Selecting and installing suitable analogue instruments is an important requirement in designing a process plant. However maintaining these instruments to ensure efficient and correct operation is an ongoing requirement within the plant. At the centre of all process plants is a DCS / PLC system which performs actions based on operator inputs and measurements received from instruments installed in the plant. Of all the process instruments in a plant, those with analogue actuation and control have significant importance because they are used to regulate and control the operating point of the plant.
The analogue instruments available today can be broadly categorised as either being standard or smart. Standard analogue instruments allow the DCS / PLC to measure a process variable within the plant through a single electrical signal (typically 4 – 20 mA). This family of transmitters are generally calibrated in the field in order to provide the scaling of the 4 – 20 mA signal received by the DCS / PLC.
On the other hand, smart analogue instruments (such as HART Communications enabled instruments) can provide the DCS / PLC with the same 4 – 20 mA process variable measurement as the standard analogue instruments. In addition to this, smart analogue instruments can provide the DCS / PLC with other instrument specific information (such as diagnostics). For a HART enabled instrument, the additional information is obtained through the same 4 – 20 mA signal without losing the measured process variable. This is achieved by using the Frequency Shift Keying standard which superimposes the additional information as a digital signal on the top of the 4 – 20 mA signal. Hence no additional wiring is required to obtain this information.
When comparing these two families of analogue devices from a plant operations perspective, the smart instruments offer superior process control and monitoring within the plant. They essentially provide confidence and certainty that the instrument will do what is asked of it during operation. However this is not the only advantage the smart instruments have over the standard instruments.
Another benefit of using smart instruments is the ability to conduct informed and targeted maintenance on plant instruments during scheduled shutdowns. By setting up the plant’s DCS / PLC to analyse the diagnostic information available from the installed smart instruments, the system can alert operators / maintenance coordinators which valves require an overhaul or which transmitters require recalibration. This type of initiative aims to reduce the opportunity cost of conducting maintenance work on instruments which are operating nominally instead of those which do require attention. Targeted maintenance also provides confidence to operators and process engineers that their plant is operating efficiently and the process is not being influenced or suffering from instruments in a poor operating condition.
This is a small sample of benefits that can be achieved by installing smart instruments in a process plant and this discussion has just touched the surface of the true value that can be unlocked by using smart instruments.
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