Introduction to rotary seal and their selection creteria

Sealing of rotary shafts covers a very wide range of equipment and applications including domestic equipment, all types of automotive and power plant machinery, industrial pumps, aircraft gas turbines, power generation turbines and large industrial and pipeline compressors. Each application presents an individual set of demands based around the space, weight, price and reliability demands of the user. A number of individual types of rotary seal have evolved and each has an individual place based around a range of applications. Within each class of seal the range of applications can vary enormously.

Hence mechanical seals can be found on equipment as diverse as a domestic washing machine and aircraft gas turbines. Within any one application area it is quite usual for one type of seal to predominate, but at the boundaries in operating conditions between seal types then different seal types may be selected depending on the space envelope, friction, leakage and reliability objectives. For example, at low pressures there is a choice between using either a mechanical seal or lip seal. Examples include the stern shafts on ships and hydraulic pump and motor shafts. The selection will depend on individual manufacturer or user preference, and can also change as developments move the relative benefits of one seal type to another. Some of the basic seal used are listed below. Only main category is shown here as they can be classified further according to design and other criteria.

1. Lip seals
2. Mechanical seal
3. Clearance seal
4. Compression packing for rotary shafts and valves
5. Magnetic fluid seal

Selection criteria for a seal

Here is the  general criteria for selecting a seal, it may be applicable to both static and dynamic seal.

• Temperature range.
•  Pressure range.
•  Fluids to be sealed.
•  Environment.
•  Integrity of sealing required.
•  Material of counter-faces.
•  Life requirement.
•  Maintenance requirements.
•  Volume to be manufactured.
•  Assembly methods.
•  Testing and inspection criteria.
•  Historical experience of designers.
•  Degree of flexibility and relative movement of counter-faces.
•  Requirement to withstand pressure, thermal or mechanical shock.
•  Industry standards and practice.
•  Sterility and hygiene requirements.
•  Established custom and practice within a particular industry.