How to select the seal design

How to select the seal design The selection of the mechanical seal structure is an important step in the design process, and must be investigated first: ①Working parameters: medium pressure, temperature, shaft diameter and speed. ②Medium characteristic concentration, viscosity, corrosiveness, presence or absence of mechanical seal solid particles and fiber impurities, whether it is easy to vaporize or crystallize, etc. ③The working characteristics of the main engine are continuous with environmental conditions or the mechanical seals operate intermittently; the main engine is installed indoors or in the open air; the nature of the surrounding atmosphere and temperature changes, etc. ④The main engine has requirements for the allowable leakage volume and leakage direction (internal leakage or external leakage) of the seal; life-span mechanical shaft seal and reliability requirements. ⑤The host's restriction on the size of the sealed structure. ⑥ Stability of operation and production process. 1. According to the characteristics of the medium, select the weakly corrosive medium. The built-in mechanical seal is usually selected. The force state of the end face and the direction of the medium leakage are more reasonable than the external type. For strong corrosive media, because the selection of spring materials is difficult, external or PTFE bellows mechanical seals can be selected, but generally only suitable for the range of p0.2 ~ 0.3MPa. The medium that is easy to crystallize, solidify and high viscosity should adopt a large spring rotating structure. Because the small spring is easily blocked by solids, high-viscosity media will block the axial compensation movement of the small spring. For flammable, explosive, and toxic media, in order to ensure that the media does not leak, a double-end structure with sealing liquid (isolating liquid) should be adopted. The structure selected according to the above working parameters and medium characteristics is often only a preliminary plan, and the characteristics of the host and some special requirements for sealing must be considered in the final determination. For example, the sealing life of a rocket engine only takes a few minutes, but it requires absolutely no leakage in a short time. In order to obtain more effective space, the main engine on the ship often puts forward very strict requirements on the size and installation position of the seal. Another example is the drainage pump on the submarine. During the ups and downs of the submarine, the pressure changes greatly. In these cases, the standard structure cannot be selected according to the routine, but a special design must be made for the specific working conditions, and the necessary auxiliary measures must be taken at the same time. 2. According to the working parameters p, v, t selection, where p refers to the medium pressure at the sealed cavity, according to the value of p, you can preliminarily determine whether to choose a balanced structure and the degree of balance. For media with high viscosity and good lubricity, p0.8MPa, or low-viscosity, poor lubricity media, when p0.5MPa, non-balanced structure is usually selected. When the p value exceeds the above range, a balanced structure should be considered. When p>15MPa, the general single-end face balanced structure is difficult to meet the sealing requirements. At this time, a series-type multi-end face seal can be used. It refers to the circumferential speed of the average diameter of the sealing surface. According to the value, it is determined whether the elastic element rotates with the shaft, that is, the spring-rotating or spring-static structure is adopted. Generally, the spring-rotating type can be used for those less than 20-30m/s, and the speed is higher. Under the conditions of, the unbalanced mass of the rotating parts is easy to cause strong vibration, it is best to adopt a spring static structure. If the values u200bu200bof p and are both high, a hydrodynamic structure can be considered. t refers to the temperature of the medium in the sealed cavity. The material of the auxiliary sealing ring, the cooling method of the sealing surface and its auxiliary system are determined according to the size of t. The temperature t is in the range of 0～80℃, the auxiliary sealing ring usually chooses nitrile rubber O-ring; -50℃t<150℃, according to the corrosiveness of the medium, fluorine rubber, silicon rubber or PTFE Formed packing seal ring: When the temperature is less than -50 or t150℃, rubber and PTFE will produce low-temperature brittleness or high-temperature aging. At this time, a metal bellows structure can be used. When the turbidity of the medium is higher than 80℃, the high temperature is usually considered in the sealing field, and corresponding cooling measures must be taken at this time. Keywords: mechanical seals, mechanical seals, mechanical shaft seals