High Yield Pipe in B31.3 Facilities 1

[XYZ998]

Hello,

I have noticed that many companies do not utilize high yield pipe (e.g. API 5L Grade X42) in their pipe specs for facilities that will conform to B31.3. However, the same companies do utilize high yield pipe in their specs for facilities conforming to B31.8.

Is there a reason that high yield pipe is avoided for B31.3 facilities?

It seems that it would allow using STD instead of XH in certain sizes/pressure classes, which would save a bit of money.

 

[BigInch]

B31.8 uses yield strength and class location factor both as inputs to calculate the allowable stress that is an input to the wall thickness formula. Allowable stress is thus able to vary according to the environment into which it is, or will be, installed. Multiple safety factors are used. The same level of safety is not required in the middle of Wyoming as it is inside New York City.

B31.3 uses the allowable stress as defined in the code for a given material which is an input into the wall thickness formula. B31.3 allowable stresses are constant because everyone knows that the only place you can use it is in a plant environment and only one safety level is required.

What is the allowable stress given in B31.3 for API 5L X-42???

you must get smarter than the software you're using.

 

[weldstan]

B31.3 bases design with a safety factor of 3:1 on th UTS not the yield strength. X-42 is in no way considerd high yield strength pipe. The design allowable stress is 20 Ksi for both A-106 B and X42 at 400F and API 5LX grades are limited to 400 F. I know of a number of companies that have specified X60 and X65 pipe for specific services and the economic benefits.

 

[XYZ998]

Thanks for the response. I just had a look at Table A-1 in the 2008 Edition. The basic allowable stresses of A106 Gr. B and API 5L Gr. X42 are both listed as 20 ksi at 100-400ºF, which seems consistent with your response, however API 5L X52 is listed as 22 ksi at 100-400ºF.

 

[BigInch]

Actually X42 is considered as high strength, because it is not Grade B (yield 35ksi) the historic base case, typical, normal type, low carbon, plain vanilla steel.

Given normal temperatures, the B31.8 design stress allowables
for X42 are 0.4, 0.5, 0.6, 0.72 or 0.8 x 42ksi, depending on area class factor.
giving, 16,800 psi 21,000 psi 25,200 psi 30,240 psi or 33,600 psi

For API 5L Grade B, and for ASTM A106B, both yield stresses are so allowables are 35ksi x 0.4, 0.5, 0.6, 0.72 or 0.8
giving 14,000 psi 17,500 psi 21,000 psi 25,200 psi, or 28,000 psi

As you can see from a comparison to Weldstan's post, B31.8 is more conservative when inside compressor stations as the area class factor there is 0.4, or 0.5 and the allowable stress calculates to only 14,000 or 17,500 psi for 5L-B and 16,800 psi for X42, or instead of the 20,000 psi allowable stress of B31.3. Elsewhere B31.8 would give lesser wall thicknesses.

you must get smarter than the software you're using.

 

[XYZ998]

When I do the math (per B31.3)on a 6" pipe with 1/16" corrosion allowance and 1440 psig internal design pressure, A106 Gr. B is going to require that I use XH wall thickness, whereas if I use API 5L Gr. X52 I can get by with STD wall thickness.

So if my math is correct, it appears that B31.3 allows me to use the more economical API 5L X52 with STD wall for that particular size. Strikes me as odd though that I typically see A106 Gr. B with XH wall called out for this case in pipe specs.

One of my coworkers thought that A106B XH may be called out because it is more readily available, but I haven't had any trouble procuring seamless X52 piping on recent compressor stations.

 

[BigInch]

Why would you use B31.3 on a compressor station, assuming that you do have a pipeline project.

you must get smarter than the software you're using.

 

[weldstan]

Why use X52 when you could use X60? Thirty six years ago. I did a study of the yield strength of X52 pipe on over a hundred heats. Over 95% of the heats involved had Y.S. > 60 ksi. All heats exceeded 58 ksi.

 

[LittleInch]

to get back to the original question - Is there a reason?, yes there is.

As noted very well above, you get very little benefit for increased SMYS and any reduction is close to the small extra cost of higher grade material. Moreover, use of higher SMYS pipe requires use of higher strength fittings, especially flanges and tees which are not commonly available, are higher cost, longer lead time and lead to potential issues of using the wrong flange in the wrong place as one 8" flange looks pretty much like another one of the same size and schedule.

Hence any small saving, unless you have hundreds of metres of only pipe, is normally wiped out b these practical considerations. The construction contractor will not thank you for introducing multiple grades of steel when he may already have multiple type sof steel (C Stl Stl, CRA etc) to contend with.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[XYZ998]

Thanks for the insight. That helps me tremendously. I will probably stick with A106 Gr. B for the practicality reasons explained by LittleInch above.

The recent compressor stations were built to 31.8. We occasionally get involved with small gas plant projects though, so I have been reviewing pipe specs for B31.3 facilities lately in preparation for the next one.