What are some important considerations when evaluating overtemperature scenarios for gaseous hydrogen fills of Type 4 cylinders?

Computational fluid dynamic (CFD) simulations under worst-cast fueling conditions should be utilized to assist in evaluating the risk of short or long term liner or composite damage scenarios.

Starting from the inside of a Type 4 vessel: hot gas at the end of fill is usually transient in nature, where it hits a peak temperature and then immediately starts to cool when the fill stops. The liner bulk material temperature continues to heat up even as the fill has stopped, but it does not reach the peak gas temperature. A heat transfer model should be used to determine the peak bulk liner material temperature under worst-case conditions to compare to the liner softening temperature. Recognizing that the liner material’s degradation will be a result of its “time at temperature,” it can be evaluated whether a lifetime of these sporadic peaks will cause liner degradation, resulting in embrittlement (cracking) and leakage. 

Local effects should also be considered when excessive gas temperatures (>85˚C / 185˚F) are prevalent. The liner/end boss interface can be a concern when the tank design utilizes a mechanical interlocking mechanism to attach the liner to the boss. Depending upon the design of the interlock, areas of thinner liner material that the hot gas may have softened may conspire with higher permeability, resulting in the trapping of gas between the layer of plastic and the metal boss. At low tank pressures, these trapped pockets of pressurized hydrogen gas can “pop out” the interlocking plastic material and cause leakage. Tank manufacturers may be consulted to explore if this “trapped gas” scenario has been mitigated (e.g. by design or by using a venting feature in the boss).

Note: the "end boss” in a Type 4 tank is a metallic component that contains the threaded valve opening along with a means of attaching the plastic liner to form a gas tight barrier for the tank.

Moving outwards to the composite resin: the glass transition temperature (Tg) should be comfortably above 105˚C (221˚F) in accordance with industry standard CSA/ANSI HGV 2. A heat transfer model can be used to determine the peak bulk composite material temperature under worst-case fueling conditions when compared to the resin Tg. The composite bulk temperature may neither attain such a high value nor maintain it for any sufficient duration. “Time at temperature” should also be used as a means to gauge whether resin degradation is possible, but the risk may be low if the resin Tg is above 105˚C (221˚F). Tank rupture may occur if the composite properties were to degrade as a result of resin “failure,” but these instances have only been seen to occur in service where the Tg was determined to be significantly lower than 85C.