Common Faults Of Cylinders

Problem
1.
The cylinder is cast, and it must be aged after leaving the factory to completely eliminate the internal stress generated during the casting process. If the aging time is short, the processed cylinder will still deform in the future operation.
2.
The force applied to the cylinder during operation is very complicated. In addition to the static load such as the pressure difference of the gas inside and outside the cylinder and the weight of the various components installed in it, it must also withstand the reaction force on the static part when the steam flows out of the stationary blades, as well as the forces on the cylinder under the hot and cold states of various connecting pipes. Under the interaction of these forces, the cylinder is prone to plastic deformation and leakage.
3.
The load of the cylinder increases and decreases too quickly, especially when the temperature changes greatly during rapid startup, shutdown and working condition changes, the cylinder warming method is incorrect, and the insulation layer is opened too early during shutdown and maintenance, etc., which will generate large thermal stress and thermal deformation in the cylinder and flange.
4.
The cylinder generates stress during the machining process or after repair welding, but the cylinder is not tempered to eliminate it, resulting in large residual stress in the cylinder and permanent deformation during operation.
5.
During the installation or overhaul process, due to the overhaul process and overhaul technology, the expansion gap of the inner cylinder, cylinder partition, partition sleeve and gas seal sleeve is not appropriate, or the expansion gap of the ear pressure plate is not appropriate, and a huge expansion force is generated after operation to deform the cylinder.
6.
The cylinder sealant used is of poor quality, has too many impurities or is of the wrong type; if there are hard impurity particles in the cylinder sealant, it will make it difficult for the sealing surface to be tightly combined.
7.
The tightening force of the cylinder bolts is insufficient or the material of the bolts is unqualified. The tightness of the cylinder joint surface is mainly achieved by the tightening force of the bolts. The thermal stress and high temperature generated when the unit is started and stopped or when the load is increased or decreased will cause the stress relaxation of the bolts. If the stress is insufficient, the preload force of the bolts will gradually decrease. If the material of the cylinder bolts is not good, the bolts will be elongated under the action of thermal stress and cylinder expansion force during long-term operation, plastic deformation or fracture will occur, and the tightening force will be insufficient, causing the cylinder to leak.
8.
The tightening order of the cylinder bolts is incorrect. Generally, when tightening cylinder bolts, they are tightened from the center to both sides at the same time, that is, from the point with the largest sag or the point with the largest deformation. This will transfer the gap at the point with the largest deformation to the free ends in front and behind the cylinder, and finally the gap will gradually disappear. If it is tightened from both sides to the center, the gap will be concentrated in the middle, and the cylinder joint surface will form a bow-shaped gap, causing steam leakage. [3]
Causes
1.
Internal and external leakage of the cylinder is generally caused by eccentric installation of the piston rod, insufficient lubricating oil supply, wear or damage of the seal ring and sealing ring, impurities in the cylinder and scratches on the piston rod. Therefore, when the cylinder leaks internally or externally, the center of the piston rod should be readjusted to ensure the coaxiality of the piston rod and the cylinder barrel; the oil mist device should be checked regularly to ensure that the actuator is well lubricated; when the seal ring and sealing ring are worn or damaged, they must be replaced in time; if there are impurities in the cylinder, they should be removed in time; if there are scratches on the piston rod, it should be replaced with a new one.
2.
The cylinder's output force is insufficient and the movement is not smooth, which is generally caused by the piston or piston rod being stuck, poor lubrication, insufficient air supply, or condensed water and impurities in the cylinder. In this regard, the center of the piston rod should be adjusted; check whether the oil mist device works reliably; whether the air supply pipeline is blocked. When there is condensed water and impurities in the cylinder, they should be removed in time.
3.
The cylinder's buffering effect is poor, which is generally caused by the wear of the buffer seal ring or the damage of the adjustment screw. At this time, the seal ring and the adjustment screw should be replaced.
4.
The piston rod and cylinder head of the cylinder are damaged, which is generally caused by the eccentric installation of the piston rod or the ineffectiveness of the buffer mechanism. In this regard, the center position of the piston rod should be adjusted; the buffer seal ring or the adjustment screw should be replaced.
