Re: Acceptance Criteria of TOFD Method Dear Shahriyar,
We have been working in the forefront of TOFD for
almost 15 years now and have found ways to get around the
problem of codes and standards lagging behind on
technological progress for this method. Fortunately,
developments are progressing in this part of the world
now as well (Europe) despite some hesitation of some
groups as would always be the case with new technology.
The following is an extract of a publication on TOFD,
specifically adressing the subject of acceptance criteria.
You can find a lot more about many more aspects of TOFD
on our website: www.sonovation.com. Look under "Services",
then select "TOFD" and you can download the whole document
as a PDF file. Good luck!
When TOFD is used as a pre-service inspection technique, at the moment the following options are available:
· The use of TOFD for detection, in combination with conventional techniques such as Manual UT for evaluation in accordance with an existing code,by example ASME
· A job specific translation by a level III qualified person, of existing acceptance criteria in use for the conventional NDT techniques such as manual pulse echo and/or radiography to TOFD acceptance criteria
· The use of the Draft acceptance criteria developed within the Dutch acceptance criteria project by the Dutch welding institute (NIL) and KINT.
· The use of the ASME code case 2235 for wall thicknesses from ½" (12.7mm
International acceptance criteria as used for conventional NDT techniques have been used over a long time and are based on good workmanship.
Typically, for these acceptance criteria, each technique has its own set of acceptance criteria, based upon their respective capabilities and weaknesses to guarantee an acceptable quality level of the inspected construction. Economical factors, technical performance, reliability and safety classifications of the required inspection determine which technique to use.
The reliability is a function of the detection rate and false call rate, by example as shown in the KINT project. Because differences exist in the reliability of NDT techniques, technique dependent acceptance criteria have been formulated with the objective to keep the integrity after examination on a similar level. Basically it can be stated that the poorer the detection rate is with a certain NDT technique for a certain type of defect (like slag , porosity or lack of fusion) the higher the sensitivity is for rejection.
The acceptance criteria for TOFD are mostly translated from existing pulse echo- or radiography acceptance criteria. The advantage is that a new technique utilises known acceptance criteria which were proven in the past, and a further advantage is that they are familiar. In most cases a direct translation is impossible due to the reason that these acceptance criteria are required to distinguish between planar and non planar defects, which for embedded defects can not be done with TOFD. However, TOFD can reliably distinguish between porosity, surface breaking defects, cracks and embedded linear defects (be it planar or non planar). In some cases therefore, this results in a situation where the conventional acceptance criteria can not be used immediately, therefore characterisation by an additional conventional NDT technique has to be used.
This may result in a compromise to classify all linear embedded defects as volumetric. Because of the fact that also slag can have sharp edges on the upper and lower edges of the flaw, meaning that from a fracture mechanics standpoint, slag and lack of fusion are treated similarly, yet this would never be detected with conventional NDT techniques
Another problem that arises is that most conventional acceptance criteria can reject on length-information only. As we know, conventional techniques will only detect defects with a certain height (below this height they will remain undetected). TOFD will detect defects from a much smaller height, which results in a higher rejection rate if rejection on length alone is strictly applied.
To avoid this situation, an additional minimum height factor may be added to the length rejection criteria, which means that below this minimum height all types of defect are acceptable.
In 1998, the Dutch Quality, Inspection and NDT Surveillance society, KINT completed the TOFD acceptance criteria project. This project was sponsored by several major Dutch Industrial partners in the Oil and Gas, Chemical and other industries.
These acceptance criteria are based on good workmanship and details of the project are available on request.
The acceptance criteria have been tested both in a fracture mechanics model, with comparison of the results of parallel tests with both conventional techniques and TOFD in the field.
The objectives within the study to develop the criteria were:
1) To obtain similar rejection rates as with the conventional NDT techniques.
2) To obtain an equal or better quality of construction welds.
Rejection rates were proven not be significantly higher than when conventional techniques are used and the integrity of the construction is always higher than when conventional techniques are used.
A drawback of the use of these acceptance criteria is that they have only officially accepted by the Dutch authorised inspector, Stoomwezen BV but they will be proposed as European Acceptance criteria soon."
: Dear Friends,
: I'm working on TOFD method for more than one year.
: I'm searching for it's Accptance Criteria.
: If anybody knows about it, please inform me.
: Best regards,
: Shahriyar Dilfanian