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How To Choose The Mechanical Seal Design
Feb 22, 2017

The choice of mechanical seal structure type is an important step in the design process,must investigate firstly:

1,Working parameters -Media pressure, temperature, shaft diameter and speed.
2,Medium characteristics - concentration, viscosity, causticity, with or without solid particles and fiber impurities,whether it is easy to vaporize or crystallization. 
3,Host operating characteristics and environmental conditions - continuous or intermittent operation;The host installed in the room or exposed;The surrounding atmosphere properties and temperature changes.
4,The host of the seal to allow leakage, leakage direction (internal leakage or outer leakage) requirements;Life and reliability requirements. 
5,Host on the size of the seal structure restrictions. 
6,Operation and production process stability.

Firstly,according to the working parameters P, V, T selection:

Here P is the medium pressure at the seal cavity.Depending on the size of the P value, can be initially determined whether to choose a balanced structure as well as the degree of balance.For medium high viscosity, good lubricity, p ≤ 0.8MPa, or low viscosity, poor lubricity of the medium, p ≤ 0.5MPa, usually use non-balanced structure.When the p value exceeds the above range, the balanced structure should be considered.When P ≥ 15MPa, the general single-end balanced structure is difficult to meet the sealing requirements, this time can be used in series multi-terminal seal.

U is the circumferential velocity of the average diameter of the sealing surface, and determines whether the elastic element rotates with the axis according to the value of the value of U,that is using a spring-type rotary or spring-loaded structure.Generally U less than 20-30m/s can be used spring-type rotation, higher speed conditions, due to the unbalanced quality of the rotating parts easily lead to strong vibration, it is best to use spring static structure.If the value of P and U are both high, consider the use of hydrodynamic structure.

T refers to the temperature of the medium in the sealed chamber, according to the size of T to determine the auxiliary sealing ring material, sealing surface cooling method and its auxiliary system.Temperature T during the 0-80 ℃ range, the auxiliary ring is usually selected nitrile rubber O-ring;T between -50 -- +150℃, according to the corrosive strength of the media, the choice of fluorine rubber, silicone rubber or PTFE packing filler ring are available.When the temperature is lower than -50 or higher than 150 ℃, rubber and polytetrafluoroethylene will produce low temperature embrittlement or high temperature aging, this time can be used metal bellows structure.When the turbidity of the medium is higher than 80℃, it is usually necessary to consider it as the high temperature in the sealing field, and the corresponding cooling measures must be taken.

Secondary,selection according to media characteristics:

Corrosive weak medium, usually use built-in mechanical seal,the end of the force state and the direction of media leakage is more reasonable compared with the external type.For strong corrosive media, because the spring material selection is more difficult, you can use external or polytetrafluoroethylene bellows mechanical seal, but generally only applicable P ≤ 0.2-0.3MPa range.Easy to crystallize,easy to solidify and high viscosity medium,should use single spring rotary structure.Because small springs are easily clogged with solids, high viscosity media will cause small spring axial compensation movement blocked.Flammable, explosive, toxic media, in order to ensure that the media does not leak, a double-ended structure with a sealant (isolation liquid) should be used.

According to the above working parameters and media characteristics of the selected structure is often only a preliminary program,the final determination must also take into account the characteristics of the host and some special requirements for sealing.For example, the host on the ship sometimes in order to obtain more efficient space,the size of the seal and the location of the installation are often made very stringent requirements.Another example is the submarine on the drainage pump, in the submarine ups and downs, the pressure varies greatly.In these cases, the standard structure can not be selected on a regular basis,but must be specially designed to the specific working conditions, and take necessary measures

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