wall thickness of compressor stations piping

[engahmedbahgat]

hi all
until two days ago I was believing that wall thickness calculations for piping of compressor, metering and pressure reduction stations of gas transmission pipelines follow ASME B31.3 code. but when I started to skim the code; I found it states that piping of compressor stations and other facilities on the pipeline shall follow B31.8 as the pipeline itself.
so please tell me, should I calculate wall thickness of compressor, metering and pressure reduction stations with equations of B31.3 or B31.8

thank you in advance

 

[BigInch]

You are correct. See how important it is to actually read and understand the codes. B31.8 applies in general to the design of all gas containment piping components of pipeline facilities including compressor stations, but may not apply to some auxiliary facilities typically found within compressor stations, such as engine cooling water piping, hydrocarbon drain sewers and fire water, etc.

 

[LittleInch]

Depends on where you are and what code you either choose to use (should be listed in your design basis) or is imposed on you by the authorities.

I believe, but can be corrected, that in the US, the mandated code for gas compressor stations is B31.8?

However in many other locations, companies decide to use B 31.3 for all "piping" design and leave the pipeline codes to the offsite stuff. Pipeline codes should, as a minimum, continue to the pig trap and often the first valve off the pipeline.

If you've got lots of long piping runs in large diameter pipe, it can make sense to move that code break further down or even design the whole thing to a pipeline code as they all cover compressor stations.

The downside is that to get advantage of the pipeline codes, you need to use higher grade pipe. That's Ok for the pipe, as B31.3 doesn't give you much, if any, reduction in wall thickness compared to the pipeline codes, but it means you need to use high strength flange and fittings (e.g. elbows, reducers etc) material compared to a "standard" B 31.3 design. As compressor stations tend to have lots and lots of flanges, fittings and valves, the cost element of the pipe gets to be only a small fraction of the materials cost compared to a pipeline. you can also suffer some delay ( perhaps not at the moment(!)) in getting items of a higher strength compared to say A105.

So in summary, yes you can use B31.8 in those locations ( usually has a design factor of 0.6), but this might cause you more problems and increased cost / schedule when you need to buy high strength flanges and fittings. Unless you're in the US you can decide yourself.

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

 

[BigInch]

B31.8 compressor station (onshore) DF is normally 0.5, but reduces to 0.4 in city areas with buildings 4 stories or more. I extend the 0.5 (or 0.4) design factor to include meter stations and other facilities that are likely to have maintenance or operation personnel on site on a frequent basis.

0.6 is for fabricated assemblies, such as at tie-ins, or block valve assemblies in rural areas, where the area classification would not otherwise require 0.5 or 0.4 .

 

[LittleInch]

So it is. Just goes to show I don't design B31.8 inside compressor stations very often.

Has it always been 0.5? (other than location class 4)

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

 

[BigInch]

Always has been 0.5 & 0.4
In the USA I don't know of any gas transmission companies that use B31.3 for anything other than process units, such as gas treating, dehydration and liquid extraction units. Compression cooler piping, pig launchers/receivers and pipe type slug catchers and drips are CFR Title 49 Part 192 (essentially B31.8) and vessels (coalescing filters, slug catchers, etc.) are always designed to BVP VIII.

When using a DF of 0.5 the difference in wall thicknesses bewteen codes re very small. PHMSA does not object if B31.3 was used (in error) for compressor station design as long as the final design meets CFR Title 49 Part 192.

Of course you must remember that there are many design requirements for pipelines and compressor stations that B31.3 does not address at all, so when designing a pipeline system B31.3 can not really be used for anything more than determining the wall thickness of the pipe anyway.

 

[BigInch]

BTW we have discussed these issues previously,

http://www.eng-tips.com/viewthread.cfm?qid=376163

 

[weldstan]

As BigInch stated if your pipeline is governed by Part 192, you must follow B31.8 with the .5 or .4 design safety criteria, welder qualification criteria and inspection criteria. If your compressor station is within a plant and the gas is transmitted only in the plant governed by B31.3, B31.3 is the Code of design.

 

[BigInch]

Good point Weldstan. Yes many refineries, chemical process units, chemical plants using low pressure natural gas as feed stocks and lots of other types of plants have compressors where B31.3 or other design codes could apply. In order to apply B31.8 / CFR T49 P192 the primary activity should be "Transportation", as opposed to chemical processing. I would tend to call those "plant compression units", as opposed to a "gas transmission pipeline compressor station".