GASKET FACTORS

Gasket factors are very important to understand but unfortunately can be difficult to understand or are easily misinterpreted. This section contains some of the more popular versions of Gasket Factors used in determining the recommended torque for gasket installation.

EN 13555 is a working standard, much like ASME PVRC gasket factors, in the EU. It provides the testing procedures to allow persons to derive the gasket parameters: Qsmax, Q(minL), Qsmin(L), PQR, and EG so they can be used in design equations found in EN 1591-1 (Flanges and Their Joints - Design Rules for Gasketed Circular Flange Connections - Part 1: Calculation).

For a further definition of the gasket parameters see chart to right.

Gasket Parameters

Qsmax Maximum seating stress required on the gasket at a given temperature without crushing the material.

Q(minL) Minimum seating stress that is required in assembly at ambient temperature to seat gasket into the flange serrations and seal internal leakage, based on tightness class, L, and specified test pressure.

Qsmin(L) Minimum gasket seating stress required in service conditions after unloading gasket (at service temperature), so that the specified tightness class L, is maintained based on internal test pressure.

PQR m & Y m and Y values are for flange design only and are not meant to be used as gasket seating stress values in actual service. “m” is known as the maintenance factor or the multiplier. The “Y” factor is the minimum stress required (psi) over the sealing area of the gasket to provide a seal at an internal pressure of 2 psig. “Y” is not considered to be the minimum seating stress for the gasket in service. These values are used in formulas in the ASME Boiler and Pressure Vessel Code, Division 1, Section VIII, Appendix 2, to give a WM1 (minimum required bolt load for operating conditions, psi) or WM2 (minimum required bolt load for gasket seating, psi) value, based on either gasket seating or the internal pressure. The flange is designed based on the greater of these two values (WM1 or WM2). This will ensure that the flange is robust enough to maintain adequate gasket seating stress, which can decline due to flange rotation with weaker flanges when bolted up and internal pressure is introduced. These values do not take fugitive emissions into account and therefore, based on leakage, newer gasket constants Gb, a, and Gs, are being developed, based on leakage to take this into account.

This factor allows for the gasket’s effect on the load applied and the relaxation of the gasket from start (final bolt-up) and after the extended life term of the material`s intended service temperature.

EG This is the unloading moduli, which is derived from the recovery of the gasket thickness between the initial compression seating stress and unloading the gasket to ¹⁄3 of its initial seating stress.

When the final torque values are calculated using the previous gasket parameters, leakage can be classified into three tightness classes.

See chart to right.

Alternative ASME PVRC Gasket Factors: Gb, a and Gs

New gasket factors to replace the ASME Code m and Y are currently being developed by the Pressure Vessel Research Council (PVRC) and ASME. The current m and Y are difficult to replicate for non-asbestos gaskets and do not take joint leakage into account. The new approach to bolted joint design makes the tightness of the joint a design parameter. In a manner similar to that of the traditional ASME Code method, the design bolt load for a joint is calculated for operating and seating requirements from the new constants Gb, a, and Gs, and the required tightness class associated with the minimum tightness.

“Gb” and “a” provide the gasket seating load; similar to Y in the present Code. “Gs” is associated with the operating stress and is similar to the m value in the present Code. The proposed ASME constants Gb, a, and Gs give a design bolt load, obtained by interpretation of leakage test data as plots of gasket stress, Sg, versus a tightness parameter, Tp. Tp is the pressure (in atmospheres) normalized to the atmospheric pressure required to cause a helium leak rate of 1 mg/sec for a 150 mm OD gasket in a joint. Since this is about the same as the OD of an NPS 4 joint, the pressure to cause a leak of 1 mg/sec of helium for that joint is its tightness. A standard test procedure, the PVRC Room Temperature Tightness Test (ROTT), has been designed to produce the constants Gb, a, and Gs. Low values for Gb, a, and Gs are desirable while a higher value of Tp means a tighter joint.