"Impedance pipe heating? Yeah, heard of it. Not really sure what it is though.”

This response is heard too often when discussing available process pipe heating solutions. Impedance pipe heating is a long established and accepted — but poorly understood — pipe heating technology. While a simple, elegant way to heat a pipeline, impedance pipe heating technology often is viewed incorrectly and misunderstood as a method to heat process piping.

Once people learn how impedance heating works and the benefits its use brings, they are more than willing to adopt impedance pipe heating. In this article, I will provide an introduction to impedance pipe heating and offer 10 reasons to consider impedance pipe heating for your next pipe heating application.

### What Is Impedance Pipe Heating?

Impedance pipe heating is the generation of heat in a length of pipe. A low voltage AC potential is applied across the length of the pipe being heated (figure 1). Heat is generated by resistance to the flow of electric current through the wall of the pipe.

Viewed schematically, an impedance pipe heating system is represented as a simple series circuit. The appropriate amount of current and voltage are determined via calculations rooted in the application of Joule’s Law in the form of

P = I2R

and with Watt’s Law, in the form of

P = VI

Using these equations, the appropriate amount of current and voltage are calculated to generate the required amount of heat for a given process pipe heating application. (In the equations, assume P is power, I is current, V is voltage and R is resistance.)

The purpose of an impedance pipe heating system 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 product integrity, or to ensure that the viscosity of the material is kept sufficiently low to facilitate pumping efficiency.

Heat generated by impedance heating can be used to:

• Thaw and warm up products in the piping to the process operating temperature.
• Maintain the temperature of the products in a pipe
• In some cases, used to elevate the temperature of a product flowing through a pipe.

Common examples of products heated 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 moved through a pipeline, an impedance pipe heating solution can be configured to meet the most challenging process conditions of service.

Primary components of an impedance pipe heating system are shown in figure 2. They include:

• A system transformer. A dual-wound isolated type is shown.
• Control panel. It includes power and controls branch circuits, and system temperature control (digital controller or PLC).
• Temperature sensor. An RTD or thermocouple is used to sense pipe temperature.
• Secondary cabling. It connects the impedance system transformer secondary to the pipe being heated.
• Dielectric flange joints. They are located at the ends of the impedance heated pipe and provide dielectric isolation from earth ground.
• Pipe. It is installed to be electrically isolated from earth ground.
• Insulation. Insulating material is installed on pipes used for impedance pipe heating to minimize heat losses.

### Types of Impedance Pipe Heating Systems

Impedance pipe heating systems typically are configured in either of two arrangements: end feed and center tap. As the name implies, for end-feed systems, low voltage potential is applied across the entire length of the heated pipe. This creates 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 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 pipe. From there, equally balanced alternating current flows from the mid-point connection on the pipe to each end. At the ends of the pipe, 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 pipe 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 where temperature limitations can make selecting dielectric isolation materials challenging. If the pipeline is grounded at the extents of the system, a center-tap system does not require the dielectric flange joints located at these locations. (Dielectric flange joints are required on the extents of the pipe on the end-feed system type.)

Other impedance pipe heating system configurations are offered, but they all tend to be derivatives of the end-feed and center-tap arrangements. Features common to impedance heating systems include branch drops, multiple takeoffs and inline valves and devices.

In conclusion, after you have read this article, we hope you find yourself in a better position to answer the question, “What is impedance pipe heating?” With a better understanding of this pipe heating technology, you may have some installations in mind where an impedance pipe heating solution might suit your application.

### 10 Reasons to Get to Know Impedance Pipe Heating

Impedance pipe heating provides desirable benefits over other pipe heating technologies such as trace element resistive heating, water and thermal fluid jacketed piping and steam-traced pipe heating. Several of these benefits are summarized below in our Top 10 reasons to use impedance pipe heating for your next process pipe heating application.

1. It heats pipelines uniformly. There are no hot or cold spots, which prevent damaged products and fouled pipelines. The impedance heating 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 pipeline.
2. It improves process efficiency. It helps reduce process startup times from scheduled and unscheduled process outages.
3. It is reliable. It does not have any moving parts. Also, no heating elements are wrapped around the pipe or underneath the pipe insulation and jacketing. Finally, because it does not use steam, there are no leaking water-jacketed pipe connections or steam leaks.
4. It is safe. Systems are designed to NEC, CSA and IEEE standards with ground-fault systems and secondary voltages typically less than 30 VAC.
5. It is offers ease of 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 is simple to maintain, requiring minimal maintenance that can be performed by in-house personnel.
6. It can handle a range of temperature applications. Impedance pipe heating systems are used for low temperature applications for simple freeze protection and for very high temperature applications beyond 1,300°F (704°C). Operating temperatures for impedance pipe heating systems are limited only by the material limits of the pipe it is designed to heat.
7. It is efficient. Due to its resistance heating nature — and the pipe itself being the resistance heating element — impedance pipe heating is highly efficient.
8. It eliminates circulated loops. Impedance pipe heating systems maintain the process operating temperature of the pipeline whether the product is flowing or stagnant. This can eliminate the need for return loops.
9. It is cost effective. For applications for which it is suited, impedance pipe heating eliminates the need for capital equipment such as boilers, heat exchangers and pumps.
10. It is simple. Electrically, what is simpler than a low voltage AC series circuit?