Positioners

For many applications, the 0.2 to 1 bar pressure in the diaphragm chamber may not be enough to cope with friction and high differential pressures. A higher control pressure and stronger springs could be used, but the practical solution is to use a positioner.

This is an additional item (see Figure bellow), which is usually fitted to the yoke or pillars of the actuator, and it is linked to the spindle of the actuator by a feedback arm in order to monitor the valve position. It requires its own higher-pressure air supply, which it uses to position the valve.

Basic pneumatic positioner fitted to actuator pillars (valve not shown)

A valve positioner relates the input signal and the valve position, and will provide any output pressure to the actuator to satisfy this relationship, according to the requirements of the valve, and within the limitations of the maximum supply pressure.

When a positioner is fitted to an 'air-to-open' valve and actuator arrangement, the spring range may be increased to increase the closing force, and hence increase the maximum differential pressure a particular valve can tolerate. The air pressure will also be adjusted as required to overcome friction, therby reducing hysteresis effects.

Example: Taking a PN5400 series actuator fitted to a DN50 valve (see Table in Figure above)

1. With a standard 0.2 to 1.0 bar spring range (PN5420), the maximum allowable differential pressure is 3.0 bar.
2. With a 1.0 to 2.0 bar spring set (PN5426), the maximum allowable differential pressure is increased to 13.3 bar.

With the second option, the 0.2 to 1.0 bar signal air pressure applied to the actuator diaphragm cannot provide sufficient force to move an actuator against the force provided by the 1.0 to 2.0 bar springs, and even less able to control it over its full operating range. In these circumstances the positioner acts as an amplifier to the control signal, and modulates the supply air pressure, to move the actuator to a position appropriate to the control signal pressure.

For example, if the control signal was 0.6 bar (50% valve lift), the positioner would need to allow approximately 1.5 bar into the actuator diaphragm chamber. Figure bellow illustrates this relationship.

 The positioner as a signal amplifier

It should be noted that a positioner is a proportional device, and in the same way that a proportional controller will always give an offset, so does a positioner.

On a typical positioner, the proportional band may be between 3 and 6%. The positioner sensitivity can usually be adjusted. It is essential that the installation and maintenance instructions be read prior to the commissioning stage.

Summary - Positioners

1. A positioner ensures that there is a linear relationship between the signal input pressure from the control system and the position of the control valve. This means that for a given input signal, the valve will always attempt to maintain the same position regardless of changes in valve differential pressure, stem friction, diaphragm hysteresis and so on.
2. A positioner may be used as a signal amplifier or booster. It accepts a low pressure air control signal and, by using its own higher pressure input, multiplies this to provide a higher pressure output air signal to the actuator diaphragm, if required, to ensure that the valve reaches the desired position.
3. Some positioners incorporate an electropneumatic converter so that an electrical input (typically 4 - 20 mA) can be used to control a pneumatic valve.
4. Some positioners can also act as basic controllers, accepting input from sensors.

A frequently asked question is, 'When should a positioner be fitted?

A positioner should be considered in the following circumstances:

1. When accurate valve positioning is required.
2. To speed up the valve response. The positioner uses higher pressure and greater air flow to adjust the valve position.
3. To increase the pressure that a particular actuator and valve can close against. (To act as an amplifier).
4. Where friction in the valve (especially the packing) would cause unacceptable hysteresis.
5. To linearise a non-linear actuator.
6. Where varying differential pressures within the fluid would cause the plug position to vary.

To ensure that the full valve differential pressure can be accepted, it is important to adjust the positioner zero setting so that no air pressure opposes the spring force when the valve is seating.

Figure bellow shows a typical positioner. Commonly, this would be known as a P to P positioner since it takes a pneumatic signal (P) from the control system and provides a resultant pneumatic output signal (P) to move the actuator.

 Typical P to P positioner (gauges omitted for clarity)

One advantage of a pneumatic control is that it is intrinsically safe, i.e. there is no risk of explosion in a dangerous atmosphere, and it can provide a large amount of force to close a valve against high differential pressure. However, pneumatic control systems themselves have a number of limitations compared with their electronic counterparts.

