Application of hydrophobic ball valve in power plant

The steam turbine hydrophobic system is divided into high pressure hydrophobic, medium pressure hydrophobic and low pressure hydrophobic according to pressure. Mainly: steam turbine body hydrophobic; main and reheat steam inlet steam pipe; high and medium pressure main steam valve, regulating door hydrophobic; steam extraction pipe hydrophobic; door stem leakage and shaft seal system hydrophobic; small machine steam supply pipe hydrophobic; other auxiliary The system drains water. Among them, the traps commonly used in high-pressure hydrophobic systems are Y-type globe valves and metal-sealed ball valves. The operating parameters vary greatly depending on the turbine parameters. The maximum temperature is 650 ° C and the pressure is above 30 MPa.
High temperature and high pressure hydrophobic ball valve structure features
Valve body: The nominal diameter of the hydrophobic ball valve is generally ≤ DN100. The cited occasions are high temperature and high pressure difference. The quality of the small-diameter cast valve body is difficult to guarantee, so the valve body adopts an integrated forged structure (no valve cover), which avoids the leakage of the valve body and the valve cover due to the high temperature creep of the bolted material.
Sphere and valve seat: The hydrophobic ball valve requires strict shut-off. The general leakage level is required to be above ANSI B16.104 V. Now there are users who will accept the valve after 8 hours of operation and the temperature at 1 meter after the valve does not exceed 60 °C. index. In order to ensure that the valve can maintain a tight shut-off after long-term operation, attention should be paid to the design and material selection of the ball and the valve seat. At present, the ball and the valve seat are mostly HVOF supersonic flame spraying technology, spraying tungsten carbide or chromium carbide, the surface hardness can reach above HRC65, which is 1.6 times the hardness of the surfacing welder Titan 6 hard alloy. In addition to ensuring the hardness of the sealing surface, it should also be noted that the sealing surface of the valve seat and the ball should be as far as possible away from the opening of the ball passage to avoid the peeling of the sprayed layer caused by the force deformation at the ball joint.
Seat Disc Spring: The seat structure design with adjustable preload force, through the seat disc spring, auxiliary line pressure seal, provides a continuous sealing force between the ball and the seat to maintain the seal. The valve seat disc spring should have sufficient pre-tightening force, and the insufficient pre-tightening force of the disc spring is the main cause of leakage inside the valve.
Multi-stage step-down orifice plate structure: a pressure sleeve assembly with a multi-stage orifice plate upstream of the valve, the orifice plate is drilled with a plurality of small holes, which are distributed along different center lines, through which the holes are distributed The media collide with each other, thereby reducing energy and reducing the erosion of the ball on the ball and the sealing seat; the sealing valve seat is arranged with a reasonable orifice to avoid cavitation in the dead angle of the pipe at the outlet side of the valve. Scour. It should be noted that in the case where the hydrophobic time is required, such a structure is often not arranged. For example, the combined cycle power plant, its main purpose is peaking, the characteristics of the power plant is "stop at night", need to be hydrophobic every day, hydrophobic time is particularly important, the setting of the orifice plate will greatly lengthen the hydrophobic time, this is Need to pay attention.
High temperature and high pressure hydrophobic ball valve problem and improvement measures
The most common problem at the site is that the valve is stuck and does not move. One of the main reasons is that the spray coating on the sealing surface of the ball and seat is peeling off (as shown in the figure below). There are many reasons why the sprayed layer may peel off. The following points should be noted in the design and manufacturing process:

1. The sprayed layer of the sealing surface should be careful to avoid the stress concentration area.
2. The base material should have good strength and hardness to avoid large deformation under high temperature and high pressure.
3. Before spraying, the surface of the substrate should be clean, have good roughness, and should not have any stains such as oil.
Another reason for causing the valve to jam or not work is that the safety factor of the drive selected by the design is insufficient. According to the sealing test of NB/T 47044-2014 "Power Station Valve", when the medium temperature exceeds 425 °C, the valve sealing test can be carried out according to the working pressure of 1.25 times. The driving device is generally selected according to the working pressure, but here It is necessary to consider the problem of increasing the frictional moment at the sealing surface and the packing caused by the long-term non-action of the valve at high temperature, so the selection of the driving device should be noted that there is sufficient safety factor.
Conclusion
The hydrophobic ball valve produced by Qingong Valve is widely used in the power industry, especially under high temperature and high pressure conditions, and the safety, sealing and reliability requirements of the valve are strict. Therefore, the strength, material, process, assembly and other factors of the valve are fully considered in the design and manufacturing process.