A Proper Assessment
An electric heat tracing system can be used in an installation’s pipe system to compensate for heat losses and to maintain a minimum temperature. This often is important to ensure the proper viscosity of the process fluid in the pipe, or to prevent water or steam lines from freezing. Prior to designing and installing an electric heat tracing system, an engineer must carefully evaluate the application the system is serving.
Engineers have many considerations when designing a heat tracing installation, especially if the area where it is located is defined as a hazardous location - a location where flammable gases may be present. It is vital that engineers assess all their equipment and ensure that it is up to code to protect the safety of personnel, the installation and its equipment. For example, an electric heat tracing cable can be accurately tested - provided that accurate assessment methods are used - to ensure the maximum sheath temperature attained by the cable will not be so high as to cause harm to the installation through autoignition of hazardous gases present in the area.
An electric heat tracing system may be used in an installation’s pipe system to compensate for heat losses and to maintain a minimum temperature. This often is important to ensure the proper viscosity of the process fluid in the pipe, or to prevent water or steam lines from freezing. Prior to designing and installing an electric heat tracing system, an engineer must carefully evaluate the application the system is serving. Important items to check include:
Required temperature to maintain the fluid in the pipes.
Ambient temperature where the tracing system will be installed.
Heat loss from the fluid to ambient.
- Maximum pipe temperature to which the heating cable will be exposed.
In applications where the heating cables will be installed in hazardous areas, there are additional design inputs that need to be considered, including the:
Autoignition temperature of the hazardous gases that may be present in the area.
Area temperature rating of the physical location of the installation.
- Highest sheath temperature that the heating cable will reach during operation.
The sheath temperature - the temperature of the outermost heat trace cable jacket - must not exceed the area temperature rating and is an extremely important part of electric heat tracing system design.
Suppliers of heat tracing cables are required to predict the maximum sheath temperature that the cable will reach. However, given the varied methods used to measure and predict maximum sheath temperatures of mineral-insulated (MI) heating cables, there is confusion in the industry about the best approach.
Two common methods have been evaluated and identified in IEEE 515, Standard for the Testing, Design, Installation, and Maintenance of Electrical Resistance Trace Heating for Industrial Applications. IEEE 515 requires that manufacturers verify their ability to predict sheath temperatures using either the pipe test or the plate test.
Benefits of the Plate Test vs. the Pipe Test
There is some confusion in the industry as to the best approach to measure and predict maximum sheath temperatures of mineral-insulated heating cable (shown in a cutaway view).
Many pipe and plate experiments were performed to determine the maximum sheath temperature of various mineral-insulated cables from the same manufacturer, ranging in size from 0.12 to 0.43" (3 to 11 mm).
Pipe Test. The pipe test consists of a 2" dia., L-shaped pipe sculpture with a valve located along the horizontal section and various flanges located along the length of the pipe. The heating cable is installed on the surface of the pipe according to the manufacturer’s instructions. Thermocouples are located at anticipated hot spots and attached to the heating cable sheath, with another thermocouple located on the pipe itself in close proximity to the thermocouple on the heating cable sheath. After several of these thermocouple pairs are located along the length of the pipe, the entire assembly is insulated with 2" fiberglass insulation. The heating cable is energized at the required wattage and the pipe temperature is allowed to stabilize. After stabilization, the heating cable sheath temperature is recorded from all thermocouples, and the highest recorded temperature is the maximum sheath temperature of the cable at the given operating conditions.
With the pipe test, it was difficult to obtain reproducible temperatures, and the hottest spot on the pipe varied from test to test and pipe to pipe. After reviewing the variations in the results, it was concluded that the pipe test was not a suitable method for consistently and accurately determining the maximum cable sheath temperature. Considering the implications for hazardous areas where a fire or explosion may occur, it is critical to have accurate sheath temperatures.
Plate Test. The plate test uses a metal plate that is air cooled, electrically heated and well insulated. In the procedure, the cable is laid on the plate over a trough to simulate a common field condition where the heating cable is not in direct contact with the pipe. Thermocouples are located on the cable sheath at the center of the trough to measure the sheath temperature and also on the plate itself to measure the plate temperature. The plate temperature is monitored and controlled by the heating and cooling channels throughout the test. The assembly is insulated on all sides and on the top and bottom.
After many trials, it was determined that the plate test produced more accurate and repeatable results with several benefits. It provides a simple, consistent method to install the cable and removes the installer-related variability from the test. The cable sample simply is placed on the plate over a predetermined location and clamped down, without any bending or manipulation of the cable required. The trough depth of ~0.20" (5 mm) between the cable and the plate is highly controlled, represents the worst case condition, and is not dependent on the technique of the installer.
In the authors’ opinion, the plate test proved to be a more repeatable, cost effective and reliable method of predicting maximum sheath temperature and ensuring the intent of the industry standards is met.
In conclusion, engineers can rest assured that most heat tracing equipment manufacturers take a conservative approach to prevent hazards. For instance, the area temperature ratings and the temperature at which a fire can occur often are intentionally understated by the industry to build precaution into electrical installation designs. Additionally, even if the maximum temperatures are reached, the required insulation and cladding around the heating cables offer another layer of protection.
The plate test has proven effective in predicting maximum sheath temperatures for mineral-insulated cable in an accurate and repeatable manner. Adoption of the plate test within industry standards as the single acceptable test method is the most logical next step to ensure consistency across the industry. PH