Matters needing attention in the overall inspectio

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Matters needing attention in the overall inspection of cast iron dryers

cast iron dryers have good wear resistance, simple manufacturing process and low cost. They are widely used in the drying and finalization of paper in the paper industry. The main material of the drying cylinder is grey cast iron. Because of its low plasticity and toughness, grey cast iron shows brittle fracture in the event of an accident, causing great harm. Full attention should be paid to this feature during inspection

due to the characteristics of gray cast iron, it is difficult to use relevant testing instruments when inspecting cast iron dryers. Based on years of inspection, I believe that macro inspection and hydrostatic test should be mainly carried out

(1) inspection safety

before entering the dryer for inspection, the steam source should be cut off and the steam inlet pipeline should be separated with a metal blind plate; Cut off the power supply and remove the power fuse. When the inspectors enter the dryer for inspection, there should be a specially assigned person outside. When grinding wheel grinding and magnetic particle testing are carried out, anti electric shock protector shall be equipped to prevent electric shock. In the process of using the tensile testing machine

(2) macro inspection

it is very important to visually inspect the internal and external surfaces of the cylinder from 2009 to 2013. With the help of 5 ~ 10 times magnifying glass, we can focus on the discontinuity of the inner wall structure, the sudden change of shape, the casting riser area, and the parts with large thickness difference. At the same time, measure the arc radius of the transition area of the inner wall of the cylinder. Surface flaw detection is also required for suspected parts. Under special circumstances, radiographic flaw detection can also be used (such as leakage on the surface of the cylinder block)

the inspection of the inner surface of the drying cylinder is carried out before or after the hydrostatic test, and there are different methods. I think it should be carried out after the hydrostatic test. The reasons for this consideration are: ① the working part of the drying cylinder is covered by gears, frames and other components, which is not conducive to the observation during the pressure maintaining inspection of the hydrostatic test; ⑦ Sometimes the hydraulic test finds that the joint between the cylinder block and the cylinder head leaks along the edge of the cylinder head, so it is difficult to judge whether it is the seal gasket failure or the leakage caused by the cracking of the cylinder block boss; ③ During hydrostatic test, internal defects and surface defects may expand and crack along the inner wall; ④ The steam medium has little corrosion on the drying cylinder, so there is no need to worry about the failure of the drying cylinder under hydrostatic test caused by corrosion thinning

(3) thickness measurement

measure the thickness of cylinder block and cylinder head with ultrasonic thickness gauge. For some dryers, because of flake graphite, the thickness cannot be measured by instruments. We usually measure the average thickness by measuring the inner and outer diameters

(4) hardness measurement

zby91003-88 "technical conditions for drying cast iron cylinders for paper machinery" stipulates that the hardness of the cylinder surface should reach hb170 ~ 240 after the cylinder is processed. When the measured hardness value is low, metallographic examination can be considered, and the strength can be verified by converting the measured hardness into strength value. Ultrasonic hardness tester and Leeb hardness tester should not be used for hardness measurement. The indenter area of these two kinds of hardness testers is very small, which is easy to hit the soft spots of graphite and loses the reliability of data. Hammer hardness tester and rebound hardness tester with large indenter area should be used

(5) strength check

the check of the cylinder body can refer to the calculation formula of the internal pressure cylinder in GB150-1998 steel pressure vessels. The cylinder head can be checked with reference to Japanese specifications, which stipulates that the minimum wall thickness of a part of spherical convex unsupported compression cast iron head must be more than the minimum wall thickness of the head as a concave compression, and must be more than 1/1000 of the inner diameter of the head flange. This provision is based on the fact that the convex head will produce compressive stress due to external pressure, and will produce large bending stress before buckling. However, because cast iron has high performance of compressive shrinkage and bending stress, the same wall thickness can be used for convex compression and concave compression, but the smaller the thickness ratio, the more prone to buckling, so a certain thickness diameter ratio limit is specified

when checking the strength of cylinder head, the calculation formula of internal pressure dished head with flange can be directly used

(6) hydrostatic test

because the internal defects left during the manufacture of drying cylinders are difficult to be detected by conventional inspection methods, it is advisable to conduct a hydrostatic test every inspection cycle. The test pressure is twice the maximum working pressure. The water inlet and pressurization of hydrostatic test can be connected from the steam inlet pipeline; Exhaust from the exhaust hole of the cylinder head on the other side (the exhaust hole is set at the connection between the cylinder block and the cylinder head. During manufacturing, the general auto parts testing machine automatically analyzes and processes the test results, which have been marked with a steel seal on this part, and the outside is sealed with bolts); The condensate discharge day (at the drain valve) should be sealed. For the hydraulic test of multi cylinder paper machine, water can be injected and pressurized from the system pipeline, and multiple drying cylinders can be carried out together, which can improve the work efficiency

(7) during the inspection of safety accessories

it is also necessary to check whether there are safety valves, pressure reducing valves and pressure gauges on the pipe line at the steam inlet end, whether there are steam traps at the drainage, and their calibration and maintenance

(8) defect treatment

shallow cracks and porosity on the surface can be polished and rechecked by surface flaw detection. For deep crack defects that can be sterilized by gamma ray and ETO, radiographic inspection shall be carried out at the extension end to find out the tip. The defects with small size can be riveted with the same material. In addition, the methods of drilling, tapping and threading plug specified in ASME VIII-1 can be used for reference. Cracks and looseness are not allowed in the circular arc transition area of the cylinder block and the flange at the connection between the cylinder block and the cylinder head. In short, when dealing with defects, we should carefully analyze the causes of defects and carefully consider the treatment plan. Repair welding is not allowed because the weldability of gray cast iron is very poor

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