PRV for high pressure H2 recycle loop

[tgmcg]

I am looking for a PRV suitable for protection of a high-pressure hydrotreater recycle loop operating between approx 3200 psig inlet and 2600 psig outlet. Process gas is refinery hydrogen with MW between 3-6. The wrinkle with this application is that we would like to relieve to a lower pressure area of the system operating at about 2600 psig. The PRV should be of the modulating type. This lower pressure system is protected by it's own PRV to the plant relief system.

There is currently no PRV on the 3200 psig end of the system. Protection is provided by assuming that the hydraulic pressure drop through the system, when added to the PRV set @ 2650 psig, will NEVER exceed equipment/piping MAWP. There are no block valves in the main process stream, but there is a huge packed bed reactor vessel and several heat exchangers. I have little confidence in anyone's ability to predict pressure drop for 2-phase flow though a packed bed with absolute precision.

We'd prefer to find a more predictable and direct method of overpressure protection that will not result in unit shutdown.

We would install this "bypass PRV" in parallel with the existing compressor anti-surge control valve.

Some years back, a PIC was installed on the compressor discharge to override the anti-surge valve, however, this controller was "de-nutted" without anyone being aware that it was there for overpressure protection. An advantage of using a PRV is that it would always be treated as an overpressure protection device.

 

[Montemayor]

tgmcg:

What you are describing is a process control need - not a pressure relief device per se. This application is a common one in the process industry - it's used quite a lot in applications such as hydraulic systems.

I would advise you to consider that what you call a "PRV" (Pressure Relief Valve) is exactly that - in a Process Sense. What you seem to want is a back-pressure control device - which is a process control valve. You haven't once mentioned over-pressure as an emergency hazard. If your need to relieve the over-pressure is due to an emergency or a process upset, then you have an API 520 or 521 situation. What you describe seems to be nothing more than process pressure fluctuations that require pressure control - such as a back-pressure control valve that dumps excess pressure into a lower pressure sink. I would apply a standard control valve loop based on what you describe.

Pressure relief devices ("PRVs") should never be used as control devices. They were never meant nor designed for that application. They simply relieve (dump) the overpressure to a safe reservoir that is usually the atmosphere - which can be relied upon to always be there.

 

[tgmcg]

Montmayor,

Thank you for your thoughts.

What we require is overpressure protection....not control. Past attempts to install a common discharge pressure controller acting on the anti-surge recycle valve and turbine speed, as I believe you're suggesting, ended up in them being disabled....along with the anti-surge controller...but that's another story. PRV's are more strictly managed, and should not be subject to being disabled. The only question is finding a suitable device.

So, what I'm looking for is a self-contained PRV...pilot or spring operated... capable of protecting reactor loop equipment by means of relieving...in a modulating manner...back to compressor suction upstream of the cooler/KO vessel. We require modulation action so that the entire unit does not come off line in the event of an abnormal pressure excursion. This PRV would not be used for routine pressure control.

 

[TangoCleveland]

tgmcg,

It doesn't really sound like a complicated situation. I'd suggest contacting your Flowsafe, Anderson-Greenwood, or other major pilot RV rep. You basically want a PRV that will have a constant backpressure on the discharge side. You probably have a huge flowrate so you may need multiple valves. You'll need to tell them gas properties, flowrate, temperatures and pressures.

Larry

 

[tgmcg]

TangoCleveland,

Thanks for the suggestion. I'd already contacted A-G to begin exploratory discussions and am waiting for their senior applications specialist to phone back.

I am posting here to see if anyone has any experience in the same or similar application. Generally speaking, the maximum recycle flow isn't huge, maybe 50-70 mmscfd. The issue, I suspect is finding a PRV capable of handling a 3,000 psig of backpressure.

It's not rocket science, but might necessitate a completely custom valve.

Regards,

Tom

 

[Ashereng]

capable of protecting reactor loop equipment by means of relieving...in a modulating manner...back to compressor suction upstream of the cooler/KO vessel

From your description, my first impression is also one of back pressure control.

In most gas pressure safety relief scenarios, a gas PSV "pops" whereas a liquid PSV "modulates" (these are quasi subjective descriptions).

To me, I think the process requirment is to maintain a certain maximum backpressure (with a back pressure controller) and any excess pressure is to be relived to the compressor suction, but at a certain pressure (with a pressure reducer). So, the answer is a back pressure reducer and then a pressure regulator in series, with a PSV in between?

Then again, maybe I missed the point entirely.

Comments?



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[tgmcg]

Ashereng,

Although a simple discharge pressure control loop would obviously work, and has been tried on other reactors, our concern is that the control loop might not be treated over the long haul as a pressure relief device. Whereas a relief valve is treated as a safety device and would be far less susceptible to anyone deactivating it, say 5+ years from now when nobody remembers why the control loop was installed in the first place. The referenced pressure control loop was, in fact, deactivated at some point in the distant past.

The moral of the story is...for protection against overpressure, use a pressure relief valve. And that's what I'm looking for.