Taking the mystery out of actuators
As with any solution for a specialist field, technical terminology and acronyms are part of everyday communication in the industry, and the water treatment sector is no different. Steve Penney throws light on some main actuator definitions.
In short, an actuator is an electrically powered device that converts a control signal into a rotary or linear movement, which, via a valve unit, changes the flow of a fluid.
Actuators generally comprise the following parts: power and control cable connector, motor controls with local control station, control unit, motor, gearcase and valve interface (see Fig 1). The specification of an actuator is driven by the needs of the valve application and environment but basically two types are used - part-turn and multi-turn actuators.
A part-turn actuator typically turns 90Ú and is used most extensively to drive butterfly valves, but also have application controlling ball valves and plug valves. The multi-turn actuators - it typically turns ten to 200 revolutions - are primarily used for gate valves (both wedge and knife varieties) and penstock valves (channel or sluice construction). Linear actuators are seldom used in the water processing industry.
The are a number of variations which are generally available for both part-turn and multi-turn actuator applications. These also satisfy particular design or environmental needs.
Modulating actuators are ideal for providing real-time flow. Many actuators are used in simple open / close applications. However, increasingly real-time flow control is required in process applications. Addition of a feedback control loop in the actuator control circuitry means the actuator can continuously vary or modulate the valve to adjust fluid flow.
As the actuator is being driven harder, higher-duty cycle motors are specified and increased drive tolerances reduce backlash to give finer positional control.
By using variable speed actuators (VSAs), water hammer can be avoided. Water hammer is a well known side-effect of the rapid opening or closing of valves. In order to reduce this problem, actuator manufacturers can modify the valve open and closure rates.
Known as soft start, the use of variable speed actuation also has the benefit of being kinder to valve seats, thus ensuring greater valve life and minimal downtime. Changes to the actuation speed are programmable and therefore can be tailored to specific applications.
Fewer wires and enhanced communication can be achieved through bus systems. It was in the early 1990s that digital solutions - known as "bus" or "fieldbus" - first made their mark in actuation systems. Not surprisingly, a market that was coping with the set-up and maintenance challenges presented by hard-wire systems was ready to embrace a simpler and more effective option.
There are a number of different bus communication protocols used within the water industry. Profibus DP represents the most common choice for European applications, with DP used as short-hand for decentralised periphery.
Actuators already have internal gearing but where large torques are required, for
example to operate large butterfly valves or where the actuator drive shaft needs to be perpendicular to the valve shaft, a supplementary gearbox can be used.
Although used on all actuators, tripping torque and limit switches are worthy of additional comment. A limit switch is simply a device used to feedback when the valve has reached a predetermined position - usually fully open or fully closed. These devices can be mechanical or electronic in nature and are usually fitted within the actuator.
Greater feedback can be provided by the use of a position sensor, which is generally via a potentiometer fitted within the actuator gearcase. Position sensing is a prerequisite for a modulating actuator as defined above.
The tripping torque setting is primarily a means of protecting the valve and actuator in event of an obstruction causing the valve to fail to open or close. Once the preset torque has been reached, the actuator switches off and an alarm signal can be sent through the control circuit. In the case of gate valves, the actual close point is often determined by the tripping torque, which determines how hard the gate is driven against the valve seat.
Explosion-proof actuators are generally not used in water processing plants as generally do not have hazardous areas requiring ATEX compliance. However, the exception to this can be in sewage treatment works where the generation of explosive gases can require explosion-proof devices.
Typically zone 1 or 2 hazardous areas are designated and the actuators used are designed with an explosion proof housing which prevents excessive heat or a flame path to the outside of the actuator casing. Additionally, contacts are embedded in a synthetic material cast into a surrounding metallic frame.
As a result, when the plug cover is removed, the inside of the actuator remains sealed, preventing ingress of dirt and moisture. In other respects, the actuators are similar to the non-explosion proof products.
There are a large number of variables when specifying an actuator. The main guideline, though, is to check that a modular approach to actuator construction has been used as part of the product design. Modularity ensures adaptability and, as such, the needs of the plant engineer can be accommodated without adding significant cost and delivery time penalties.
Steve Penney is with Auma Actuators.
T: 01275 871141.