Phased array and TOFD in lieu of RT for pre- and in-service inspections

15/03/2013

As the restrictions in the use of industrial radiography (RT) have increased over the years, the need for alternative weld inspection methods and techniques, such as time-of-flight diffraction (TOFD) and phased array (PA), has become more evident.

In this article, Norbert Trimborn, Strategic Business Leader, Advanced NDT at SGS, discusses the replacement of RT by PA and TOFD. Information is given about the inspection requirements for pre-service and in-service inspections. Based on these requirements, SGS has developed a PA inspection system for the inspection of boiler tube welds. The advantages and disadvantages of this system are discussed by giving information about general advantages, combined with an overview of the differences in the detection and sizing of different types of defect.

Generally, new developments in NDT are driven by improved performance of the new technique and cost reduction.

For pre-service inspection, cost reduction is generally given more consideration than improved performance, the rationale being that inspections are acceptable as long as welding satisfies the requirements of the applicable codes and standards.

For in-service inspection, the performance and accuracy of the inspection is considered more critical, as the risk of failure between shutdowns should be as small as possible, therefore requiring a higher probability of detection and more accurate sizing of the indication.

When phased array and time-of-flight diffraction techniques are

performed correctly, the quality of performance is superior when compared with radiography. This means that the confidence level of the structural integrity of products inspected with TOFD or PA is higher than for those inspected using radiography. The inspection costs of PA and/or TOFD are typically higher than the costs of conventional RT. However, because dangerous radiation is not a factor when using the advanced techniques, there can be huge savings associated with the avoidance of the disruption of works within the area. In addition, the speed of PA and TOFD shortens the overall inspection time and also eliminates the lost time associated with RT. An inherent factor of using a more accurate inspection technique is the higher repair rate due to the more stringent acceptance criteria of PA and TOFD. Increased confidence in the integrity of the product offsets the relatively small rise in repairs, thereby decreasing any significant influence to total costs.

It is important to note that pre-service inspections are not intended to detect all defects that may lead to failure. This is an impossible reality, as no NDT technique has a 100% detection rate. In order to test at 100% accuracy, each component would require individual acceptance criteria based purely on its unique ‘fit for service’ acceptability, as operation conditions, material, alloys and other factors vary.

For in-service inspection, however, a higher detection rate is necessary in order to prevent failure prior to the next scheduled shutdown. This can only be realised when supplementary NDT techniques are used in parallel.

In-service inspection places less importance on normal acceptance criteria based on the size of a detectable indication. When critical defects are detected in such an inspection, the remaining lifetime of a component must be calculated. This remaining lifetime calculation is the main reason for this type of inspection, as it is not intended for the detection of weld defects unreported during the initial NDT inspection.

Pre-service inspection requires acceptance criteria to confirm that welds are meeting the requirements of the relevant codes and standards and that the workmanship is acceptable. Here we have several options:

Use ASME acceptance criteria, for example CC2235-9 for pressure piping and vessels. The problem with this is that the acceptance criteria were developed in a wall thickness range starting at 12.4 mm, although technically it is no problem to use TOFD and PA from 4 mm and upward.

Use European codes and standards. Here, the minimum wall thickness is 6 mm.

Define your own acceptance criteria. These may be based on: the confirmation of acceptable workmanship; the determination of fitness for purpose of the product.

SGS has developed a weld inspection system using phased array for boiler tube welds.

The system has the following features:

It covers pipes from 21.3 mm (0.84 in) to 114.3 mm (4.5 in) OD;
It operates within 13 mm (0.5 in) clearance (on all standard pipes), permitting inspections in limited access areas;
It can hold two phased array probes for complete weld coverage in one pass;
It can be configured to make one-sided inspections for pipe-to-component evaluations;
The design provides stable and constant pressure around the full circumference of the pipe;
It can be manipulated from one side of a pipe;
The spring-loaded scanner can be used on ferromagnetic and non-ferromagnetic pipes;
It has an encoder resolution of 32 steps/mm;
There are two encoders on the scanner.

System benefits include:

No health & safety implications with using ionising radiation;
Other work in the vicinity need not be suspended during inspection;
The elimination of radiation protection measures;
An equal or higher reliability of examination in comparison to radiography;
High cost savings;
100% coverage of the weld;
Fast inspection is carried out in near real time;
It is possible to characterise and size many defects;
Two-dimensional sizing;
Direct assessment after the test;
Possible to produce both hard- and soft-copy results.

Drawbacks include:

Tube wall thickness has to be >4.5 mm for detection and characterisation of defects and bore chamfering must be handled with care;
It requires operators skilled in the phased array technique and these are not always readily available;
It does not quite match radiography in the detection and characterisation of inclusions and gas pores.

A comparison with RT in this particular application produced the overview presented in the Table below.

In conclusion:

PA dramatically reduces health & safety implications;
PA offers advantages in saving time and money;
The advantages and limitations of each technique have been presented;
Radiography offers benefits for the detection of inclusions and porosity;
PA can detect all defect types and characterise them in accordance with acceptance criteria.

Table 1. Comparison between inspection performances of PA and RT for a boiler tube inspection

	Phased array		Radiography
	Detectability	Characterisation	Detectability	Characterisation
Porosity	Acceptable	Acceptable	Good	Good
Inclusions	Acceptable	Acceptable	Good	Good
Lack of root fusion	Good	Good	Poor	Poor
Lack of side-wall fusion	Good	Good	Poor	Poor
Weld toe crack	Good	Good	Acceptable	Good
Excess penetration	Acceptable	Acceptable	Good	Good
Wormhole/piping	Acceptable	Acceptable	Good	Good
Weld root crack	Good	Good	Acceptable	Acceptable
Root concavity	Good	Acceptable	Acceptable	Acceptable
Lack of root penetration	Good	Good	Good	Good

www.sgs.com/ndt

Note: The views expressed in this article are those of the author. The British Institute of NDT does not guarantee the completeness, timeliness or accuracy of the information presented and shall not be liable for any loss or injury arising from its publication. Healthy discussion is encouraged and letters should be emailed to ndtnews@bindt.org

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