Selecting An Electro Pneumatic Converter For A Control Valve

An electro pneumatic converter converts 4-20 mA into a 20-100 kPa pneumatic signal. That sounds like it will suit any valve, and it will not. The converter has to be selected against the site conditions, and getting that wrong produces a valve that never reaches full travel.

The spring range decides the selection

The converter should be used according to the site conditions, which means the spring compression coefficient of the pneumatic control valve connected to it has to be 20-100 kPa. If the converter is used beyond this range, the full-open and full-close actions of the regulating valve cannot be realised.

Why this is different from a positioner

A valve positioner takes feedback from the stem and will drive towards the demanded position within its capability. A converter has no such feedback. It produces a pressure proportional to the signal and nothing more, so the electrical converter must be carefully selected rather than adjusted into place afterwards.

Air circuit pretreatment

Just as an audio amplifier has requirements for its power supply voltage, the converter has requirements for the instrument air that powers it. The instrument air pressure entering the converter is required to be stable near 0.14 MPa.

Where the filter regulator belongs

The air inlet of the converter should be equipped with an air filter and a pressure reducing device, so that the instrument air entering the converter is constant and free of impurities. Upstream conditioning at the header is not a substitute: the pressure has to be right at the instrument.

A selection checklist

Confirm the input signal, confirm the actuator spring compression coefficient, choose the output range from the published table or coding system, choose the electrical termination the installation uses, confirm the area classification, and fit a filter regulator at the inlet.