The most common nondestructive tests used in boiler inspections include the following:
1. Magnetic particle testing (MT)
is used to detect surface and nearsurface flaws by observing any discontinuity that shows in the pattern of iron particles that are applied to the test piece. Because a magnetic field is used on the test piece, these tests are applicable only to ferrous materials. The shape of the test piece also determines whether it can be MT tested.
2. Liquid penetrant testing (PT)
also detects surface cracking in a component, and it's not dependent on the magnetic property of the material or its shape. The PT test detects surface flaws by capillary action of the liquid dye penetrant, and it is only effective where the discontinuity is open to the component surface.
3. Ultrasonic testing (UT)
is used extensively for measurement of the wall thickness of tubes. The loss of material due to corrosion and erosion is a primary cause of failure of pressure parts. In many areas of the boiler where corrosion and erosion are expected, a baseline UT test is conducted. In subsequent tests, these test results are compared with the baseline information, and the life of the component can be projected. Therefore, at planned outages, tubes having a high rate of wear can be replaced, which prevents an unscheduled outage due to tube failure.
4. Radiography or x-ray testing (RT)
is the most important nondestructive test during the field erection of a boiler. It is also used in assessing the condition of pressure parts such as piping. The major disadvantage of radiography is excessive exposure to radioactive rays and the potential harmful effects to personnel. Protective procedures must be used to minimize any potential effect.
Extract from STEAM PLANT OPERATION 8TH EDITION
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