Communicating Diagnostic Data in a Legacy Environment
A pulp and paper mill in North America recently solved a nagging problem with a misbehaving magnetic flowmeter by using a wireless device to report diagnostic information that was not coming through the wired I/O. The mill’s instrumentation and control system architecture is largely conventional, primarily using wired, 4-20 mA field instruments and valves that are controlled by programmable logic controllers (PLCs) and distributed control systems (DCSs). More recently, the engineering staff installed a new WirelessHART network (IEC 62591) in the powerhouse area to facilitate learning about wireless instrumentation and the practicality of deploying additional field devices without the normal wiring costs. An Emerson Smart Wireless Gateway device located on the Ethernet plant information network provides communication with the wireless network. Host devices located on the plant information network can access and integrate real-time data from the wireless devices for improved mill operations.
The problem device was in the waste treatment area of the mill which is typically about 500 ft from the powerhouse area where the wireless network is based. The waste treatment area treats various effluents from the mill and makes them environmentally friendly and safe for discharge. The field devices installed at the waste treatment area include various 4-20 mA with HART instruments for measuring process variables such as pressure, temperature, flow, and level that are directly wired to a PLC. The PLC is configured to display the process values at an operator console in a control room. Both the control room and the instrument shop are located several hundred feet from the field instrumentation.
A magnetic flowmeter configured to measure effluent flow in the waste treatment area suddenly became a focus of attention for the mill personnel. This particular flowmeter started to experience random, intermittent failures that caused it to report unacceptable variability in the effluent flow rate. In a typical reporting scenario that was repeated two to three times a month, the instrument technician would receive the dreaded phone call in the wee hours of the morning from the area operator informing him that the control room computer screen was showing the effluent flow rate to be dangerously close to the allowable limit. Shortly after the call, the instrument technician would have to reach the process area where the flowmeter was mounted, read the value directly from the flowmeter display, and verify that the effluent flow rate was normal, within range, and stable. Conflicts would then arise when the instrument technician would see stable effluent flow values at the flowmeter that would not be consistent with the information available to the operator at the control room. There was no obvious process or environmental condition that would trigger the intermittent failure. Being random and intermittent in nature, it was not possible for the mill personnel to re-create the failure for diagnostics and testing.
This problem was treated seriously by mill management due to the risk of potential violations of environmental regulations and the risk of causing a mill upset by taking corrective action based on faulty information. The situation was proving to be beyond the skill set of the instrument technicians, and negatively impacting production while the operator attempted to resolve whether the effluent flow rate had truly changed, or was stable but falsely being displayed on the control room monitor.
Tracing the cause
The mill personnel working with Emerson Process Management experts agreed that multiple factors may have contributed to the intermittent failure problem. Magnetic flow sensors, in general, are very sensitive to electrical noise. The random and intermittent nature of the failure generally suggested that a potential ground loop noise problem may exist with the flowmeter caused by two or more devices connected to a common ground through different paths. As many experienced troubleshooters know, investigations of intermittent failures are often very costly and time consuming. While the flowmeter had the capability to generate diagnostic data, the network could not access it, which is a common problem in many process industries.
Mill managers considered various solutions for resolving the problem, examining capital and labor costs, time to implement, resources required, risk/reward analysis, process downtime required, and so forth. The alternatives were grouped into three general categories:
• Extensive replacement of assets
• Limited replacement of assets, and
• Enhancing the functionality of existing assets.
The first was the most costly and consumed the most amount of time to implement. It included replacement of process equipment assets (such as pumps, pipes, etc.) and addition of new I/O multiplexing cards to interface the flowmeter to the DCS instead of the PLC.
The limited replacement option focused on replacing the flow transmitter, flow tubing, and rewiring the flow loop.
The third option to enhance the functionality of existing assets emerged as the most attractive and easily affordable since the mill had already installed a WirelessHART network within 500 ft of the flowmeter.
Virtually all HART instruments can provide access to diagnostics and process data, yet this valuable information is often unused since many older legacy systems are wired to receive 4-20 mA signals but are not equipped to receive HART communications. Many directly wired HART multiplexer products are available to access this data. However, in this case the wired approach was dismissed as too expensive, complicated, and time consuming compared to the WirelessHART approach. Upgrading traditional 4-20 mA field devices with Emerson’s THUM Adapter is an easy and cost-effective strategy to access the stranded diagnostic and process information. It can retrofit any two- or four-wire HART device, without special power requirements, to enable wireless transmission of measurement and diagnostic information in parallel with the process variable. It can be mounted directly to the device transmitter or separately. Such devices operate as components of the self-organizing wireless field network, delivering field intelligence to enable improvements in quality, safety, availability, operations, and maintenance costs.
In this application, the location of the effluent flowmeter in the waste treatment area was not optimum to facilitate reliable communication between the THUM Adapter and the wireless network installed in the powerhouse. To improve reliability of communication, the device was mounted separately from the flowmeter at an elevated level with improved sight lines, thereby creating a wireless antenna for future wireless devices.
Implementation was simple given that the necessary infrastructure was already in place. In addition to the wireless gateway, the plant had also added an Emerson Asset Manager System (AMS) platform, so adding one more device to the system was a minor effort when compared to the other options. Once the THUM was activated with the diagnostic option, the AMS device manager had remote, on-line access to the diagnostic and process information. This allowed an instrument technician to validate the flowmeter measurement from the plant control room rather than making trips to the field.
Tracking it down
Since the wireless transmitter is powered by the 4-20 mA HART device, it is isolated from spurious ground-loop signals that may get introduced into a piping system from many different sources. Thus, if a ground-noise-induced spike appears on the wired flowmeter signal, it is not automatically transferred to the wireless transmitter output.
The AMS is able to monitor continuously for potential ground faults and wiring faults in the magnetic flow transmitter from a remote location. An alert monitor can be configured to be triggered when the process noise exceeds a threshold.
Adding the ability to access HART data via the wireless adapter allows technicians to work with the AMS suite to troubleshoot field devices from their desk so they can optimize maintenance schedules, minimize downtime, and reduce the time spent in hazardous areas. Online access to device diagnostics allows users to monitor devices continually and predict a potential problem before it causes a plant shutdown. It can also improve maintenance efficiency since outdated practices can just as easily cause excessive maintenance activity. Some industry experts estimate that as many as 75% of control valves installed in the field are routinely removed from service for performing unnecessary maintenance. Using diagnostic data coupled with an appropriate analysis platform such as AMS ValveLink software can easily enable digital valve controllers to support in-service valve testing, alert monitoring, and valve position trending to avoid unnecessary maintenance and improve maintainability.
The mill is planning to use an OPC link in the powerhouse to monitor the totalizer function built into the magnetic flowmeter. Additional variables such as analog output may also be accessed via OPC. Such information enables performing an overflow analysis of the effluent by validating the total number of gallons passing through the flowmeter.
Wireless instrumentation and wireless extensions of wired networks offer the ability to add new measurement points to previously hard-to-reach, hazardous, or unaffordable locations throughout the mill, reaching previously inaccessible information. Such capabilities can provide the means to avoid undesirable situations like a costly environmental spill or an unplanned shutdown before they happen. In this particular case, the mill realized quality improvements, reduction of unplanned downtime, and lower maintenance costs by eliminating the intermittent failure problem with minimal investment. With no ground noise issues to contend with, the improved accuracy and reliability of the effluent flow measurement improved operation and safety of the waste treatment area.
Steven Moore is a metrology technician supervisor for Emerson Process Management.