Understanding the Complexity of Heat Tracing Systems
Misinterpretation of the information provided by heat tracing monitoring and control systems is common.
Electronic monitor and control systems are generally accepted as having a place in the heat tracing application arena. Questions surrounding these systems do not relate to electronic versus mechanical differences, but focus on what features are necessary and what configurations are needed.
From a simplistic perspective, present day heat trace microprocessor-based temperature control and monitoring systems were developed based on the concept of mechanical thermostat replacement. It is true that the systems presently on the market have high accuracy, repeatability and most importantly, diagnostics to warn of potential application disruption. But, what do you need and receive from these units that is not available from mechanical thermostats?
Temperature Alarms. These alarms serve as a prewarning of potential process disruptions and insulation deterioration. However, the main drawback of these alarms is that they are not system wide. The indicated status and associated alarms are valid only at the point of sensor installation.
Current Draw Alarms. Current draw alarms monitor basic system integrity. Verifying electric heater system functionality with current monitoring is only truly effective when considering constant wattage style cables such as series heater circuits. In these cable types, the current draw is definable, and therefore predictable. Any significant deviation in operational measured current would reflect an alteration of the heater's ability to deliver the designed power. If a cable cannot deliver the designed power, it cannot satisfy the required heat input during extreme conditions. Problems with this alarm relate to cable types such as self-regulating or power limiting, which will alter current draw during normal operation.
System intelligence is not built into controllers to automatically predict acceptable tolerance based on measured temperature. Although this function is theoretically possible, how complex would the system have to be for the readings to be meaningful? System parameters such as installed length, cable down turn ratios as well as numerous temperature readings over the entire system would be required. Coupled with historical base line readings, a system operational footprint is required to provide tight tolerance. Although this is technically possible, does the average heat trace user want this much complexity? Typically not.
Simplicity seems to be a lost art. Whether dealing with PC-based programs or industrial instrumentation, systems and requirements are becoming more complex while product life cycles are getting shorter.
Most systems will benefit from continual improvement and enhanced capabilities, but there must be a balance between system enhancements and how easily they are understood and maintained by the end user. Over the years, systems have become equipped to provide more and more information. Interpreting and understanding the true meaning of the information has begun to require a higher skill level. Misinterpretation or undo concern are becoming the norm.
What enhancements and added system attributes are beneficial to the end user, and how do you implement them without detracting from a system's usefulness and reliability? To answer this question, look at the industry changes during the past few years. There has been growth in computerization usage and familiarity, coupled with a reduction in full-time, long-term staff. Most facilities are looking at a significant reduction in full-time staff including a loss of senior employees who know where and why enhancements have been made. Thus, the continuous growth of in-house knowledge is severely curtailed.
One area that must be addressed in this new environment deals with the issue of how to get information to a central point where it can be accessed by relevant parties. From a practical perspective, the cost of the actual monitor and control device can be swamped by the installation cost. The expense of running conduit and wire can be significant. If cabling can be reduced or eliminated, the actual cost of ownership of a heat tracing system can be reduced without affecting performance or features.
Two philosophies for reducing cable requirements exist. One deals with locating the controller in the field and communicating the information from that device via a shared communication line back to a central graphical interface. This issue addresses communication between a central PC or DCS and intelligent field devices, and it is well accepted in industry. The second philosophy deals with transmission of the measured field parameters back to a central control mechanism utilizing existing cabling.
When looking at transmission of measured field parameters, review the signals required to monitor and control a heat tracing system and where these signals can be generated. You will see that temperature, current and ground fault reflect the basic signals. With regards to any temperature responsive cable, current monitoring is not as beneficial a factor as with constant wattage style cables.
The temperature of the system being protected is the one signal that must be measured in the field, under insulation. Typical measurement is with three-wire RTDs. This signal must get from the pipe or vessel back to the device making the decision about out of tolerance alarms and control. Typically, this is done with dedicated shielded triad cabling. But, what if you could eliminate this cabling and get the signal back to the monitor and control device?
Approximately 10 years ago, two controller manufacturers tried to utilize power cabling to transmit the measured temperature back to the decision device. One firm met with major failures and the other firm met with limited success. The basic concept of both companies was sound: superimposing an information carrier signal on top of 60 Hz standard power distribution wiring to transmit the temperature back to the power distribution center. At the distribution center, the signal was stripped off and interpreted in relation to the actual field-measured temperature. The reasons for the lack of overall success and industry acceptance are many, but overall the systems did not provide satisfactory reliability and universal functionality.
New Systems on the Market
With the general advances in heat tracing technology, a more robust and reliable system is being introduced to the marketplace. With this package, virtually all RTD wiring can be eliminated for heat tracing applications. Some minor restrictions apply, but with system planning and forethought, this system is applicable to most new installations as well as many existing ones.
Systems are becoming more complicated. At the same time, plants are seeing reductions in both the quantity of staff as well as the overall long-term knowledge base of that staff. A natural progression is needed to tie the more complex systems with the support structure required to interpret the information being generated. This may be entirely an in-house system with a PC-based package that ties the system together or one that allows for remote access by an external expert who can access and interpret the information.
By having the right information in the right person's hands, in a format he or she is comfortable with, intelligent decisions can be made. These changes will further enhance the ability of an internal, external or contract expert having the ability to either support or completely handle the total heat tracing needs of one or many plants.