Hi Joe,
With regards to new fabrication, I guess the acceptance criteria depends on the type of fabrication and the governing code. The contract documents will specify the applicable code. In very short detail, in AWS D1.1, the Structural Welding Code, inspection is covered in Section 6. Table 6.1 is the table for visual acceptance criteria, i.e. undercut, porosity, undersized, cross section, etc. Included in Section 6 is the acceptance/rejection criteria for ultrasonic testing. Figure 5.24 in Section 5, the fabrication section shows acceptable/unacceptable fillet weld profiles, and also for groove weld profiles in butt joints. Also, the AWS publishes a hardback book titled Welding Inspection that would be a valuable guideline.
Hi Scott,
I think he was asking for an industry standard "Acceptable Rejection Rate", which sorta sounds like a contradiction in terms.
:) (acceptable rejection)
Hi Joe,
I'm not sure sure where you would find such a number to determine if the rejection rate is unacceptable unless it is specified in your customer's specs. I tend to get a feel for how the rejection rate is going as I'm inspecting. When my comfort level decreases, do to finding the need for lots of repairs, I tend to inspect to a greater degree to assure myself and the customer a good inspection. I find in our customer specifications wording like... Visually inspect 100% of all welds, UT 25% of all full pen welds, unless the rejection rate reaches 5%, then UT 100% until rejection rate improves. Similar wording in regards to MT'ing the fillet welds. The Piping industry may have a particular number as a standard, I don't know. Maybe some of the piping industry regulars can post more about that.
John Wright
Hi,
OK John, I see. I don't believe that there is an industry standard for acceptable rejection rates. As you mentioned, at least from a structural fabrication standpoint, the job specifications dictate the percentages of NDT welds to be tested, and the required increased inspection percentages if the specified rejection rate is exceeded. In our shop, we visually inspect 100% of all welds, and although the contract documents may specify otherwise, we UT all complete penetration welds. Our in house rejection percentage rates among individual welders are monitored to determine whether someone remains a welder or is pulled from production and retrained.
theoretically a 100% visual inspection only becomes an 80% visual inspection because we all know that thins can be missed as far as quality I belive the only way to 100% everyting is to mag them dye tem radio them ultra them and so on.
the frequency of inspection is not the same as probability of detection (POD). 100% inspection refers to how many welds are inspected, and not the probability of finding all flaws. I think it is pretty well recognized that no inspection method will find all types of flaws and that even specific flaws the method is designed to detect will sometimes be missed, even if 100% of the welds are inspected. The key is to know what kinds of flaws are really a problem in a specific type of weldment, how large the flaws can be before they become a problem, and what methods will have a very high probability of finding those flaws of that size.
Hi campro,
I think the orginal poster had something different in mind. I agree with you and Bill that not everything can be caught due to the weaknesses and strengths of the various NDT methods you mentioned(ie. RT vs UT).
What we were discussing is really repair rates. In other words if the customer had wording in the contract concerning the amount of inspection that read...UT 25% of all full pen welds unless the rejection rate (welds that need to be repaired) reaches 5% or more, then UT 100% of all full pen welds until the rejection rate improves. At this point if the rejection rate (repairs) improve, you would fall back to only UT'ing the 25% of the full pen welds as they come through. The 25% is not 25% of the joint, it's 25% of the number of welds to be inspected. You would still UT 100% of the joint in the 25 out of a 100 welds that come through.
Clear as mud?
John Wright
I don't believe any of the codes give an acceptable rejection rate. Each code requires every weld to be 100% acceptable using the specified evaluation methods.
From a productivity point of view for fabrication, and depending on the nature of the work, a 5% weld rejection rate is considered too high and 1- 2% is expected. The estimators put the percentages into the bid to cover the cost of anticipated rework.This does not come from any particular source, but are simply some numbers that some people tend to use.
I have no idea on acceptable percentages for maintenance work. Those jobs are estimated witha budget and extra work is usually T&M. Every situation is different so it's difficult to give a weld reject rate. All you can do is "guestimate" how much a weld would cost to repair and how many repairs it takes to erase profitability.
Maybe some others can give you some better info.
Chet Guilford
I'm thinking along the same lines as John...You will find a lot of discussion on this subject since rejection rates are code specific. The acceptance criteria for pipeline welding for example is not as strict as for pressure vessels. A reject rate of 5% may be average for a particular pressure vessel shop, but this number would be very high for pipeline work. Also consider that not all welding receives a 100% examination. Some companies tolerate rework more than others and some companies have a more skilled workforce.
There are two trains of thought in calculating the reject rates - straight percentage (1 weld fails out of 10 welds = 10% reject rate) and the linear percentage (total discontinuity length compared to total length of weld). The second method, of course is always lower.
Most of our clients are 2% or below for shop reject rates. One in particular is around 15% (and it has been that way for 15 years). Field connections, tie in welds etc. can be higher due to conditions. I’m interested to see some other replies from different areas of the country or world.
There was a related question asked and discussed briefly back on June 8. The question is listed as "Piping Repair Rates" . The answer is probably closely related to the type of material being welded, the code being used for construction and inspection criteria, shop versus field welding, welding process, etc. My response in that thread was with regard to field SMAW of steel pipe to specifications based on API 1104.
A customer of my company currently has a rejection rate of 2%, if it rises above this it requires an immediate corrective action to be started. All of our welds are 100% full pen and 100% Uted.
I believe you need a statistical process control approach.
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