What is the difference between an actuator and a positioner?

Actuators are essential components in many industrial processes, enabling precise control and the automation of valve operation. Actuators play a pivotal role in regulating fluid/gas flow, maintaining ideal pressure levels, and ensuring optimal process conditions.

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Actuators

In Block 5, ‘Controls Theory’, an analogy was used to describe simple process control:

• The arm muscle and hand (the actuator) turned the valve - (the controlled device).

One form of controlling device, the control valve, has now been covered. The actuator is the next logical area of interest.

The operation of a control valve involves positioning its movable part (the plug, ball or vane) relative to the stationary seat of the valve. The purpose of the valve actuator is to accurately locate the valve plug in a position dictated by the control signal.

The actuator accepts a signal from the control system and, in response, moves the valve to a fully-open or fully-closed position, or a more open or a more closed position (depending on whether ‘on/off’ or ‘continuous’ control action is used).

There are several ways of providing this actuation. This Module will concentrate on the two major ones:

• Pneumatic.
• Electric.

Other significant actuators include the hydraulic and the direct acting types. These are discussed in Block 7, ‘Control Equipment: Self-Acting Controls’.

Pneumatic actuators – operation and options

Pneumatic actuators are commonly used to actuate control valves and are available in two main forms; piston actuators (Figure 6.6.1) and diaphragm actuators (Figure 6.6.2)

Piston actuators

Piston actuators are generally used where the stroke of a diaphragm actuator would be too short or the thrust is too small. The compressed air is applied to a solid piston contained within a solid cylinder. Piston actuators can be single acting or double acting, can withstand higher input pressures and can offer smaller cylinder volumes, which can act at high speed.

Diaphragm actuators

Diaphragm actuators have compressed air applied to a flexible membrane called the diaphragm. Figure 6.6.2 shows a rolling diaphragm where the effective diaphragm area is virtually constant throughout the actuator stroke. These types of actuators are single acting, in that air is only supplied to one side of the diaphragm, and they can be either direct acting (spring-to-retract) or reverse acting (spring-to-extend).

Reverse acting (spring-to-extend)

The operating force is derived from compressed air pressure, which is applied to a flexible diaphragm. The actuator is designed so that the force resulting from the air pressure, multiplied by the area of the diaphragm, overcomes the force exerted (in the opposite direction) by the spring(s).

The diaphragm (Figure 6.6.2) is pushed upwards, pulling the spindle up, and if the spindle is connected to a direct acting valve, the plug is opened. The actuator is designed so that with a specific change of air pressure, the spindle will move sufficiently to move the valve through its complete stroke from fully-closed to fully-open.

As the air pressure decreases, the spring(s) moves the spindle in the opposite direction. The range of air pressure is equal to the stated actuator spring rating, for example 0.2 - 1 bar.

With a larger valve and/or a higher differential pressure to work against, more force is needed to obtain full valve movement.

To create more force, a larger diaphragm area or higher spring range is needed. This is why controls manufacturers offer a range of pneumatic actuators to match a range of valves – comprising increasing diaphragm areas, and a choice of spring ranges to create different forces.

The diagrams in Figure 6.6.3 show the components of a basic pneumatic actuator and the direction of spindle movement with increasing air pressure.

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Direct acting actuator (spring-to-retract)

The direct acting actuator is designed with the spring below the diaphragm, having air supplied to the space above the diaphragm. The result, with increasing air pressure, is spindle movement in the opposite direction to the reverse acting actuator.

The effect of this movement on the valve opening depends on the design and type of valve used, and is illustrated in Figure 6.6.3.

There is however, an alternative, which is shown in Figure 6.6.4. A direct acting pneumatic actuator is coupled to a control valve with a reverse acting plug (sometimes called a ‘hanging plug’).

The choice between direct acting and reverse acting pneumatic controls depends on what position the valve should revert to in the event of failure of the compressed air supply. Should the valve close or be wide-open? This choice depends upon the nature of the application and safety requirements. It makes sense for steam valves to close on air failure, and cooling valves to open on air failure. The combination of actuator and valve type must be considered.

