EN 13555

Gasket Constant Definitions

PQR A measure of creep relaxation at a predefined temperature. It is the ratio between the gasket stress after relaxation and the initial gasket stress. The ideal PQR value is 1. The closer the test value is to the ideal value, the lower the loss of gasket stress.
Qmin(L) The minimum required gasket stress at ambient temperature for a certain leakage class L when the seal is first installed.
QSmin(L) The minimum required gasket stress for a certain leakage class L in service.
QSmax The maximum gasket stress that may be applied on the gasket, without damage or intrusion into the bore, at the indicated temperatures. It depends on the temperature and the gasket thickness.
EG The recovery (elastic behavior) of a seal at load reduction and is related to the modulus of elasticity. It depends on the applied gasket stress, the seal thickness and the temperature.

 

General Test Method Description

PQR Creep Relaxation is measured at different temperatures, initial gasket stress, seal thickness values and flange stiffness values. The seal initially is exposed to the predefined gasket stress, then the temperature is increased and maintained for four hours. The residual gasket stress is then measured.
Qmin
QSmin		 A load is applied to and removed from the seal in predefined increments, with the leakage being measured constantly. The internal pressure is usually 40 bar (test gas: helium).
QSmax
EG

The gasket stress is increased cyclically and then reduced to 1/3 of the previous gasket stress. The seal thickness is then measured. The test is repeated at various temperatures.

The EG value is calculated from the load reductions and thickness changes. For QSmax, a sudden drop in seal thickness indicates failure. If a sudden drop occurs, the value of the loading step before failure is taken. In case no failure occurs, the maximum possible gasket stress of the test equipment is taken. The identified value is then used as the initial stress in a PQR test to verify the final QSmax under constant loading.

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