Electric heat tracing systems have maintenance requirements. While this article is written with industrial pipe tracing system in mind, the basic ideas can be applied across any type of system for tanks, as well as for commercial applications for roof and gutter deicing or snow melting. What is most important is that the maintenance crew understands the individual components of the system - and their functions within the system - to perform proper maintenance and prevent costly system downtime.
In addition, insulation and weatherproofing are required to make the system work as intended. Most systems are designed with a 10 to 20 percent safety factor for system heat losses. That means there is enough heat to maintain the process at temperature, but there is not enough heat to raise the temperature of the system in a shut down condition. The following 10 tips that follow should provide a better understanding of these systems and help maximize system performance through better maintenance service.
1. Start the Selection Process Early
Don’t wait until cold weather sets in to perform pre-winter system tests and inspections. June, July and August are the best months to perform these tasks. The weather is better, visibility is better, and personnel mobility is better. Most of all, it leaves enough time to identify potential problems and pinpoint solutions before a costly system freezeup. Many heat tracing equipment manufacturers have service organizations that can assist, supervise or perform preseason tests and inspections. Also, just as preseason is the best time to perform system tasks, repairs are easier to handle in warmer, drier weather conditions.
2. Inspect Your Existing Electric Heat Tracing System
Heat trace systems comprise many different components that are critical to the proper operation of the entire system. The cable is important but other components such as insulation, weatherproofing and connection/termination components play important roles in system operation too.
A good visual inspection and component assessment are necessary.
- All insulation should be checked to be sure it is in place and dry. Wet insulation will result in an inability to maintain system temperature.
- All connections and terminations should be checked to ensure the wiring is tight, proper grounding is in place and the connections are dry. Moisture in connection and termination accessories is a common source of system failure.
Moisture ingress can be prevented by using the proper connection accessories for the cable and following the manufacturer’s installation instructions. These connection kits are designed specifically by manufacturers to protect the cables and prevent moisture problems. Failure to use and install them correctly will lead to system performance issues. Third-party approvals and certification of the cable system also are dependent on use of the approved and certified cable connection kit. Failure to use the approved kit will result in the loss of a certified system.
3. Buy a Megohmeter
One key piece of equipment needed for proper system maintenance is a megohmmeter, commonly called a megger. Meggers are used to test the insulation resistance of the cable jackets to ensure that there are no short circuits to ground or short circuits to the piping system. When selecting a megger, look for one capable of producing 2,500 VDC in 500 VDC increments. It should have an analog or digital readout of the megohm value tested. Follow the manufacturer’s instructions regarding the best test method for each cable type and the minimum allowable megohm reading.
4. Test Each Electric Heat Tracing Circuit
Each circuit should be tested for megohm reading, end-of-circuit voltage and stabilized current draw. Megohm readings should be taken for each cable per the manufacturer’s instructions. Cables should be isolated from the power wiring at the cable power connection kit to ensure that megohm results are for the cable alone.
End-of-circuit voltage should be checked next. This check should be performed at the end of each cable circuit to ensure that the power wiring within the cable is good all along the length and that the proper voltage is present to the end of the circuit.
Finally, stabilized circuit current should be checked for all circuits. For self-regulating cables, allow the circuit to stabilize for at least 20 minutes. The current reading and the pipe and ambient temperatures should be recorded after the stabilization period. The proper cable output per foot is determined by dividing the current rating by the overall length of the cable and then multiplying by the applied voltage.
Self-regulating cable will change its output vs. the pipe and ambient temperature. Use your manufacturer’s data sheet to determine if the cable is producing the proper output by comparing the calculated watts per foot to the data in the pipe temperature charts provided. Typically, self-regulating cables’ nameplate ratings are derived from their power output at 50°F (10°C).
Constant wattage cables will have the same output regardless of pipe temperature. With constant wattage cables, using the current reading divided by the circuit length and multiplying by applied voltage will give you the cable output in watts per foot.
5. Repair Any Failed Heat Tracing Circuits
Once the cables have been tested and inspected, it is likely some circuits will need repair. The best time for repairs is right away and, of course, in the warmer, drier weather of July and August. Replace cables with a like model, manufacturer and power output. Your heat trace provider can help you identify the product you have and the correct replacement.
