Clarification on Compressor Station MAOP 2

That 10% number causes more confusion than any other entry in any standard in the world.

The standard is that safety devices must lift at or below MAOP. They must be sized such that a credible scenario cannot cause the pressure to exceed 110% of the MAOP while the PSV is open.

So if a PSV can pass 1 MMCF/d and you have a credible scenario that will cause inflow at a 2 MMCF/d rate for 20 minutes followed by a rapid decline to below 1 MMCF/d. If the volume of the piping and vessels within the facility is such that the brief period of 2 MMCF/d (while the PSV is relieving 1 MMCF/d) will not raise the system pressure above 110% of MAOP then the valve is properly sized.

Most of the time you size your PSV for at least 100% of the credible inflow rate so the pressure will decrease during the PSV-open event. The 10% allowance was for those situations with clearly defined transient inflow greater than PSV setting.

I'm sure that there are times where this is important, but it could never be worth the confusion it has caused.

David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Everyone is entitled to their own opinion, but not their own facts" Patrick Moynihan

Thanks for the clarification David!

Leave it to government trained people to write statements that are confusing to understand.

Have a swell & sudsy cerveza.

Keep the gears rotating!
-pmover

I went to a major pipelines compressor station that had a 720 MAOP based on flanges. They set theier standard spring operated relief valves at 792!!

So even a 50+ year old pipeline company can be confused.

Hi guys. Well SOMEBODY IS CONFUSED, but I don't think its me. Its MAOP + 10 %.

pmover's quote of CFR 192.169 is exact.
In combination with p 169, see part 201, where the set pressure is specifically addressed and the allowed maximum condition is plainly stated.

Reading this immediately took me by surprize, as I have never limited PSV set pressures to MAOP. Based on many years working directly for and with a number of major USA gas companies, Northern Natural Gas, Enron, Transco, TransTexas, Texas Eastern, and working on many projects at engineering companies specializing in design of both gas and liquid pipelines for all types of gas pipeline transporters from gathering systems to city gates, none of which limited PSV settings to MAOP, I have to disagree with your interpretation.

Looking directly to the text of the gas pipeline regulation and noting the highlighted lines of paragraph 201, of Part 192.201, I think its obvious what pressure setting is applicable,

[/quote]
§ 192.201 Required capacity of pressure
relieving and limiting stations.
(a) Each pressure relief station or
pressure limiting station or group of
those stations installed to protect a
pipeline must have enough capacity,
and must be set to operate, to insure the following:
(1) In a low pressure distribution system,
the pressure may not cause the
unsafe operation of any connected and
properly adjusted gas utilization equipment.
(2) In pipelines other than a low pressure
distribution system:
(i) If the maximum allowable operating
pressure is 60 p.s.i. (414 kPa) gage
or more, the pressure may not exceed the maximum allowable operating pressure plus 10 percent, or the pressure that produces a hoop stress of 75 percent of SMYS, whichever is lower;
[/quote]

OK, so you may ask, to what pressure does "the pressure" refer? "My logic is as follows,
Well, it absolutely could not be MAOP, so it must be referring to either PSV set pressure, or it is saying that it is the pressure in the system during a relief event. In either of those 2 cases the end result is that the maximum pressure IS = MAOP + 10% and, if it refers to "set pressure", or "relief event pressure", is immaterial, as BOTH have values greater than MAOP. As such, settings for relief valves CAN be higher than MAOP.

What about that part "75 percent of SMYS", what then? See section 192.739. This is especially interesting here, since sub (b) specifically mentions "MAOP + 4%" is the "pressure limit". Again, not specifically limiting PSV setting to MAOP.

That interpretation is not contradicted by 192.169 Compressor stations: Pressure limiting devices.

(a) Each compressor station must have pressure relief or other suitable protective devices of sufficient capacity and sensitivity to ensure that the maximum allowable operating pressure of the station piping and equipment is not exceeded by more than 10 percent.

Click to expand...

Now, even though CFRs are law, B31.8 could play a part in that law by reference, so checking the provisions of ASME B31.8, once again IMO the limit is MAOP + 10%. In Section 843.44 pressure Limiting Requirements in Compressor Stations, 441 shows,

Pressure relief or other suitable protective devices of sufficient capacity and sensitivity shall be installed and maintained to ensure that the maximum allowable operating pressure of the station piping and equipment is not exceeded by more than 10%

Click to expand...

Here reading that carefully, one we note that actual PSV set pressure is not directly addressed and that all pressures are simply limited to MAOP+10%, and this does not contradict the interpretation above that a PSV setting could indeed be higher than MAOP, as long as MAOP + 10% is never exceeded, and that provision applies to compressor station factilities too.

