Actuator Exercises (Part 1): Composition, Valve Types and Fail-Safe Selection

1. What role does the actuator play in an automatic control system?

In a process control loop the actuator receives the controller command, converts it via the actuating mechanism into angular or linear displacement, and operates the regulating element, changing energy or material entering and leaving the plant — automatic control. No control system is complete without it. If sensors are the system's sense organs and the controller its brain, the actuator is the hands doing the work.

2. What are the parts of an actuator and their functions?

Two parts: the actuating mechanism and the control valve (regulating element) (Fig. 8-1 shows a pneumatic actuator). The actuating mechanism is the mover, producing thrust proportional to the control signal to stroke the valve; the valve is the regulating part, producing displacement or rotation under that thrust to modulate flow directly.

3. How are actuating mechanisms classified?

By energy source: pneumatic, electric, hydraulic. (1) Electric actuators run on mains power — energy is easy to obtain, action is fast, signals transmit fast and over distance, and they pair naturally with digital equipment. They are in development and ascendance, a promising type. Drawbacks: complex structure, higher price, smaller thrust; and generally unsuited to fire/explosion-risk duty unless explosion-proof construction is used, which can meet the requirement. (2) Pneumatic actuators run on compressed air — simple, reliable and stable, powerful, easy to maintain, intrinsically fire/explosion-safe — hence widespread in petroleum, chemicals, metallurgy and power, especially explosive-duty chemical processes. Drawbacks: lag, unsuited to long transmission (within 150 m), no direct digital connection. Hydraulic actuators are rarely chosen today at home or abroad.

4. What control valves are common, and their pros and cons?

Many varieties exist, but by plug motion they divide into linear-stroke and quarter-turn classes — linear valves move the plug along a line; quarter-turn valves rotate it. Common constructions are shown in Fig. 8-2. (I) Linear-stroke regulating elements: 1. Single-seat globe valve — one plug and one seat (Fig. 8-2b, c): simple, low leakage (even tight shut-off), small allowable differential; suits clean media needing tight shut-off at low differential — watch the allowable differential lest the valve not close fully. 2. Double-seat globe valve (Fig. 8-2a) — two plugs and seats, ~20–25% more capacity than a same-bore single seat; forces on upper and lower plugs cancel, but both rarely close simultaneously: large allowable differential but larger leakage — suits clean media at high differential without tight shut-off demands, not viscous or fibrous media. 3. Angle valve (Fig. 8-2d) — right-angle body, simple flow path, low resistance; suits high differential, high viscosity, suspended solids and particles. Normally bottom-in side-out for good stability; at high pressure side-in bottom-out prolongs plug life but hunts at small openings. (II) Quarter-turn regulating elements: 1. Butterfly valve (Fig. 8-2g) — a disc rotating on a shaft controls flow; parts are body, disc, shaft and seal. Simple, small, light, cheap, large capacity — ideal for low differential, large bore, large gas flows and suspensions — but leaks more. Its characteristic resembles equal-percentage up to 70°; beyond 70° operation is unstable and the characteristic poor, so butterfly valves normally work within 0–70°. Widely used in petroleum, gas, chemicals and water treatment, and in power-station cooling-water systems. 2. Cam-flex valve (eccentric rotary valve, Fig. 8-2i) — a new-type valve whose spherical plug centreline is offset from the shaft; the shaft swings the plug eccentrically into the seat front-and-below. Small, light, reliable, easy to maintain, versatile, low flow resistance — suits viscous duties, performing well on lime and slurry.

5. What is the valve flow coefficient, and what does it depend on?

The flow coefficient is the volume or mass of fluid passing through the fully open valve per unit time — it expresses the size the valve must have per process requirements. For incompressible fluids, energy conservation readily yields the flow equation; flow depends on fluid type and properties, operating conditions, plug/seat dimensions and more, so the coefficient is defined under fixed conditions: KV is the hourly volume (m³) passing the valve at 100 kPa differential with fluid density 1,000 kg/m³.

6. How to choose air-to-open versus air-to-close, and controller direct/reverse action in a single-loop system?

The rule: on signal-pressure loss the process and equipment must stay safe. If an open valve on air loss is safest (flow must continue), choose air-to-close; if a closed valve is safest (flow must stop), choose air-to-open. In any loop, controller, valve and process must combine into negative feedback. Procedure: determine the process direction from how the manipulated variable affects the controlled variable; set valve fail action from process safety; then the three elements combining to "negative" fixes the controller action. Example — a level loop with an air-to-open valve: define +A for air-to-open (-A air-to-close) and +B for rising controlled variable when the valve opens (-B falling). Then A×B="+" selects reverse-acting controller; A×B="-" selects direct. Here (+A)×(-B)="-": choose direct-acting.

7. What is the basic construction of an electric actuator?

It receives 0–10 mA DC or 4–20 mA DC from the controller and converts it to output-shaft angular or linear displacement to stroke the valve. Two parts — servo amplifier and actuating unit — as in the block diagram (Fig. 8-3). The controller signal drives the motor through the servo amplifier; a reducer turns the valve, and a position transmitter feeds stem travel back to the amplifier, forming a position-follow system. Position feedback ensures the input converts accurately into stem travel.

8. What are the main control-valve constructions, their features and applications?

(table)

9. What are air-to-open and air-to-close pneumatic actuators, and how to choose?

As signal pressure rises, a valve that opens is air-to-open; one that closes is air-to-close. The choice follows process safety: if full-open is the dangerous state, choose air-to-close; if full-close is dangerous, choose air-to-open.

10. Outline the functions and main types of electric actuators.

The quarter-turn type receives 4–20 mA from the controller and converts it into angular or linear displacement operating valves, dampers etc. for automatic control. Main types: quarter-turn, linear and multi-turn. The quarter-turn actuator, motor-driven, converts the DC input into rotation (0–90°) — suiting butterfly valves and dampers. The linear type turns a motor, reduces speed and outputs linear stroke, operating single-seat, double-seat, three-way and other linear valves. The multi-turn type opens and closes multi-turn valves such as gate and globe valves, generally for local and remote operation.

Editor's notes: Figs. 8-1 to 8-3 were not preserved in the web version; in Q10 the original "4–20A" should read "4–20 mA".

Related Reading

• Actuator Exercises (Part 2)
• Electric, Pneumatic and Hydraulic Actuators Compared