01 December 2003

Flow control in hazardous environments

BY Patrick Lowery and Richard Thompson

Its 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 failureone mechanical and one electricalbefore 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.

The intrinsically safe device relies on mechanical, self-regulating pressure balancing valves in conjunction with a precision internal flow restrictor. The flow controllers set point materializes via a pneumatic signal by a newly developed, high-resolution electronic pressure controller and flow computer outside the hazardous location. A single passive, intrinsically safe pressure and temperature transducer employs the flow feedback.

INTRINSIC SAFETY STANDARDS

The two most stringent intrinsic safety standards with respect to explosion or spark prevention are the Class 1, Division 1, and the Class 1, Division 2, specifications as defined by the National Electric Code Article 500 standards in the U.S.

The term intrinsically safe refers to the fact that any device in the explosive environment, subsequently referred to as the intrinsic zone, cannot store energy or have operational energy requirements that exceed the energy required to combust flammable or explosive gases, fuels, or particles in the presence of an oxidizer such as air or oxygen.

One important requirement for Class 1, Division 1, applications is there cannot be a situation where the device undergoes a mechanical failure or fault that releases ignitable gases or vapors at the same time as another electrical failure, which would become a source of electrical ignition. Say a flow control device accidentally bursts, thereby releasing ignitable gas, where the explosion also exposed a bare circuit board, electrical sensor, or wire. The Class 1, Division 2, specification does not contain this added clause.

In practice, electronics used in Class 1, Division 1, applications are termed intrinsically safe circuits, whereas Class 1, Division 2, applications are termed nonincendive circuits. The term nonincendive refers to circuits that, under normal operating conditions, have no arcing, sparking, or exposed surfaces that operate hotter than the auto-ignition temperature of a surrounding hazardous or

explosive atmosphere. The electrical energy used in nonincendive circuits may be sufficient to ignite the hazardous atmosphere, but unless there is a fault, there is no ignition mechanism.

COMMON MASS FLOW CONTROLLERS

There are three common mass flow control methods: thermal mass, Coriolis, and pressure-based mass flow controllers (both sonic orifice and differential pressure). The most common mass flow controller on the market today is the thermal mass flow controller. It employs two thermal sensing elements that relate a temperature difference to a given amount of heat, hence mass, that is being transferred from one element to another. Therefore, the thermal mass flow controller relies on the heat transfer properties (specific heat and thermal conductivity) of the particular gas going through it to sense the mass flux.

The device relies on heating of the capillary tube with a high watt density to allow enough heat transfer by the gas to be detected. Therefore, it is very difficult for the thermal element to meet intrinsic safety requirements, due to the simultaneous fault criteria Class 1, Division 1, applications specify. Simply put, if the sensor tube fails, the explosive gas will come in contact with the heated filaments and could cause an explosion due to high temperature or high-energy discharge of the filaments.

Regardless of the specific metering or feedback mechanism, all of these respective mass flow controller technologies rely on a proportional control valve in the form of a solenoid or piezoelectric actuated valve. Because the solenoid is an inductive device, it requires a fair amount of energy to move the valve against a flow- or pressure-generated force. The piezoelectric actuator relies on the piezoelectric effect, which involves the isolinear expansion of a material (at the atomic level) in the presence of an electric field. Although the overall energy consumption of these valves is very low, the voltages and electric fields needed across the piezoelectric material are high, which can complicate the design of the power amplification circuitry for intrinsically safe applications. As a result of these fundamental electrical requirements, they are inherently difficult to use in an intrinsic zone or hazardous area because of possible high-energy discharge with the valve or the associated electronics. IT

Patrick Lowery is vice president at FlowMatrix Inc. in Carlsbad, Calif. Richard Thompson was formerly with Siemens Applied Automation in Bartlesville, Okla.

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