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Control Technologies

Implementation of this variety of control algorithms and associated field devices comes at a cost and consequently requires that some form of return on investment be realised. This return is normally economic, though it can also be in the realms of safety, environment or public policy. Almost all economic justifications are based on the ability to either increase the capacity of a unit operation without the addition of more capital equipment, or by improving the quality of the resulting product. Both conditions are met because of a reduction in the variability of the process. Figure 2 shows a typical normal distribution curve and the confidence factor in per cent that a given condition will be within a certain ‘distance’ of the mean. As shown, greater than 99% of all variability in a process under normal conditions should be within three standard deviations of the mean or average value. This same condition is the basis for all quality charts and the justification for process control because process control reduces the variability in the process.

Figure 2: Characterising Variation

Manual Control

As indicated earlier, manual or open loop control allows the process to run with minimal intervention and consequently this mode of operation will have the greatest variability/standard deviation.

Pneumatic Control

Pneumatic control consists of installing local controllers only to provide regulatory control. Since these controllers are not connected to a central host system/computer, it is not possible to do more complex control than cascade control. As a result, multivariate control and optimisation and their associated benefits are foregone.

The generally accepted rule is that migrating from manual or open loop control to closed loop control provides an improvement by reducing the standard deviation by half.

Local pneumatic control is typically used on remote locations or small skid-mounted operations, such as a gas well head or chemical injection pump. However, with the advent of more recent technologies such as fieldbuses, the economics and benefits of more tightly integrated control is now possible and improved significantly.

Analogue Control

Improvements in electronics made the widespread distribution of electronic devices possible in the 1960s and 1970s. These transmitters and field devices operate from an analogue current that varies between 4mA and 20mA. By connecting all these signals to a central computer system, it is now possible to not only improve the accuracy and response of the metering and controlling technology but also to control the interaction between the various parameters and implement higher levels of operation such as multivariate control and optimisation.

Because of this higher accuracy, the standard deviation of the process has also been reduced, though not as significantly as the move from manual to closed loop control.

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