Set Pressure of Safety Relief Valves

Why is there too much pressure in the fuel gas line?

It is operated at the optimum pressure. The line is connected between Central LPG plant and Compressor Stations and the RV is on the header supplying fuel gas to one of the Compresssor stations.

What has changed since the plant was originally commissioned with the current relief valve pressure setting?

What does the Process Description document have to say on the pressure setting and why is there a desire to alter it? What has caused the desire?

We have several other relief valves on the fuel gas system supply lines to compressor stations which are operating at the same pressure but the RVs are set at a higher pressure, 400 psig, which is the design pressure for the system. For some unknown reason this valve is set at 350 and there are no ducuments available unfortunately. Data sheets for the other valves are available, but we are looking for some acceptable justification to re-set the pressure.

As I stated in my other post, the gas code is very semantic (and it needs to be).

Although it may not seem logical the reliefs at the station are NOT there to protect the pipeline, they're there to protect the station. So as illogical as it may seem, the compressor relief set points can be above that of the pipeline (after all you already have the required reliefs in the pipeline that serve to protect it!).

You could walk through all the definitions, classifications, and calcs in the code in an attempt to change the max operating pressure (and it CAN be done), but I think you will end up finding everything is as it should be. Again, the set pressure of your reliefs weren't pulled out of thin air or just arbitrarily picked. They were set carefully to get the most capacity out of the pipe and still provide the required levels of safety to meet code.

The other relief valve I mentioned about, which are set at a higher pressure are also on the same piping system, but on a separate branch. This is what is confusing to us.

I agree. If it were a new plant, the relief valve set pressure can be a minimum of 10% above max operating pressure i.e. = 330 *1.1 = 363 psig, and the maximum of piping design pressure i.e. 400 psig, or anywhere in-between.

If there is no other limitation then the set pressure can be set from 363 to 400 psig.

To change the setting on an existing relief valve require consideration of orifice area. Generally, for the same design flowrate higher set pressure need smaller orifice and smaller valve compared to a low pressure set point.

the relief valve set pressure can be a minimum of 10% above max operating pressure i.e. = 330 *1.1 = 363 psig, and the maximum of piping design pressure i.e. 400 psig, or anywhere in-between.

No. This is wrong. The Gas Pipeline Code is specific, it states relief valves are set to a MAXIMUM of 110% of the max allowable operating pressure. i.e. the relief valve set pressure <= 1.1 * MAOP

ChaoticIntellect,

This is a typical "error" in interpreting the code. I have encountered it before also.

No gas code in my knowledge, such as ASME, API520 etc., recommend RV set pressure higher than the MAOP. It is not safe!

However, codes differentiate between:

1. Set pressure - pressure at which the RV will start opening, and

2. Accumulated pressure - pressure during rated relieveing flow scenario, typically 10% for process and 21% for fire case

So, SP <= MAOP, and AP = SP x overpressure (1.1 or 1.21)

Hope it makes it clear.

Earlier you wrote...

I agree. If it were a new plant, the relief valve set pressure can be a minimum of 10% above max operating pressure i.e. = 330 *1.1 = 363 psig, and the maximum of piping design pressure i.e. 400 psig, or anywhere in-between.

Then you write...

No gas code in my knowledge, such as ASME, API520 etc., recommend RV set pressure higher than the MAOP.

Clearly a contradiction. But I am not misinterpreting. From the Gas Pipeline Code,:

"Each pressure relief ... must be set to operate... If the maximum allowable operating pressure is 60psi or more, the set pressure may not exceed the MAOP PLUS 10% ..."

There is no misinterpretation here... it clearly states RV set pressure <= 1.1 * MAOP

There are semantics and definitions galore in dealing with gas systems.

Here's my guess... Your pipe, according to the manufacturer, may have a design pressure of 400psig (or if you look it up on a table for the materials and class of pipe). In a gas system, there are many "If-then" statements which alters the maximum pressure you as the operator can place on the pipe. One of the more common ones de-rates it to 75% of the design.

Overpressure protection is supposed to be set at no more than 110% of this calculated maximum.

So in your case...

400 psi (design) x 0.75 (derate) = 300 psi (max working pressure)

300 psi x 1.1 = 330 psi (relief valve set point)

Your numbers are slightly different, but close, so it sounds like your system falls into one of those "if-thens" and that everything is designed the way it is supposed to be. If the valves are lifting, either they're not set at 350 psi or you're operating above that.

I can guarantee though that those numbers are NOT arbitrarily pulled from a hat.

If it is popping then it is working

Have you confirmed that 350 really is the set pressure?

If it is working fine then you might have an upstream regulation issue.[]p

Alternatively you could remove the spring and replace it with a bit of pipe and wind it up nice and tight whilst enjoying a cigarette.

The set pressure, as you call it, should be lowered to below 330. This would be the pressure relief. The over-pressure relief sounds like it's doing it's job.

spring safety valves have a range, each range is defined by it's spring.

Contact the manufacterer for the spring you need for your setting.

