There are several different processes that require a product to flow through a pipe from one piece of equipment to another. A certain level of heat is often required for this process piping, and this heat must be at very high levels for many applications.
Furthermore, these temperatures must be maintained throughout the complete length of a given pipe. Depending on the media that is being transferred, failing to maintain high temperature levels (sometimes well over 1000°F) can lead to crystallization, blocking, organics buildup and, ultimately, the shutdown of the entire system.
If the material running through the pipe does solidify and create blockage, there is an entirely new set of problems to address concerning the pumps, valves and various other components. All of these issues will lead to a scenario where the whole process will need to be shut down so the pipes can be cleaned out. Between the maintenance expense and the lost production time, it is essential for a plant’s budget to keep its process piping at a specific temperature.
At lower temperatures, a common solution involves some variation of heat trace. However, this solution is not viable when dealing with high temperatures greater than 1000°F.
Organics Building Up
One of the more common issues when process piping is not properly heated is the buildup of organic material inside of the pipe. Once temperatures drop, certain chemicals will react before being evacuated from a process through piping. This reaction can leave organic material to build up on the inside of the pipe. By not keeping the pipes heated to a high enough level, this material will build up on cold surfaces.
That is not to say that organic buildup isn’t also an issue in extremely high-temperature processes. For example, one byproduct of a high-temperature oven can be carbon material in the exhaust. This carbon material will be burned off in a thermal oxidizer. As the exhaust moves from the oven to the thermal oxidizer, however, a thick carbon-based material will build up on cold surfaces of the interconnected pipe system. Over merely a few weeks, this can clog up a pipe, requiring shutdown of the process and several days of maintenance work. This buildup can be virtually eliminated by maintaining the surface temperature of the pipe at the same level as the exhaust coming out of the oven.
Another common example is molten salts, which are often used as a heat-transfer fluid in high-temperature applications (1000°F or higher). However, molten salts have the disadvantage of a high freeze-point temperature. It is important to maintain the salt above its freeze point to avoid solidification. This is important to understand and monitor because when it is being circulated through the process, it will solidify if there is a cold pipe surface that it encounters. As it builds up over time, this can most certainly clog the pipe and decrease the life of process components, leading to downtime and, ultimately, impede production.
In order to ensure that this does not happen, it is critical to monitor and control the temperature of the process piping. One of the key words here is "control." It is not as simple as cranking the heat up on this process piping to be sure that certain organics don’t solidify or the pipe doesn’t freeze. Careful attention must be given to the specifications of the pipe itself, especially at high temperatures.
Depending on the pipe material and schedule, a pipe will have a specific temperature rating that cannot be exceeded. Additionally, if the pipe and/or the flange connection get too hot, not only is the structural integrity affected, but the pipe pressure rating will be de-rated. If pipe temperature is not monitored and controlled, there can be instances where the temperature exceeds design and ultimately causes problems with the life of the pipe and the rating of the flanges.
With time and monetary implications associated with monitoring the temperature of process piping, it is important to understand the solutions available. For small sections, one can typically just use high-temperature band heaters. These are cost-effective heaters that can do the job for limited sections of pipe. As the pipe becomes significantly longer, however, this approach becomes less ideal. Band heaters are only 3-4 inches wide and can only cover a nozzle and 1 or 2 feet. Anything more than that and it becomes cumbersome to wire up and connect them to terminal boxes, making it very difficult to manage and maintain. For the amount of time required to wire and put everything together, high-temperature blankets are a more cost-effective solution.
High-temperature blanket heaters can maintain temperatures up to 1200°F. These types of heaters can cover long lengths of pipe with just one connection point and can include integral thermocouple sensors to monitor various locations of the pipe. Blankets are often custom-built for a specific application and can be designed to fit odd shapes like flanges and valves. Versatility also plays a big part in why this approach is gaining popularity among those in charge of maintaining high temperatures for process piping. Blankets can be easily removed for maintenance and then reassembled when maintenance is complete.
For even higher temperatures, another possible solution is using ceramic-fiber radiant heaters. Ceramic-fiber heaters have operating temperatures greater than 2000°F. These are also custom-made for the application but are more rigid in design than high-temperature blanket heaters. Ceramic-fiber heaters use radiant heat to transfer energy from the heater to the pipe surface. The heater is designed to fit around the pipe, creating an oven-like environment and providing uniform energy to the surface.
With any of these solutions, it is important to monitor and control the pipe temperature to avoid exceeding the pipe design temperature as well as protecting the heater from over-temperature. Choosing the right sensor(s) and placing the sensor(s) in the correct location is critical. In applications above 1000°F, a type K or type N thermocouple is typically a good choice. SCR power control is important to eliminate temperature overshoot. An SCR can pulse power to the heater multiple times a second to offer more precise control and reduced thermal stress on the pipe and on the heater, leading to a longer heater life.
Finally, a high-quality PID controller is required to take input from the process sensor(s) and output to the SCRs. A high-end multi-loop process controller can take several sensor inputs for both control and monitoring, load current input for troubleshooting and network communications to ensure control technicians have real-time access to the process from the control room.
When considering a heating solution for process piping, the first concept to understand is the characteristics of the application and accuracy of heat applied to the application. Start with the media flowing through the pipe, design temperature and pressure, voltage and wattage required, and environmental conditions. The next step is determining the budget available to achieve these goals. Together, these pieces of information will help guide decision-makers in choosing the complete heating solution that best meets their needs.