into account the steel enclosure volume increase, the young's modulus of the steel to account for the volume increase from the liquid pressure increase. The pressure increase therefore will some what less than that estimated by fully confined calc by either the compressibility or the bulk modulus.