Many plants have legacy control systems with analog input (AI) and analog output (AO) channels that cannot bring in the HART digital signal superimposed on the 4-20mA analog signal. She notes:
Control systems that are HART compatible can interpret the primary variable from the HART field device (HART systems will also be able to get digital data and not just primary variable), but analogue systems will not be able to understand the digital data – for example, the device tag and descriptor fields. The analogue systems are unable to read the secondary or tertiary variables from the field device, if it is a multivariable transmitter, and the diagnostic data.
HART multiplexer devices can route the signals from these devices in two directions–one to the control system I/O and the other to an asset management system. She notes the downside of this approach:
…is not a simple procedure, requiring the rewiring of marshalling cabinets which can pose a risk to current operations and be both time-consuming and costly.
…retrofit on any two or four-wire HART instrument, allowing the device to transmit up to four variables and HART status information at a user-configurable update rate. The data is sent wirelessly, via a gateway, to an asset management system, whereby the information can be interrogated by operators and maintenance staff.
Catrine shares how remote monitoring improves maintenance activities. Without access to the diagnostics, there are:
…huge inefficiencies in the way many process level devices are maintained, including routine checks and preventive maintenance on equipment when it’s not needed. If the infrastructure is not in place to communicate the digital HART data to the control room or maintenance area, regular trips to the field may be the only option.
She cites as examples [hyperlinks added]:
…Rosemount 5300 Guided Wave Radar and 5400 Non-Contacting Radar devices can both transmit data to produce an Echo Curve graphic to any host that supports enhanced EDDLs. The Echo Curve is an important troubleshooting tool as it provides full insight to the installation echoes and how they behave over time. Understanding the performance of devices gives a much clearer picture as to when maintenance is required.
Regulatory changes have caused some plants to have to monitor on-line what previously was monitored manually, such as tank overfill protection. In these cases, she notes:
Often, tanks are located remotely and the cost of installing wired devices is very high. Here, wireless presents the obvious solution.
Catrine closes with the point:
Whether you want to install a new measurement point without the high cost of wiring or you would like access to stranded diagnostics and device health information from existing HART instruments, the ability to connect IEC 62591 WirelessHART adapters with process level devices is enabling the full range of HART functions to be accessed in a cost effective way.
Give the article a read if your level measurements do not provide the diagnostic information back to the operations and maintenance staff.