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