Impedance pipe heating is a long-established and accepted — but poorly understood — pipe heating technology. While a simple and elegant way to heat a pipeline, impedance pipe heating technology often is viewed incorrectly and misunderstood as a method to heat process piping.
Fortunately, once people learn how impedance heating works and the benefits its use brings, they are more than willing to adopt impedance pipe heating and leave behind other pipe-heating methods. This article will provide an overview of impedance pipe heating and provide several tips to keep in mind when considering the use of impedance pipe heating for your next process pipe heating application.
What Is Impedance Pipe Heating?
Impedance pipe heating is the generation of heat within a length of pipe. Heat is generated by resistance to the flow of electric current through the wall of the pipe caused by the application of a low voltage AC potential across the length of the pipe being heated (figure 1). Viewed schematically, impedance pipe heating is represented as a simple series circuit. Impedance heating is designed through calculations rooted in the application of Joule’s Law, in the form of
P = I 2 R
and with Watt’s Law, in the form of
P = V I
where P is power, I is current, V is voltage and R is resistance. The appropriate amount of current and voltage are determined to generate the required amount of heat for a given process pipe heating application.
FIGURE 1. Impedance pipe heating is the generation of heat in a length of pipe. Shown here is a schematic of a simple impedance pipe heating system.
The purpose of impedance pipe heating is to provide heat to a liquid material or gaseous vapor flowing through a pipeline. Many products and ingredients transferred through pipelines must be maintained at a minimum temperature to ensure the integrity of the product, or that the viscosity of the material is kept sufficiently low to facilitate pumping efficiency. Heat generated by the impedance heating system can be used to thaw and warm up product in a pipe to the process operating temperature. Impedance heating also can be used to maintain the temperature of the product in a pipeline or, in some cases, elevate the temperature of a product flowing through a pipeline. Typical examples of products heated successfully with impedance pipe heating include asphalt, petroleum products, chemicals, waxes, molten metals, glues and adhesives, chocolate, shortenings, corn syrup, palm oil, peanut butter and honey. Regardless of the product being transferred through a pipeline, an impedance pipe heating solution is a good solution to “keep it moving.”
Primary components of an impedance pipe heating system include (figure 2):
- System transformer, a dual-wound isolated type.
- Control panel, which includes power and controls branch circuits. It provides system temperature control via a digital controller or PLC.
- Temperature sensor, which can be an RTD or thermocouple used to sense pipe temperature.
- Secondary cabling, which connects the impedance system transformer secondary to the pipe being heated.
- Dielectric flange joints, which are located at the ends of the impedance heated pipe and provide dielectric isolation from earth ground.
- Pipe, which is installed to be electrically isolated from earth ground.
- Insulation, where the insulating material covering pipe heated with impedance.
FIGURE 2. There are several components in an impedance pipe heating system. This figure details the typical impedance system components.
How Many Types of Impedance Pipe Heating Systems Exist?
Impedance pipe heating systems typically are configured in one of two arrangements: end feed and center tap. End-feed systems, as the name implies, utilize a low voltage potential applied across the entire length of the heated pipeline, creating a simple series electrical circuit through which the impedance system’s secondary current flows from the system transformer’s X1 to X2 (figure 3). The simpler of the two arrangements, end-feed systems are best used for low to medium temperature maintenance applications up to around 300°F (149°C).
Center-tap systems are configured such that the impedance system transformer’s secondary X1 is applied to the electrical resistive mid-point (or center) of the impedance heated pipeline. From there, equally balanced alternating current flows from the mid-point connection on the pipeline to each end. At the end of the pipeline served by a center-tap impedance system, connections are made back to the system transformer’s X2 connection.
From an electrical schematic point of view, the center-tap system represents a parallel circuit through which the system’s secondary current flows equally through each side of the impedance heated pipeline from the X1 mid-point connection (figure 4). Center-tap systems are best applied to longer point-to-point pipeline runs and higher temperature applications. Note that a center-tap system does not require the dielectric flange joints located at the extents of the system such as are present on the end-feed system type — provided that the pipeline is grounded at each of these two points. Center-tap impedance pipe heating systems are a good choice where temperature limitations can make selecting dielectric isolation materials challenging.
