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Failure Analysis and Preventive Measures of Sanitary Diaphragm Valve

by:Mayer     2021-08-17
Sanitary diaphragm valve failure analysis and preventive measures The main components of diaphragm valve are valve head (upper valve cover, lower valve cover, spring tray, spring), valve stem, adjusting nut and valve body. The rated working pressure of the diaphragm is 0.69~0.75 MPa. After the oil pressure is established on the upper part of the diaphragm, the diaphragm moves downwards, pushing the valve stem downward while overcoming the spring force to close the valve; the spring rebounds when the upper part of the diaphragm loses pressure Drive the valve stem to move up, and the valve opens quickly. The steam turbine adopts a digital electro-hydraulic control system, in which the hydraulic diaphragm valve (hereinafter referred to as the diaphragm valve) is a key component that connects the control oil protection circuit and the lubricating oil circuit for emergency interruption. When the brake is manually opened or the main engine is overspeed, the lubricating oil system is critically interrupted. When the oil is drained, the upper oil pressure of the diaphragm valve is released, and the valve stem moves up quickly, so that the safety oil of the control system and the return oil are connected to release the pressure, the automatic main valve and the regulating valve are quickly closed, and the unit shuts down. 2 Failure phenomenon (1) Leakage on the sealing surface of the valve cover. The valve head of the diaphragm valve is composed of upper and lower valve covers and diaphragms fastened by bolts. The lubricating oil system of the main engine is introduced into the chamber between the upper valve cover and the diaphragm to cut off the oil critically. This sealing surface has continued to leak since the unit was put into production. The effect of multiple treatments is not good. (2) Diaphragm rupture causes the oil pressure to disappear and the diaphragm valve opens. The diaphragm of the diaphragm valve was ruptured, and the lubricating oil flowed to the high-temperature pipeline, so the machine had to stop the machine and replace it with a new diaphragm. A subsequent inspection revealed a hole with a diameter of 3 mm in the area where the diaphragm and the spring tray were in contact. 3 Reason analysis (1) The adjustment door (used for pressure relief) to adjust the oil pressure is a needle valve. Due to the influence of vibration and other factors, it will gradually close during operation, causing the oil pressure to rise slowly. The diaphragm is operated at an overpressure of 0.80~0.85 MPa, which accelerates the damage of the diaphragm. In addition, when the main engine was doing an overspeed test, the number of revolutions rose to about 3 300 r/min. Due to the untimely adjustment, the oil pressure of the diaphragm valve once rose to 0.9 MPa, causing a short-term overpressure. (2) The designed width of the upper and lower valve cover sealing surfaces is too small. The inspection found that the effective sealing width (extruded deformation part) of the leaking diaphragm was less than 5 mm overall, and the narrowest part was only 2 mm. (3) The thickness of the upper and lower valve cover plates is only 5 mm. According to the effective diameter of the upper valve cover 240 mm and the oil pressure 0.8 MPa, the pressure is about 36 170 N. Opening the mouth further narrows the sealing surface and intensifies leakage. (4) The installation process of the diaphragm is unreasonable. The maintenance personnel are not familiar with the structural principle of the diaphragm valve. When installing a new diaphragm, they did not take measures to loosen the adjusting nut to make the spring lose its force, but proceeded with the diaphragm valve open. Because the spring tray is in a high position, the diaphragm is in contact with the upper valve cover, but there is a 20 mm gap with the lower valve cover. In order to fasten the three together, they must be forcibly connected with a long rod bolt (the purpose is to overcome the elastic force of the spring). At this time, the bolt hole of the diaphragm is pulled and deformed, which is easy to cause damage, and the position of the bolt hole is relative to the valve cover The bolt holes shifted to the inside and became elliptical, the sealing surface became narrower, and the sealing effect became worse. (5) There are bubbles in the colloidal part of the diaphragm, and there is layering inside, and the maintenance personnel did not check carefully before replacement, and failed to find out in time, leaving hidden troubles after installation. (6) Accidental damage to the diaphragm. The diaphragm is not properly protected during storage and installation. The diaphragm contacts corrosive liquids or collides with sharp parts of tools and parts (such as spring trays with raised corners), squeezes, damages, or scratches the surface of the diaphragm. Failure to deal with it in time, causing potential leakage hazards. 4 Preventive measures (1) Carefully inspect the goods to prevent the use of unqualified products. The diaphragm should be kept and stored properly; during the installation process, the diaphragm should not be bumped or scratched; check the upper valve cover and spring tray to eliminate burrs and keep the contact parts smooth. After the above treatment, the diaphragm valve operates well, ensuring the long-term safe operation of the main equipment. (2) Prevent the diaphragm from over-pressure operation. Operators should strengthen monitoring, and should immediately notify maintenance personnel to calibrate the pressure gauge when the oil pressure exceeds the standard. After confirming that the oil pressure has risen, the adjustment door should be slowly closed to stabilize the pressure within the rated range. Special attention should be paid to assign special personnel to monitor the oil pressure on the spot and adjust it in time during the speed-up test of the unit and the over-speed test of the main engine. (3) Transform the upper and lower valve covers. After demonstration and experimentation, while keeping the safety oil pressure of the control oil system unchanged and not affecting the characteristics of the diaphragm valve, the bonnet was newly processed by imitating the original part design, the thickness was increased to 10 mm, and the width of the sealing surface was expanded inward to increase to 10 mm. This measure was implemented on No. 1 and No. 2 units in 2003 and 2004, with good results, and no leakage has occurred so far. (4) Improve the diaphragm installation process. Before replacing the diaphragm, first measure and record the exposed size of the adjusting nut, and screw it out to make the spring free and the spring tray in the lowest position. Now that the diaphragm is installed, there is no gap anymore, and the bolts can be tightened symmetrically and evenly. After installation, screw the adjusting nut upwards to the original position.
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