How to select the mechanical seal

How to select the mechanical seal:

The mechanical shaft seal should be selected according to the operating conditions at the shaft seal location.

These important factors must be considered when selecting a mechanical shaft seal:

• Shaft seal diameter

• Type of pumped medium

• Temperature

• Sealing pressure

• Shaft speed of rotation.

Shaft seal diameter:

The shaft seal diameter must be selected to fit the pump shaft. If no seal with the required diameter is available, the shaft diameter can be changed with a bushing. 

Type of pumped medium:

The chemical resistance of the shaft seal materials to the pumped medium has to be considered.The viscosity of the pumped medium affects the lubrication and leakage of the seal. The viscosity of most media depends on the temperature.A single shaft seal can be used for a dynamic viscosity below 2500 cP (centipoise). For a higher viscosity, a back-to-back seal arrangement should be used.

Temperature:

The elastomeric parts of the seal must be able to withstand the temperature of the medium around the seal. This might be different from the temperature of the pumped medium. If the temperature is above the boiling point of the pumped medium, lubrication is poor. This must be considered when selecting seal design and materials.

Sealing pressure:

The sealing pressure is the pressure around the seal. For high pressures, a balanced seal should be used.

Shaft speed of rotation

If the speed of rotation is low, shaft seals with hard/hard material pairings might produce noise because the lubricating film in the seal gap is extremely thin. At speeds above 15 m/sec, a balanced seal with a rotating seat must be used to reduce seal unbalance.

In addition to these operating conditions, the content of abrasives and additives in the pumped medium might be relevant to consider when selecting seal ring materials.In some instances, the space available for the shaft seal is also an important factor.When selecting the right sealing arrangement around the mechanical shaft seal, also take into account the content of abrasives and the risk of build-up of wearing particles on the atmospheric side as well as the health hazards, explosion risk and toxicity of the pumped medium.