Reciprocating compressor package pipe schedule 1

[pyMorty]

Hi,

I´m working on a spread sheet to calculate required gas pipe diameter for given P1, P2 and flow (I´m ussing Weymouth, for some reason is the one has given more accurate results according to my fiel of job which is gas compression).

I´m putting also the recommendation for pipe schedule, and using as example the schedule 40, which allowable stress, according to ASME B31.3 table K-1 is around 1900 psig @ up tp 400 deg F, and comparing the results with P&ID´s of 3-stage reciprocating compression packages we own, operating at suction 30 psig, discharge 1000 psig, flow of 6 MMSCFD, sweet gas, I got the following doubt:

Why do the packager set first interstage piping schedule 80 while having working pressure at that stage no higher than 200 psig (or 300 psig considering hydro test)? which could easily allow to use sch 40. They just use schedule 40 at first stage. Note that max expected temp is 266 deg F before entering gas cooler and process gas is sweet. Could that be an offset for standard minimum corrosion allowance of 0.0625"?

Whit this I´m not moving forward withe recommending schedule 40 for services of let´s say 700 psig.

Also, could I considering allowable stress as MAWP of the piping? or maybe this is were I´m getting the incorrect parameter.

 

[mk3223]

It's not clear what are the design conditions for the systems which were either with the Sch 80 compared with the Sch 40?

Basically, the MAWP is the operation/design parameter of the system, and the allowable stress is the design parameter of the material property. How could they be the same?

 

[pyMorty]

I meant to say why do the packager set schedule 80 where it could easily be schedule 40 according to normal operating conditions and max expected (300 psig max). That doubt is keeping me from suggesting schedule 40 for applications of way more pressure, 900 psig for example.

 

[LittleInch]

Pressure isn't the only force/ stress in action here. Thermal expansion, extremal forces, vibrations etc etc.

The difference between sch80 and 40 cost is negligible. Sometimes you need a bit more metal, especially at relatively small pipe sizes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pyMorty]

Thank you LittleInch.

On the piping connecting the compressor, for example suction, which wouldn’t be subjected to as many forces as it would be in the compression skid, what percentage under the piping max allowable stress would you allow to normally operate?

 

[MickMc]

You do not mention pipe diameters as it is common to have a minimum schedule for small bore piping for mechanical strength e.g.operators standing on skid piping.

 

[MJCronin]

I agree with MickMc ...

How can you even have this discussion without mention of Pipe Diameters and pipe materials ?

Practical matters such as pressure rating reduction when cutting pipe threads are somehow missed in this thread ....

Schedule 40 has very little pipe wall available after common threads are cut.... Sched 80 is almost always specified.

What about corrosion allowance ? ..... Will there be zero corrosion in your wet discharge piping ?..... Explain

What about the 12.5% mill tolerance ?.... What about when the piping contractor runs into the engineer's trailer and asks if he can "cold bend" piping instead of using welded elbows ?..... What if that same contractor wants the engineer to approve mitered elbows of "stab-in" intersections ?

What about when additional wall thickness is required of the run pipe at a tee where a relief valve is located ???

Schedule 40 probably will not be acceptable ... but we don't know because there is no pipe diameter

https://www.littlepeng.com/single-post/2017/07/21/ASME-B313-Sample-Wall-Thickness-Calculations

Is this the OPs first assignment with piping design ?

Larger diameter CS piping at higher temperatures ..... Sched 40 ....no way !

MJCronin
Sr. Process Engineer

 

[pyMorty]

Thanks for your comments guys. I started as a general question, but you are right, in the case I had on mind the following are the specifics:

Flow: 6 MMSCFD
Temp. 95 deg F
Sweet gas, no tolerance for H2S required
Suction: 10" sch 40, 30 psig
Firs stage discharge: 6" sch 40, 144 psig, 277 deg F
Second stage suction (post cooler): 140 psig, 118 deg F, 6" sch 40
Second stage discharge: 395 psig, 257 deg F, 4" sch 80
Third stage suction (post cooler): 386 psig, 118 deg F, 4" sch 80
Final discharge (post cooler): 1149 psig, 277 deg , 3" sch 80. (actually one page of the P&ID states all the lines to be sch 40, even final discharge, but should be an error)

So, I appreciate the hints you have given me, the 12.5% mill tolerance one is a baseline I didn´t know. Also I learnd about pulsation studies resulting in provisions for extra pressure peaks.

 

[1503-44]

Weymouth is usually not appropriate for small diameter, short length flow. It was developed for large diameter pipeline use, 20" and above.

You might use sch 40 for normal pipe runs, but not on the compressor skid.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[pyMorty]

Thank you ax1e.

Yeah, I´ve heard/read that a lot, Weymounth not being that appropriate, but for some reason it's the one that best accommodates to the real conditions I´ve been involved with, which are precisely the ones it shouldn´t accomodate, piping ranging from 2 to 12" ID, pressures from 20 to 3500 psig, flows from 0.5 to 40 mmscfd. I´ve crossed my calcs with the reality and have been pretty much accurate. I´ve added the fittings k value to compensate for theoretical extra length and get equivalent lenghts, that has helped to get more accurate results.

Anyway I´ll try to add the calcs with another equation as well.

 

[1503-44]

Well they say it's better to be lucky than good. If Weymouth gives you the right answers, use it... this time. Just don't count on it always being right.


“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[EdStainless]

Vibration and fatigue from pulsation are big factors.
Compare the value of the pipe to the compressor, going up a size is nothing in BOM.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[pyMorty]

Thanks for the advise ax1e.

Thank you EdStainless, for sure that clarifies a lot on why to go up on size/schedule on a compressor package even under same base pressure and flow conditions than an offskid pipe.