What is Ultrasonic (UT) thickness gauging?

What is UT Gauging?

UT (Ultrasonic Testing) gauging is a NDT Technique used for measuring the thickness of material (plate, pipe, forging or casting or any other material shape) using the contact pulse-echo technique at temperatures not to exceed 93C [200F]. UT gauging technique is used a lot in oil & gas sector where corrosion mapping of piping’s, vessels and other equipment is part of inspection plan on regular intervals.

thickness gauging 1 What is Ultrasonic (UT) thickness gauging?

How does ultrasonic thickness gauge work?

Ultrasonic thickness gauging applies the principle of UT contact pulse-echo technique. A sound beam is sent to the material to be tested and this sound beam is reflected back from the other side of the material. The time taken by the sound to travel is calculated by the gauging machine and converted to the distance ( in millimeter or inches). Conventional Ultrasonic Testing machine can also be used for UT gauging with a 90 degree probe instead of using a UT gauging equipment as shown in above figure.

UT principle What is Ultrasonic (UT) thickness gauging?

What is the minimum thickness limit for manual UT?

Generally, by considering standard probes a minimum nominal thickness of 6mm and pipe diameters of above 2” NB is recommended to be inspected using the UT gauging or ultrasonic testing. Before starting the work, calibration of UT gauging equipment is mandatory to ensure the inspection or check of the actual material thickness used on the job. Any wrong data can lead to component failure in-service which could have a lethal substance or liquid or vapors.

How accurate is ultrasonic thickness measurement?

The accuracy of thickness measurement on clean surface materials can achieve high accuracies but this is not the case for in-service materials/fixed equipment’s etc. like live pipelines, vessels, columns, heat exchangers in oil and gas industries.

List of the affected  factors can be considered to achieve highest accuracy is

  1. Test surface condition
  2. Reflecting surface condition
  3. Flaw reflecting face morphology
  4. Temperature of the component
  5. Velocity  of the component
  6. Coupling medium to suit the test surface conditions.

In addition to above  the accuracy of the thickness will depend on  accurate calibration, probe type and size, resolution capability of the flaw detector(equipment).

Can we do UT gauging below 6mm?

Yes, under certain situations UT for material thickness less than 6 mm can be performed. In such scenarios,  UT probe choice is very important which should evaluate to decide its thickness measurement abilities, delay line and surface resolution of the probes also play significant role as per job requirements.

UT gauging procedure

Following are the simple steps in UT gauging procedure based on the component thickness, diameter, surface condition etc.

  1. Select right probe , cable, equipment, and standard calibration block.
  2. Do all the connections and switch on the equipment.
  3. Set the initial echo to zero.
  4. Select right technique like single echo, single echo delay line or multiple echoes.
  5. Do the range calibration on the standard calibration block.
  6.  Check resolution, sensitivity , horizontal linearity, and vertical linearity.
  7. Select appropriate couplant as per surface conditions.
  8. Apply couplant on the test locations and start scanning the surface and record the readings.
  9. Prepare the report.

UT Gauging report

UT gauging report should include the location map for gauging. For piping, it shall have the piping sketch, gauging locations and direction as North, south, east & west. Additional direction can also be given.

UT Gauging standards

  1. ASTM E797 and
  2. EN 14127

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