In ultrasonic testing, high-frequency sound waves are transmitted into a material to detect imperfections in material properties. The range has been found to be between 10 Hz to 20 Hz frequency Sound waves above 20 KHz are commonly referred to as Ultrasonic. The most commonly used ultrasonic technique is pulse echo, whereby sound is introduced into a test object and reflection(echos) from internal imperfections or the part’s geometrical surfaces are returned to a receiver. The Ultrasound waves are generated by piezoelectric transducers, which convert electric oscillations into mechanical vibrations and vice-versa. A short brust of ultrasound is introduced into test material so some or all of the energy is reflected by discontinuities. The reflection of the ultrasound energy is a function of the ratio between the acoustic impedance of the discontinuity and the base material. The greater the impedance ratio the more sound energy will be reflected. The principal of ultrasonic testing is operates on the principal of time-of-flight measurement. This principal utilizes the precise timing of the transit time of a short brust of ultrasound energy, through a material under test. The ultrasound waves travel to the far side of the test piece and reflect back to the transducer/probe and a measurement is obtained. Basically, three types of transducers/probe are available for different types of applications:
- Straight beam probe (normal probe)
- TR probe (Dual crystal probe)
- Angle beam probe
As the wave travels through the material, it may get reflected, refracted, scattered or transmitted depending upon the condition within the material. From a defective location, the waves get either reflected or attenuated. These signals are picked up by the transducer and recorded for display as time pulse-height-pattern on a screen of a cathode ray tube (CRT). Spacing between pulses and height of pulses are interpreted in terms of relative location and size of the flaw in the material.
Three important techniques of Ultrasonic Testing are:
- Pulse-echo Technique
- Through Transmission Technique
- Resonance Technique
In the field of Ultrasonic Testing, interface is referred to a surface joining two mediums (materials) of different acoustic properties. Whenever ultrasonic waves strike at an interface between two materials, it undergoes reflection, transmission and/or mode conversion depending upon the angle of incidence and Acoustic impedance.
OXIDE SCALE MEASUREMENT
Superheater tubes carrying steam in fossil – fired boilers operating at temperature above 900º F (482º C) are subject to failure by creep – rupture. These tubes from an internal oxide layer that inhibits heat transfer through the wall and causes the tube metal temperature to increase over time. This test method describe how oxide scale thickness measurement can be used to assess the temperature history of Super Heater Tubes.
MEASUREMENT OF STEAM SIDE OXIDE SCALE GROWTH
During service an oxide layer will grow on the inner surface of the boiler tubes. The growing oxide layer acts as a thermal barrier and consequently temperature of the tube material will rise even if the surrounding flue gas temperature remains constant. This oxide induced temperature rise will reduce the life of the tube due to overheating. The thickness of this oxide scale can be used to estimate the remaining creep – rupture life of a tube when seen in conjunction with the data from the operating parameter of the unit and the wall thickness measurement. This calculation assumes that the oxide scale deposited is proportional to the total time the tube is operational. The amount of oxide scale build-up increases the temperature in the tube and accelerates creep rupture failure. Periodic measurement of scale thickness allows a plant operator to imate remaining tube service life. This also assists in identification and replacement of tubes that are approaching the failure point.
The Oxide Thickness is known at the beginning and at the end of a service time interval, it is possible to imate the rise in temperature and thus evaluate the life time reduction due to this phenomenon. The high frequency ultrasonic testing method can be applied non – destructively to measure the steam side oxide scale thickness from the outside to a sufficient accuracy for the purpose of Remaining Life calculation of super heater tubes. The application of this method is described in detail below.
The data needed for these calculations are: -
- Length of the service period in hours from the start or most recent acid cleaning
- Oxide scale thickness in mm at the end of the service period
- Material thickness of the tube in mm
- Steam temperature / tube wall temperature
- Internal steam pressure