PERCENTAGE NDT, NDT LOT & PENALTY SELECTION - A REVIEW BASED ON WELD JOINTS [1]

WELD JOINTS - PERCENTAGE NDT, NDT LOT &

 PENALTY JOINT SELECTION. 

(A review based on weld joints in process piping. )
PART - 1

INTRODUCTION

Those who are familiar with process piping will also be familiar with the topic of percentage NDT of weld joints, NDT lot concept and NDT penalty for weld joint repairs.

We will discuss the concept based on radiography testing of weld joints in process piping . (Which is applicable to other NDT methods as well.)

All weld joints are to be defect free. For some piping class (piping class defined in client specifications based on  process, severity of damage in case of leaks etc) the percentage of NDT can be 0%, 10%, 20%, 50% and 100 %. 

(Please note that visual examination of the weld joints is not considered in this discussion, which will be 100% in the case of welded joints)

% NDT BASICS

The two extreme cases can be ignored that is 0% and 100% NDT.

Take the case of 10% NDT. It means that out of 100 joints welded, 10 joints to be tested by NDT; or in simple terms, out of 10 joints welded one joint to be tested by NDT.

Take the case of 20% NDT. It means that out of 100 joints welded, 20 joints to be tested by NDT; or in simple, out of 20 joints welded one joint to be tested by NDT.

NDT LOT-BASICS

We will discuss in detail with the concept of 10%NDT.

In 10% NDT, We have to make a lot of 10 joints and radiographic testing of one joint out of these 10 joints will be done. If that one joint is accepted the entire lot is accepted. If that one joint is rejected more joints to be radiographed from the lot .

Below is the excerpt from ASME 31.3 regarding the lot sampling for NDT.

LOT CONCEPT IN WELDED JOINTS

How the lot is to be made?

The concept of lot is a statistical concept and well introduced by quality experts for the mass manufacturing process and used widely in industry in production process using mass producing machine's.

The concept when used in mass producing manufacturing has the advantage that there is a limited number of fluctuating parameters and that can be easily identified and can be controlled. Thus the concept of lot reduced the cost and time for quality and gives good results.

One of the premier EPC organisation  of India , M/s.ENGINEERS INDIA LIMITED (EIL) , specification says that the lot should cover WPS and welder or both.

Most of the other EPC and PMC organisations are following the same concept for the lot system.

So based on WPS, weld joints made by a welder in a day will be grouped as a lot and a lot number will be given. If sufficient number of joints are not welded by a welder, in a day, in a particular wps (to make a lot) , welded joints from consecutive days done by the same welder using the same WPS will be grouped and the lot will be filled. 

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

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

Click Here for Next Post in this series

Disclaimer: Photos are taken from different websites. If this violates any copyright rules this will be removed.If any of the photos or contents violates copyright rules please inform me through 13hareesh13@gmail.com. This blog is meant for educational purpose only. If it is used for any other means without the written consent from the author, the author will not be responsible. Author does not guarantee the accuracy of data used in this blog.All the information, data, photo/image, calculations presented in this blog should be used at the sole risk of the user. 

The blogger is not liable or responsible in any way for any damage, losses or costs arising from use of the blog for any purpose whatsoever or whosoever it may be. The blog is to be used at your sole risk and responsibility.