Liquid in a gas flow instrument FAQ

Liquid in a gas flow instrument FAQ

Gas flow controllers and meters are designed to flow only gases, but accidental liquid ingress into a gas flow device can happen in most environments. 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.

Here, we explain what to do when liquid gets into your Alicat device. We also present a case study demonstrating how two gas mass flow technologies, thermal and differential pressure based (DP), respond to liquid ingress.

What to do if liquid gets into an Alicat DP gas flow instrument

  • 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.
  • If you’re worried about liquid ingress, choose your device accordingly. Anti-corrosive meters and controllers are made with 316L stainless steel components, selected specifically for their corrosion resistance with 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.
Switching from thermals to DP instruments to prevent frying electronics

Liquid ingress causes thermal mass flow electronics to fry

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. However, the thermal mass flow controllers regulating this process kept breaking due to an ingress of liquid ammonia.

Alicat differential pressure mass flow controllers installed at a water treatment plant to prevent failures due to liquid ingress.

Alicat differential pressure mass flow controllers installed at a water treatment plant to prevent failures due to liquid ingress.

Thermal technology works by introducing small amounts of heat into a gas flow stream, proportional to the flow rate. Since liquids conduct heat better than gases, even a small amount of liquid in the flow stream will rapidly cool the heating elements. When the temperature regulator detects this drop in temperature, it draws more heat to compensate. Due to the high specific heat of a liquid, this results in the regulator drawing in a significant amount of heat. The power required to draw this much heat fries the electronics.

Alicat DP instruments don’t rely on heat

Most Alicat instruments use DP based technology. These work by converting turbulent gas flow into laminar flow, measuring the pressure differential across the laminar flow path, and combining that with known viscosity information and measured temperature and pressure to calculate the gas flow rate.

When liquid gets into these controllers, there is no need for temperature compensation. The water treatment plant was therefore able to replace their thermal units with Alicat DP instruments. When liquid ammonia got into these devices, they could simply be purged with a high-pressure flush of ammonia gas. Immediately afterwards, they could once again be used in the sanitizing process.

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