Internal Inspection of Tubes
Furnace tubes are internally inspected from the ends for evidence of coke adherence or fouling or for reamer damage. Excessive thinning of the rolled section results in roll relaxation and header leakage. A light held at one end of the tube illuminates the tube very well for an observer at the other end. Inspect all tube ends for corrosion and cracks on the end and in the rolled section. Check for excessive bulging behind the rolled section and flaking or chipping in the rolled section. Both indicate excessive rolling of the tube. Where tubes are sealwelded into headers, inspect the seal weld for cracks and porosity.
Tubes may be internally gaged to establish the maximum inside diameter by using the Shaler Gauge (sometimes called the El Segundo or through Gauge). The wall thickness may be calculated by the formula:
W1 = W0 – E (2 W0 – D)
W1 = Final Wall Thickness (inches)
W0 = Original Wall Thickness (inches)
E = Eccentricity as percent of total loss occurring on one wall. A
chart is usually available for each furnace.
D = Gaged OD – ID (inches)
Tube wall thicknesses may also be established by the use of an internal ultrasonic tube gage or by external ultrasonic gaging.
The eccentricity factor may be calculated using the formula:
E = (t – t1) /[2 t – (t1 + t2)]
t = Orig Wall (inches)
t1 = Measured Min Wall (inches)
t2 = Measured Wall Thickness Opposite t1 (inches)