The gasket characteristics are determined in a defined procedure under laboratory conditions under idealised test conditions. This test is based on the corresponding compression, leakage and relaxation tests in accordance with EN 13555. The gasket characteristics must adequately describe the sealing and deformation properties of the gasket. According to the new concept of flange calculation in accordance with EN 1591-1, the gasket parameters are included in the flange calculation to ensure the tightness and strength of the flange connection. For example, limit values for the surface pressure are required for both the installation and operating conditions, which specify the permissible range (see Table 1).
Table 1: Flange gaskets characteristics
| Gasket characteristic | EN 13555 | Description | Impact | Use in flange calculation | Notes on characteristic value |
|---|---|---|---|---|---|
| Assembly surface pressure | QA [MPa] | The gasket surface pressure in the assembly condition | Design (flange, gasket type, bolts) | Degree of utilisation of the bolt; Material selection of the bolts | The assembly surface pressure is not determined in EN 13555 |
| Minimum surface pressure in installed condition for required tightness class L | Qmin(L) [MPa] | The minimum level of surface pressure required for lea kage rate class L on assembly. The minimum gasket surface pressure on assembly required at ambient temperature in order to seat the gasket into the flange facing roughness and close the internal leakage channels so that the tightness class is to the required level L for the internal test pressure. The value essentially depends on the medium and the internal pressure, but also significantly on the required leakage class. | Temperature; Gasket width; Gasket height; Roughness of the sealing surfaces; Internal pressure p; tightness class L; Fluid | Determining the minimum sealing force in the installed state | A low Qmin(L) value is desirable. |
| Minimum surface pressure in operating condition for required tightness class L | QSmin(L) [MPa] | The minimum level of surface pressure required for leakage rate class L after off- loading. The minimum gasket surface pressure required under the service pressure conditions, (i.e.) after off- loading and at the service temperature, so that the required tightness class L is maintained for the internal test pressure. QSmin(L) is always smaller than Qmin(L). | Gasket width; Gasket height; Roughness of the sealing surfaces; Internal pressure p; Assembly surface pressure QA; Sealing class L; Fluid; Temperature; Stress duration; | Determination of the minimum sealing force in operating condition | A low value is desirable; A small difference between two Qmin(L) values with different leakage rates taken into account is also desirable, as this indicates a low sensitivity to unloading during operation. |
| Maximum surface pressure when assembled | Qmax [MPa] | The maximum surface pressure during installation with which the gasket may be loaded without damage occurring. | Gasket width; Gasket height; Roughness of the sealing surfaces; | Determination of the maximum permissible sealing force in the installed state | The maximum surface pressure Qmax is obtained when the permissible degree of utilisation of the flange, bolts or gasket is reached in the flange calculation in accordance with EN 1591. |
| Maximum surface pressure in operating condition | QSmax [MPa] | The maximum surface pressure that can be safely imposed upon the gasket at the service temperature without damage. The maximum surface pressure that may be imposed on the gasket at the indicated temperature without collapse or “crush”, compressive failure, unacceptable intrusion into the b ore or damage of the stressed area of the gasket such that failure was imminent. | Temperature; Gasket width; Gasket height; Roughness of the sealing surfaces; | Determination of the maximum permissible sealing force (bolt load) in operating condition | A high QSmax value is desirable. |
| E-modulus (secant modulus) | EG [MPa] | Describes the springback (elastic behaviour) of a gasket when the load is removed. This is the additional change in thickness of the gasket or sealing element due to creep between the completion of the loading and the end of the test period. | Assembly surface pressure QA; Gasket height; Temperature; | Determination of the change in sealing force between assembly and operating condition | A low EG value is desirable. |
| Creep relaxation factor | PQR [–] | Creep relaxation factor, the ratio of the residual and initial surface pressures. This factor to allows for the effect of the imposed load on the relaxation of the gasket between the completion of bolt-up and long term experience at the service temperature. | Temperature; Gasket width; Gasket height; Roughness of the sealing surfaces; Assembly surface pressure QA; Stiffness C of the flange system; | Determination of the change in surface pressure or sealing force between mounting and operating conditions | PQR ≤ 1; High PQR value is desirable. |
| Creep relaxation | ∆eGC [mm] | ΔeGc is the thickness change of the gasket determined by creep and or relaxation from initial surface pressure to final surface pressure in the test PQR. |
Tightness classes
The basis for the gaskets characteristics are the defined tightness classes, which are based on certain specific leakage rates. The sealing characteristics are determined under different test conditions (different test pressures and pressures) in the internal pressure test and assigned to the tightness classes. On the basis of this assignment, the corresponding specific leakage rate of the flange connection can be guaranteed by the mathematical proof that the limits of the sealing characteristics are complied with.
The tightness classes L1.0, L0.1, L0.01, …) represent the maximum limit for a specific leakage. If a tightness class is to be complied with by a gasket, the leakage under the defined conditions must not be higher than permitted by the tightness class.
Table 2: Tightness classes
| Tightness class | Specific leakage rate [mg / (s•m)] Helium * |
|---|---|
L1,0 | ≤ 1,0 |
| L0,1 | ≤ 0,1 |
| L0,01 | ≤ 0,01 |
| L0,001 | ≤ 0,001 |
* “m” means gasket circumference length
Test condition EN 13555
EN 13555 is the test standard for determining the gasket characteristics. These gasket parameters are used for the calculation of flange connections in accordance with EN 1591-1 in order to provide proof of tightness for the flange connection. Flange calculations with proof of tightness require reliable gasket characteristics.
The EN 13555 standard has defined standard test conditions:
- Test gas: Helium
- Test pressure: 40 bar
- Test temperature: Room temperature
- Surface pressure: min. 5 MPa, max. 160 MPa;
The standard also contains various surface pressures up to which the gasket is loaded in order to carry out a leakage measurement. After loading, the leakage is measured. This is followed by unloading (in several stages) in order to measure the leakage here too. The standard contains surface pressures from 5 MPa to max. 160 MPa. The test-flanges must have a size accordingly, chosen from the standard EN 1092-1 (DN40/PN40) or EN 1759-1 (NPS 4″/Class 300).
A flange connection according to DIN EN 1092 is considered to be (permanently) technically tight according to the current state of the art if a calculation verification according to EN 1591-1 or finite element analysis (FEM) for a leakage class L0,01 can be provided (TA-Luft edition August 18, 2021, VDI 2290 edition June 2012).
Gasket manufacturers are responsible for publishing the characteristic values for their gaskets at the relevant test conditions.
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