Pagina di dettaglio dell'applicazione
Measuring Internal Oxide Scale in Boiler Tubes
 Application: Measurement of thickness of internal oxide scale in steam boiler tubing.
Recommended instruments: Model
37DL PLUS thickness gage with oxide measurement software option,
EPOCH XT or
Epoch LTC flaw detector, or
EPOCH 4/EPOCH4 PLUS flaw detector with wave analysis software option
Background: The very high temperatures found inside steam boilers (in excess of 1500 degrees Fahrenheit or 800 degrees Celsius) can cause the formation of a specific type of hard, brittle iron oxide called magnetite on the inside and outside surfaces of steel boiler tubing. At very high temperatures, water vapor will react with the iron in the steel to form magnetite and hydrogen according to the formula:
3 Fe + 4 H
20 = Fe
3O
4 + 4 H
2
The speed of this reaction increases with temperature. Oxygen atoms will diffuse inward through the magnetite layer, and iron atoms will diffuse outward, so the scale continues to grow even after the tube surface is completely covered.
Magnetite scale acts as thermal insulation on the pipe, since the thermal conductivity of scale is only about 5% that of steel. When heat can no longer transfer efficiently from the flame through the tube into the steam inside, the tube wall will heat up to temperatures beyond the intended operating range. Long term exposure to overly high temperatures, combined with the very high pressure inside the tube, leads to intergranular micro-cracking in the metal and to creep deformation
(a slow swelling or bulging of the metal), which in turn eventually leads to tube failure by bursting.
A secondary issue is oxide exfoliation, in which pieces of oxide scale break off (usually due to thermal stresses during boiler startup or shutdown). These hard pieces will be carried by the steam flow into the turbine, where over time they will cause erosion damage.
The growth of magnetite scale and the associated metal damage are primary limiting factors with respect to boiler tube service life. The process begins slowly and then accelerates, for as the scale grows thicker the tube wall becomes hotter, and that in turn increases the rate of both scale growth and metal damage. Studies in the power generation industry have indicated that the effect of scale is relatively insignificant up to thicknesses of approximately 0.012" or 0.3 mm,
but that beyond that thickness the negative effects of scale increase rapidly. Periodic measurement of scale thickness allows a plant operator to estimate remaining tube service life and to identify and replace tubes that are approaching the failure point. Ultrasonic testing provides a quick and nondestructive method for measuring scale. Hand-held, portable instruments such as the Model 37DL PLUS
thickness gage or the EPOCH series flaw detectors can measure internal oxide layers down to a minimum thickness of approximately 0.125 mm or 0.005 inch.
Typical internal oxide measurement with 37DL PLUS thickness gage and M2017 delay line transducer (zoomed)
Typical internal oxide measurement with EPOCH XT flaw detector and M2017 delay line transducer, showing delay line echo at left of screen For specific transducer recommendations and instrument setup information, contact Panametrics-NDT, a business of Olympus NDT Inc at 781-899-2740
Further detailed information on the subject of oxide scale and its effects on boiler tubing may be found in the book
Metallurgical Failures in Fossil Fired Boilers by David N. French (John Wiley & Sons, 1983). |
Products used for this application
Epoch 4 | The standard-featured EPOCH 4 flaw detector offers great measurement capabilities and software options such as the fully encoded B-scan. |   |
37DL PLUS | Advanced Panametrics-NDT corrosion gauge with large A-scan display and sophisticated alphanumeric data logger. Features include Thru-Coat, Echo-to-Echo, B-scan acquisition, Emat, and optional Oxide/Scale measurement. |  |
EPOCH LT | A truly handheld, affordable flaw detector with many standard features and software options. This Panametrics-NDT unit is only 1.5 inch thin (38 mm). |  |
EPOCH 4PLUS | The advanced EPOCH 4PLUS with color LCD offers more measurement features and software options than any other Panametrics-NDT flaw detector. |  |
EPOCH 4B | Combines low cost and many practical measurement feature. This Panametrics-NDT flaw detector offers some software options. |  |
EPOCH XT | The Panametrics- NDT flaw detector offers EN-12668 compliance, multi-color LCD, multiple battery options, host and client USB ports. Dynamic DAC/TVG is standard |  |
EPOCH LTC | Rugged handheld flaw detector: IP67 and EN-12668 compliant, weighs only 2.12 lbs (0.96 kg), full color VGA display, USB On-the-Go Port for PC. Dynamic DAC/TVG is standard |  |
|
|
|