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3. Specify the performance required
Each factor defining the required performance should be specified in terms that are as objective as possible. Thus, where appropriate, numerical information should be given, terms must be explained and vagueness avoided. Then the boundaries of satisfaction must be defined for each of the chosen factors. This helps the designer to decide which seals meet the requirements in each respect. The boundaries must be numerical whenever possible. When subjective judgements are necessary a means of comparison must be established.

Seal performance is greatly affected by the pressure difference between the two fluids being separated and the amount and speed of any relative movement at the seal face. The required life of the seal and the permitted leakage of one fluid to the other are important performance criteria. It may also be important to be able to detect leakage. Some seals must be located to prevent unwanted movement.

The choice of seal size and the dimensions of the sealing surfaces are inter-related and affected by the function, size and performance of the machine. Typically, shaft diameters or flange diameters together with the associated linear and geometrical tolerances should be specified.