Inspection and Analysis of High Temperature Fasteners of Steam Turbine Sets in Power Plants

introduction

Turbine high temperature fasteners are suitable for cylinders, valves, valves and steam pipe flanges with working temperature (referred to as steam temperature) greater than 400 °C (including single-head threaded bolts, studs and equal-length double-headed screws) Column), nut and washer. High-temperature fasteners are generally made of alloy steel, which is subjected to tremendous stress and tightness during operation. The large-scale domestic cylinders are connected by bolts to the upper and lower cylinders to ensure that the cylinders are tight and leak-proof, ensuring high-temperature and high-pressure steam in the cylinders. acting. Therefore, it is of great practical significance to discuss the installation and maintenance process of high temperature fasteners.

1 High temperature fastener installation points

1.1 Cleaning and inspection. Carefully clean the bolts, nuts and washers, remove the anti-corrosion oil, and perform macro inspection on the bolts and nuts. The surface and thread parts are required to be smooth and smooth. There should be no dents, cracks, burrs and other stress-induced defects. After the completion, black lead powder should be applied to the surface of the part (note that excessive black lead powder should be blown off with compressed air) to prevent corrosion.

1.2 Spectroscopy, physical and chemical and metallographic examination. 1 Alloy steel parts shall be subjected to 100% spectral review, and the part of the spectrum analysis shall be selected at the end of the bolt. 2 pairs of bolts ≥ M32: a. Perform 100% ultrasonic flaw detection (magnetic powder, coloring and other methods may be used if necessary), and cracks and defects affecting strength shall not exist. In the ultrasonic inspection, the mark shall be marked on the corresponding position at both ends of the bolt, and the length of the bolt shall be measured by an ultrasonic instrument and recorded as the reference length of the creep measurement of the bolt; b. Perform a 100% hardness test, the inspection site is the bolt end face or the polished rod; c. Metallographic sampling of 20CrlMolVNbTiB bolts; d. Grain size and histomorphometry can also be considered if necessary. 3 Carry out other inspection items requested and entrusted by the owner.

1.3 Pairing number. In order to prevent thread seizure and reduce wear, the nut material should be lower than the bolt by 1 grade and the hardness value is low by HB20-50. In order to prevent the bolt or nut hardness difference caused by random pairing from being too large or too small, the bolts and nuts should be paired and numbered according to the hardness measurement value during installation. The number should be marked with a steel number on the bolts and nuts that are conspicuous and not easily damaged, and that are not subject to force and are not easily damaged.

1.4 Initial installation and measurement. Regulatory requirements: bolt threads should be screwed into the wire hole or nut, and the first wire is lower than the nut or the plane of the wire hole 0.5-1 mm; after the nut or wire bolt is screwed, the nut or There should be a 3 mm gap between the top of the wire hole and the end of the bolt. For this reason, the lengths of the wire holes of bolts, nuts, cylinders, valves, etc. should be carefully measured to calculate whether the gap meets the requirements. If it does not match, the bolt end face can be removed for a certain length (pay attention to restore the chamfer of the end after machining). After the measurement is completed, carefully check the thread of the wire hole to ensure that the thread is partially clean and free of defects such as burrs.

1.5 Inspection and grinding of the gasket. On modern large-scale units, high-temperature fasteners use spherical washers, which can greatly eliminate the additional bending stress of the bolts. During the installation process, the focus is on the bearing surface of the washers and nuts and the contact between the washers and the bearing surfaces of the cylinders/flanges. The main method is to use the red dan and the feeler gauge to check the gap. If the contact is found to be uneven, after the permission is obtained, the bearing surface of the grinding nut and the bearing surface of the cylinder and the flange are used. The gasket and nut should be checked multiple times during the inspection. If the contact deviation can be offset, the processing workload can be greatly reduced.

1.6 Installation. Before the final installation and tightening of the bolts, the bolts should be carefully inspected for the last time to remove debris and ash, especially between the threads. At the same time, carefully clean the holes on the cylinders and valves. All sundries and stagnant water. Bolts and nuts shall have no damage to performance and reliability. The bolts, nuts and washers shall be paired during installation. The bolt thread should be coated with molybdenum disulfide high temperature anti-seize agent, but the anti-seize agent should not be used for nickel-containing alloy steel bolts (copper-containing graphite lubricant can be used). Both cold and hot tightness should start from the maximum (central) of the vertical arc of the cylinder, and then proceed symmetrically to both ends. Since the tightening force of the bolts will be reduced after the adjacent bolts are tightened, the cold and hot tightness are both It should be tightened twice. When the bolt is hot tight, the heating position is the center hole of the bolt, avoiding direct heating of the thread and nut portion of the bolt.

