Liquid in a gas flow controller: How bad is it, Doc?

mass flow controller in water treatment gas sparging

Alicat Mass Flow Controllers at a water treatment plant solved a big failure problem.

A regional water treatment plant feeds ammonia gas into their water system—in precise doses with chlorine—to sanitize 60 million gallons of drinking water per day. But their controllers (not from Alicat) were breaking due to an ingress of liquid ammonia. The problem was the thermal differential sensors inside the meters. The sensors in ordinary thermal MFC’s are programmed to raise the gas temperatures to a specific range. Liquids conduct heat better than gases, and the presence of even a small amount of liquid rapidly cools the elements inside. Detecting the drop, the temperature regulator will draw more and more power to heat them back up—quickly frying the electronics.

Accidental liquid ingression in a gas flow controller can happen from many causes. Environmental monitoring stations take measurements in all kinds of weather. Rain, condensation, and melting snow are all potential sources. Fuel cell systems produce moisture by mixing oxygen and hydrogen in their process, a situation fraught with risks for accidental exposure. Or, a fluid tank that’s getting gas sparging may have a backflow event.

There is a cure: differential pressure

Unlike thermally-based instruments, Alicat flow meters and flow controllers operate on the principle of differential pressure. Inside the instrument, gas flow turbulence is eliminated by using laminar flow elements which ensures that the gas flow stays in the laminar flow regime. The difference in pressure between the two ends of the laminar stack is measured and used with viscosity to calculate the flow rate.

The water treatment plant got tired of buying new meters every time fluid got into their devices. They replaced their thermal instruments with Alicat meters. Now, when the liquid ammonia gets into their meters, they simply purge the system with a high-pressure flush of ammonia gas, then resume their sanitizing process.

Dry it, then flush it out.

If your Alicat meter/controller gets liquid into it, don’t delay. Liquids may cause the dissimilar metals to interact if left inside too long. If you can, put the assembly under vacuum conditions. We recommend a hard vacuum (0.25 psi or less) and some gentle heat (50° to 60°C). Together, they will encourage the liquid to evaporate out of the small channels in your instrument.

If the liquid has suspended solids, drying the liquid may leave grit, and solutions can leave a residue. There’s a small chance that the contamination is heavy enough to interfere with the laminar flow conditions needed for accurate measurements. In that case, the instrument can be flushed with isopropyl alcohol. Our service department can disassemble, clean and recalibrate for you. If you need service, you can request a return authorization, or call our customer service experts to make arrangements.

If you have one of our controllers with 316L stainless steel components—selected for their resistance to corrosion—you’re in the best possible situation. They are virtually identical, physically, to our liquid flow meters and have the highest degree of tolerance for liquids. [This might be a good time to ask: Did you know we make liquid flow meters and controllers?]

Many water treatment plants and air quality monitoring entities have switched from ordinary thermal controllers to Alicat’s differential pressure controllers because they had run into ingress problems. It’s one of the lesser-known benefits of our products, but if your process is at risk of backflow events, it’s a good reason to choose Alicat mass flow controllers and meters.

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