Additionally,
It is usually contended that MAOP is possible to be exceeded during transient events, as under the definition of Maximum Operating Pressure found in B31.8 section 213, MOP is defined as the highest pressure under a "normal operating cycle". If relief events are not included in "normal operating" cycles, then MOP is allowed to be exceeded during relief events. What relief event could possibly be included in a "normal operating cycle", when relief valves are designed to protect against "overpressure" protection? IMO, it is impossible to include overpressure within the definition of "normal" operating pressures. The concept of exceeding MAOP for transient events defined under maximum time ranges is recognized in various international codes, such as DNV, and many major oil/gas company pipeline design guides as well. Under the time limit definitions, it is hard to categorize relief events as normal operations, as they are almost always short term events. B31.3 recognizes higher PSV set pressures than MAOP. B31.3 on which the US Army process piping guide is based recognizes relief valve discharge as transient effects. EM1110-1-4008 5 May 1999, see example in Chapter 3, where relief valve set pressure is 2780, but maximum allowed operating pressure is only 2650, that representing a 5% increase over MAOP.

None the less, and allowing for misinterpretations to become entrenched in hand me down company standards in some manner or another, I made an extensive search through Office of Pipeline Safety records to find out if this question has arisen before and I have found the following interpretation by the Acting Associate Director for Pipeline Safety Regulation,

The allowable override of 10 percent of the MAOP is included in the regulation

Click to expand...

As this OP deals with the specific application of set pressures to pipelines, I was wondering what the opinion is for other applications and spent quite a number of hours searching the net and turned up with this. Apparently the same rather convincing logic can be found for set pressures above working pressure based on capacity for vessel design, which I share and at the least this proves I'm not alone in my opinion, and no matter how hard this evidence is, there are some die-hards that still maintain PSV settings maximums should be MAOP of the vessel. Read the article here, http:/
Well, don't know if I convinced you, but I'll stick with allowing PSV settings over MAOP when necessary. It appears to me that OPS has clarified it very well.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

dcasto,
I see that all the time. I also see PSV's set at MAOP on top of rupture disks set at 110%. When I ask "WHY?" I get "the rupture disk is to keep trash out of the PSV and rupture disks are supposed to be set at 110%".

The last time I had this conversation was with a Major Oil Company's Technical Authority on Relieving Systems. This person was the final word on PSV's in one of the biggest companies in the world.

My son was working for a local Vessel Fab shop doing PSV testing in the field. He asked me why three operators in this basin have completely different requirements for setting releiving devices. One company has the Rupture Disk (independent nozzle on vessel) at MAOP and the PSV at 110%. Another has the PSV at 90% and the Rupture Disk at 100%. The third has the PSV at 110% and no rupture disk. I tried to explain that even smart people can do stupid things. Luckily he changed jobs before he had a chance to relate my sage advice to any of the operators.

There is a lot of confusion out there.

David

See what?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

BigInch,
Everything you wrote in that tome above supports my position. None of it requires any particular set point for a safety device, it all relies on the operator to install adequate safety devices to ensure that the system never exceeds 110% of MAOP. An operator is not wrong if he sizes a PSV for at least maximum flow during a credible scenario and sets the PSV at 110%. He is also not wrong if he sizes it so that a defined increase in pressure with the PSV open will not allow pressure to increase above 110%.

The article also supports my position. The thing that is poorly understood is what the maximum possible flow rate is during a Credible Scenario. The article touches on it with the box toward the end where he says

CODE AS WRITTEN

Capacity based on Set Pressure + Allowable Overpressure

Click to expand...

If you know what the maximum inflow is during a high-pressure event, then you can use the 110% pressure to calculate the relieving rate and size/set your valve for that. I never have enough confidence in my maximum inflow rate to either use 110% to calculate flow or to set the relieving pressure at 110% of MAWP.


The confusion comes from taking the language above and thinking that it allows PSVs to always be set at 110% of MAWP. It doesn't say that, it leaves the relieving scenario up to the operator and requires the operator to provide whatever safety devices set at whatever pressures will ensure that the pressure stays below 110% of MAOP during an actual over-pressure event.

I don't know about you, but I never have enough confidence that I have nailed the maximum flow during all credible scenarios close enough to set the PSV at values above MAOP. Maybe you are better at defining Credible Scenarios than I am.

David

I'm glad you agree. I was apparently confused by your second sentence where you said, "The standard is that safety devices must lift at or below MAOP." I don't see anything about lifting at MAOP in any of those references.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

BigInch,

Lets limit the debate to gas pipelines for now. In my example the use of a spring load relieve atoumatically says that the pipeline will exceed it's MAOP as the valve will NOT relieve the pressure at its set point, instead it will reach full capacity at 10% over pressure. If you put a pilot operated valve in the opens to 100% at set point, then you can set the pressure at 110% of MAOP.

NEXT, nowhere in the regulations is a relief valve even required. Most people use switches or such to isolate the line. It is the operators responsibility to assure that the isolation system works and that transiets or time lags in the isolation system do not allow the line pressure to exceed 110% of MAOP.

For a spring loaded relief, I would think something like 1.05 x MAOP would be appropriate, given any actual pressure still did not exceed 110%.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

BigInch,
OK, I did misspeak on the first sentence. The standards say that the operator is required to ensure that the pressure does not exceed 110% of MAOP. Satisfying that proscription is the obligation of the operator and the job of engineers. I do it by sizing relieving devices at MAOP or less and using the calculated flow rate at the set point, this is probably very conservative, but I'm old and don't like breaking things.

David

192.169 apparently does require relief valves in compressor stations.

Anyway, let's try to answer pmover's specific question, which I would say is, MAOP for all compressor discharge and connected piping is 2500 psig.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)