Optimizing the industrial electric heat trace engineering process
The application of heat to piping, equipment, or instrumentation to offset heat loss due to a lower ambient temperature, heat tracing is a process that can be applied through several methods, including electricity, steam, glycol, and even hot oil. Providing a basic foundation of the most common heat trace systems and components electrical professionals will likely encounter in industrial applications, this article will focus specifically on electric heat tracing and discuss the makeup of the system, methods for optimization, and tools available to assist in the estimating and design process.
While the general intent of heat tracing is to provide a maintained temperature, there are two commonly designed systems that handle the majority of applications. These are tracing for winterization (commonly referred to as freeze protection) and tracing for process maintenance. Winterization tracing is designed to protect the product from freezing and is typically designed to operate when the ambient temperature falls below a certain level — commonly 40°F to 50°F. Winterization tracing is typically installed on piping, equipment, and instrumentation where water (or other product with similar properties) may be exposed to freezing conditions (Photo 1). A winterization tracing system is simplistic in both design and control.
Heat tracing for process maintenance is much more complex and requires far more design considerations. This method is commonly used where higher temperatures must be maintained, and more exact temperature control is required. With process maintenance tracing, the various product flow paths must be considered during the design stage, which often results in the need for additional heater circuits and more specialized control methods. Process maintenance heat tracing is regularly used to maintain a higher temperature for the purpose of reducing product viscosity and preventing wax or hydrates from forming in the product.
Although there are numerous types of heat trace cables available, they are generally of either parallel or series resistance design. The most common parallel constructed heater cable is the self-regulating type. These cables (Photo 2a) incorporate two bus wires that are surrounded by a semiconductive polymer matrix that is high in carbon content. As the temperature of the matrix increases or decreases, a change in the chemical composition takes place, which results in a higher or lower resistance and more or less heat output. This self-regulating characteristic allows the cable to adjust its watt output at any point along the entire length of the heater circuit, helping to eliminate hot or cold spots. Self-regulating cables are used in a wide range of applications, including applications involving nonmetallic piping and equipment. Featuring a parallel design, these cables may also be cut to any length without changing the overall resistance of the cable.
Series resistance-type heater cables (Photo 2b) use isolated single or multiple resistive conductors to create a heating circuit. A voltage is applied to the conductors, and power output is determined by a combination of the length of the cable and the overall resistance of the conductor. The power output of these cables is relatively constant, and they do not exhibit self-regulating characteristics. Series resistance cables are available with flexible, polymer outer-jackets as well as with a metal jacket commonly known as mineral insulated (MI) cable.
Proper installation of the series resistance cable is critical. As the power output is partly determined by the overall length of the cable, the cable should be installed at the designed length. If the cable is cut too short or too long, the overall resistance will have changed, resulting in a cable that provides either too little or too much heat output. When designing a series resistance heater cable for a hazardous area, caution must also be exercised as these cables do not self-regulate — and may have exceedingly high sheath temperatures during operation.
The next step in the heat trace process is determining the type of control method required for the application. Control of the heat trace system is typically achieved by one of three methods. These include mechanical thermostats, power distribution panels using ambient-sensing thermostat control, or the more advanced microprocessor control. The most simple and cost-effective control for heat tracing is through the use of a mechanical thermostat (Photo 3). This device is normally pipe-mounted, using a capillary bulb to sense the pipe wall temperature and provide basic “on/off” temperature control.
A typical mechanical thermostat will monitor only the temperature, and no alarm functionality is provided. One disadvantage of the mechanical thermostat is that, without alarming, you may not realize there is a failure. If failed-closed, the thermostat will continue to energize the heater cable in a “runaway” condition. This could also present a serious issue, depending on the type of heater cable used, product being heated, and possibly area classification. Another drawback is local access to the thermostat. As the thermostat is mounted to the actual pipe being heat traced, it will often be located in an area that’s not easily accessible. Direct access to the thermostat is required to adjust the temperature set point and provide access for replacement, if required.