Typical I to P converter

To alleviate this, additional components are available to enable the advantages of a pneumatic valve and actuator to be used with an electronic control system.

The basic unit is the I to P converter. This unit takes in an electrical control signal, typically 4 - 20 mA, and converts it to a pneumatic control signal, typically 0.2 - 1 bar, which is then fed into the actuator, or to the P to P positioner, as shown in Figure bellow

Pneumatic valve / actuator operated by a control signal using I to P converter and P to P positioner

.

With this arrangement, an I to P (electrical to pneumatic) conversion can be carried out outside any hazardous area, or away from any excessive ambient temperatures, which may occur near the valve and pipeline.

However, where the conditions do not present such problems, a much neater solution is to use a single component electropneumatic converter / positioner, which combines the functions of an I to P converter and a P to P positioner, that is a combined valve positioner and electropneumatic converter. Figure  bellow shows a typical I to P converter / positioner.

  typical I to P converter / positioner fitted to a pneumatic valve (gauges omitted for clarity)

Most sensors still have analogue outputs (for example 4 - 20 mA or 0 - 10 V), which can be converted to digital form. Usually the controller will perform this analogue-to-digital (A / D) conversion, although technology is now enabling sensors to perform this A / D function themselves. A digital sensor can be directly connected into a communications system, such as Fieldbus, and the digitised data transmitted to the controller over a long distance. Compared to an analogue signal, digital systems are much less susceptible to electrical interference.

Analogue control systems are limited to local transmission over relatively short distances due to the resistive properties of the cabling.

Most electrical actuators still require an analogue control signal input (for example 4 - 20 mA or 0 - 10 V), which further inhibits the completion of a digital communications network between sensors, actuators, and controllers.

Digital positioners

Sometimes referred to as a SMART positioner, the digital positioner monitors valve position, and converts this information into a digital form. With this information, an integrated microprocessor offers advanced user features such as:

• High valve position accuracy.
• Adaptability to changes in control valve condition.
• Many digital positioners use much less air than analogue types.
• An auto stroking routine for easy setting-up and calibration.
• On-line digital diagnostics.
• Centralised monitoring.

*Using digital communications protocols such as HART^®; Fieldbus, or Profibus.
The current industrial trend is to provide equipment with the capability to communicate digitally with networked systems in a Fieldbus environment. It is widely thought that digital communications of this type offer great advantages over traditional analogue systems.

  Digital positioner

Selecting a pneumatic valve and actuator
In summary, the following is a list of the major factors that must be considered when selecting a pneumatic valve and actuator:

1. Select a valve using the application data.
2. Determine the valve action required in the event of power failure, fail-open or fail-closed.
3. Select the valve actuator and spring combination required to ensure that the valve will open or close against the differential pressure.
4. Determine if a positioner is required.
5. Determine if a pneumatic or electric control signal is to be provided. This will determine whether an I to P converter or, alternatively a combined I to P converter/positioner, is required.

Rotary pneumatic actuators and positioners
Actuators are available to drive rotary action valves, such as ball and butterfly valves. The commonest is the piston type, which comprises a central shaft, two pistons and a central chamber all contained within a casing. The pistons and shaft have a rack and pinion drive system.

In the simplest types, air is fed into the central chamber (Figure bellow), which forces the pistons outwards.

The rack and pinion arrangement turns the shaft and, because the latter is coupled to the valve stem, the valve opens or closes.

When the air pressure is relieved, movement of the shaft in the opposite direction occurs due to the force of the return springs (Figure bellow).

It is also possible to obtain double acting versions, which have no return springs. Air can be fed into either side of the pistons to cause movement in either direction. As with diaphragm type actuators, they can also be fitted with positioners.

 Spring return rotary pneumatic actuator

Air supply
An adequate compressed air supply system is essential to provide clean and dry air at the right quantity and pressure. It is advantageous to install an individual coalescing filter / regulator unit ahead of the final supply connection to each piece of equipment. Air quality is particularly important for pneumatic instrumentation such as controllers, I to P convertors and positioners.

The decision to opt for a pneumatically operated system may be influenced by the availability and / or the costs to install such a system. An existing air supply would obviously encourage the use of pneumatically powered controls.