The selection of mechanical seal structure type is an important step in the design process, which must be investigated first: ① Working parameters - medium pressure, temperature, shaft diameter, and speed. ② Medium characteristics - concentration, viscosity, corrosiveness, presence of solid particles and fiber impurities, whether they are easily vaporized or crystallized, etc Working characteristics and environmental conditions of the host - continuous or intermittent operation; The host is installed indoors or outdoors; The nature of the surrounding atmosphere and temperature changes, etc Requirements for the allowable leakage amount and leakage direction (internal or external leakage) of the host for sealing; Requirements for lifespan and reliability. ⑤ The limitation of the host on the size of the sealing structure. ⑥ Stability of operations and production processes.

1. Select based on working parameters p, v, t

Here, p refers to the pressure of the medium at the sealing chamber. Based on the magnitude of p, it can be preliminarily determined whether to choose a balanced structure and the degree of balance. For media with high viscosity and good lubricity, p ≤ 0.8MPa, or for media with low viscosity and poor lubricity, p ≤ 0.5MPa, non-equilibrium structures are usually selected. When the p-value exceeds the above range, a balanced structure should be considered. When p>15MPa, it is difficult for a single end balanced structure to meet the sealing requirements. In this case, a series multi end seal can be used.

υ refers to the circumferential velocity of the average diameter of the sealing surface. Based on the value of υ, it is determined whether the elastic element rotates with the axis, that is, using a spring rotating or spring stationary structure. Generally, for υ<20-30m/s, a spring rotating structure can be used. Under higher speed conditions, due to the unbalanced mass of the rotating part, strong vibration is easily caused, so it is best to use a spring stationary structure. If the values of p and υ are both high, a fluid dynamic pressure structure can be considered.

T refers to the temperature of the medium inside the sealing chamber. The material of the auxiliary sealing ring, the cooling method of the sealing surface, and its auxiliary system are determined based on the size of t. Within the temperature range of 0-80 ℃, nitrile rubber O-ring is usually used as the auxiliary sealing ring- 50 ℃≤ t ＜ 150 ℃, depending on the strength of the corrosiveness of the medium, fluororubber, silicone rubber, or polytetrafluoroethylene formed packing sealing rings can be selected. When the temperature is ＜ -50 ℃ or t ≥ 150 ℃, rubber and polytetrafluoroethylene will undergo 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 ℃, it is usually considered as high temperature in the sealing field, and corresponding cooling measures must be taken at this time.

2. Select based on the characteristics of the medium

For media with weaker corrosiveness, internal mechanical seals are usually used, and their end face stress state and medium leakage direction are more reasonable than external seals. For highly corrosive media, due to the difficulty in selecting spring materials, external or PTFE corrugated tube mechanical seals can be used, but generally only applicable within the range of p ≤ 0.2-0.3MPa.

For media that are prone to crystallization, solidification, and high viscosity, a large spring rotating structure should be used. Because small springs are easily blocked by solid objects, high viscosity media can hinder the axial compensation movement of small springs.

For flammable, explosive, and toxic media, in order to ensure that the media does not leak outside, a double end structure with sealing liquid (isolation liquid) should be used.

The structure selected based on the above working parameters and medium characteristics is often only a preliminary plan, and the final determination must also consider the characteristics of the host and certain special requirements for sealing. For example, the sealing life of a rocket engine only takes a few minutes, but it requires absolute no leakage in a short period of time. Sometimes, in order to obtain more efficient space, the main engine on a ship often puts forward very strict requirements for the size and installation position of the seal, such as the drainage pump on a submarine, which undergoes significant pressure changes during the submarine's sinking and floating process. In these cases, standard structures cannot be selected as usual, and special designs must be made for specific working conditions while taking necessary auxiliary measures.