Tips! How the packing seal is converted into a mechanical seal

Packing seal is an ancient sealing structure. In the previous water lifting machinery, the method of packing cotton yarn was used to block the leakage. The packing seal is obtained because of its simple structure, low price and wide source. Favored by many industrial sectors. Mainly used for dynamic seals such as process machines and moving parts of equipment in the equipment industry, such as rotary shaft seals of vacuum pumps, centrifugal pumps, compressors, mixers and reactors, and plungers or piston rods of reciprocating pumps and compressors, as well as screw valves. Seal between valve stem and stationary body. Depending on the working conditions and the properties of the medium, mechanical seals include high temperature and low temperature resistant mechanical seals, high pressure and corrosion resistant mechanical seals, particle-resistant mechanical seals and mechanical seals for easily vaporized light hydrocarbon media, etc. Use mechanical seals with different structural forms and materials. The tight contact force of the packing seal comes from the axial pressure of the gland on the packing, which causes the packing to generate radial expansion force, resulting in a large friction between the packing and the bushing, so the worn bushing needs to be replaced frequently. In order to remove the friction heat between the packing and the shaft or bushing, there must be a certain amount of leakage, resulting in the loss of cooling water. In a word, the advantage of packing seal is that it is simple and easy to operate, but the disadvantage is that the maintenance workload is large, and the power loss is also large, and because it always has a certain leakage, it is not suitable for conveying flammable, explosive, toxic and precious liquids. . Therefore, mechanical seals tend to replace packing seals. However, most packing seals can be refitted into mechanical seals. The steps are as follows: 1. Understand the condition of the refitted machine, including the rotational speed, shaft diameter, radial runout, and axial string of the machine. volume, vibration, positioning and installation. In particular, it is necessary to understand the size of the packing seal cavity, which mainly refers to the inner diameter and length of the seal cavity to ensure the space required for the installation of the mechanical seal. When the size of the sealing chamber is not enough, either increase the inner diameter of the sealing chamber, or remove the shaft sleeve or reduce the outer diameter of the shaft sleeve. For some old-fashioned pumps, due to the small size of the seal cavity, considering the interchangeability and standardization of mechanical seals, the method of expanding the size of the seal cavity of the pump should be adopted as much as possible. If the length of the sealing chamber is not enough, the static ring gland can be thickened to make a concave bottom structure, and the axial length of the sealing chamber can be lengthened, or the shoulder on the impeller side of the sealing chamber can be turned through. When the accuracy of the original machine does not meet the installation requirements, it is necessary to improve the support and connection method of the rotating shaft of the machine. If the reaction kettle is modified with a mechanical seal, when the verticality of the flange datum surface of the kettle mouth and the center line of the rotating shaft is out of tolerance, and the deflection of the rotating shaft is large, an intermediate support or a kettle bottom tile is generally added. In addition, in order to facilitate installation and adjustment, the flange of the kettle mouth can be enlarged, and the positioning stop can be pulled out, and the gearbox can be directly fixed on the flange of the kettle mouth. In addition, careful estimation or actual measurement of the pressure in the seal chamber is important for retrofitting. According to general experience, for a single-stage centrifugal pump, when the pump inlet pressure is zero, the pressure at the sealing chamber is about 1/3 to 1/2 of the pump outlet pressure. When there is pressure at the pump inlet, the pressure at the sealing chamber is about Half the sum of the pump inlet pressure and the outlet pressure. For the sealed chambers at both ends of the multistage centrifugal pump, generally one end is connected to the inlet, and then connected to the other end through the balance pipe, so that the pressure at the sealed chamber is similar to the inlet pressure. For the double-return pump, the sealing chamber is generally connected to the inlet, and the pressure of the sealing chamber is the inlet pressure. Since the estimation error is generally large, the better method is the actual measurement. 2. Understand the condition of the sealing medium. In order to convert the packing seal into a mechanical seal, it is necessary to understand the condition of the sealing medium, including whether the state of the sealing medium contains suspended particles, the corrosiveness of the medium, viscosity, specific gravity, pressure, and temperature. According to these conditions of the sealing medium, the structure, material and cooling and lubricating structure can be selected correctly. 3. The choice of mechanical seal structure, a key issue is to correctly select the mechanical seal structure. For this reason, it is also necessary to understand the current domestic standards. Only when the standard type cannot meet the requirements of use, can another non-standard type mechanical seal be designed.