Varying Reference Gain in PAUT Set-up Files Impacts Indication Assessment

Varying Reference Gain in PAUT Set-up Files Impacts Indication Assessment

#PAUT  #Calibration  #Ultrasonic testing  #Phased Array Ultrasonic Testing  #Reference gain

 Key Learning Point: 

“Using a changed Set-up file (Reference gain) affects the size of the surface indication and may cause confusion in equipment monitoring result.” 

Background:

     Phased Array Ultrasonic Testing (PAUT) to detect discontinuities in weld and base metal of the Deaerator head to shell section, which is made of stainless steel with a thickness of 8 mm. Due to the geometry constraints of the equipment, a small PAUT probe was required for the inspection, as shown in Figure 1 

Figure 1 PAUT probe footprint size [1] 

The PAUT probe with small footprint used have 3 types as follows:

1. 10L32A10 : (High frequency type) This type of inspection probe is not suitable for stainless steel materials inspection because it is very difficult to distinguish signals of coarse grain material and small defect.

2. 2.5L32A10 : (Low frequency type) The frequency range of this type is too low, results in low sensitivity of the monitoring signal.

3. 7.5CCEV35-A15 : This type of inspection probe designed for small defect inspection.

Based on the PAUT Probe list provided by Contractor A, Contractor A has chosen to use the 7.5CCEV35-A15 for inspections. This probe is specifically designed for detecting small defect, making it suitable for detecting small indication with internal surface of equipment. However, if the operator sets the equipment incorrectly and uses excessive gain during the setup, it could lead to a situation where the operator mistakenly interprets the signal as a defect within the equipment or assumes that the defect is growth.

In the first inspection, the operator from Contractor A detected many surface indications.

In the second inspection, the operator from Contractor A found that some of the surface indications had growth. The operator believed that these surface indications had growth. From further investigation by conducting an internal inspection, it was found that the surface indications were actually grinding marks inside the equipment as figure 2. 

Figure 2 The image reveals the grinding marks inside the equipment. 

After this incident, the team conducted an investigation to determine the cause and found that there were differences in the software setup between the first and second inspections. 

The difference reference gain is due to different calibration file setup, which means that the setup files used are not the same.

What are the effects of different Reference gain values?

1. Small surface indications become larger (as shown in Image 3).

2. Non relevant small surface indications become clearly surface indication (as shown in Image 4). 

Figure 3 Changes in the reference gain increased size of the surface indication.

Figure 4 Changes in the reference gain cause signals from small grinding marks to appear as indications.

Lesson learned:

“Check setup file and information before inspection and use the previous setup data, or verify the data before inspection.”

1. The setup file must be the same as the previous setup for monitoring inspections. If there are any changes, the setup file must be verified before inspection.

     Setup verification step:

·      Every time the PAUT operation is performed, the PAUT operator must have the previous PAUT reports before performing the operation.

·      Every time the PAUT operation is performed, the PAUT operator must have the previous technique sheet document before performing the operation.

·      The monitoring inspection report should include both the previous PAUT setup and the current PAUT setup for comparison and investigation result. 

Figure 5 Example of information in technique sheet document. 

2. Personnel performing SCC by Phased Array Ultrasonic Testing must be PASS qualified the SCC course from GCME Qualification Center.