Springs can loose their strength during time depending on the enviroment, your problem indicates a simmilar defect, or a wrong spring.

Hi,

I only have oil hydraulic experience but all RV's have a characteristics which means pressure rises with increased flow and I would assume your RV is the same. You need to obtain the Pressure/Flow curve for your particular unit and adjust your setting accordingly. If the curve is not flat enough you will have to look around and see if you can get a better one.

Regards

Oliver Dunthorne

Hi,

Why you do not return to the P&ID. or consult the manufacturer for such data.

Are you sure no one swap this RV with another.Is there a log book for daily records?

Good luck

We have an excellent record and the current set pressure conforms with the P & ID. Operations want to operate this line at the optimum pressure and don't ask me why, as they are the owners of the plant. All I want to know is whether we can safely increase the set pressure without violating the code, either by changing the spring or the valve itself.

Based on the information provided I suggest you to reset this safety valve for 400 psi immediately. Frequent popping is dangerous for fuel gas lines. Remove the valve from the system and connect it with test jig to confirm the present setting and adjust spring tension for 400 psi.

That has to be one of the worst suggestions I've ever heard... unless you want your insurance to drop you and to be held liable for any accidents that occur (probably to the extent of criminal charges if someone dies as a result).

From one of the OP's comments...

... the current set pressure conforms with the P & ID.

The RV set pressure conforms to the P&ID (what the engineer said it should be). You're right in that it means either the RV setpoint has drifted (possible, but these are supposed to be verified and adjusted about every 18 months) or the system is being operated higher than what it should be (likely, as we already know the owners are trying to stretch the capacity of the system).

I agree with the suggestion of testing the RV to ensure it is lifting at the design pressure and not before, but I don't agree with arbitrarily setting the relief above what the design documents state.

The pressures on the P&ID are calculated and set by by the engineer who followed the code. Unless something major has changed (such as a major decrease in population density enough to change the pipe location classification) I will guarantee you can't increase the pressure without violating code.

Has the "MAOP" been established yet in this discussion? It seems that 330 psi is an optimum pressure, not a maximum pressure. Even so, 330 x 1.1 = 363 psi. So maybe it's no wonder that 350 psi reliefs sometimes lift.

For all we know (owing to the lack of documentation), maybe sometime in the past someone stuck a 350 psi relief on this system because that was what was available at the moment, even though a higher setting was allowable.

A coordinated evaluation of all this might be helpful, but perhaps difficult if original documentation has been lost (as could happen in fires, floods, etc.)

This thread sounds like naval engineers discussing the basic design of the Titanic. Basing decisions on bypassing essential safety requirements on 'what the owners want' or 'raising the RV setting immediately because it is popping too often' (!). There is a lot of skepticism regarding the desired outcome of just raising the RV setting, but the OP does not want to hear the criticism, just wants a legal justification for doing what the owners want. Who are they?

GA. 'what the owners want' and 'legal justification for doing what the owners want'... really intriguing.

'raising the RV setting immediately because it is popping too often' can be legally done. RV can be set up to 10% higher than the MAOP which is 363 psig.

See at # from OP. "The other relief valve I mentioned about, which are set at a higher pressure are also on the same piping system, but on a separate branch" My question to OP is:

What is the legal justification for setting other RV's at 400 psig? Can't you follow same justification for this case too?

Thank you for your valuable comments. We see there is a hidden justification for which we cannot find any documentation. The is the issue we are trying to resolve.

If the owner is serious about exploring a change, then the cheapest way to do everything "by the book" is to go back to the engineering firm that designed the system. Chances are they have the design calc's in some file. Pose your questions to them, let them review the original design and come up with a conclusion.

I might seem negative in some of my responses and to answer the heart of the matter, yes, the code allows for uprating a system. However, it is NOT just an arbitrary change that allows an owner/operator to set it higher just out of convenience or some "logical reason". There are calculations to be done.

Some areas of engineering have facets that rely on the experience of the engineer to determine a parameter and it becomes more of an art than a science. However, for your situation there is very little art in these types of designs, so there is little room for interpretation. Two independent engineers, one with 40 years experience and one with 5, would likely come up with the exact same numbers.

The things that the code looks at is the strength of the pipe, how it is used (transmission, collection, distribution, etc), and the population density of the area it is used. Can you see why I would say there is likely only ONE reason that would allow you to uprate the system?

This is just my perception. I've designed gas systems and am basing what I say on that experience. I do make assumptions (1) the engineers who did your system followed the code and (2) didn't add in arbitrary safety factors. Assumptions can be wrong. If the owners are serious about trying to make a change, go to the original design engineers, spend the money, and get the answer. If you do, I'd be interested in hearing what they have to say... keep us updated as to what happens!

In the mean time, you already have the advice to check the setpoint of the valve. It's an easy thing to do and might allow you a little more wiggle room to operate.

Thanks for your valuable input. I have refered this matter to our engineering division and shall keep you updated on the outcome. Thanks once again for everyone.