Improving measurement and process performance using smart instruments and asset management systems
Developments in smart instrumentation and asset management systems are offering a raft of new opportunities for today’s process operators.
Global process industry losses amount to $20 billion, or five percent of annual production, caused by unscheduled downtime and poor quality. ARC estimates that almost 80 percent of these losses are preventable and 40 percent are primarily the result of operator error.
Smart instrumentation combined with asset management offers the opportunity to minimise these losses. Smart instrumentation first appeared in process and power plants in 1983. Since then, significant advances in sensor and microprocessor technology have resulted in a new generation of intelligent field devices offering more information than users could have dreamed of in the past.
Smart instruments in the field measure or directly affect single or multiple plant variables, contain a microprocessor for processing data, and are commercially available “off the shelf.” These instruments include not only sensors for measurements and communications, but also actuators, valves, motor variable speed drives, and other control equipment. They allow operators and engineers to gain more useful information about the process and the device itself which was previously locked away.
Today’s plant engineers and operators have access to such functions as power management, maintenance systems, process automation, asset optimisation, and safety systems. Standards such as NAMUR NE107 are steadily improving the Human Machine Interface (HMI), making it easier to commission, configure, and manipulate instrument parameters.
Benefits of smart instruments
Smart instruments are characterised by:
- Fast, bidirectional digital-communication capability
- Enhanced sensor, electronics, and process diagnostics
- Increased measurement accuracy under varying operating conditions
- Better record keeping
- Capability for wireless communications.
Process engineers are no longer limited to a process variable measurement from a unidirectional 4-20 mA analogue signal. Intelligent instruments in fieldbus networks offer remote configuration and calibration, data beyond process variables, diagnostics, and much more. These systems are decreasing the cost of process instrumentation while providing increased informational value.
The key benefits of smart instrumentation include:
- Scaled process variable: No further scaling is needed outside of the instrument, reducing complexity and the possibility of introducing error
- Self-validation/status: Indication of instrument’s state and health, alerting operators to a change in quality of measurement and potential problems
- Tag-number: Clear P&ID identification of the device within the network, reducing potential errors
- Description: A written definition of the instrument and its application more clearly identifies its role
- Time stamp: Provides a real-time record of process variable information
- Serial number: Can be synchronised with remote instrument life-cycle management systems and maintenance information
- Traceable validation: Indication that device calibration is valid, often addressing ISO 2001 Chapter 7.6
The development of bus communications has drastically increased the amount of transmissible information. Also, bidirectional communication of digital information can take place between a field device and a system, and between field devices.
To make the most of communication improvements and to satisfy more advanced needs, big changes are taking place within field devices, especially those with wireless capabilities.
Maximising plant assets and reducing unplanned plant shutdowns have increasingly 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 communications pathway back to the host control system were created.
Typically, existing installed instruments have a built-in HART communication protocol, normally used during instrument commissioning. With the arrival of wireless standards, such as WirelessHART, wireless adapters can now be fitted to existing HART instruments, providing a cost-effective and secure communication pathway back to remote condition monitoring applications. With the arrival of battery powered standalone devices, this capability has been further extended, opening up new opportunities for quickly and safely installing wireless devices in both safe and hazardous areas.
Current estimates indicate that only 10 percent of the 30 million HART instruments installed since 1989 have a digital pathway back to the host. Remote digital access would allow operations and maintenance to take full advantage of this stranded instrument information. WirelessHART adapters for field instruments eliminate significant rewiring costs and provide an ideal solution for secondary monitoring applications. Recovered information could include, for example:
- Multivariable process data
- Instrument condition monitoring
- Degrading valve performance
- Sticking valve
- Analyzer calibration required
- Low level of pH calibration buffer stock
- Instrument over-pressure counter
- Mass flow and totaliser
- Mass flow and density/temperature
Wireless communications can improve plant uptime in three steps. Initially, the instrument identifies a fault and sets an internal alert. Then an application that monitors conditions reads the instrument alert via the WirelessHART network. The asset management system generates a fault report based upon severity. Finally the maintenance or remote support team connects to the field instrument and drills down via HART tools such as DTM (Device Type Manager) to diagnose the fault and arrange repair.
The use of smart and wireless technologies considerably increases the range of information from field instruments. In addition to the measured value, status and alarm messages provide valuable information about plant conditions as well as the reliability of the measured values.
Developments in multi-parameter sensor technology, where one field device detects multiple measured variables, are opening up a raft of new opportunities for measurement across a plant. A traditional analogue transmission system requires one cable for each measured variable. Bus communication supports multivariable transmission. So the field device can transmit all measured variables detected via a single cable. The same goes for control signal transmission to a positioner for an actuator or control valve. Using a bus communication system enables the transmission of multiple data such as control signals, limit signals, and valve opening signals.
