The ROTT test is a combined compression, relaxation, and helium leakage test at room temperature. It consists of three parts.

The first part is the compression test. This yields values for the modulus of elasticity (EG) and the maximum allowable gasket stress (Qsmax) at different levels of temperature.

The second part represents the changes in gasket stresses due to creep/relaxation after installation. This is represented by the factor PQR. At least two different levels of temperature are required for the first two parts of the ROTT test. The DeltaV-Seal was tested at ambient and at 400℃.

The third part is the leakage test, where the gasket is loaded and unloaded in a cyclic manner. The internal pressure of 40 bar is held constant and the leakage rate is measured throughout the test. From this, the minimum gasket stress in service (Qmin(L)) is obtained.

Purpose:

To determine the flange design parameters, the minimum required gasket stress in the assembly and operation (Qmin(L)), the E-modulus (EG), the maximum allowable gasket stress (Qsmax), and the creep relaxation factor (PQR).

Compression Tests

Test conditions:

Temperature levels at ambient and at 400℃.

Procedure:

The desired test temperature of the gasket is raised under initial gasket stress. Then cyclic compression and recovery loadings on the gasket at progressively higher surface pressures are carried out until the gasket collapses or the maximum load of the test machine or the maximum gasket stress specified by the manufacturer is reached.

Acceptance criteria:

Not applicable (informative)

Recordings:

The maximum load of the testing equipment of 800 MPa was reached at both levels of temperature, and no collapse of the gasket can be recognized. Also, no distinctive feature is visible which would indicate damage to the gasket material. The modulus of elasticity at ambient temperature increases steadily with increasing gasket stress. Good repeatability of the double test is noticeable. At 400℃ a lot of the values if the modulus of elasticity could not be determined. Hence dummy values were set at 180 GPa.

Result:

The maximum allowable gasket stress at both ambient and 400℃ is set to

Qsmax = 800 MPa.

Creep Relaxation Tests

Test conditions:

Temperature levels at ambient and at 400℃.

Procedure:

A stiffness of 500 kN/mm, which is typical for a PN-designated flange, was chosen for this test. The test consists of loading the test gasket until the initial load is achieved. Then it is held constant at this load level for 5 minutes before the temperature is raised to the desired test temperature. The test temperature is then held constant for a period of 4 hours. After the 4 hour period, the remaining gasket load after relaxation is noted.

Acceptance criteria:

Not applicable (informative)

Recordings:

Good repeatability of the double test is noticeable.

Result:

Leakage Test

Test conditions:

The temperature at ambient 25℃, internal pressure of 40 bar with helium as the medium.

Procedure:

The gasket is loaded and unloaded in a cyclic manner with measurement of the leakage rate at several effective gasket stress levels. 200 MPa was the initial gasket stress. Further, the gasket was loaded to different levels from 200 to 800 MPa and reduced down to 200 MPa for all loading-unloading cycles.

Acceptance criteria:

Not applicable (informative)

Recordings:

The leak rate is decreasing with increasing gasket stress up to 600 MPa and stays at a very low leak rate level. All leak rate values below 1.1 · 10^-10 mg/(s·m) were set to dummy values of 1.1 · 10^-10 mg/(s·m).

During the unloading cycles, the leak rate is increasing only slightly. In all unloading curves, no drastic increase in the leakage rate (or sudden blowout) is observed.

Result:

Tightness class L0.000001 (1.00E-06) was reached when the gasket stress raised to 295 MPa or 254 MPa, respectively. Hence the minimum gasket stress in assembly for this tightness class is set to Qsmin(L0.000001) = 295 MPa.

Did this answer your question?