Reasons and solutions for leakage of packing valve stem seal

Graphite packing accounts for approximately half of valve failures in thermal power generation systems due to valve stem seal leakage. In general, if the valve stem is pressed too tightly during operation, electric and pneumatic valves are prone to jamming, and the valve cover bracket and valve stem of manual valves are prone to breakage. If the packing is pressed too loosely, especially when the oil circuit valve, high-pressure water supply valve, and steam valve are opened and closed, the medium is prone to leakage. It is difficult to maintain and repair, and the medium is prone to accidents such as fire or injury caused by spraying. When leakage occurs in key parts of the valve during operation, it often leads to the forced shutdown of the entire unit. Therefore, solving valve leakage is of great significance for the stable operation of the unit. The appropriate depth of the filling box design is the key to ensuring the sealing of the packing. If the depth of the packing box is too deep, there will be too many packing rings and too thick packing. The graphite disc root will exert excessive pressure on the surface of the valve stem, increasing the friction force between the valve stem and the packing. When opening and closing the valve for torque protection, the uneven distribution of frictional force along the axial direction causes severe wear of the valve stem and leakage of the packing. If the depth of the packing box is too shallow, the pressure of the packing gland is directly proportional to the pressure inside the medium. Under the "maze effect", the leakage pressure of the medium inside the packing decreases gradually. When the pressure inside the medium is greater than the pressure, if the pressure of the medium at the end of the packing leaks, the graphite packing will inevitably cause packing leakage. The figure shows the relationship between the diameter of the valve stem and the depth of the packing box under different medium pressures, based on actual maintenance experience.

 

Some valves with graphite packing pads are not designed with packing bases. Under the repeated impact of the medium, the packing decreases continuously, causing leakage. Therefore, it is necessary to consider designing packing pads in the design. The gap between the valve stem and the packing gasket is generally between the packing gasket and the packing box. If the gap is too large, long-term operation can also cause packing leakage.

To achieve good sealing performance, it is necessary to increase the clamping force of the graphite packing gland to maximize the radial force at the ring packing near the end of the gland. In fact, only a few rings of packing near the gland act as a seal. If the gap between the valve stem and the packing gland is too large, the packing is compressed and squeezed out during the opening and closing process of the valve, causing valve leakage. The gap between the packing box and the gland is generally that between the graphite packing valve stem and the cover. The valve stem must have sufficient strength and toughness, be resistant to medium corrosion and abrasion, and have good processability. In addition, to improve the wear and corrosion resistance of the valve stem, surface treatment such as nitriding, chrome plating, or surface quenching must be carried out. Prevent medium leakage caused by valve stem bending, corrosion, and surface pulling. The selection of packing is to fill the space of the valve packing box to prevent leakage when the medium passes through the valve stem and packing box space. Graphite packing should have the characteristics of good sealing, low friction coefficient, low corrosion, good elasticity and plasticity, certain strength, and high chemical stability. The recommended fillers for the steam water system of thermal power plants are shown in the table. When using, corresponding fillers can be selected based on the temperature and pressure of the medium.

 

Improper installation of graphite packing can also lead to valve stem seal leakage. In addition, during valve maintenance, it is necessary to strengthen the sealing and grinding of the valve stem. In case of packing leakage, the valve will be moved to the fully open position. At this time, the sealing surface of the valve stem and valve cover will form an upper sealing pair, which can eliminate leakage and replace the packing graphite packing safely during valve operation.