Solution
1. For the joint surface of the cylinder with large deformation or serious air leakage, the method of scraping the joint surface should be adopted
If the deformation of the upper cylinder joint surface is within 0.05mm, take the upper cylinder joint surface as the reference surface, apply red lead or stamp blue paper on the lower cylinder joint surface, and scrape the lower cylinder according to the traces. If the deformation of the upper cylinder joint surface is large, apply red lead to the upper cylinder, use a large ruler to grind out marks, and grind the upper cylinder flat. Or use mechanical processing to level the upper cylinder joint surface, and then use the upper cylinder as a reference to scrape the lower cylinder joint surface. There are generally two methods to scrape the cylinder joint surface: ⑴ For bolts that are not tight on the joint surface, use a jack to slightly push the upper cylinder back and forth, and scrape according to the coloring of the red lead on the lower cylinder joint surface. This method is suitable for high-pressure cylinders with strong structural rigidity. ⑵ For bolts that are tight on the joint surface, check the tightness of the joint surface based on the feeler gauge, measure the value and the marks of the pressure, and scrape the joint surface. This method can eliminate the influence of the cylinder sag on the gap. 2. Use appropriate cylinder sealing materials Because there is no unified national standard and industry standard for turbine cylinder sealants, the raw materials and formulas are different, and the product quality is uneven; when choosing turbine cylinder sealants, you must choose a regular manufacturer with a good reputation in the industry and guaranteed product quality to ensure the tightness of the cylinder after maintenance. 3. Local repair welding method
Since the cylinder joint surface is eroded or corroded by steam, use appropriate welding rods to fill the grooves, use a flat plate or a ruler to grind out the marks, and scrape the weld bead and the joint surface in the same plane. When the cylinder joint surface is deformed or the gas leakage is serious, weld one or two 10-20mm wide seal tapes to eliminate the gap on the joint surface of the lower cylinder, and then measure with a ruler or buckle the upper cylinder, and apply red lead to scrape until the gap is eliminated. The process of this operation is also very simple. Preheat the cylinder to 150℃ before welding, and then perform segmented back welding or jump welding at room temperature. Use austenitic welding rods, such as A407 and A412, and cover with asbestos cloth to keep warm and cool slowly after welding. After cooling to room temperature, grind and scrape.
4. Coating or spraying of cylinder joint surface
When the cylinder joint surface leaks a large area and the gap is about 0.50mm, in order to reduce the workload of scraping, the coating process can be used. Use the cylinder as the anode and the coating tool as the cathode, and repeatedly brush the electrolytic solution on the joint surface of the cylinder. The thickness of the coating depends on the size of the gap between the cylinder joint surfaces, and the type of coating depends on the material of the cylinder and the scraping process. Spraying is a method of using a special high-temperature flame spray gun to heat the metal powder to melt or reach a plastic state and then spray it on the treated cylinder surface to form a coating with the required properties. Its characteristics are simple equipment, convenient operation, and firm coating. After spraying, the cylinder temperature is only 70℃-80℃, which will not cause the cylinder to deform, and a heat-resistant, wear-resistant, and corrosion-resistant coating can be obtained. It should be noted that the cylinder surface should be polished, degreased, and roughened before coating and spraying, and the coating should be scraped after coating and spraying to ensure the tightness of the joint surface.
5. Method of adding pads to the joint surface
If the local gap leakage of the joint surface is not large, a copper mesh of 80-100 mesh can be used to reduce its hardness through heat treatment, and then cut into an appropriate shape, laid on the air leakage of the joint surface, and then matched with a cylinder sealant. If the gap of the joint surface is large and the leakage is serious, a groove with a width of 50mm and a depth of 5mm can be opened on the upper and lower joint surfaces, and a toothed pad of IGr18Ni9Ti can be embedded in the middle. The thickness of the toothed pad is generally about 0.05-0.08mm greater than the depth of the groove, and can be adjusted with a stainless steel gasket of the same shape.
6. Methods for controlling bolt stress
If the deformation of the cylinder joint surface is small and very uniform, new bolts can be replaced at the gap, or the preload of the bolts can be appropriately increased. Tighten from the middle to both sides at the same time, that is, tighten the bolts from the largest sag or the place with the largest deformation. Theoretically, the preload of the bolt can be calculated using the formula d/L≤A, but since the data and measurement methods of this calculation are still under research and have not been promoted, it is mostly determined based on experience within the maximum allowable stress of the bolt.
7. Polymer material methods used in the new era
With the further development of technology, polymer composite materials have gradually been successfully applied in cylinder maintenance. Compared with traditional means, polymer composite materials have excellent temperature resistance, good pressure resistance, and more outstanding sealing performance, and have good plastic deformation. They will not solidify when heated and the sealing film will not be damaged, thus ensuring the sealing of the sealing surface of the machine part; in addition, it is easy to clean, and the used sealing surface can be easily wiped off with anhydrous ethanol or acetone without adhering to the sealing surface; due to its excellent performance, it is gradually favored by more and more cylinder companies.