Examples of uses for multivariable detection and transmission include:
- Monitoring the condition of the steam heat tracing of differential pressure transmitters by ambient temperature information
- Detecting clogging in impulse lines by static pressure information
Many other pieces of information can also be used to expand measurement and control capabilities. Combining multiple sensor systems in a single pressure transmitter permits simultaneous measurement of differential pressure, absolute pressure and, via an external sensor, process temperature. Additionally, the sensor's internal temperature is measured and recorded for service and diagnostic purposes. The sensor temperature and the absolute pressure can be used to eliminate environmental effects on the sensor.
Improving DP flowmeter accuracy
A single multivariable DP instrument can measure gauge or absolute pressure, differential pressure, and temperature, overcoming the problems associated with multiple instruments by reducing pipe intrusions and the opportunities for leaks while facilitating regulatory compliance.
Three sources of error exist in a DP flow measurement, specifically:
- Minimising transmitter errors
- Minimising errors in gas and steam caused by pressure and temperature variations
- Minimising primary element errors.
Minimising all three sources provides the best accuracy and repeatability.
Based on their experience with traditional analogue systems, many users believe that the transmitter is no longer important when it comes to improving DP flow measurement performance. They believe the transmitter is a 3 to 5 percent device over a 3:1 flow turndown, and that the orifice plate is the main source of error. However, new smart transmitters can dramatically improve performance by taking into account the effects of the various sources of pressure variability and flow errors that can affect DP flow measurement.
Recalculating these components based on the flow rate and temperature significantly improves performance, and can greatly extend the flow range that can be measured accurately with DP Flow. Recognising these issues explains how DP orifice flow measurement can improve from a 3 to 5 percent device to a better than 1.0 percent device.
Equipment uptime for continuous production represents an important factor in improving process plant productivity and overall profitability. Smart instruments can play a key role in optimising the maintenance function toward this end.
Coal pulverising and rotating machinery provide good examples of the benefits of asset management principles. In coal pulverising operations typical of power plants, plant maintenance sometimes has to deal with problems associated with the long impulse lines that transfer pressure to remotely mounted pressure transmitters.
The lines may plug as often as once a week and even once a shift in some cases. A small air purging system in the sensing line may be present to provide positive pressure, attempting to keep the coal out of the sensing line. Even then, this may not be sufficient to stop the problem. Wet coal following a rainstorm, for example, will invariably lead to plugged lines, creating dried ‘mud’ which has to be drilled out by maintenance technicians.
Once impulse lines are plugged, reliability of measurement becomes questionable. Smart pressure transmitters equipped with Plugged Impulse Line Detection (PILD) can quickly alert maintenance departments to measurement problems. On sensing a plugged impulse line, the transmitter displays a diagnostic message while sending a digital and/or analogue alarm.
This capability protects the transmitter while offering predictive diagnostics of the pressure measurement loop. The operating condition of critically important rotating machinery can be monitored continuously. Permanently installed sensors make it possible to communicate vibration information continuously. Vibration levels of support machinery can also be measured periodically in the field by plant personnel using portable equipment.
In both cases, health management software processes the data, providing a complete picture of the operating condition. The ability to overlay frequencies, and match fault frequencies to peaks, allows trained personnel to efficiently analyse the data. Alarm reports enable decision makers to quickly evaluate a situation and take appropriate action to prevent a breakdown.
The aim of the Namur NE107 recommendation is to summarise how to make use of diagnostic data from field devices to support operators to take appropriate actions as required. ABB smart instruments follow the NAMUR “Traffic Light” standard for identifying fault levels, which can be adapted by the customer, depending on the application.
The user must be able to interpret the fault levels and formulate an appropriate response to a diagnostic event. Reactions to a fault in the device may vary, depending on the user's requirements.
Focused asset management supports maximum productivity while incurring minimum costs. Productivity is maximised by fast, reliable start-ups, by adopting predictive maintenance strategies to assure reliability of essential production assets, and by using field-based information and diagnostics to identify and avoid potential trouble. Careful planning and execution of plant turnarounds minimises their duration and extends intervals between them.
A predictive maintenance program can be expected to bring a 1 to 3 percent improvement in product throughput, generating enough additional revenue for payback in three to six months.
Despite the numerous benefits of smart instrumentation, there still remains a long way to go before the benefits of field-based intelligence are fully embraced throughout the process industries. However, growing pressure from all the key areas impacting on business today, from tough trading conditions and health and safety issues to energy costs and environmental concerns, are all driving plant operators to look for ways to work smarter. Intelligent instrumentation can help them do that.
As a manufacturer and supplier of intelligent instrumentation encompassing everything from pressure and temperature measurement through to smart positioners and water and process analysers, ABB is well-placed to advise on the expanded range of possibilities offered by today’s technologies. For more information, email firstname.lastname@example.org or call 0870 600 6122 ref. ‘smart instruments’.
Published in Valve User Magazine Issue 34
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