Table of Contents ECNDT '98 Session: NDT of Welds

NUMEX SOFTWARE FOR USEABLE FILM CALCULATION IN DOUBLE WALL GIRTH WELD RADIOGRAPHY M.B. Hoppenbrouwers, Röntgen Technische Dienst bv, P.O. Box 10065, 3004 AB ROTTERDAM, THE NETHERLANDS, telephone +31 10 2088208, fax +31 10 4158022, e-mail rtd@rtd.nl, internet: http://www.rtd.nl

TABLE OF CONTENTS

Abstract Introduction RADIOGRAPHIC TECHNIQUES DOUBLE WALL TECHNIQUE CALCULATION METHOD NUMEX FUNCTIONS CALCULATION EXAMPLE ENQUIRIES

ABSTRACT

Radiographic inspection procedures for circumferential girth welds in pipeline are stipulated in numerous national and international standards. Interpretation of these codes lead to mutual differences in inspection procedures between NDT service companies, their clients and the accrediting organisations. To standardise interpretation of the codes, the Numex software package was developed. Numex calculates the number of exposures and other relevant radiographic parameters subject to the applicable code. Inspection quality and personnel skills will benefit from this development. Since 1994, inspection companies in the Netherlands use the Numex software for inspection planning and technical support. It has been accepted as a national industry standard and is expanded with international codes to support internationally operating companies. The software has been updated to operate under Windows 95 to promote a more widespread use.

INTRODUCTION

Radiographic procedures for inspection of circumferential girth welds in pipelines are prescribed in several national and international codes. Calculation of radiographic parameters, such as the required number of exposures per weld, using a specific radiographic technique, can be a laborious task. Non-uniform interpretation of codes gave rise to differences between NDT companies and differing procedures between NDT companies.

Automation provides a practical aid to standardise the calculation work. Numex (Number of Exposures) software has been developed with the intention of internal use, but shortly after its introduction, it was accepted as a national standard and was spread amongst international branches and colleague NDT companies. Recently, the Numex programme has been issued in a MS Windows 95 version and it has been expanded with various European standards. It offers NDT companies, their clients and the accrediting organisations certainty that correct procedures have been followed.

STANDARDS AND PROCEDURES

NUMEX includes international standards ASME V, BS2910, DIN54111 A/B , EN444 and Dutch T0201.

RADIOGRAPHIC TECHNIQUES

Every detailed double wall technique, described in a radiographic standard, is a compromise between various radiographic parameters and an acceptably short exposure time. Relevant parameters are focal spot/source size, kV value, object thickness, focus/source film distance, number of exposures, film type and film density. All parameters are input in the Numex program and the number of exposures according to the applicable code is calculated.

DOUBLE WALL TECHNIQUE

The double wall technique uses an external X-ray/gamma source to obtain a single or double wall image on a film attached to the other side of the pipe (figures 1 and 2).

Figure 1 Film and source of radiation outside: double wall, double image	Figure 2 Film and source of radiation outside: double wall, double image

Numex will produce an error message, when incorrect examination parameters are input. Errors in procedures or inspection planning will be signalled in an early stage and adequate measures can be taken.
Relevant technical parameters are presented to assist the radiographer in following the right procedures. This will have a positive effect on technical abilities and personnel skills.

CALCULATION METHOD

In the codes DIN54111 A/B, BS2910, EN444 and T0201, the number of exposures is calculated, based on two criteria:

Figure 3 Increase in penetrated wall thickness under varying irradiation angle

Maximum increase of penetrated wall thickness:
DIN54111 A	maximum increase 20%
DIN54111 B	maximum increase 10%
EN444 A	maximum increase 20%
EN444 B	maximum increase 10%
T0201	maximum increase 20%
BS2910	maximum increase 10%
Minimum film-focus-distance (FFD):
DIN54111 A	minimum FFD = 7.5 * s * d2/3 + d
DIN54111 B	minimum FFD = 15 * s * d2/3 + d
EN 444 A	minimum FFD = 7.5 * s * d2/3 + d
EN 444 B	minimum FFD = 15 * s * d2/3 + d
T0201	minimum FFD = 7.5 * s * d2/3 + d
where:	s = focal spot / source size, d = nominal wall thickness

The British Standard BS 2910 accepts a different approach. In this code, no minimum or maximum values are prescribed, but an optimum technique is recommended. Formulae are given for a number of cases, where there is discrimination between:

