PRV sizing for a gas blow-by scenario

[Richard62]

I am trying to validate the sizing case for an existing PRV located at the top of the column which is located downstream the depicted system (please see attached sketch).
The sizing case of the PRV is the gas blow-by through the depicted drum (an HP separator in a naphtha hydrotreating unit). In the event of level loss, the incoming gas passes through the bottom outlet and enters the column (stripper), which makes the PRV at the top of the column to open.
There is also an ESDV at the bottom exit of HP separator which is activated (closed) when the level of the drum is very low (interlock system).
However, in my oppinion, the described scenario (gas blow-by) occurs due to a double failure: fail at level control loop and also a fail of the interlock system.
In this case, shall the gas blow-by scenario be taken into account on the sizing of the PRV at the stripper column ? Is there a SIL classification of the IS which allows us not to consider this scenario ?

We appreciate any advice.

 

[Chance17]

This is how I would approach the problem. I'll admit not all professionals view the solution as I.
It is credible for the IS to fail. This is the single contingency.
If so, you should not take credit for favorable action by the control valve.
Try to determine the precontingency control valve opening (normal-average flow).
Then calculate the gas blowthru at the reduced CV opening.
The overall impact is the gas blowthru will be less than 100% CV open. So the IS does reduce the potential safety impact.

 

[ajs1972]

You need to consider that scenario in your PHA and determinate the risk level in case of over pressure. The SIL will be determinate in function of the risk level.

You can do a LOPA analysis and to determinate if is necessary the PRV or not according to PFD (probability of failure on demand of the DCS controller and safety interlock)and the frequency of the overpressure scenario.

 

[dcasto]

you need the MAOP of the drum and MAOP of the Stripper The difference is the differential pressure across the valve. Then use the Valves Cv to get the gas flow through the valve. That mass/ volume is another design point for the PSV on the stripper.