Impedance pipe heating systems are, in general, very simple. That’s one of their huge advantages over competing technologies such as steam jacketing and heat trace cable. As with so many things in life, though, “the Devil is in the details,” as they say.
In this article, we’re going to focus on the details of a quality control system: what technologies are typically involved and what are typical customer options.
Power must somehow flow from the control panel out to the pipe being heated. The major components typically consist of: fused disconnect or circuit breaker (1 per system), system contactor, power controller (usually a solid state relay (SSR) or silicon controlled rectifier (SCR)), transformer, and finally the pipe itself.
Fused Disconnect or Circuit Breaker
Standard devices that provide short circuit and overload protection to the associated system. They reside inside the control panel. Each individual system should be equipped with one. They also provide the ability to isolate individual systems without affecting any other systems being fed out of a common control panel.
A properly designed system should include an appropriately sized contactor, especially when using electronic devices such as SSRs or SCRs as the primary control device. These contactors are “on” whenever the associated impedance pipe heating system is enabled without any safety alarms. They provide a backup to the SSRs and SCRs, especially since those devices typically fail in the “on” state when they do fail. Lower quality basic systems will ONLY utilize this contactor for both safety and control. The downside to that design is that the contactor will wear out significantly faster, and if it does fail in the “on” state and a high limit controller is not part of the control system, then there is nothing to prevent a run-away heating condition.
This is typically an electronic device, such as a solid state relay (SSR) or silicon controlled rectifier (SCR). But can also be an electro-mechanical contactor. As noted above, some lower quality basic systems may not even utilize a separate power controller, and do everything utilizing the system contactor. An SSR is the electronic equivalent of a contactor, except it has absolutely no moving parts. It is strictly an on/off device; when it is on, 100% power is being delivered to the heated pipe via the impedance system transformer. A system equipped with SSR power control is useful for most pipe heating systems, when desired temperatures are within +/- 4 °F of set point. If more precise control is necessary, then an SCR is utilized. An SCR can control the amount of power being delivered to the pipe at any instant. In this case, temperatures can be controlled to within less than +/- 1 °F of set point.
A transformer is necessary to convert the voltage coming from the facility power distribution system to that required to properly heat the pipe. A quality impedance pipe heating transformer is custom engineered and built for a specific system by a reputable manufacturer. They should typically include multiple primary taps for adjustments to field conditions.
The control system is responsible for making decisions as to when and (in the case of SCR based systems) how much power to apply to the pipe. At its very basic, the control system consists of a temperature detection device and a temperature controller. Other components may be included or required, such as the measurement and display of the current flowing through the pipe, and ground fault detection.
This is a device, such as an RTD, that is mounted directly to the pipe underneath the insulation. It provides feedback to the system temperature controller regarding the current temperature of the pipe. If using quality pipe of uniform wall thickness, only one temperature detection device is required per system.
Pipe Current Measurement
While not a requirement, it is very useful to measure and display the amount of electric current flowing through the pipe when the heating system is energized. While this can be used for strictly informational purposes, it can also be used for diagnostics and for shutting down the system if excessive current is measured due to a short circuit of the system.
Ground Fault Detection
Ground fault detection systems measure the amount of electric current flowing from the system through a ground path instead of being constrained to the pipe. The NEC requires ground fault detection on systems where the operating voltage equals or exceeds 30VAC. Ground fault detection helps protect the system components from short circuits, as well as protecting personnel if they come into accidental contact with an energized system and become the path the current is flowing through.
This is one place where there can be a huge amount of variability. The most basic systems utilize a separate stand-alone temperature controller for each system. Functionality is limited by the controller chosen. Some may only be able to control on/off type systems, while others may have an analog output for use with SCR based systems. Some have communication capabilities for remote control and motoring; the more basic units do not.
For installations that have a large number of systems controlled from one location, it quickly becomes more cost effective to utilize a PLC with an HMI instead of separate stand-alone temperature controllers for each system. This allows many systems to be combined into one small control unit with an interface that can be customized for each installation. Depending on the selection of hardware, communications capabilities are typically inherent, allowing for integration with larger overall plant systems.
Banner-Day has a long history of designing and implementing impedance pipe heating systems using all of these technologies. Safety and quality are of paramount importance to us: all of our control panels are UL/ULC Listed, and all of our systems meet appropriate codes, such as NEC Chapter 427, NFPA 70E, and IEEE 844. Please contact us if you have any questions or would like to consider implementing an impedance pipe heating system in your facility.
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