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보일러 튜브의 내부 산화 스케일 측정


The very high temperatures found inside steam boilers (in excess of 1500 °F (800 °C) 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; as the scale grows thicker, the tube wall becomes hotter and that, in turn, increases the rate of 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 in. or 0.3 mm, but that beyond that thickness the negative effects of scale increase rapidly. Periodic measurement of scale thickness enables 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. Handheld, portable instruments, such as the 38DL PLUS® thickness gage or the EPOCH® series flaw detectors, can measure internal oxide layers down to a minimum thickness of approximately 0.2 mm or 0.008 in. using the commonly recommended M2017 transducer (20 MHz delay line). The specialized M2091 transducer, which is a 20 MHz shear wave probe, can be used along with shear wave couplant to measure down to approximately 0.152 mm or 0.006 in. In all cases, the coupling surface must be smooth and, in some cases, surface preparation will be required.

oxide scale 38DL PLUS screen
Typical internal oxide measurement with thickness gage and M2017 delay line transducer (zoomed)
Oxide Set-up with EPOCH 650
Typical internal oxide measurement with a flaw detector and M2017 delay line transducer, showing delay line echo at left of screen

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).

Olympus IMS

EPOCH 6LT 휴대용 초음파 결함 탐상기는 한 손 작업에 최적화되어 있으며 로프 접근 및 고소 작업이 필요한 고휴대성 응용 분야에서 탁월한 성능을 제공합니다. 가볍고 인체 공학적 디자인으로 사용자의 손에 단단히 고정시키나 로프 접근이 필요한 응용 분야에서는 다리에 묶을 수 있습니다.
EPOCH 650은 다양한 응용 분야에 대한 우수한 검사 성능과 유용성을 갖춘 재래식 초음파 결함 검출기입니다. 이 직관적이고 견고한 장비는 인기 있는 EPOCH 600 결함 탐상기 후속작이며 추가 기능을 갖추고 있습니다.
38DL PLUS는 고급 초음파 두께 측정기입니다. 내부 부식 확인 용도의 이중 요소 탐촉자를 사용하며, THRU-COAT 기술 및 에코 대 에코를 비롯한 기능들을 갖추고 있습니다. 단일 요소 변환기를 사용하여 매우 얇거나 매우 두꺼운 또는 다층 재료의 두께를 매우 정확하게 측정할 수 있습니다.
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