2 replies to this topic

[Jtor1988]

I'm a process engineer attempting to calculate the pressure increase of a few liquid chemicals due to thermal expansion within a pipe. The purpose is to determine if the selected pipe will withstand the expansion and whether or not a PSV is necessary. I'm using methanol and ethanol as similar substitutes for the chemicals of concern. I'm worried that my calculations are incorrect because with a temperature increase from 75F to 100F, I'm resulting in an increase of over 1000psi. The equation I'm using is from API 521. 

 

 

I've had a very difficult time determining the isothermal compressibility coefficient, so I feel as though that is where the equation is going wrong. The values I have are both roughly 8.0E-06 1/psi for this coefficient. Does this seem correct? Is there another section of the equation that may be causing such a large increase? Any advice on this calculation is appreciated. 

 

Thank You.

[ankur2061]

Jtor1988,

 

Wikipedia gives a isothermal compressibility for ethanol as 110E-11 1/Pa. If you convert this to USC units the value comes out to be 7.58E-12 1/psi. The link is provided below:

 

https://en.wikipedia...Compressibility

 

In the above equation per my understanding, α_V is the volumetric expansion coefficient defined with units of either 1/K or 1/R. For ethanol the value given in wikipedia is 750E-06 1/K which when converted to USC units gives a value of 417E-06 1/R. Refer the link below:

 

https://en.wikipedia...ermal_expansion

 

So either there is a confusion in understanding the terms in the equation or you are getting the wrong results because the units are not consistent while being used in the equation. Do a recheck of your calculations for unit consistency. Most of the times this turns out to be the problem.

 

Regards,

Ankur.

Edited by ankur2061, 16 January 2016 - 01:54 AM.

[MrShorty]

I realize I am a few days late to the discussion. Why do you think a pressure increase over 1000 psi is incorrect? I note that NIST's EOS for methanol (http://webbook.nist....hemistry/fluid/ ) predicts well over a 1000 psi increase (about 2000 psi increase) along the 0.8 g/mL isochore from 75 to 100 F. It is not at all the same equation as the API equation you are using, but it seems to show a similar pressure increase.