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 engineers 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® line. 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.
To the best of our knowledge, no practical study has been conducted to date, and we were interested in obtaining such data for our soft gasket materials from TESNIT® BA line.
Table 1. Explosion limits and ignition temperatures of gases.
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 leakage rates are tested using nitrogen (N2), the leaked gas volume is measured at room temperature (23°C), 32MPa surface pressure and 40bar internal pressure, e.g. with a gas burette, mass flow detector or differential pressure method. The “DVGW criterion” applies here to a leakage of <0.1mg (s/m) for a sealing thickness of 2mm.
- 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 afore mentioned 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 L0.01 tightness class for He could serve to tightly seal other gases too.
But does that also apply to hydrogen?
New test bench 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: DONIT system for determining H2 leakage.
DONIT Gasket Materials for He & H2 Gases
Gasket permeability to He or H2 was performed on representative soft gasket materials: TESNIT® BA-U and TESNIT® BA-SOFT. These exclusive materials from DONIT are based on aramid fiber with NBR.
TESNIT® BA-U is a renowned brand that is well-suited for gas (air, methane, propane, butane) installations, while the latest innovation, TESNIT® BA-SOFT, combines excellent adaptability and sealability with very good thermomechanical properties.
Mass and volume leakages of TESNIT® BA-U and TESNIT® BA-SOFT were determined under 30 MPa of surface stress and 5-40 bar internal pressure (Fig. 2). Figure 2 initially shows the comparison of leakage values of He and H2 as mass and volume leakage of the material TESNIT® BA-SOFT. The gasket was installed with 30MPa surface pressure. The leakage was measured at different internal pressures ranging from 5 to 40bar. In both diagrams, the H2 leakage at 40 bar internal pressure is slightly, but not significantly higher than the He-leakage.
The result reflects the slightly larger kinetic diameter of H2 in comparison to of He (Table 2). This would actually lead to higher leakage of He in comparison to H2. In reality a higher rate of effusion of hydrogen, might be the reason for the opposite performance and and the observed higher leakage rates of H2.
Table 2: Data of some fluids.
Fig. 2. He (dashed line) and H2 full line) – leakage curves for TESNIT® BA-SOFT (red) and BA-U (blue) at 30MPa installation surface stress.
Figure 2 also shows the direct comparison between TESNIT® BA-U and TESNIT BA-SOFT. All sealing materials were installed under identical conditions (30MPa surface pressure).
InterestinglyTESNIT® BA-SOFT displayed a 1000-times superior sealing performance to TESNIT® BA-U with mass leakages of ca. 10-5 and 10-2 mg(s/m), respectively.
In a trial simulating a sub-optimal installation of TESNIT® BA-SOFT (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 TESNIT® BA-U which was installed under the optimal conditions (i.e. 30 MPa); this demonstrates the practical functionality and advantages of TESNIT® BA-SOFT.
Fig. 3. He (dashed line) and H2 (full line) leakage curves for TESNIT® BA-SOFT at different installation surface stress.
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.
We have established a safe and valuable H2 leakage testing method and applied it to TESNIT® BA-U and TESNIT® BA-SOFT 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.
TESNIT® BA-SOFT gasket material proved its superior sealing performance as it exhibited a 1000-fold lower mass leakage vs TESNIT® BA-U ‒ 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 TESNIT® BA-SOFT in H2 sealing applications. The experiment has shown that BA-SOFT still has the same leakage level as conventional established sealing materials at low surface pressures (poor installation conditions). This potential of fault tolerance also provides significant risk reduction and increased safety for hydrogen applications.