2. Maintenance points for high temperature fasteners

2.1 Disassembly

For cylinder bolts, the wall temperature of the adjustment stage should be <80 °C, and the valve and pipe flange temperature <100 °C can be dismantled. The method of disassembling the nut is similar to the method of installing the nut. Inject the lubricant into the thread 4 hours in advance (bolster loosening agent or kerosene can be used). When the hot tightening bolt is removed, first use the electric heater to heat the bolt. After the bolt is extended, use the rigid wrench. Loosen the nut loosely and remove the nut. When the cold bolt is removed, use the appropriate torque to move the nut back and forth, and use a copper rod to knock, so that the nut is loosened and then removed to prevent the loose nut from being threaded due to threading. Pull or break the bolt. The bolts to be removed shall be stored according to the number, and the nuts, bolts and matching washers shall correspond one by one and be marked. The order of removing the bolts should be symmetrical to the two sides after selecting the part with the largest gap in the gap after referring to the previous installation and maintenance records, so as to prevent the other bolt threads from being subjected to excessive force after the bolts are removed. For the bolts of the cylinder and the valve joint surface, special attention should be paid to the process during the disassembly to prevent damage to the thread. First, spray the bolt loosening agent on the threaded joint surface of the cylinder, then tap the bolt end and the screw part with a copper hammer, and turn the bolt back and forth while tapping until the bolt can be easily unscrewed.

2.2 Cleaning and inspection

1 Use special custom-made taps to ream the nut thread and the thread of the planting hole to remove the burrs on the surface of the bolt and nut and the scale which may fall off; 2 Try to install all the nuts and bolts, it should be easy to rotate Swing out, for the bolts of the wire, the bolt verticality should be checked after the fitting; 3 the bearing surface of the nut, washer and cylinder/valve should be inspected carefully. If there is damage, it should be treated by the method of research or consideration. Replace the gasket spare parts; 4Measure the length of the bolt according to the creep measurement mark, make a record, as the initial value of the bolt fastening and the calculation basis of the creep elongation; 5 For the more difficult to disassemble, the steam leakage or abnormality in the middle facet The bolts should be inspected to determine if the bolts can continue to be used.

2.3 Spectroscopy, physicochemical and metallographic examination

1 Perform a spectral review of all alloy steel high temperature fasteners. 2 pairs of bolts ≥ M32: a. Perform 100% non-destructive testing; b. Perform a 100% hardness check (the inspection site is in the middle). 3 Metallographic examination of bolts with unacceptable hardness, and impact toughness check if necessary. 4 When monitoring the bolt life loss according to the amount of creep deformation, a representative bolt should be selected as the monitoring bolt or 20%, and the creep elongation should be detected during overhaul.

2.4 Repair

1 Thread damage, use triangle oil stone for cleaning and grinding, if necessary, machining. 2 The hardness is exceeded, the metallographic examination is carried out, and if necessary, the impact toughness is checked and the heat treatment method is used to recover. 3 Analyze the crack type. If it is only a surface crack, continue to use after grinding to eliminate the crack, otherwise it should be scrapped. 4 Creep value exceeded the standard, according to DL439-1991 "Technical Guidelines for High Temperature Fasteners in Thermal Power Plants" for discrimination. 5 Bolts are bent. When the bending is small, the heat treatment is corrected. If the bending is large, the spare parts need to be replaced. 6 Washers are worn or indented, ground or replaced. 7 Female thread is damaged, repaired with taps or machining, and spare parts can not be replaced when repairing. 8 The thread of the wire is damaged, and it is repaired by tap or machining. If it cannot be repaired, it should be contacted by the manufacturer. If necessary, use reaming and tapping to re-supply the special-shaped bolt. After all the high temperature fasteners have passed the hardness, the bolts and nuts should be renumbered according to the hardness value.

2.5 Inspection of replacement parts

The replacement high-temperature fastener inspection method is consistent with the installation requirements. It should also be stamped according to the requirements, and the corresponding original data should be sorted and archived.

2.6 Reloading

The reassembly requirements and procedures for high-temperature fasteners are the same as the installation, but it is important to note that the original length of the bolt should be the length measured during inspection, not the original length of the bolt during installation. At the same time, since the cylinder is inevitably deformed during operation, it should first be tightened from the bolt at the maximum position of the gap, and then tightened symmetrically to both ends. The maximum position of the gap should be determined by the combination of the cylinder during inspection.

Conclusion

In short, once a major damage accident occurs in a steam turbine plant, it takes a long time to repair the power. Whether it can avoid serious equipment damage accidents and reduce the severity of equipment damage is directly related to equipment maintenance technology, operation technology and the correctness of the accident judgment and treatment methods.

references

[1] Su Yukui, Ding Yonggang. Installation and Maintenance Methods of High Temperature Fasteners for Steam Turbine Sets[J]. Mechanical Engineer, 2009.09.

[2] DL439-91 thermal power plant high temperature fastener technical guidelines.

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