Operating pressure very close to set pressure

[jimbob1974]

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?

 

[Ashereng]

You also need to be aware of blowdown in a pressure safety valve (I am assuming you are in gas service?).

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
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[StoneCold]

Maybe we can help you in a different way. I agree with Ashereng you are too close. But why? What are the operating conditions, temp, pressure? What is in the piping? What is the piping made of? Flanged?

Sorry, just lots of questions no magic fix. But it should get you thinking.

 

[jimbob1974]

Unfortunately, we're retro-fitting the relief valve to an existing system with these design and operating conditions.

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.

 

[Ashereng]

jimbob,

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?

 

[UmeshMathur]

jimbob1974:

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?

 

[djack77494]

I couldn't agree more with Umesh. All too often, we're pushed to the edge and beyond in application of engineering judgement. Convenient "worst case" scenarios are then thought up to justify the desired endpoint. Anything beyond that is deemed unrealistic, impossible, or otherwise dismissed.

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

 

[xcrosby]

Make sure that your clearly specify the conditions to the pressure relief supplier. Your conidtions are extreme and exceed the recommendations of just about every type of presssure relief valve that I am aware of. I would suggest that you specify that the set pressure be held tighter than normal tolerance. Typically the tolerance is +/-3%. I would suggest that you specify a tolerance of +3/-0%.

 

[UmeshMathur]

Doug's point is well taken: in the interest of continued emplyment, refusing to do your job is not recommended.

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.

 

[jcaiken]

I would add that a HAZOP Review of the plant would highlight such problem areas and enable them to be documented in an open way acceptable to both client and contractor. The close-out of actions raised by the HAZOP team would require both the client and contractor to sign-off the solutions as acceptable.

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.