Operating pressure very close to set pressure
Operating pressure very close to set pressure
(OP)
We've got an existing system that is operating at up to 97.5% of relief valve set pressure. We're trying to specify the best new valves to protect this system.
The relief valves will be installed on pipework only (i.e. no large "receiving" volume to damp out pressure spikes) and the system is pressure controlled which could result in small required relief loads due to mis-control.
Our initial thoughts are to install a modulating action pilot relief valve for the following reasons:
- Seat tightness should minimise atmospheric emissions and damage to valve seats due to simmering (which we'd expect from a spring/bellows operated valve); and
- Modulating action should allow small relief loads to be catered for without resulting in the valve repeatedly cycling (as would be expected with a pop action pilot since we only have a small upstream process volume).
Does anyone have any clever ideas/technologies that would suitable for such a situation?
The relief valves will be installed on pipework only (i.e. no large "receiving" volume to damp out pressure spikes) and the system is pressure controlled which could result in small required relief loads due to mis-control.
Our initial thoughts are to install a modulating action pilot relief valve for the following reasons:
- Seat tightness should minimise atmospheric emissions and damage to valve seats due to simmering (which we'd expect from a spring/bellows operated valve); and
- Modulating action should allow small relief loads to be catered for without resulting in the valve repeatedly cycling (as would be expected with a pop action pilot since we only have a small upstream process volume).
Does anyone have any clever ideas/technologies that would suitable for such a situation?





RE: Operating pressure very close to set pressure
Most valve have a blowdown of about 3.5% of set pressure. You are within that. So, you may have a problem. A pilot safety valve will have less blowdown than a conventional, but I am not sure of the numbers. You will need to take a look at the valve datasheet.
By the way, operating at 2.5% of set pressure is usually not a good idea. You are too close. If you can't get around this situation, you will continue to have simmering.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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RE: Operating pressure very close to set pressure
Sorry, just lots of questions no magic fix. But it should get you thinking.
RE: Operating pressure very close to set pressure
The piping system is flanged and constructed from low temperature carbon steel. The major restriction on the system design pressure is a large number of existing process vessels (mol sieves, heat exchangers, separators, etc.).
Our client has squeezed the margin between operating and design pressure over a number of years and want to continue operating in the same manner. The problem I've got is to install a relief valve that will adequately protect their system without leaking or getting damaged.
The client doesn't see the way they're currently operating as an issue and it would take a lot of capital investment to re-engineer the system to give us a bigger marging between operating and design pressure.
RE: Operating pressure very close to set pressure
Maybe some process info would help with understanding our problem, along with process fluid.
Back to blowdown.
If you operate above the blowdown, the PSV will not close after it lifts. There is no way around this.
Pilot PSV/PRV also have a blowdown, it is just smaller. Again, if you are operating above blowdown, the valve will not close again. If your pilot PSV has a blowdown less than your 2.5%, then it will work. If not, then it won't. There is not magic about this - it is a limitation.
By the way, you don't need to re-engineer anything. The simple solution is capital investment only. Replace equipment with a higher rated pressure equipment. If you are at ANSI 300#, go to 600#. No re-engineering required, just more money. But I think this is not what your client wants to do.
You may have wringed out as much from the existing equipment as you can.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Operating pressure very close to set pressure
From what you say, it seems clear to me that your client is skirting with disaster. All major chemical engineering systems can exhibit large deviations dynamically from the steady-state or design condition. Pressure surges are quite common in systems with large gas inventories (compressor startups, trips, valve dynamics or failure, and so forth). Leaving virtually no margin and failing to upgrade the downstream hardware (as recommended by Ashereng) creates potential safety hazards that would normally be considered quite unacceptable.
Such risky policies have resulted in numerous well-publicized accidents, as everyone knows. In my view, professional engineering ethics require that such a serious issue be presented formally and in plain language to the client. How would you feel if there were some adverse event later and your team was then hauled up and cross-questioned vigorously about why you didn't formally advise an upgrade to the downstream hardware?
RE: Operating pressure very close to set pressure
Well, you can't fight your management or clients on issues like these. Just offer the best design you can, and if it's not accepted and you're given an alternative (looser) scenario to design to, then document the source and, if you disagree, document your objections. Then, proceed with the requested design (which will probably be a lot better than the design that would result if you removed yourself from the whole process).
I know the above could be considered "selling out". I am definitely not suggesting any violations of codes or even recommended practices. I'm strictly talking about "grey areas", subject to interpretation and judgement. Though I'd be interested in reading any comments to the contrary, this is my opinion and I sticking to it.
Doug
RE: Operating pressure very close to set pressure
RE: Operating pressure very close to set pressure
However, document your position unambiguously and in writing and make sure it is NOT buried deep within the archives and out of sight of your management (i.e., those whose necks would be on the line if there was to be a screw-up later). There are ways to write such position papers that are at once polite, compelling, and demand a proper airing of the issues and their informed resolution.
RE: Operating pressure very close to set pressure
Normally I would not advocate that a HAZOP be used as part of the design/engineering process in this way (my preference is that a HAZOP should only be used as a check-stage once the engineering process is otherwise complete) but its use in your case may be appropriate.
Note that HAZOPs are starting to have a legal status (particularly following the Longford fire in Australia) and the attendees are expected to perform to their best endeavour.