Does the HSP have experience with hydrogen/oxygen flow control devices (i.e., mass flow controllers) into a vacuum chamber and can the Panel suggest a reliable solution for R&D efforts (0-50 slpm) and for up-scaling? To date, the project in question has tried two types of MFCs: thermal based and differential pressure-based flow measurement devices. Both types have suffered from two issues: 1. The accuracy of metering drifts in a matter of months. 2. After several months, the valve starts to leak gas into the process line even when closed. The next option is to trial Coriolis measurement-based devices. But these are a significant step up in cost and have not been readily available for the pipe currently used in the project’s R&D efforts. The MFCs are used to control the flow of hydrogen or oxygen into a reaction chamber. The chamber has a vacuum pump that maintains a level of vacuum and continuous flow through the chamber. The flow rate of these devices in the project’s R&D environment is 0-50 slpm. However, the project is up-scaling for a pilot program and will require larger flow rates in the future. Any direction would be greatly appreciated.
While HSP members have limited experience with MFCs in experimental setups, the Panel does not consider them to be reliable to provide a positive flow shutoff. For safety, a shutoff valve in series is recommended. Projects will also need to consider hazardous electrical rating and location when flowing H2. Regarding Coriolis mass flow measuring devices, Coriolis flow meters measure mass rate changes by oscillating a flow tube and measuring tube distortion response. Measurement resolution is better with heavier, denser materials. Since hydrogen is the least dense gas, the response is much less than with denser materials. They are also expensive and very challenging to operate at low mass flow rates.
The HSP is aware of a few used with electrolyzers for flow confirmation at the 500 slpm range but lacks feedback on how well these worked. The project should consider working with the supplier regarding this application. There is a special MFC for vacuum application that has been observed to work well, the only difference is the valve on this is toward vacuum. On the other hand, MFCs made for pressures of atmospheric or above could be used in slight vacuum (up to 10 psia) and low flow rates (1-2 slpm) but are not very accurate due to the expansion of gas at the exhaust of the valve; it depends on the compressibility of that particular gas.
The recalibration time recommended is 2 years. If there is a valve leak, check the inlet-outlet valve opening pressure ratings on the spec sheet. From a safety perspective, it is not recommended that an MFC be relied on to provide a leak-tight seal against hydrogen. Further, MFCs are not accurate if the gas has other constituents not originally included in the calibration. For instance, humidification of fuel cell supply gases would significantly change the accuracy. Also, given the inherent risk in mixing H2 and O2 in a vacuum chamber, the HSP recommends a rigorous, multi-party, hazard analysis.