Liquid ingress into a gas flow controller: how bad is it?
Gas flow controllers and meters are designed to flow only gases, but accidental liquid ingress into a gas flow controller can happen in most environments: environmental monitoring stations take measurements in all kinds of weather, and flow devices may be subject to rain, humidity, condensation, and snow melt. Fuel cell systems themselves produce water by mixing oxygen and hydrogen in their process. Fluid tanks such as chemical or biological reactors into which gases are sparged may be subject to backflow events.
Depending on the specific flow devices at hand, liquid ingress may be a minor inconvenience, or may result in requiring a whole new set of devices.
The problem: Thermal MFC electronics fry if they get wet
A regional water treatment plant feeds ammonia gas and precise doses of chlorine into their water system to sanitize 60 million gallons of drinking water per day. But the mass flow controllers regulating this process kept breaking due to an ingress of liquid ammonia. Every time the controllers got wet, they needed to be replaced.
The issue was caused by the thermal differential sensors which power the flow controllers. Thermal mass flow controllers work by introducing small amounts of heat into a gas flow stream, with the amount of heat correlating with flow rate. However, liquids conduct heat better than gases, so the presence of even a small amount of liquid in the flow stream will rapidly cool the heating elements.
The temperature regulator will detect the temperature drop, and draw more heat to compensate. Liquids have much higher specific heats than gases, so the liquid will continue to absorb the heat without noticeably warming, and the regulator will continue to draw more heat. The power required to draw this much heat will fry the electronics.
The solution: differential pressure MFCs don’t rely on heat
Unlike thermally-based instruments, flow meters and controllers which rely differential pressure measurements to measure and control flow, rather than on heat, will not fail if they get wet.
Most Alicat instruments operate on this principle: inside the device, turbulent gas flows are converted to laminar flows. The devices measure the pressure differential across the laminar flow path and combine it with known viscosity information and measured temperature and pressure to calculate the gas’s flow rate.
The water treatment plant therefore replaced their thermal mass flow controllers with Alicat differential pressure mass flow controllers. When liquid ammonium wettened the flow meters, the meters didn’t need to be replaced: they could simply be purged with a high-pressure flush of ammonia gas. Immediately afterwards, they could resume their role in the sanitizing process.
If a flow controller gets wet, flush and dry it immediately
If you notice liquid ingress into your Alicat meter or controller, don’t delay. While liquids will not harm the electronics or sensor in you flow device, they may cause the dissimilar metals to interact if left inside too long.
If possible, put the device under vacuum conditions. We recommend a hard vacuum (0.25 psi or less) and some gentle heat (50° to 60°C), to ensure all liquid will evaporate out of the small channels in your instrument.
If the liquid has suspended solids, flush the device with isopropyl alcohol. Liquid solutions may leave residues as they dry or evaporate, and any grit which is left in the flow path may interfere with the laminar flow conditions needed for accurate measurements. Flushing the device with alcohol will ensure there are no leftover particulates.
If solid particles have interfered with the laminar flow elements, return the device for servicing. Our service department can disassemble, clean and recalibrate a device for you. If you need service, request a return authorization, or call our application engineers at (520) 290-6060 to make arrangements.
If you’re worried about liquid ingress, choose your device accordingly. Anticorrosive meters and controllers are made with 316L stainless steel components, selected specifically for their corrosion resistance and have the highest degree of tolerance for liquids. If liquid ingress is becoming liquid flows, it may make most sense to switch to liquid flow meters and controllers.