B31.8 and Seamless Mill Tolerance 1

[KernOily]

Hi guys. For a B31.8 line that uses semaless pipe, do I add the mill tolerance into the calculation for wall thickness?

I think I know the answer, but I want to hear what you all think.

Thanks guys! Pete

 

[KernOily]

Think I found the answer, but let me have your comments.

§804.5 Dimensional Tolerances and Definitions

nominal wall thickness, t: the wall thickness computed... (blah blah) Under this Code, pipe may be ordered to this computed wall thickness without adding allowance to compensate for the underthickness tolerance permitted in approved specifications.

To me, that says I do not have to add the 12.5% mill tolerance. What say ye?

 

[SJones]

We say aye.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[BigInch]

For B31.4 and 8, the wall tolerence is ignored for stress calculations, if you order the pipe in compliance to the code's referenced specifications. What you see in the pipe dimension tables is what you use for stress calculations. If you want a 0.375" wall thickness, you order for example, 12" Pipe (12.75" OD) x 0.375" wt SEAMLESS API 5L X60, then you use 0.375", without making any further adjustments for production variations, in all the code's stress formulas.

Example for calculating allowable Pressure based on maximum permitted hoop stress, (assuming no temperature or joint factor reduction)
P = 2 * wt * SMYS * DF / OD, for a DF area of 0.72
P = 2 * 0.375" * 60,000 psi * 0.72 / 12.75" = 2541 psig

You can ask the pipe mills to produce pipe to criteria other than what is stated in the standard (API 5L, or other) specs that might include asking for a perticular custom tolerance, but be prepared to pay for it.

Independent events are seldomly independent.

 

[racookpe1978]

SO, if you've made some calculation from your temperature and pressure to require .375 wall thickness; then order .375 wall thickness per API, you get 0.375 wall thickness - using your example.

What would be missed - I think! - is corrosion allowance, right?
That is, your calculation of "required wall thickness" must include the corrosion allowance (if any) applicable to your given plant and circumstances BEFORE you go look up the available seamless pipe wall tables.

 

[pipe15]

maybe I missed something, but if you calculate the required wall thickness of a pipe to be 0.360" (for example), including any allowances for threading or erosion, etc., you will not be able to order standard wall piping or .375 nominal wall pipe because the vendor is allowed to supply the pipe at 0.328" wall which is the 12.5% mill tolerance value.

the vendor can sell standard wall pipe that is .328" thick and still call it standard wall pipe, but it will not meet your requirement.

you can specify min wall pipe, but that brings with it other aspects that are not all obvious too.

 

[BigInch]

So you searched for wall thickness tolerance and did not read the whole API 5L specification. If you read the whole spec you would know about the weight tolerance!

If you arrived at a 0.360 wall thickness, you may order any standard wall with a thickness greater than that, ie. 0.375" and use it as if it was 0.375 The wall thickness tolerance would permit some wall thickness measurements to be as low as 0.328, however because of the additional WEIGHT TOLERANCE (typically +10 to -3.5%, in some cases even less, SEE API 5L TABLE 10) it would not allow the wall thickness of the whole pipe to be furnished at 0.328". A weight tolerance of -3.5% applied to a 12.75" OD x 0.375" wall thickness pipe would only allow the average pipe wall thickness measurement to drop to as low as 0.361", not nearly as low as your 0.328"

Independent events are seldomly independent.

 

[KernOily]

Guys thanks for the replies and the discussion - I appreciate it! Pete