Often, heat tracing manufacturers will recommend completely replacing cables that require repair. Their experience has shown that trying to splice new sections of cable with old, or trying to identify which sections of the cable may be reusable, is expensive and often delivers less than desired system performance once complete. The initial cost to replace an entire circuit maybe higher than a repair, but the results are reliable. In the long run, system performance will be more predictable and consistent.
6. Document Inspection and Test Results
Document all inspection and test results. Many manufacturers provide maintenance logs as part of their operation and inspection (O&I) manuals. These logs are designed to give the operator and maintenance crew all of the information required to describe the system’s performance and serve as a baseline for testing and troubleshooting if problems occur. When inspecting and repairing a heat trace system, time and care should be taken to document several the performance expectations, design, layout, equipment and operational characteristics. At a minimum the log should contain:
- Line identification.
- Circuit identification.
- Cable type and model.
- Cable wattage/voltage.
- Circuit length.
- Megger test results.
- Current draw results.
- End-of-circuit voltage results.
- Lot code for the installed heat trace.
- Pipe temperature setpoint.
- Actual pipe temperature.
- Test and inspection dates.
- Notes on failures, fixes and modifications.
A record should be kept from initial system commissioning through the life of the pipeline.
7. Add a Controller to Manage Your Heat Tracing Cable Temperature
One common misconception about self-regulating cable is that temperature control is not needed to make the heat trace system work. While it is true that the system will operate without control, it is not efficient. Moreover, without temperature control, there is no way to monitor and manage system performance.
Heat tracing systems are designed so that the pipe temperature can be maintained at a specified minimum ambient. However, the temperature is rarely, if ever, that low in most installations; therefore, there is always extra heat flowing into the process. Yet this may not be needed. For instance, in an application where heat tracing is used for freeze protection system, heating is not needed when the ambient temperature is above 40°F (4.4°C). Yet without temperature control, self-regulating cable will produce some power all the way to a pipe temperature of around 150 to 160°F (66 to 71°C). This means energy is being wasted.
Also, in any application, without some form of control, there is no indication that the heat trace is working at all. Some installations use end-of-circuit lights to indicate power to the heat trace. Bear in mind that all the light indicates is whether voltage is present at the end of the circuit. There is no indication of proper system performance.
A control system composed of RTD temperature sensors and microprocessor control will provide indication of all of the critical heat trace parameters such as:
- Process setpoint.
- Actual process temperature.
- Circuit current.
- Circuit voltage.
- Ground fault status.
These systems are cost effective to install and also save energy by using only the proper amount of power to maintain the process setpoint. In addition, such systems provide early indication of potential system problems to allow some repairs before replacement is necessary.
8. Monitor Heat Tracing Cable Performance
Stepping up to an RTD and microprocessor-based control system will help maximize performance and efficiency and add circuit-monitoring functions. These systems monitor critical parameters such as process temperature, circuit current, circuit voltage and ground fault status. Data can be logged and trends can be monitored. Changes to these values can indicate potential performance issues early on and allow the tracing system user to plan cost-effective repairs that eliminate excessive and expensive downtime. System alarms can be set that indicate whether the parameters are drifting outside acceptable ranges. These systems also can be tied into process or building management systems to optimize performance and enhance energy savings.
9. Train Operators on Your Heat Tracing System
Because different types of heat tracing cable require different approaches, each maintenance professional should be trained in the operation of self-regulating, constant wattage and mineral-insulated cables. Personnel should be familiar with all connection accessories and their operation and installation, as well as any controls or monitoring equipment used in the system. Proper safety training should be carried out for handling electric circuits and test equipment, in addition to any mechanical requirements the plant may have for circuit access and personal protective equipment. All plant regulations for access and personal protection should be followed.
Most manufacturers of heat trace systems provide factory or on-site training programs for maintenance and troubleshooting. These training courses can save many hours of headache and downtime and are well worth the time invested.
10. Consider Outsourcing Heat Tracing Maintenance
Another option to ensure proper system performance is to contract with your equipment provider to handle system maintenance requirements. These contracts can be written to provide as little as supervision of your crew all the way through a complete turnkey service for inspection, testing, documentation and repair.
These tips should provide insight into the development and implementation of a more effective maintenance program for the heat trace system. Whether the task is handled in-house or outsourced, these tips should help ensure proper heat trace system performance.
This article originally appeared in the September 2010 issue of Process Heating with the headline, "Heat It Up!"
|See the web-exclusive sidebar, "Installation Tips for Heat Tracing Cable," for diagrams that show specific installation locations for some components.|