All,

I am working on a PSV study for a natural gas processing plant. I started working on the fire sizing or the PSVs in the hot oil system. The facility uses Therminol-55 for the heating fluid.

These PSVs are set at 150 psig with relief pressure of 181.5 psig (1.21*150 psig). The Therminol-55 datasheet lists the pseudocritical pressure as 191 psia (site atmospheric pressure is 13.3 psia) so the relief pressure exceeds the pseudocritical pressure and fluid would be in dense phase. Based on the Therminol-55 datasheet, I used the alternative methods for supercritical relief in API 521 6th edition 4.4.13.2.4.4 to calculate relief rate and direct integration from Annex B in API 520 pt I to calculate orifice size.

The problem I encountered is that I am getting very different results depending on which equation I use from 4.4.13.2.4.4. I am using a time step of 3 seconds and total time is adjusted until I obtain peak flow rates. Equation 14 results in larger relief rates at later time steps, so at a greater temperature. Equation 15 results in smaller relief rates at earlier time steps and lower temperatures. When I use 520 pt I Annex B to calculate the mass flux through the PSV, I am obtaining different values due to the difference in starting temperature from the two equations in 4.4.13.2.4.4. When using equation 14 with higher relief temperature, the mass flux is much less than if using equation 15. The larger relief rate and lower mass flux when using equation 14 results in a much larger relief valve.

Has anyone else encountered this issue? I am not sure which equation from 521 4.4.13.2.4.4 to use as I obtain different results from both. The biggest difference I see between the two is that in equation 14, the volume to be relieved is calculated based on the heat input and resulting enthalpy and density changes instead of equation 15 where the mass to be relieved is calculated just based on the initial volume and the density changes.

**Edited by therossboss, 15 July 2022 - 11:11 AM.**