Specialized microprocessor-based monitor and control systems provide an alternative to conventional thermostats and general-purpose, solid-state temperature controllers for critical process plant freeze protection and temperature maintenance systems.

Electric heat tracing too often is an afterthought following the design and installation of a piping system designed to transfer critical fluids at a target temperature from one location in a plant to another. Until recently, conventional practice was to specify and install a mechanical thermostat, adjust the setpoint and walk away. While mechanical thermostats generally performed well for low temperature and noncritical applications, this approach has several drawbacks in a modern process environment. With a mechanical thermostat:

• Setpoint dial does not provide precise temperature settings.

• Deadband typically varies within +/-10^oF of dial setpoint.

• Make/break contact wear leads to poor response and increased deadband drift.
• Service life and reliability are directly linked to contact wear.

• Explosionproof enclosures are required in hazardous locations.

• Relatively short capillary tube (less than 10') requires that the device be mounted at tracer power point on pipe. This position often is physically difficult to access.
• Thermostat switch loads generally are limited to 15 A and 240 V or less.

• Visual indication or monitoring of heat tracing system functionality is absent.

Mechanical thermostats have no means to alert users that the heat tracing system cannot meet design conditions due to heater cable performance degradation or damage, a damaged thermostat capillary, tripped breaker or reduced thermal insulation performance. Historically, this lack of performance has been first noted by line blockages or products being off specification. In addition, mechanical devices seldom are located in easily accessed locations. Because the capillary bulb must be mounted on the pipe, it is restricted to installation within 7 to 10' of the pipe. Also, the components must be physically protected against mechanical damage that would render the unit inoperative or significantly inaccurate.

In 1990, several designers and manufacturers began building electronic heat tracing controllers by combining general-function, off-the-shelf components. While these modules were an improvement over mechanical thermostats, lack of functionality and design consistency made each module a hybrid custom assembly largely unrelated to other, similar equipment installed in the plant. Operators simply were not equipped to handle the variety of custom packages. In addition, some designers of hybrid systems tried to adapt components not specifically designed for installation in the harsh environment of petrochemical facilities, leading to control system failures.

Other attempts to add heat tracing circuit functionality feedback included "end of circuit" lamps, which actually provided no information other than that the heater cable was connected to power and the lamp filament was not broken. Similarly, ammeter panels confirmed the heater cable was powered but, particularly in the case of self-regulating heater cables, provided no information on the health of the tracer. Neither approach truly indicated operating status or warned of developing problems with the heat tracing.

Early Microprocessor Systems

In the early 1990s, the first microprocessor modules designed and manufactured specifically to continuously monitor and control heat tracing cable systems were introduced. While somewhat primitive by today's standards, these control/monitors provided significant improvements over technology being utilized at the time. The first microprocessor-based controls provided important status displays as well as programmable control and alarm features. For example, the maintain temperature could be programmed with a fixed 5^oF or less deadband. In addition, the system displayed continuous temperature status with high/low temperature alarms; operational current status with high/low alarms; and ground fault leakage status with high alarm.

The control/monitor modules were available with solid-state relays, which were not subject to wear failure like mechanical relays and, when properly designed and installed, provided high levels of reliability and long life. Either three-wire 100 ohm RTDs, Type J thermocouples or thermistors could be used, which offered a level of temperature control accuracy and repeatability unavailable with mechanical thermostats.

More importantly, the control/monitor modules could be located remote from the trace pipe and actual heat tracing sensor, so operators could observe a large block of heat tracers from a centralized location. Heat tracer maintain temperatures also could be revised or modified at the module when process conditions changed. Some diagnostic procedures could be implemented without going to the actual pipe location and, in many cases, the need for scaffolding and a "hot work" permit when the tracer was in a hazardous location was eliminated.

For the first time, process plant operators and engineers had the tools to manage an entire tracing system from a defined location. The introduction of microprocessor-based temperature control/ monitors reduced operating costs, increased efficiency and improved reliability.

Temperature Control/Monitors Improve

In the past three to five years, process plants have begun to standardize on microprocessor-based temperature status and surveillance modules. Such equipment has refined and advanced heat tracing management. Tracer status modules are available in standard configurations ranging from 1- to 50- point monitoring configurations. Several features ensure good control.

Data Display Modules. A back-lit LCD panel displays operational temperatures, currents and alarm status as well as programmed parameters. Separate LEDs indicate present and alarm status, power to the heat tracing circuit(s) and pending alarms affecting the system.

Alarm Relay. Alarm relay modules are provided with a normally open, held-closed, solid-state alarm relay rated 1 A at 240 VAC, providing fail-safe interrogation. A secondary alarm relay and inverse operation also are available in some modules.

Automatic System Verification. One of the major drawbacks of mechanical thermostats is the absence of system functionality feedback. Identification of nonfunctionality is accomplished by either the process going off specification, discovery of line blockages or manual system checks. By contrast, most control/monitor modules have an integral feature to cycle the system at regular, predetermined intervals. By monitoring operational current and ground leakage, system abnormalities can be readily identified prior to system disruption.

Ground Fault Monitoring and Trip. In the last few years, the National Electrical Codes for North America have mandated the monitoring of system ground leakage current for all industrial heat trace systems. To satisfy this requirement, most microprocessor-based systems now incorporate ground fault monitoring as a standard component. By contrast, mechanical systems require an additional external device such as ground fault breakers.

Current Clamping. This feature controls startup current and limits the electric tracer thermal output for temperature-sensitive applications using solid-state relays to limit the maximum power to the heat tracer.

Soft Start/Ramped Voltage. Many control/monitors also incorporate soft-start capability. Designed to minimize current at startup, this feature is valuable when using self-regulating tracers, which have a high startup current when energized at any ambient.

Electrical Classification. Because the control/ monitor module typically utilizes solid-state relays and components, certification agencies have approved the microprocessor-based module for installation in Class I, Division 2, Zone 2 (hazardous) locations without requiring cast or explosionproof-rated enclosures. Therefore, the modules can be located in easy to access, unprotected environments in the plant. The user is no longer restricted to locating the control within 7 to 10' of the pipe or in remote, protected environments such as motor control centers.

Communications Package. Another option is a sophisticated control and monitor package to interface with the remote, field-mounted tracer control/monitor modules. Graphical representation of installed tracers operational status, alteration of operational and alarm parameters, and operational and alarm report generation are typical features. The communications package is configurable for specific user installations and provides full heat tracing intelligence, including a graphic isometric display of imported user piping/equipment isometric drawings, for immediate visual understanding of a systems operating status.

Each module can be operated as a stand-alone unit or be connected to a data highway for remote, centralized control and supervisory functions. In addition, it can be wired directly to an industrial PC, distributed control system (DCS) or host computer to provide real-time, online monitoring.

Summary

Historically, electric heat tracing has been controlled by simple bulb and bellows thermostats. These devices continue to garner a significant portion of the heat trace system control market. The benefits of these devices lie in their lower initial capital cost and simplicity. The drawbacks are, however, significant from both a functionality as well as a long-term cost of ownership perspective.

There are many options available to control heat tracing circuits, and there is no single optimal controller selection. The initial installed cost as well as the need for tightness of control and potential cost of system nonfunctionality should be considered to determine the most appropriate heat trace controller for each application. With all of the latest technology advances, the end user has been presented with many previously not available. Simply continuing existing habits does not allow for simple changes which can yield significantly better operational and long-term results with minimal, if any, cost impact.

Links

• Tyco Thermal Controls