During the commissioning phase of a chemical plant in Texas, technicians found a 4-20 mA pressure transducer calibrated for 60 psi at 20 mA in a location that called for 300 psi. All of the field devices had been preconfigured and calibrated before installation. What was the cause of such an error?
The simplest explanation is that it was a case of mistaken identity. Because multiple devices of the same type are common at plants, this device might have been placed at the wrong location. That would mean that the unit configured for 300 psi is installed at another, unknown location. Or, perhaps, the device was set up incorrectly by the factory or installation crew. Regardless, it would take further investigation to pinpoint the cause — and additional commissioning time.
In a second scenario, the device comes preconfigured and calibrated. The configuration is valid but an “as found” test reveals the device is out of tolerance and requires calibration. This shift in calibration may have occurred during shipment or installation. During installation, the most common causes for such a problem are over-tightening, simply dropping the device or, in some cases, incorrect orientation.
HART-enabled 4-20-mA field devices can prevent errors like these. In such devices, the 4-20 mA analog channel is supplemented by a digital channel supporting read/write access of all device data. With HART-enabled software, field devices are configured and calibrated after installation, minimizing errors and commissioning time.
The power of HART
But how can you justify the extra cost of HART? Certainly take reduction in commissioning time into account in return-on-investment calculations. The abundance of data available in every HART device also can minimize installation, operations and maintenance costs; this alone can provide a payback of less than one year. In addition, consider other potential benefits. Online HART systems:
• are simple, high-value, low-risk propositions that are easy to use and maintain.
• allow diagnosis of problems in real-time and prediction of maintenance cycles based, for example, on valve closures rather than the calendar.
• can keep accurate records that meet the compliance and traceability requirements of OSHA and the FDA.
• can help optimize raw material use by enabling tighter process formula control.
• can check for faulty equipment. Every output from a HART device includes diagnostic information. An indication of a problem can prompt action, either automatic or manual, depending upon the criticality of the device. For instance, the device may be monitored at a higher rate to gather trending information or linked to annunciators or predictive maintenance algorithms.
• enable, in batch operations, the rescaling of field devices remotely as process needs change, while confirming the change to the host system to ensure a closed-loop quality control system. Conventional and proprietary systems usually lack this feature.
Quite simply, integration of HART field devices with existing plant control, safety and asset management software leads to increased productivity at lower costs.
According to the ARC Advisory Group, Dedham, Mass., 40-45 million field devices are now installed worldwide — approximately one-half are analog and another one-fourth are HART (Figure 1). So, there is a pre-existing knowledge base among operations and maintenance personnel when upgrading 4-20-mA devices to HART. And these are the people who would use the hardware and software tools, such as handheld communicators and plant asset management software, for communicating with HART devices.
However, there is one overwhelming reason to consider HART for plant retrofits or expansions: the investment in copper wiring already in place. Using a rule-of-thumb of $1 per foot of copper wiring in conduit and assuming 5,000 devices at an average distance of 300 feet, the investment in wiring is $1.5 million. With those same wires used for analog control, you can have read/write access to all HART device data over the existing infrastructure.
The place to start
Implementing a HART strategy begins with the field devices. You must identify the critical areas in the process and decide whether to layer a HART communication channel over the analog channel or combine a HART bus with other field buses.
In a system consisting of thousands of devices, a relative few will be deemed critical. These devices are necessary for controlling the product quality and should have higher reliability and, usually, better accuracy than others. (In the last five years, a new measure of reliability has emerged, the Safety-Integrity-Level (SIL) rating; in general, the higher the SIL rating, the greater the reliability.) These devices may also have a higher level of diagnostics, which can provide more information about the status of a device if monitored.
You probably already have an inventory of field devices. So, first, determine which are HART-capable or could be retrofit with HART devices. Then, identify what HART information is available and what advantages having that information can bring. Every HART device provides 35-40 data items, such as its serial number, calibration date, secondary measurement values or error conditions. In addition, there are device-dependent variables like the totalized values in a flow meter that could be cleared between batches.
At the same time, determine which measurement functions could be combined into one device. Some HART devices, for example, are capable of measuring temperature and pressure. This reduction in the number of field devices provides initial and continuing costs savings.
Making the most of the data
Having HART device data available in the field is advantageous for technicians with handheld communicators and calibrators. Perhaps more importantly, bringing those data into the control room gives operators the ability to monitor and address malfunctions and alarm conditions — and minimizes trips to the field and their associated costs. The data are also useful when creating maintenance schedules with audit trails for the devices.
There are two methods to acquire HART data from field devices: a HART multiplexer-and-host combination that monitors hundreds of devices, and a HART input/output module that teams an analog I/O module with a HART interface.
