Permeability tests with Helium and Hydrogen on Soft Gasket Materials
For some time, hydrogen (H2) has been widely advocated as the future optimal alternative fuel. If H2 were to substitute methane even partially in this sector, H2-running systems will become commonplace and their leak-proof certification a must. Therefore, DONITengineers undertook the creation of a reliable and practical database purpose-made for the H2 barrier/permeability property of the company materials’ portfolio, namely the TESNIT® brand. This would result from actual real corresponding leakage performance measurements using H2 as the test medium, which would then correlate with the ones obtained with the helium (He) conventional test medium.
In the sealing sector, gasket materials should be, by definition, compatible with the transiting or contained fluid and should withstand the operating conditions. These requirements cannot be overstated in the case of corrosive, hazardous, or flammable fluids. In fact, H2 is one of the most difficult gases to contain and prevent from leaking. Besides, knowing that the explosive limit of H2 gas is approximately ca. 20% lower than that of methane and combined with a lower ignition temperature (Table 1), this translates into a significantly higher explosion risk for the former and a greater hazard for the installation and operating site. And naturally, one cannot tolerate a greater gasket leak of H2 than of methane. Therefore, it is crucial to determine the actual gasket material sealability for H2 itself.
Table 1. Explosion limits and ignition temperatures of some gases.
To the best of our knowledge, not many practical studieshave been conducted to date, and we were interested in obtaining such data for our TESNIT® gasket materials.
Conventional Standards for Gasket Leakage Measurements
Considering gasket functionality in applications, conventional fluid leakage tests are conducted according to established standards typified by the following ones:
- Acc. to DIN 3535-6, the specific leak rates are tested using nitrogen (N2), the leaked gas volume is measured at room temperature (23°C), 32 MPa gasket surface load under 40 bar internal pressure, e.g. with a gas burette, mass flow detector or differential pressure method. The “DVGW criterion” applies here to a leakage of <0.1 mg/(s*) for 2 mm thick gasket materials.
- In DIN EN 13555, the leak of He gas (as the test medium) is measured at room temperature (RT). Accordingly, the obtained Qmin values represent the surface loads required for the gasket installation, whereas the Qsmin simulate the leakage in-service conditions. As this technique has recourse to a sensitive mass spectrometer as the detector, very low leaks can be detected. Gasket data according to DIN EN 13555 are the bases for the flange-calculation acc. to EN 1591-1, and allow the validation of flanged connections for TA-Luft.
- In TA-Luft and VDI 2440‒VDI 2200 (VDI 2440/2200) and following a defined heat treatment cycle applied to the gasket, the He gas (as the test medium) leak is measured at RT under 1 bar (internal pressure) and 30 MPa of gasket surface load. Here also this technique employs a mass spectrometer for the fine leakages with a sensitivity equivalent to the latter test.
Note that all the aforementioned standard test methods call upon either N2 or He gas as the test medium (non-H2 environments). In general, it is commonly accepted that if the sealability threshold is determined for He or N2, this would be valid as well for methane; in practice, it is admitted that a gasket material with an L0.1 tightness class for He could serve to tightly seal other gases too.
But does that also apply to hydrogen?
New Test System for Determining H2 Leakage
From DONIT R&D Application Engineering labs, a gasket testing setup for H2 permeability emerged in 2021 (Fig. 1). Its design is based on a further improvement of the VDI2440/2200 standard. This latter well-established standard can now be extended to the testing of sealing materials for their H2 permeability and resistance.
Fig. 1: test system for determining H2– leakage
Gasket Materials for He & H2 Gases
Gasket permeability to He or H2 was performed on representative soft gasket materials: TESNIT® BA-U (FA-Standard) and TESNIT® BA-SOFT (FA-New Generation). These materials are based on aramid fiber with NBR.
FA-STANDARD stands as a representative gasket material with low compressibility of ~ 6-9% , well-suited for gas (air, methane, propane, butane) installations, while FA-NEW GENERATION, stands as a representative material with compressibility of ~25%, high adaptability and sealability with very good thermomechanical properties.
Mass and volume leakages of FA-STANDARD and FA-NEW GENERATION were determined under 30 MPa of surface stress and 5-40 bar internal pressure. Figure 2 shows the comparison between leakage values of He and H2 as mass and volume leakage of FA-NEW GENERATION. The gasket was installed with 30 MPa surface pressure. The leakage was measured at different internal pressures ranging from 5 to 40 bar. In both diagrams, the H2 leakage at 40 bar internal pressure is slightly, but not significantly higher than the He-leakage.
Having an isolated view on the larger kinetec diameter of H2 compared to He, one would expect a lower leakage of H2 , while with an isolated view on the higher effusion rate of H2 compared to He, leakage of H2 would be higher. Our results show that the effusion rate is the governing factor.
Table 2: Data of some fluids.
Fig. 2. He (dashed line) and H2 (full line) leakage curves for FA-NEW GENERATION (red) and FA-STANDARD (blue) at 30 MPa installation surface stress.
Interestingly, under 30 MPa of surface stress, FA-NEW GENERATION displayed a 1000-times superior sealing performance to FA-STANDARD with mass leakages of ca. 10-5 and 10-2 mg/(s*m), respectively.
In a trial simulating a sub-optimal installation of FA-NEW GENERATION (i.e. under 15 MPa of surface stress), a significant increase in the leakage was observed (Fig. 3). However, this leakage still remained within the same level as that of FA-STANDARD which was installed under the optimal conditions (i.e. 30 MPa); this demonstrates the practical functionality and advantages of FA-NEW GENERATION.
Fig. 3. He (dashed line) and H2 (full line) leakage curves for FA-NEW GENERATION at different installation surface stresses
Prospects:
DONIT is committed to providing reliable and practical performance data concerning its gasket materials, especially for the ones which were purpose-built for high-risk applications. For the sake of increased safety, reducing the H2 emissions in an installation using it is a top priority due to the hazardous nature of this fluid.
Aa safe and valuable H2 leakage testing method was developped and applied to FA-STANDARD and FA-NEW GENERATION gasket materials as examples. This method, which is based on a further improvement of the VDI2440/2200 standard, offers concrete and reliable data for the evaluation of gasket materials destined for sealing H2. This work demonstrates that the determination of mass or volume leakage according to EN 1591-1 standard with He as the test medium, is valid as well for H2 medium.
FA-NEW GENERATION gasket material proved its superior sealing performance as it exhibited a 1000-fold lower mass leakage vs FA-STANDARD ‒ the latter exemplifies a common material within its category of applications. In addition, in a trial simulation, the former displayed higher tolerance to poor installation conditions. Therefore, we recommend the use of FA-NEW GENERATION in H2 sealing applications.
