A New Method for Treatment of Internal Leakage Defects in High Temperature and High Pressure Electric Gate Valves

After setting the valve stroke position, the valve electric test switch, the valve test switch process, the staff found that the door rod temperature has increased, the electric device over torque action, the staff found that the valve stem threaded sleeve stuck in the The valve stem is half-open and close; then the valve disintegration inspection found: the stem and the stem thread sleeve stuck, the stem has been bent, the threaded rod cover is damaged, the valve flap and seat sealing surface (Machine side) has a deeper strain, the upper part of the contact between the valve disc and the valve seat are about 0.6 ~ 1.2mm gap. Reason analysis When the valve is set after the trip, the electric shut down, due to the closure of the trip switch does not work, the trip has not stopped after the place, the valve closed under tight conditions, continue to close the valve, the new replacement of the electric device turns a large number, Electric shut-off valve is more effort (nameplate hand wheel rotation of 250 laps, in theory, the electric device to turn 250 laps, the valve door screw thread only a circle), resulting in torque does not move; when the valve is closed to no longer close, Over torque to move, the door due to the valve is too tight and cause bending; valve (wedge single gate) caused by the top of the cone by the extrusion deformation, the valve flap and the upper contact gap after the formation of the valve; Sealing surface scratches. When the electric re-open the valve, due to a large number of revolutions of the electric device, the motor load is small, the electric valve can still be pulled out from the valve seat, the temperature rise of the door rod sleeve, eventually resulting in the stem and the door rod thread sleeve Door stick bending at the bend, the electric device over torque action stop. Treatment method According to the valve damage, the replacement of the valve stem and threaded rod door sleeve, the valve seat and the valve sealing surface grinding repair, the sealing surface and found that the upper contact position on the boiler side actually 1.2mm gap , The turbine side also has a gap of 0.6mm, which means that the seat and valve plate taper has been seriously inconsistent, affecting the tightness of the valve, resulting in valve leakage. Purchased valve disc spare parts do not match the seat taper, this fatal flaw indicates the valve may face scrapped conditions, the need to replace the entire valve can be solved, and replace the valve requires a longer duration and cost. In order to solve this defect, only consider the site to repair the valve, as the main steam electric main steam valve is a single valve wedge gate valve, then to solve the problem only to repair the valve flap with the same seat taper. To make repairs, you must accurately measure the taper. How to measure the taper? The use of measuring instruments Because of the deep inside the valve, the human hand is difficult to reach the bottom of the seat, so you can not accurately measure the valve seat high and low, and the measured point of the distance, it can not accurately measure the valve seat Taper, after careful consideration, in order to repair the valve work developed a construction program. Step 1: In order to solve the problem of the taper of the valve seat and the valve disc, firstly, two shutter molds slightly larger than the valve disc are manufactured and then suspended on the contact surface of the valve seat. Four small iron bars are used to hold the shutter mold It is completely in contact with the valve seat, the welding will be four small iron bars with two models uniform welding firm, to be taken out after cooling, measured with a protractor gate model taper. Step 2: the valve flap according to the taper of the valve plate model turning, the carved good valve flap into the seat for repair, repairing the method can be used Hongdan contact; when the taper is basically the same, within the valve flap Concave surface of the car is also the same taper surface, used as the sealing surface after welding turning datum. Step 3: The valve sealing surface surfacing. Welding electrodes can be used according to the temperature of the valve to be selected, welding electrodes for welding are semi-ferritic high-chromium steel and nickel-chromium molybdenum silicon two categories, mainly for the working temperature is lower than 600 ℃ high-pressure valve Sealing surface surfacing; or using cobalt-based hard sealing surfacing electrode: D802, D812. In the welding process of sub-welding, to prevent excessive welding temperature caused by disc deformation. Surfacing work and working conditions are very complex, surfacing must be based on the different requirements of the appropriate selection of electrodes. Different workpieces and welding electrodes to use a different surfacing process, in order to obtain satisfactory surfacing effect. D802, D812 cobalt-based welding electrodes welding should pay attention to the following points: ① welding before the electrode to be bake about 200 ℃ for more than 1 hour and then perform welding; ② welding as far as possible the use of short arc, and the electrode and the workpiece to maintain vertical; ③ According to the size of the valve and the base metal, choose 300 ~ 600 ℃ preheating, welding should adopt a small current short arc; ④ after welding should be 600 ℃ ~ 700 ℃ 1 hour after the tempering or slow cooling of the workpiece immediately Put it in dry and hot sand box or slow cooling grass ash to avoid cracks; ⑤ surfacing layer to be rough grinding, if found defects, according to the above steps for welding and slow cooling. Matching matters needing attention: the thickness of the valve sealing surface surfacing to ensure that after the turning of not less than 4mm, valve sealing surface after welding, the lathe in accordance with the reserved reference plane for roughing, roughing after the metal coloring Penetrant inspection Check the sealing surface welding situation (with or without cracks and pitting), such as unqualified, after turning to welding process to re-welding, until qualified. Qualified valve flap sealing surface hardness inspection, hardness should not be less than HRC45 °, the same sealing surface hardness difference is not greater than HRC2 °. When the hardness of unqualified, the lathe can be fine car, fine car good valve to be repaired on the scene and the valve seat, the method can be used light transmission, red lead contact, kerosene leakage method to check the gap between the taper, and Can be used grinding and turning the fine way gradually and gradually until repair qualified, and then the valve after the qualified heat treatment to improve its hardness.