Maximum operating Pressure (MOP) for a pipeline as per ASME B31.8

[look4advice]

Dear all,

the point is: one pipeline is to be designed according to B31.8 and is protected by an upstream PSV.
Knowing that usually the PSV overpressure is 10 % of the set pressure, how can I correctly define the design pressure of the pipeline, i.e. should I use as design pressure the PSV set point or should I increase that value by 10%?
Let me make an example: Suppose that the pipeline is expected to operate normally up to 91 bar g, with high pressure alarm at 93 bar g and high high pressure shut down at 96 bar g with the PSV set point at 100 bar g: can, as per ASME B31.8, the pipeline be designed for 100 bar g or the design pressure must be 100 x 1.1 = 110 barg, as actually, once the PSV is fully open, the pressure in the pipeline can be higher than 100 bar g up to 110 bar g?

In other words, with ref. to this example, does the code B31.8 require to consider in the design pressure also the increase of pressure due to the PSV relief?

Thanks to all.










Thanks to all.

 

[dcasto]

The MAOP pressure and the mechanical spring type PSV set point are equal. You could install a pilot operated relief set at 110% of MAOP.

Why do even use a PSV, they are not required, you can use a ESD or MOV valve.

 

[Ashereng]

One reason is that a PSV does not need a motive source.

A MOV needs power.
A ESD usually need instrument air or gas, unless you go with a self-contained hydraulic (in which case you need to refile the hydraulic fluid once in a while).

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[dcasto]

They have 91 bar of energy available.

 

[Ashereng]

Process Gas many not be suitable for use as Instrument Gas?

"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?

 

[dcasto]

The whole city of Chicago IL is at risk then beacuse most of the natural gas delivery points use gas to run their controllers and ESD's.

 

[Ashereng]

Sales gas are usually of very good quality, and most are suitable. I am thinking more of gas straight out of the well, before processing.

My gas is usually wet, sour, and with solids.

"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?

 

[toffeeblock]

The orignial post is a classic case of design pressure meaning different things to on-plot process design and the connected pipeline design. This happens on many jobs when Process and Pipelines groups often speak at cross purposes.
If I can rephrase the problem as an example:

1. On-plot compressor normal operating discharge pressure (ANSI B31.3 piping) = 90 barg (say)
2. Compressor discharge piping design pressure = 100 barg
3. Hence PSV set pressure on piping is 100 barg. (Alarms and trips hopefully act before these lift, but are not relied on)
4. Hydrotest pressure is 1.5x piping design pressure = 150 barg
5. Note that PSV "accumulated" pressure (API521 and Process engineering term for when PSV will be fully open under relief case) = will be 1.1x set pressure, ie. 110 barg.

Question:
Following the ANSI B31.8 code for the export scraper/launcher and export pipeline, what is deemed the design pressure of:
A. Scraper Launcher (100 or 110 barg?)
B. Export pipeline (100 or 110 barg?)
C. What is the set pressure of the scrapers PSV (which is normally installed in ANSI B31.3 piping? (100 or 110 barg?)

 

[zdas04]

Toffeeblock
That is an excellent re-statement of the problem, and the basic question only matters if you are designing to very close to the design conditions.

In my world, I would design the pipe at 100 bar. The flange could be ANSI 600, material group 1.1 at 50C for a flange rating of 100.2 bar, but I would use ANSI 900 material group 1.1 at 50C for a flange rating of 153 bar. I'd pick my pipe for hoop stress around 50% of SMYS. Some might call this overkill, but when you look at the percentage difference in cost between arbitrarily limiting your hoop stress and allowing 100% you'll see its pretty small.

In short, if you're designing to the limits of the material then this is an important question. Otherwise it is just words.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[look4advice]

Thank you ZDS04 and Toffeblock for your replies; but still my doubt is there: what does the code B31.8 require in this case, i.e. can the design pressure of the pipeline be 100 bar g or it is mandatory to use a design pressure equal to 110 bar g to be in full compliance with with B31.8?

looking4advice

 

[dcasto]

You design to 100 barg that is the MAOP. Don't care about flanges or pipe material other than at the design point you comply with 31.8 with all the material factors, design location, weld or seam factors that go into the equation set out in 31.8 and the proper hydrotest that goes with it. You can operate the line at 100 barg every second of the day.

The overpressure protection must fully stop the pressure from exceeding 110% of the design pressure which is the MAOP or 75% of the SMYS, which ever is lower.

 

[look4advice]

dcasto,

thank you for your reply. This is also my interpretation of B31.8, that is, a pipeline can be designed for a pressure that then will be the MAOP for that pipeline and, in addition, it is in full compliance with B31.8, the fact that the PSV protecting the pipeline and therefore set at the MAOP (which is the highest allowable value for the PSV set point), will protect the pipeline with an increase of pressure over the set point value but not higher than 10 % of the set pressure.

This is to say :
a) PSV set point must be < or = MAOP of the pipeline.
b) Actual pressure of the pipeline with PSV fully open can
be < or = 110 % (max) of MAOP

 

[dcasto]

Don't forget the 75% of SYMS. This won't be a problem for class 2,3,4. (class 1 MOAP is 72% so that means you can only go to 103% of MAOP) OR if the PSV is pilot operated.