Can I take credit if there is a manual valve before relief valve in HAZOP?

[prchick1984]

I don't know if I'm over thinking this, so I thought I would reach out.

I'm doing the HAZOP for a new plant we're constructing. The other engineers have installed valves before a pressure transmitter and a conservation vent. They want to take credit in the HAZOP for the high pressure interlock on the transmitters and for the vacuum relief on the conservation vent, but I didn't think we could because there is a manual valve before both of them.

Can we take credit for these even with a manual valve in the line? Would it be okay if we said that the valve is locked open?

 

[jari001]

With respect to using a "locked open" designation to mitigate the situation where a valve is inadvertently closed, it takes a very robust administrative process to track and ensure these valves are in the proper position, the valves haven't been tampered with, and unlocked and relocked without a proper safety analysis/permit. There are valve car seals that are used in this effort, but the admin burden or a culture that won't take this seriously make this option less viable.

I've been in situations where the inclusion of the valve means less credit can be taken and I've been in situations where the process safety guidelines of the company said no credit could be taken. If you describe the situation and the consequences of the high pressure event, you can get better input.

 

[prchick1984]

There are two scenarios I'm looking at:
1.We have a reactor where if you add a raw material too quickly or loose cooling, you can have a reaction that created CO2 that can build up in the reactor and over pressure it.
2. On the same reactor, we have nitrogen blanket on it and the nitrogen has to go through a regulator first. If the regulator fails, the nitrogen pressure is high enough to over pressure the reactor.

In both cases, the safeguards listed are the high pressure interlock on the pressure transmitter as well as the conservation vent. We also have a rupture disc on the reactor and there is no valve before that thankfully. The conservation vent and pressure transmitter are the elements that have the valve and I know I've had instances where someone left a valve before a transmitter and gave bad readings.

We've used control locks on valves before, but they were meant for noncritical things and have had issues where the valves weren't closed as they were supposed to be.

 

[0707]

In some designs we have double relief valves sectioned by a manual valve. One of the relief works as spare and the manual valves have the function to section them, to permit calibration or repair of the reliefs.

luis

 

[georgeverghese]

The language used here is a little confusing - what credit are you assuming with the manual valve before the PRV - conservation vent? Any manual valves before PRV should be locked open and operated only with work permit from the shift operations supervisor, and such valves should be marked as such on plant PIDs.

 

[prchick1984]

In our HAZOP, to take credit for the relief valve and pressure transmitter as a safeguard for the above scenerios.

 

[georgeverghese]

Yes you can, provided you have the required operating safety procedures built into the plant work culture.