When a large amount of winterization tracing is designed, it is common to provide control with a dedicated power distribution panel. This is very similar to a standard power distribution panel but includes an ambient sensing thermostat and mechanical contactor for control. The thermostat will sense when the ambient temperature falls below a certain level (typically 40°F to 50°F for winterization tracing), at which time the contactor will energize the panelboard. This type of control panel can normally be further enhanced with monitoring capabilities for voltage and current. For more exact temperature control, a microprocessor-based control panel may be considered (Photo 4). This is also similar to a standard power distribution panel but incorporates advanced control modules and pipe-mounted RTDs to allow for monitoring, control, and alarming of both winterization and process maintenance heat tracing.
Pipe-mounted resistance temperature detectors (RTDs) are used for temperature sensing of process maintenance heat tracing and provide real-time feedback for close tolerance temperature control. Using RTDs establishes a centralized location of the control panel, with placement possible in either an electrical room or outdoors in the operational unit. From this central location, any of the heater circuit temperature set points or alarm parameters may be adjusted. Typical alarm functionality for this type of panel includes both high and low temperatures, high and low current, RTD failures, and ground fault current leakage. Control methods that are commonly available with this type of advanced panel are on/off, ambient sensing, and proportional control.
During the early stage of a project, all of the information required to develop a complete heat trace design is rarely available. On large projects, it’s common to employ the services of a heat trace consulting firm to assist with developing the heat trace scope and determining estimated costs. Essential information that should be provided to the heat trace vendor include all relevant specifications, a scope of work, available piping and instrumentation diagrams (P&ID), area classification drawings, complete piping line list, equipment list, instrument list, and piping lengths (either estimated or from piping isometrics). There are additional documents that may be required as the scope of work dictates. If power distribution is to be included, the heat trace contractor will also require plot plans and equipment/instrument location plans.
Because much of this information is either unavailable, incomplete, or in a constant state of revision at this early stage of the project, lack of information will often result in a heat trace estimate being based on many assumptions, which is normally not representative of the end product. As the project progresses and the required information becomes available, this frequently results in a substantial increase to the heat trace scope of work, anticipated power loads, and overall system cost. These growths may also result in a significantly negative impact to the project schedule.
To minimize potential heat trace load and system cost overruns, consider involving the heat trace contractor as an additional resource during the estimate stage. This person can provide critical knowledge and guidance at this early stage of a project. When requested, it is common for the contractor to offer his assistance to review or advise on specifications, helping to estimate power loading and system cost. While the heat trace contractor will typically provide this assistance at no cost, it’s important to understand that this is a symbiotic relationship. Not only does this provide the contractor with an opportunity to further develop a professional relationship with the engineer, but also the firm is now in a position to request the opportunity to bid on the heat trace scope of work. The time the contractor has spent assisting with the heat trace estimate will ultimately provide him with the in-depth and thorough knowledge of the project that will allow him to offer a bid consistent with the specifications.
During the estimate stage, there are options that should be explored in an effort to optimize the heat trace system and minimize overall system cost. These include reviewing the type of heat trace to be installed, the thermal insulation system specified, and the option of incorporating a “turnkey” approach if installation services are required. Recommendations may also be made at this stage regarding the use of electric, steam, or other types of tracing.
The thermal insulation is highly critical to the overall heat trace design, and its importance is often overlooked. When the selection of thermal insulation for heat traced piping is made, rarely is consideration given to what type of insulation would best optimize the heat trace design. The thermal conductivity — also known as “k value” or “thermal resistivity” — varies greatly between the different types of insulation. Changing from the specified type of insulation to an alternate insulation can have a dramatic effect on the heat loss. Under certain design conditions and types of insulation, it is possible to have a reduction in heat loss by nearly one-half. This situation results in less heat input being required and potentially less heater cable being installed. Depending on the overall impact, this can result in a significant reduction of heater circuits, control panels, and power distribution required. This single item is often the most effective method for reducing overall system power requirements and system cost.
