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Selection Of Mechanical Seal Design
- Aug 18, 2018 -

The choice of mechanical seal structure type is an important step in the design process, such as the mechanical seal for the kettle:

Working parameters - medium pressure, temperature, diameter and speed.

Properties of substances - concentration, viscosity, corrosion, no solid particles and fiber impurities, easy to vaporize or crystallize, etc.

Continuous or intermittent operation;The main engine is installed indoors or in the open air;Ambient air quality and temperature change.

Requirements for allowable leakage volume, leakage direction (internal leakage or external leakage) of the sealing machine;Life and reliability requirements.

Transaction host limits the size of the sealing structure.

Stabilization of handling and production processes.

1. Select types according to work parameters p, v and t

In this case, p refers to the medium pressure at the sealing chamber. According to the size of p value, it can be preliminarily determined whether the mechanical seal for the pump chooses the balanced structure and the balance degree.For medium with high viscosity and good lubricity, p < 0.8mpa, or medium with low viscosity and poor lubricity, p < 0.5mpa, non-equilibrium structure is usually adopted.When the p value exceeds the above range, consideration should be given to using balanced structure.When p > is 15MPa, it is difficult to meet the sealing requirements for a single end face balance structure.

Nu is refers to the average diameter of circumferential velocity on sealing surface, according to the size of the nu value to determine whether elastic element with the axis of rotation, namely mining with rotary spring or spring stationary type structure, general nu < 20 ~ 30 m/s spring rotary can be used, under the condition of higher speed, due to the imbalance of the rotating parts quality easy cause strong vibration, had better use spring static structure.If both p and altitude are high, fluid dynamic pressure structures can be considered.

T refers to the medium temperature in the sealing chamber, and determines the material of the auxiliary sealing ring, the cooling method of the sealing surface and its auxiliary system according to the size of t.Temperature t within 0 ~ 80 ℃, the auxiliary seal ring usually choose NBR O ring;- 50 ℃ or less t < 150 ℃, according to the corrosive medium strength, can choose fluorine rubber rubber, silicone rubber or ptfe forming packing seal: temperature < - 50 t 150 ℃ or higher, or rubber and ptfe can produce low temperature embrittlement or high temperature aging, metal bellows structure can be used at this time.Medium turbidity higher than 80 ℃, usually in the field of seal is to consider as high temperature, the corresponding cooling measures must be taken at this time.

2. Select types according to media characteristics

The less corrosive medium, usually adopts the built-in mechanical seal, and the stress state of the end face and the leakage direction of the medium are more reasonable than that of the external seal.For highly corrosive media, it is difficult to select spring material, so external or ptfe bellows mechanical seal can be used, but generally only within the range of p < 0.2mpa ~ 0.3mpa.

Medium that is easy to crystallize, easy to solidify and high viscosity should adopt large spring rotating structure.Because small springs are easily blocked by solid materials, high viscosity media will hinder the small spring axial compensation movement.

Inflammable, explosive and poisonous media, in order to ensure that the medium does not leak out, should adopt a double end surface structure with sealing liquid (isolation fluid).

The structure selected according to the above working parameters and media characteristics is often only a preliminary scheme, and the final determination must also take into account the characteristics of the host and some special requirements for sealing.For example, the sealing life of a rocket engine takes only a few minutes, but it requires absolutely no leakage for a short time.Sometimes, in order to obtain more effective space, the main engine on a ship often puts forward very strict requirements on the size and installation position of the seal, such as the exhaust pump on the submarine, and the pressure changes greatly in the process of submarine sinking and floating.In these cases, the standard structure cannot be chosen according to the routine, but special design must be made for the specific working conditions, and necessary auxiliary measures must be taken.