Intro4u2u

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.

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 6.6.18a), 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 6.6.18b).

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.
Fig. 6.6.18 Spring return rotary pneumatic actuator Fig. 6.6.18 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.