PERCENTAGE NDT, NDT LOT & PENALTY SELECTION - A REVIEW BASED ON WELD JOINTS [2]

WELD JOINTS - PERCENTAGE NDT, NDT LOT &

 PENALTY JOINT SELECTION. 

(A review based on weld joints in process piping. )

PART - 2

IS THE NDT LOT SYSTEM EFFECTIVE IN THE CASE OF WELD JOINTS?

Welding is a "Special process" as defined by ISO standards. 

What is a special process ?

"Special processes are those which cannot be verified after the process without destructive testing."

There are a lot of uncertain parameters included in the production of a  welded joint. Some of them are,

1. Bevel angle- bevel angles may be different to different joints. Though all joints will be within limit, there will be variations.

2. Root gaps may be different.

3. Components used for fit-up may be different and for the same components also heat number/batch number will be different. 

4. Cleanliness of the joint will be different.

5. Location of the joints will be at different elevations and different locations.

6. Electrode used for welding may be from different batch, different manufacturer, from different storage conditions including different baking batches.

7. Welder psychological and ergonomic issues.

8. Current,  voltage, fitness of machinery during welding.

From the chances of the above uncertainties, will the concept selecting a lot based on WPS and welder will make a sense ?

From the concept of lot, the lot should contain items that are almost identical in all respects. Then the concept of lot will be successful.

If we found that the first piece from the lot is accepted, then the entire lot is accepted and we can say that the lot is covered or lot is closed.

If in that lot a defect is found, further iteration to be done. The guidelines for further iteration as per ASME B31.3 is as follows.

We will analyse a situation of welded joints.

Situation:

Welder A welded with WPS1 on two lines Line1 and Line2.

Line1 is a 10% NDT line and Line2 is a 20% NDT line.

Welder A welded 5 joints on 10% line (Line1) and 3 joints on 20% line (Line2) using WPS1 in one day

The next day he welded another 5 joints on 10% line (Line1) and 2 joints on 20% line (Line2).

The joints of two different days made as a lot for welder A, one of 10% lot covering the joints of 10% line spec (10 joints) and 20% lot covering the lines of 20% lot (5 joints).

The 20% got accepted with a weld joint randomly selected and it was welded on the second day.

Now a defect is found in the 10% lot and the defect was porosity. Further sampling was done with two randomly selected joints and it was from the second day and the entire 10% lot also got accepted.

Now on investigation, it is found that the porosity is due to improper baking of electrodes on day 1.

This raises some questions

Is the lot acceptance criteria which we have done or followed generally in the industry, is an effective one?

Is defect investigation needed?

What about the other joints welded on the same day by the same welder and same WPS?

Is there any other welders who used the same baking batch electrodes on that day?

Will be continued in the next post.....

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