Hello everyone,
we have the following situation as shown in the attached sketch:
The distillation column C-1 can be overfilled with liquid when valve V-3 is closed. The liquid (density = 600 kg/m³) then flows over into B-1, and at some point the entire system is liquid-full. Feed pump P-1 has a shut-off pressure of around 18 barg, so the pressure rises in the system. Relief valve RV-2 opens, but is too small to handle this relief load, so the pressure continues to rise until RV-1 opens (the allowable accumulation of B-1 is exceeded at this point due to the additional hydrostatic head of 1,2 bar). RV-1 is much bigger than RV-2 and can handle the relief load.
In order to prevent the pressure in B-1 from exceeding the allowable accumulation, it was proposed to change the set pressure of RV-1 to 13,8 barg (15 barg minus 20m hydrostatic head), so RV-1 and RV-2 would open at more or less the same time.
If however a vapor relief case occurs (e.g. excessive heat input by reboiler E-1), RV-1 will always open first (hydrostatic head of gas is negligible), which somewhat defeats the purpose of having multiple relief valves, as usually the smallest relief valve should be set to the lowest pressure (risk of chattering?)
I have the following questions:
Is this a widely used relief valves design, or has anyone designed/operated something like that?
Can the set pressure of RV-1 be changed to 13,8 barg as mentioned above, or should there be a larger difference between set pressures ? (we can go as low as 13 barg without causing problems during normal operation)
Are there any standards/guidelines that describe such a system and how the set pressures should be staggered?
I only found section 5.4.2.2 (Multiple Device Installation) of API 520 Part 1, but this seems to be applicable to multiple devices that are installed on the same vessel, not different ones. Also, the applicable code here in Germany makes no distinction between single or multiple device installation and only allows 10% accumulation, not 16%.
Any help would be appreciated
Attached Files
Edited by PhilippM, 24 January 2020 - 07:42 PM.