Other impedance pipe heating system configurations are available, but they tend to be derivatives of the end-feed and center-tap arrangements. It is common for impedance heating systems to include branch drops, multiple takeoffs and inline valves and devices.
FIGURE 3. End-feed systems utilize a low voltage potential applied across the entire length of the heated pipeline
Benefits of Using an Impedance Pipe Heating System
Impedance pipe heating provides many desirable benefits over other pipe-heating technologies such as trace element resistive heating, water and thermal fluid jacketed piping, and steam-traced pipe heating.
Simplicity. Electrically, what is simpler than a low voltage AC series circuit?
Cost Effectiveness. Impedance pipe heating systems do not require capital equipment such as boilers, heat exchangers and pumps.
Circulated Loop Considerations. Impedance pipe heating systems maintain the process operating temperature of the pipeline whether product is flowing or stagnant. Depending on the process, this may eliminate the need for return loops.
Heat Transfer Efficiency. Due to its resistance heating nature and the pipe itself being the resistance heating element, impedance pipe heating is efficient.
Temperature Range. Impedance pipe heating systems are used for low temperature applications for simple freeze protection and for very high temperature applications ranging beyond 1,300°F (704°C). Operating temperatures for impedance pipe heating systems are only limited by the material limits of the pipe it is designed to heat.
Installation and Maintenance. Employing simple methods to isolate the pipeline from ground, an impedance pipe heating system can be applied to any pipeline requiring heating. It also is simple to maintain, requiring minimal maintenance performed by in-house maintenance personnel.
Safety. Systems are designed to NEC, CSA and IEEE standards with ground fault systems and secondary voltages typically less than 30 VAC.
Reliability. There are no moving parts and, even better, no heating elements wrapped around the pipe, underneath the pipe insulation and jacketing and no leaking water-jacketed pipe connections or steam leaks.
Process Efficiency. Use of these systems helps reduce process startup times from scheduled and unscheduled process outages.
Uniform Pipeline Heating. Using impedance heating, there are no hot or cold spots, preventing damage to products and fouled pipelines. Impedance system’s secondary current flows evenly through and around the circumference of the pipe, resulting in the even application of heat along the entire length of the impedance heated pipeline.
To help achieve these benefits, there are several elements that should be considered during project design and installation. These include the following 10 tips for successful application of impedance pipe heating. Project engineers and skilled trades installation personnel should consider each of these when implementing an impedance pipe heating solution. Doing so will significantly enhance the project’s prospects for success.
FIGURE 4. The center-tap system represents a parallel circuit through which the system’s secondary current flows equally through each side of the impedance heated pipeline from the X1 mid-point connection.
10 Tips for Successful Application of Impedance Pipe Heating
- Impedance pipe heating systems are best applied to pipelines of uniform size and wall thickness.
- Impedance pipe heating systems are best applied to pipelines whose entire length is located in a uniform ambient environment.
- Impedance systems are best applied to pipelines with uniform insulating material and thickness.
- Allowing sufficient allowance for thermal expansion and contraction of the pipeline at the supports helps avoid nuisance system ground faults.
- For threaded pipe joint applications, an electrically conductive thread compound such as graphite paste is recommended. Teflon-based thread compounds are not acceptable for impedance pipe heating applications and should be avoided.
- Insulation and jacketing penetrations should be configured with sufficient
- clearances to avoid nuisance system ground faults.
- When practical, the impedance system transformer should be installed near the pipeline it is heating. Doing so minimizes the length of system secondary cables, reducing installation cost and improving overall system efficiency.
- The impedance system secondary cable terminals should be installed on the bottom of the pipeline for outdoor applications to minimize exposure to weather.
- Impedance system secondary cables are best routed along the top centerline of the pipeline and secured with nonferrous cable strapping.
- Impedance system secondary cable must never be installed in ferrous conduit.
After having read this article, you should find yourself with a deeper understanding of impedance pipe heating, its benefits and how to best configure for success. With that better understanding, it is hoped, comes interest in exploring an impedance pipe heating solution for your next process pipe heating application.