HART’s digital communication is intended for process control. The digital signal can be slow, especially when handled via old multiplexers. Where faster response times are necessary, the analog signal is used to read measurement values. Machinery control and programmable logic controller applications that require millisecond response times call for higher-speed digital buses.
Accumulating, storing and accessing the data of hundreds of field devices require an application server running a database program known as asset or device management software. That program manages and documents device configurations and calibrations, stores device history and provides audit trail information. Here are some key features to look for in this type of software:
• support for the complete service life of a device, from installation/commissioning and maintenance through decommissioning.
• support for all HART or conventional field devices so you can continue to purchase the best devices regardless of manufacturer.
• the ability to track, record and maintain all activity with field devices so you have an audit trail for compliance with corporate policy and agency rules, such as FDA’s 21 CFR Part 11. The software should provide the necessary documentation and security. It should document when devices are put in service and when devices are retired from service; it should provide an audit trail that lists all changes in configuration and calibration to field devices. Password protection should ensure that only qualified technicians work on devices and also the authenticity of signatures for calibration signoff.
• support for OPC for exchanging data.
• integration with a maintenance, repair and operation (MRO) system to schedule work, needed resources and paperwork such as work orders.
• integration with handheld communicators and calibrators to eliminate “clipboard” commissioning and maintenance. This can reduce labor costs by as much as 50%.
Of course, the software should have the ability to grow as your plant adds product lines and as manufacturers introduce new field devices.
Getting up and running
In a plant with device management software, HART devices are installed in a preconfigured state. Once communication is established, nonconfigurable data, like serial number and firmware revision, are uploaded from the devices while user-configurable data, like zero and span, are downloaded to the devices.
This streamlined installation procedure is especially useful when duplicating a production line. Duplication or cloning can extend right down to the device configurations and those data can be downloaded to the field devices. This can cut the commissioning time in half.
However, because of their extreme flexibility in configuration to allow for the best fit, HART devices are complex to configure. The ultimate configuration of options needs a close review to ensure the devices will handle operating conditions properly and, in the event of a failure, do so in a safe and predictable manner.
The 80/20 rule applies to a plant’s lifetime costs: 80% of the total costs are associated with operations and maintenance and 20% relate to installation and commissioning. The benefits of integrating HART field devices with device management software are even more apparent over the life of the plant.
The greatest impact comes from reduced labor for the repetitive task of configurations. These are performed remotely from the control room or maintenance shop and do not require a visit to the field device, which may be installed in a difficult-to-reach or hazardous location.
Maintenance schedules become more predictive as device lifecycle management software tracks historical data of the devices. Examining trends of equipment performance and scheduling repairs before a problem occurs lead to less downtime. However, this requires a change in the maintenance mindset from being driven by the calendar to being driven by device information. Operators and technicians will require training on how to properly analyze the data, plus management must make a commitment to follow through on the preventive or predictive maintenance needed.
Troubleshooting can be done remotely. When a field inspection is necessary, the technician already will have performed remote diagnostics. And when a device needs replacement, its configuration can be cloned into a new one.
Many field devices remain in service for years. Replacement can pose a configuration issue. Devices having the same model numbers from the 1990s and today have different firmware as HART has evolved from Revision 4 through Revision 6 and manufacturers have added features and capabilities. As a result, the old configuration will result in a “device mismatch” with the new device. “Map” features enable automatic mapping of the old configuration to the new device. The user can update devices with minimal downtime, which can be especially important for device failures and unexpected shutdowns.
Extending the benefits
There is a wealth of information about plant operations in the database created by device management software. Sharing that information with an MRO system is only the beginning. Figure 2 shows how a common database with open standards can link all the software tools of the plant, integrating operations and maintenance with enterprise tools. This leads, for example, to efficiencies for data backup and device ordering.
The software applications that can be linked together include:
• the device management system. It configures and calibrates field devices and tracks/documents changes.
• valve management. This software exercises valves, tracks closures and gets valve signatures.
• the plant automation system. This is the actual control system and acts as supervisor and watches for device failures and error flags.
• MRO software. It communicates with business management software and issues work and purchase orders.
• engineering tools. This is the software used to design control systems.
• the human-machine interface. It provides the operator interface to the monitoring and control software.
• batch historian. It keeps process formulas constant and tracks changes.
Integrating HART device-management software into the enterprise provides benefits to plant operators, technicians, engineers and managers. A fully integrated HART plant allows commissioning of field devices and upload/download of parameters to calibrate and troubleshoot devices, either remotely or locally; all of this is documented to show compliance with company and regulatory requirements.
HART integration pays for itself with productivity gains, maintenance savings and more efficient and accurate recordkeeping.
Dan Prugar is director of marketing and sales for Meriam Process Technologies, Cleveland. E-mail him at [email protected].