Beyond the heat trace design, a common area for inefficiency and risk involves the installation of the thermal insulation. When installation services are required for the heat trace system, it is not uncommon for the insulation to be overlooked. Often, the installation of insulation on heat traced items will be requested from an insulation specific contractor. Heat trace contractors routinely provide and install insulation on heat trace installations. With the insulation included as part of their scope of work, it allows for a more efficient and seamless work flow and shifts responsibility to the heat trace contractor for ensuring that the installed heater cable is not damaged.
One final consideration is to verify the coverage of the heat trace warranty. The heat trace contractor may provide only limited warranty coverage if the installation of associated insulation is done by another contractor.
Once a project moves into the detailed engineering stage, the continued involvement of the heat trace contractor will further ensure a successful heat trace system. While preparing the bid package for heat trace on larger projects, it has become routine to include the provision for in-house support by the heat trace contractor. This should include, at minimum, a project manager and several qualified heat trace designers. Having the contractor in-house allows for a more direct involvement with the numerous disciplines from which he will have to interface for information. This will enhance communications, allowing the contractor to be informed of and involved with new project developments as they occur. The overall result is a more efficient working relationship that benefits both the contractor and engineering firm and establishes a coordinated effort that will be essential to the success of the project.
Specialized heat trace software is commonly used to assist in the design effort. Most heat trace companies have either developed their own proprietary design software or use the software of the heat trace cable manufacturer they represent. These specialized programs allow the heat trace designer to input basic design criteria to calculate possible tracing solutions. More advanced software has recently been developed that incorporates data directly from the project 3D model, allowing for a much reduced heat trace design schedule. Essentially, certain data files from the 3D model are extracted and can be provided to the heat trace contractor for input into this program. Within the program, many of the basic heat trace design functions are automated.
A key advantage of this type of program is the reduction in time for design and creating the heat trace deliverables. From this program, the heat trace contractor is able to generate deliverables at a fraction of the time normally required. Typical deliverables include heat trace isometric drawings, line lists, load calculations, material take-offs, and heater circuit power locations. This method also greatly reduces typographical and other similar type errors that are common when generating heat trace drawings in the traditional manner. Without this software, heat trace isometrics are created as new drawings by manually entering data, typically using the piping spool drawings for information. This method is time consuming and much more susceptible to input error.
A successfully completed design does not guarantee a successfully completed heat trace system. Even the most well-designed heat trace systems can be headed for trouble if not installed as specified and instructed by the heat trace design contractor. If the installation is to be performed by a third party, one option that may be available to minimize risk is to request that the heat trace contractor provide a qualified individual to supervise the installation. Depending on the contractual requirements, this individual may be expected to perform many roles. These could include: advising on scope of work and installation issues; verifying the proper installation of heat trace material, thermal insulation, and power distribution equipment; providing any training that may be required to operate the system; and assisting in the commissioning and start-up of the heat trace system. This supervision not only helps ensure the system is installed as designed, but also may be required, depending on the type of warranty.
Most heat trace companies, if not involved in the installation, will only provide a basic warranty of design and material. It’s not uncommon for the heat trace contractor to offer a more comprehensive warranty if requested; however, supervision of the installation will typically be required. The availability and extent of any warranty, along with the availability of supervision to oversee the installation, should be addressed as early as possible. Preferably, this should be requested at the estimate stage. This is a time when most heat trace contractors are willing to aggressively negotiate additional requests in an effort to successfully win the project heat trace award.
Because heat tracing is a specialized field, these services are better left to a qualified heat trace contractor. Heat trace design can involve much risk if performed by inexperienced persons; thus, most engineering firms look to the heat trace contractor for this guidance. This relationship and how it is managed is highly important. If the task of managing the heat trace contract falls to a junior engineer, he is unlikely to be aware of the traps to avoid or the importance of addressing critical issues that are often overlooked. With a basic understanding of the heat trace requirements, along with guidance from a qualified heat trace contractor, one will be better prepared to handle the challenges that will ultimately lead to a successful and safely designed heat trace system.
Schultz is an experienced heat tracing and electrical designer employed by Fluor Enterprises, Inc., Sugar Land, Texas. He can be reached at: randal.schultz@fluor.com.