It’s difficult to provide a closed-looped control of fluid flow in an intrinsically safe zone, because the device must meet certain electrical resistance, capacitance, and inductance requirements. It must have more than two independent means of failure—one mechanical and one electrical—before it ignites. The most stringent of these specifications is Class 1, Division 1 (or Zone 0, internationally) applications where there is explosive gas or liquid.

BY: Patrick Lowery and Richard Thompson

It’s difficult to provide a closed-looped control of fluid flow in an intrinsically safe zone, because the device must meet certain electrical resistance, capacitance, and inductance requirements. It must have more than two independent means of failure—one mechanical and one electrical—before it ignites. The most stringent of these specifications is Class 1, Division 1 (or Zone 0, internationally) applications where there is explosive gas or liquid.
Flow controllers show up in nearly all industries that require accurate delivery of gas or liquid media to a chemical process. They manage the rate of flow into a process by using a flowmeter integrated with either a manual or servo-controlled proportional control valve. To make processes faster, better, and cheaper, flow controllers need to be electronically modulated, so the process loop can become closed. These processes appear inside a potentially explosive atmosphere, or they require the control of an explosive gas or liquid. Applications include in situ process analytical sampling systems in petrochemical plants, manufacturing process gas and chemical control, and hydrogen gas and fuel control for fuel-cell delivery.
Most electronic closed-loop flow controllers employ a flow feedback sensor and electromagnetic or piezoresistive proportional valve. In practice, there are only two ways you could use a flow control device inside a potentially explosive atmosphere. You could segregate or separate the device by means of a physical barrier such as a sealed, purged enclosure. But the more elegant approach is to
design the flow control device to be intrinsically safe, meaning there will never be enough electrical energy available to create a spark (or high-temperature event) and ignite the explosive atmosphere in the presence of air or oxygen.

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