Electronic vs. Pneumatic Actuation in Pharmaceutical Plants

Advantages of Electronic vs. Pneumatic Actuation in Your Pharmaceutical Plant

Recognition of the cost/benefit ratio of high quality electric actuation in the U.S. is on the rise.  Pneumatic actuators were cost-effective at one time, but they are no longer well-suited for modern electronic controllers.  They require interfaces which add expense and degrade the signal from the high quality control outputs now available.

In an effort to meet the needs of DDC processor control, Belimo has introduced a precision direct coupled actuator which is more reliable than the pneumatic hybrid (actuator, positioner and transducer), has a lower first and life cycle cost, and has all the advantages of pure electric (outside use and no compressor problems). Belimo electric actuators provide simpler installation, control features which pneumatic cannot provide, maintenance-free longer life, flexibility for changes, and a positioning resolution as high as 160:1 – accuracy as high as the DDC electronic output signal.

Importance of Accuracy

The Belimo 2 to 10 VDC (4 to 20 mA with 500 Ω load resistor) control resister is an international standard used by all control manufacturers.  The worst case hysteresis is 1%.  The Belimo electric actuator is positive positioning using a precision feedback circuit with an op amp comparing signal with position feedback.  No degradation of accuracy will occur during the 20 year life of the actuator.

Hysteresis

The gradual operation damper area is from 50%-20% of that recommended for a 2-position operation because the hysteresis of the pneumatic actuator is 1.5 PSI over the 5 PSI spring range.  Further loading of the actuator will increase the hysteresis.  Since the signal is also the power source it must increase beyond setpoint to break free. As a result the stem jumps forward and overshoot occurs. On the return stroke the signal must drop far enough below the spring force to allow it to overcome the resistance. With a 4” stroke actuator a 1” hysteresis is normal.

Pneumatic Hunting

Energy waste and tenant dissatisfaction are inevitable by-products of pneumatic hunting.  Since pneumatic actuators, even one with a positioner, is completely subject to the forces opposing it,  the non-linear dynamic torque loading of a damper as it moves through its rotation will cause the actuator to hunt.  At the point where maximum torque is required, pneumatics will hunt for position, resulting in under and over shoot at the max torque threshold. This can also occur any time there is a substantial change in torque through a small range of rotation.  This issue enhances at higher air velocities or static pressures.

The Belimo electric actuator is inherently positive positioning.  It utilizes an internal potentiometer to verify its position with respect to the control signal. It simply obeys the relationship between control signal and potentiometer between ± .05 volts DC regardless of opposing forces acting on it.

Reliability

Pneumatic Actuation:  The transducer, compressor and other interfaces needed with the pneumatic often fail well before the actuator itself.  Transducers cannot handle the oil and dirt even with regular maintenance.  The compressor is particularly maintenance intensive and the source of oil which contaminates the air lines.  Fittings are a source of leaks and maintenance.  The drier is a refrigeration machine with normal maintenance problems.  The dew points of the instrument air are as low as 35° when new and rises to 45° when the condenser is dirty. If it isn’t cleaned soon after that point, it will burn out the compressor. Auto blow-downs can fail leading to water problems. Refrigerant leaks as the machine ages eventually leading to water entering the system.  The oil removal filter must be maintained regularly. Coalescing filters must be sized carefully or oil carryover occurs.

Electronic Actuation:  The Belimo electric actuator has the pneumatic actuator’s durability but without the difficult interfaces and alleviates many maintenance items included in the pneumatic structure.  The pneumatic actuator has 23 maintenance items vs. 3 with the electric actuator (wiring, DDC panel and transformer).  Belimo electric actuators have an average life of 15-20 years under normal conditions and a proven published failure rate of only .3% over the 2 year warranty period.

 

*Adapted from Belimo’s DOC5.4-10.99-5M- “Electronic or Pneumatic Actuation?”

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