Treatment measures for leakage of steam turbine high pressure bypass valve
(HY-industry technical centre)

Firstly, check whether the valve is closed. If the valve stroke indication is not adjusted correctly (if the indication reaches zero, but it is not closed), the relationship between the stroke and the indication of the valve should be re-adjusted correctly to ensure that the indication is zero and the valve can be closed tightly. The valve clamp is mostly caused by the bending of the valve stem. The valve stem should be measured and corrected. If necessary, replace the new valve stem. If the valve drive mechanism fails, check the repair drive mechanism to ensure its normal operation.

For the leakage caused by the damage of the sealing surface of the high-pressure bypass valve, the following different repair processes should be used according to the specific damage conditions:

  • For the lighter defects with a valve sealing surface defect depth of less than 1 mm, the method of grinding the valve sealing surface and turning the valve core in the field is generally adopted. Before the grinding work, the valve core and the valve seat should be accurately surveyed, and the valve seat should be ground according to the dimension of the drawing. The grinding machine is made of cast iron material, and the size of each part must be ensured when processing. The centering plate of the grinding tire is also processed according to the size of the valve casing to ensure the concentricity of the grinding tire and the original valve seat, so as to ensure that the sealing surface does not wear off. When grinding, first remove the defects on the sealing surface of the valve seat with a paste of 80 to 100 mesh, then replace a new grinding head, finely grind with a 400 mesh polishing paste, and polish. Finally, the turning valve core and the valve seat are used for research, and the red powder is used for inspection to ensure 100% contact between the valve core and the valve seat sealing surface, and the contact line is required to be uniform.

  • For the more serious defects with valve sealing surface defect greater than 1mm, since the sealing surface of the valve seat surfacing is only 3~5mm at most, if the grinding is directly carried out, it is very likely that the sealing surface surfacing will be seriously thinned or even exposed. The valve seat matrix material makes the valve leakage more serious. Therefore, at this time, only the method of re-surfacing the sealing surface can be used.

During the maintenance of the unit, it was found that the sealing surface of the high-pressure bypass valve was damaged to a depth of 1.25 mm. The high-pressure bypass valve is produced by American CCI Company. The valve seat sealing surface is made of stellite 6 cobalt-based alloy, which is an alloy composed of elements such as Cr, W and C. The specific measures taken during the overhaul are: before the resurfacing of the sealing surface of the valve seat, the degreasing and derusting treatment is carried out, and the defects such as cracks and peeling on the original sealing surface are completely removed by machining; cobalt is selected. Base alloy surfacing electrode is welded by manual arc welding method; preheating of weldment before welding, preheating temperature is 300 °C ~ 600 °C; in surfacing, valve seat maintains horizontal position, continuous surfacing 3-4 layers Immediately after welding, 680 ° C ~ 750 ° C high temperature tempering, and then machining; after tempering and machining, then 950 ° C ~ 1000 ° C air quenching, and finally the valve core and valve seat according to process requirements . After the above process, the operation practice shows that the high-voltage bypass is restored to the original normal working state.

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