• double wall, single image or double wall, double image techniques
• X- / gamma- ray sources
• wall thickness below 20 mm and above
• fine grain and medium speed film
• for isotopes 'end on' and 'side on' application

The allowed deviations from these optimum values are not prescribed but subject to agreement between contracting parties. In BS 2910 it is written 'The basic philosophy of weld radiography is to recommend a technique that gives the best possible chance of detecting small planar flaws such as cracks.' (p. 4). If this 'standard' is handled strictly, this may lead to more stringent rules than other codes. In a more lenient approach, however, differences in radiographic examination procedures may occur in practice. This would create a situation we are trying to remedy with the Numex programme. In the ASME V code, the number of exposures is determined empirically, based on two criteria:

Film density:
for isotope sources	film density between1.8 - 4.0
for X-ray	film density between2.0 - 4.0
Minimum film-focus-distance (FFD):
WT < 50.8 mm	minimum FFD = (2.0 * s + 1) * d
50.8 mm ( WT ( 76.2 mm	minimum FFD = (1.3 * s + 1) * d
where:	s = focal spot / source size d = nominal wall thickness

The ASME V code contains ambivalence in the calculation of the number of exposures using the film density criterion. Allowances in film sensitivity and film contrast hamper exact calculation of the required number of exposures. This standard, also leaves space for the contracting parties to use the standard to their advantage.

NUMEX FUNCTIONS

Numex calculates a number of relevant parameters, which may be of help in radiographic inspection procedures.

Numex asks the user to enter the following input parameters:

Inspection code:	ASME V, BS2910, EN444, DIN54111 A/B or Dutch T0201
Pipe size:	Outer diameter and wall thickness in schedule size, millimetres or inches.
Source:	X-ray / Iridium-192 / Cobalt-60 source
Technique:	Double wall single image or double wall double image
Film type:	Film class G1 to G4 (C1 to C6 in the EN444)
Film-focus-distance:	variable

Numex performs a check to verify if the selected combination of input parameters is conforming the selected inspection code. Numex calculates the following output parameters:

• Number of exposures for each pipe size
• Useable film length
• Exposure time in Ci*min for Iridium-192 and Cobalt-60
• A table of exposure numbers for standard pipe sizes

Figure 4 : Numex command window with general operating functions

Figure 5 : Output window with calculation results

Figure 6 : Table format output with number of exposures for combinations of schedule diameters and wall thickness

Additional calculation results are:

• Non-rounded number of exposures with corresponding useable film length to estimate the margins in the inspection set-up
• Opening angle of the X- / gamma-ray beam according to the selected inspection code. This is used as a check whether or not the used X-ray tube or gamma camera with collimators is accepted in the codes.
• An error message in misuse of the code. The programme checks against the following criteria:
  • Allowed inspection technique
  • Limit to the penetrated wall thickness
  • Minimum film-focus-distance
  • Criterion for normal penetrated thickness
  • Allowed film type

CALCULATION EXAMPLE

The operation of Numex is demonstrated with a simple example. At the start of the programme, the command window is opened (see figure 4). A new file may be opened or an existing file imported. One of the languages English, German or Dutch is selected. The input parameters may be entered in general measures (mm, inch) or schedule sizes. At the selection of one of the input buttons, the input /output window pops up (see figure 5). After the inspection parameters are entered, the calculate command may be executed. If the correct values are entered, the calculated values are shown in the output window. Precise and rounded number of exposures with the accompanying film length and opening angle are given. In case the selected values give an invalid result according to the selected norm, a warning message will be displayed. Additional remarks, which can be of help in the inspection are shown in the remarks window. A report can be generated, containing client information and the calculated number of exposures for the given set-up. Standard save and printer functions are included in this window.

In practice, it is considered useful to generate a table of schedule sizes and wall thickness with the respective number of exposures for a certain technique. These tables may be consulted instead of running Numex for each individual inspection order. The table option is incorporated in Numex. An example of the table output is shown in figure 6. Not allowed combinations with the selected technique are left blank. The number of exposures for valid combinations are given in round numbers.

ENQUIRIES

Further information on Numex software and a free copy may be obtained at the Technical Advice and Support Department of Röntgen Technische Dienst bv, Rotterdam or of a local branch office.