Wireless Integration for Better Performance of Process Instrumentation a report by Gareth Johnston Global Wireless Product Manager, Measurement Products, ABB
In today’s economic climate, maximising plant assets and reducing unplanned plant shutdowns has become a focus for reducing costs and maximising productivity. Currently, potentially valuable information acquired by process instruments is often left stranded in the field. This information could be monitored if a suitable communications pathway back to the host control system were created. This article will describe a wireless solution to unlock the stranded information and release it to higher-level asset management applications.
Information Left Stranded in the Field
Over the past few years, intelligent field instruments have benefited from the use of more powerful and low energy microprocessors to offer more features and information. Instruments now have the ability to diagnose both device and process connection conditions to improve availability. Instrument configuration can be carried out via a selection of remote communications options including digital fieldbus and highway addressable remote transducer (HART) 4–20 mA, or via a local interface such as a ‘Through The Glass’ menu system at the instrument display. There are also multivariable instruments with several process variables available from fewer process connections. It is surprising, therefore, that little use is made of this information.
HART communication is the dominant communication protocol used in process industry today; however, it is often used only during the commissioning of field instruments. Typically, a HART hand-held tool is connected directly to the field instrument, where parameters such as range and transducer type can be set. Once the hand-held tool is disconnected, all the remaining instrument information is left stranded within the instrument unless a pathway back to the host is available. It has been estimated that only 10 % of the 30 million HART instruments installed since 1990 have a pathway back to the host; an example would be skid modules on a production plant. A production plant may have between 15 and 20 serial networks from a skid module back to the host control system. Each skid would incorporate control equipment but would normally not provide remote communications for the associated field instruments. If an instrument communications network could be retrofitted, then operations and maintenance could take full advantage of this stranded instrument information for functions that include the following.
Process Monitoring
Some instruments, such as flow meters, often hold other process data such as totalisers, density or temperature, all of which are useful for process monitoring.
Condition Monitoring
Process connection issues (e.g. plugged impulse line detection for differential pressure flow or control valve condition monitoring by
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signature analysis) and instrument issues (e.g. analyser consumable usage or analyser probe condition) are monitored by the instrument, where an alert is set and made available via remote communications.
Remote configuration
An engineer is able to remotely access the instrument configuration to make alterations or manage a central configuration database.
The value of the information stranded within the instrument will largely depend upon the asset it is measuring or controlling, and also upon the capabilities of the instrument itself. Some analysers, for example, contain buffer fluids to allow periodic calibration; these buffer tanks will need refilling and this requires a maintenance alert. Other plant assets with moving components (e.g. drives or valves) would also have the ability to predict maintenance requirements based upon activity.
Upgrade to Wireless and Unlock Process and Asset Information
It is possible to modify an existing 4–20 mA HART loop to add remote communications access with the use of a wired HART multiplexor. Access to the instrument HART data is via a serial link from the multiplexor to the asset management system. As the multiplexor is connected in series with the 4–20 mA, then the best location for it is in the marshalling cabinet where each instrument signal would be connected in turn.
The addition of HART multiplexors (see Figure 1) to an existing installation is possible; however, there are issues to be aware of:
• The most recent panel wiring diagram should be correct and available. • •
Adequate space should be available in the marshalling cabinet.
It may be necessary to wait for a plant shutdown as each loop requires rewiring.
A wireless solution can provide remote instrument access without modification to the marshalling cabinet by the use of a plug-in wireless adapter at the instrument itself (see Figure 2). The wireless adapter can be powered via the 4–20 mA loop using energy harvesting techniques so no battery is required. Such a solution would provide a low-cost, point-by-point means to add instruments to a wireless network communicating information back to the host system.
The wireless adapter is often connected at a spare instrument cable gland and becomes part of a mesh network back to a gateway device connected to the host system. In the case of the skid application, the gateway would be located near to the control equipment (programmable logic controller [PLC] for example) using the Ethernet link back to the host.
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