Fig.1 Linkage retrofit: (a) before (b) after

Electric actuators — linear-stroke and quarter-turn — are indispensable in automatic control systems. With their mechatronic design, full-range microprocessor control and micromechanics, they offer small size, low weight, high torque, high reliability, accurate positioning and smooth operation; in service they consume little energy, last long, cost little, and support bumpless manual/automatic changeover. Their task is to convert the controller's signal proportionally into linear or rotary motion to drive control valves, dampers and other final elements, with the microprocessor handling signal transmission, control-law synthesis, parameter switching, status indication and control output.

At our company, the industrial boiler feed-water and make-up water control systems and the calcination rotary-kiln cooling-water system use electric linear actuators; the kiln's induced/forced draft systems and the electrolysis gas-cleaning damper control use electric quarter-turn actuators. They have raised automation, reduced workload and misoperation losses, and extended equipment life. Over years of service, however, we also encountered problems — our retrofits and lessons are summarized below.

1. Troubleshooting

1.1 Feed-water/make-up valves fail to shut tightly, boiler level abnormal (linear actuators), affecting boiler temperature. Three root causes were identified:

(1) Mechanical wear of the round valve plug — long-term reciprocating motion wears the plug so it no longer fills its seat, causing leakage. Remedy: replace the plug.

(2) Rotation of the pushrod linkage, shortening the pushrod stroke so the valve cannot close tightly. As shown in Fig. 1, we inserted a spacer of equal thickness between the two linkage discs and fastened them with bolts — this extended the pushrod stroke, closed the valve tightly, and lengthened the threaded connection between disc and pushrod, eliminating rotation. In recent years manufacturers have replaced the disc with a two-half threaded slotted block, clamped in operation. With such actuators, to prevent pushrod drop-out, choose a suitable maximum travel value in addition to the appropriate given travel range.

(3) Output current mismatched to valve position (quarter-turn types too; same remedy): per site conditions, run the actuator manually to the full-open and full-close limits and repeatedly adjust the "span" and "zero" potentiometers until the actuator operates correctly.

1.2 Unsteady operation, repeated oscillation, excessive impact — over time this severely wears the valve stem and plug, shortens equipment life, and destabilizes boiler feed-water and damper opening. Remedies:

(1) Adjust the damping potentiometer until operation and indication are fully normal; for large-amplitude oscillation, swap the speed-sensor connections.

(2) Where open/close is normal without special accuracy demands, reduce the zero current slightly and increase the span current slightly — this buffers the pushrod's impact against both limits and reduces mechanical wear.

1.3 No operation or difficult operation — severely affects production, raises workload, and can create safety hazards. Check from two directions:

① Disconnect the automatic control signal and drive the actuator with the handwheel — if it still will not run or runs hard, the fault is generally in the reducer or the terminal controller; after checking them, the fault is normally cleared.

② If handwheel operation is normal, disconnect the actuator from the terminal controller and drive the actuator electrically with a constant signal — if it will not run or runs hard, check the servo drive board's circuit parameters or the motor; after checking, the fault is normally cleared.

2. Conclusion

Electric actuators now run widely across power, metallurgical and chemical plants; their human-centred design and convenient maintenance are appreciated by field technicians. But even the best equipment depends on people — only by continuous learning, accumulating experience on the job, and careful operation and maintenance can we make it serve us better and deliver its full potential.

We supply linear and quarter-turn electric actuators of all major brands together with spare parts, plus selection and troubleshooting support — contact us.