Figure 6.6.5 and Figure 6.6.6 show the net effect of the various combinations.

Effect of differential pressure on the valve lift

The air fed into the diaphragm chamber is the control signal from the pneumatic controller. The most widely used signal air pressure is 0.2 bar to 1 bar. Consider a reverse acting actuator (spring-to-extend) with standard 0.2 to 1.0 bar spring(s), fitted to a direct acting valve (Figure 6.6.7).

When the valve and actuator assembly is calibrated (or ‘bench set’), it is adjusted so that an air pressure of 0.2 bar will just begin to overcome the resistance of the springs and move the valve plug away from its seat.

As the air pressure is increased, the valve plug moves progressively further away from its seat, until finally at 1 bar air pressure, the valve is 100% open. This is shown graphically in Figure 6.6.7.

Now consider this assembly installed in a pipeline in a pressure reducing application, with 10 bar g on the upstream side and controlling the downstream pressure to 4 bar g.

The differential pressure across the valve is 10 - 4 = 6 bar. This pressure is acting on the underside of the valve plug, providing a force tending to open the valve. This force is in addition to the force provided by the air pressure in the actuator.

Therefore, if the actuator is supplied with air at 0.6 bar (halfway between 0.2 and 1 bar), for example, instead of the valve taking up the expected 50% open position, the actual opening will be greater, because of the extra force provided by the differential pressure.

Also, this additional force means that the valve is not closed at 0.2 bar. In order to close the valve in this example, the control signal must be reduced to approximately 0.1 bar.

The situation is slightly different with a steam valve controlling temperature in a heat exchanger, as the differential pressure across the valve will vary between:

• A minimum, when the process is calling for maximum heat, and the control valve is 100% open.
• A maximum, when the process is up to temperature and the control valve is closed.

The steam pressure in the heat exchanger increases as the heat load increases. This can be seen in Module 6.5, Example 6.5.3 and Table 6.5.7.

If the pressure upstream of the control valve remains constant, then, as the steam pressure rises in the heat exchanger, the differential pressure across the valve must decrease.

Figure 6.6.8 shows the situation with the air applied to a direct acting actuator. In this case, the force on the valve plug created by the differential pressure works against the air pressure. The effect is that if the actuator is supplied with air at 0.6 bar, for example, instead of the valve taking up the expected 50% open position, the percentage opening will be greater because of the extra force provided by the differential pressure. In this case, the control signal has to be increased to approximately 1.1. bar to fully close the valve.

It may be possible to recalibrate the valve and actuator to take the forces created by differential pressure into account, or perhaps using different springs, air pressure and actuator combinations. This approach can provide an economic solution on small valves, with low differential pressures and where precise control is not required. However, the practicalities are that: 

• Larger valves have greater areas for the differential pressure to act over, thus increasing the forces generated, and having an increasing effect on valve position.
• Higher differential pressures mean that higher forces are generated.
• Valves and actuators create friction, causing hysteresis. Smaller valves are likely to have greater friction relative to the total forces involved.

The solution is to fit a positioner to the valve/actuator assembly. (More information is given on positioners later in this Module).

Note: For simplicity, the above examples assume a positioner is not used, and hysteresis is zero.

The formulae used to determine the thrust available to hold a valve on its seat for various valve and actuator combinations are shown in Figure 6.6.9.

Where:

A = Effective area of diaphragm

Pmax = Maximum pressure to actuator (normally 1.2 bar)

Smax = Maximum bench setting of spring

Pmin = Minimum pressure to actuator (normally 0 bar)

Smin = Minimum bench setting of spring

The thrust available to close the valve has to provide three functions:

1. To overcome the fluid differential pressure at the closed position.
2. To overcome friction in the valve and actuator, primarily at the valve and actuator stem seals.
3. To provide a sealing load between the valve plug and valve seat to ensure the required degree of tightness.

Control valve manufacturers will normally provide full details of the maximum differential pressures against which their various valve and actuator/spring combinations will operate; the Table in Figure 6.6.10 is an example of this data.

Note: When using a positioner, it is necessary to refer to the manufacturer’s literature for the minimum and maximum air pressures.

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