Measuring wall thickess to use in B31.1 wall calcs

[JAYDEE23]

I want to use actual wall thickness in my min wall calcs. Can anyone point me in the direction to find out where and to what extent I need to measure my actual wall thickness to use it legitimately in my min wall calcs. Ill be asking the MFG of pipe to UT the wall for me but i want to give precise direction.

 

[Eddycurrentguy]

Jaydee,

tmin is a calculated value based upon pressure, joint efficiency, radius, stress value; are you trying to determine the maximum pressure a vessel can withstand based upon current vessel thickness?

 

[JAYDEE23]

I have pipe sitting @ the MFG lot. It does not pass calcs based on i must subtract 12.5% tolerance from the wall. I want the MFG to measure the wall and give me exact numbers. Where and to what extent does he measure? its 120ft of 8" sch160 106B

 

[SnTMan]

JAYDEE23, I'm sort of guessing this would per agreement with the owner.

Regards,

Mike

 

[JAYDEE23]

Im sure there is direction on how what is acceptable. Like testing existing piping for corrosion thinning. B31.1 refers to a grid.IV-5.3 Methods of Detection

 

[RPRad]

I have had to deal with similar situations, I am assuming however we are talking about new pipe that was manufactured below the the regular pipe schedule tolerance.

Here is what I typically have done and there are several alternate ways to tackle this as well....note that generally this is done in agreement with the regulatory authority and the owner as the pipe is technically no longer in compliance.

1) In lieu of any sophisticated inspection equipment (which a pipe mftr might have access to) I usually have a qualified ASNT inspector hand continuously UT scan the entire pipe length(s) at the 0, 90, 180 and 270 degree positions. I have him note the thickness range occurring during each scan.

2)I multiply the lowest reading obtained by 1.125 to give me a nominal thickness, if this value is greater than the largest value I have measured then I use that (measured) value instead as my nominal.

3) If my calculated minimum thickness is above the lowest thickness measured then the pipe is not usable for the application. I also look to see how much of a corrosion allowance I have (should it be pertinent) and make a determination as to what this will mean to the life of the pipe.

Generally if I see numerous wide swings in pipe thickness, especially alot of swings greater than 15%, I dont use the pipe. Also you will want to check that the pipe OD is consistent as well.

I also ensure that the above process is noted in the design calculations as you dont want somebody new (who wasnt party to the process) thinking they lost large amount of material in a short period time, five years from now or such

Alternatively you may find an ASTM specification which defines the amount of UT acceptable at time of manufacture for the pipe spec you are using.

Corroded piping is a whole other animal.

R

 

[DSB123]

RPRad,
I am not sure the pipe is out of compliance with the spec. What JAYDEE23 is indicating, if I read it correctly, is that if he does the pressure thickness calcs for tmin and then compares commercial pipe nominal thickness less the standard mill tolerance then the resulting thickness is less than his calculated tmin. So he really wants to reduce the mill tolerance to the actual "mill tolerance" of the actual pipe rather than using the specification mill tolerance in his calcs.

Providing the pipe is within the mill tolerance then it's not out of spec.

 

[JAYDEE23]

DSB123,
Correct sir, As stated in B31.1, it is allowable to use actual measured wall thickness in your min wall consideration instead of nominal-12.5%.

Im still looking for direction on the extent of measurements.

 

[RPRad]

I see, have done that too...I have used same inspection process in lieu of the mftr being involved. Good quality pipe is generally pretty consistent in its thickness so you generally dont need to tons of inspection to get a good handle on the thickness variation

This may be the spec your looking for, sorry I have never read it and dont have a copy of it, but it may have the info you are looking for


E213 Practice for Ultrasonic Testing of Metal Pipe and Tubing

R

 

[JAYDEE23]

RPRad,
We are certainly on the same page! I Looked through my ASTM specs yesterday for that one. We have all the "A" specs but none of the "E". I wonder if someone can confirm this spec does in fact cover this topic. Thanks for feedback...much appreciated..

 

[Duwe6]

To use 'actuals' you will have to UT each stick of pipe individually, 4-places around, in continous strips,or on 6-inch [or closer]centers. If you have someone in-house to do this critical work very cheaply, you will pay more for the "extra wall thickness" that you will probably find, than just buying heavier pipe. And do you want to risk the safety of your new pipe system to the work of a cheap, uncertified employee using an uncalibrated UT instrument?

The pipe distributor can have these measurements taken by a local NDE shop, using a certified Level II tech and a calibrated instrument. Then the distributor will have the UT cost, and all the extra handeling costs [remember, each stick has to be pulled out, put on blocks, and rolled until 4 or more readings can be taken in a circle, down the full length, and then put back in the bundle for shipping. These costs will be marked up 15% or more, and given to you to pay. That cost will be large.

Just buy thicker pipe.

 

[JAYDEE23]

I am aware of the options in the engineering/design phase. Without getting into all the details, this is where i am at on this project. I cant just simply buy the pipe i need at this point without heavy consequences.

That said, The vendor will be performing this for me. Before i submit the findings to my customer for review and acceptance i want to be sure it is being done to "code".

 

[Mr168]

Not sure what the application is, but if any of it is Boiler External Piping, you'll want the input of your Authorized Inspector as well.

 

[Duwe6]

There is no 'code' for the appropriate UT scanning pattern and frequency to prove wall thickness. You will have to be very specific in your instructions, and it would be prudent to have someone 'audit' the results. I would reperform some scans in random areas, and ensure that the 2nd results had good agreement with the original submitted data.

The Authorized Inspector's 'buy-in' will be mandatroy if this is B31.1 'PP' work. If it is standard B31.1 [non-stamped] or B31.3 work, whoever the owner designates at the Inspector-of-Record will have to agree with your testing methodology and results. In other words, whoever the Inspector is, you have to obtain his/her approval.

 

[JAYDEE23]

Ive got my AI approval. The customer is giving me resistance. Ive got the Mill performing it,and i will have an independent NDE house perform it also w=once it arrives at our facility. Ive asked for 8 shots around the circumference @ 6ft intervals.

 

[SnTMan]

Ah, no longer guessing now...

 

[JAYDEE23]

oh im guessing still....sounds reasonable though doesnt it..

 

[SnTMan]

Yeah, sounds OK with everybody but the customer...

 

[Duwe6]

IMHO, 8 spots around may be overkill. 4 should do fine on 8"NPS.

The 6-ft intervals are waaay too far apart for seamless pipe, will be fine for seamed pipe.

The reason 6-ft is too infrequent on seamless pipe is that this pipe was made by piercing a billet with a mandrel. If the mandrel wobbles, the wall thickness will be 'wavey', typically in a spiral pattern. I would take one 'traverse' of thicknesses at 6-inch intervals, in one 6-ft or longer section of pipe. Just take them straignt down the pipe. That will diagnose if the mandrel 'waved' during piercing.