Relief load calculations for Thermal Expansion

[ASDFqwer4]

Hi everyone,

Recently I am working on thermal expansion case for relief valve design. System is HX with crude being heated by shell side other HC. Process fluid is crude (on tube side of HX) and PSV set value is 400 psig. I have considered tube rupture - not applicable, blocked out- not applicable, fire relief load-explanation mentioned below.

In fire relief load calculation, I found out that fluid vapor properties after 10% flashing are above critical point. So the liquid is very heavy and not easy to flash, and liquid may be thermally expanding. So I assumed HoV of 40 BTU/LB and completed fire relief load calculations.

That being said, I was also considering thermal expansion as my possible relief scenario. I never came across such situation. In this calculations assuming that tube side of HX is blocked due to human error, then I was trying to find out the relief pressure using heating curve of a PRO-II. In that heating curve, I am looking for calculated pressure (412 PSIA) for maximum possible temperature (design temperature of hot side = 650 F) in order to determine whether thermal expansion is credible scenario or not.

PSV set point 400 PSIG
Relieving pressure according to 10% rule = 440PSIG

Heating Curve results
Maximum possible temperature of cold stream = design temperature of hot side = 650 F.

Calculated possible pressure is 412 PSIA


My question is, should I take the heat exchanged value ( available in HX datasheet) from hot fluid (hot which is on shell) to crude (cold which is on tube)and find the thermal relief load?


What I think about this?
I think that if the cold side fluid is blocked by any means and even if you supply maximum possible heat from hot side, the cold fluid doesn't reach 10% set pressure value. So there will not be any thermal expansion case.

Kindly reply
Thanks

 

[Latexman]

This thread has the formula for pressure rise per temperature rise in blocked-in thermal expansion scenarios:

thread124-171158


Good luck,
Latexman

 

[Chance17]

Your last sentence (not credible)is based on fluid vapor pressure.
Liquid thermal expansion is different physics i.e. liquid specific volume.
If the vessel is in fact liquid full and if the specific volume increases with temperature,
it must be concluded overpressure by thermal expansion is credible.

 

[ASDFqwer4]

Can anybody show any random example for caclulating specific volume increase? I understand that Latexman has provided a thread for the formula, but I am confused about dv/dT term.

 

[PaoloPemi]

you can calculate thermal expansion (at constant p) dV/dt, with a equation of state or equivalent methodology, for mixtures there are different correlations (including p and t corrections) which can be easily differentiated vs. t , the selection of the proper correlation depending from specific field of application (mixture characteristics , operating conditions etc.) see Properties of gases and liquids by Prausnitz et al... for additional information.
Pressure increase in a closed pipe can be estimated for example dividing thermal expansion by isothermal compressibility,
it is not difficult to calculate these values (thermal expansion and isothermal compressibility) with tools as Properties (see

http://www.prode.com)

or Refprop (see nist.gov), I do that in Excel.

 

[Latexman]

Post your specific calculation, even if incomplete, and we can try to help.

Good luck,
Latexman