It’s our last week in the THM blog series on heater controls. This week we are going to get into a topic that some people have strong opinions about – PLC’s. Some believe that PLC’s are the best way to control a heater. Some believe there are better alternatives. Some don’t know about some of the advances in PLC alternatives for heaters. And a few newbies out there are like, “what the heck is a PLC?”
PLC’s (Programmable Logic Controllers) are processors that can be programmed to facilitate and monitor any number of functions. There are many different providers today, but the most common brands are probably Siemens and Allen Bradley. PLC’s can be custom ordered to fit your specific application, with variations depending on customer requirements, number of inputs and outputs the system has to monitor/control, and whether any back-up or redundant components are needed.
For combustion systems like boilers and fired heaters, one alternative to a PLC is a combustion management system. These systems are like PLC’s, but have limited functionality. Specifically, they are designed and built to manage and run a burner/heater. As with PLC’s, there are several manufacturers of these types of systems, but the most common are Fireye and Honeywell. For smaller heaters (and boilers) these systems can be a great alternative.
At THM, we have standardized on the combustion management method of controlling our burners. We did this for 3 main reasons.
Price
There is no beating around the bush here, PLC’s can be expensive. Depending on the customer or job specific requirements, PLC’s can range from a few thousand dollars to a few hundred thousand dollars (for a few hundred thousand you could probably run an entire plant). A few of the main drivers on price for PLC’s include: model type, number of inputs/outputs, and whether you need redundant components. For a small heater with a single burner (all standard THM heaters have only 1 burner), the PLC necessary for running the system is not wildly expensive, but it does tend to run higher than a Fireye or Honeywell system.
Another component of price not directly tied to the up-front cost is the time taken to program the unit. As we will hit on in the next point, the PLC takes longer to program which is a cost of man-hours for your team.
Ease of Use
Using a PLC requires special software (which can be expensive). The software is specific to the brand of PLC being used – so different software is used for Allen Bradley than Siemens. Generally, there is an engineer responsible for programming and starting up the PLC. Again, the engineer must be familiar with the software specific to the brand of PLC being used. Many engineers have a lot of experience in only one brand of PLC/software.
The PLC is programmed and configured using a desktop or laptop computer. While certain elements of the programming can be re-used from project to project, many items are project specific and must be reprogrammed for each job.
With a dedicated combustion management system, much of these pains are eliminated. While each brand has its own software (Fireye, Honeywell, etc), the softwares are freely available to all and are not required for set-up or adjustments. Since these units are built with one purpose, controlling a burner, the units come with most of the “programming” already completed. Initial set-up of set points, light off points, etc, are usually done in the field on the system interface (HMI). Field personnel can be (and usually are) trained to make typical adjustments themselves, although it is usually recommended that a trained service technician do the initial set-up and commissioning of the heater.
Ease of Adjustments/Troubleshooting
During the plant start-up, and throughout the life of the heater, adjustments may need to be made to the heater controls. For start-up, the plant will want to make sure the heater is optimized for their current configuration. As the plant grows, or the weather changes, or any host of other reasons, the plant may want to make system changes. For example, the plant may want to adjust the ignition sequence. If the heater is run by a PLC, this will require a knowledgeable engineer to come to the jobsite with a laptop. As with the original programming, the engineer and laptop must understand and know how to use the software specific for the PLC. Most of the time, this means calling an outside company to come help make adjustments. While this may be OK during planned maintenance or shutdowns, this can be a problem when trying to troubleshoot an unplanned interruption.
Making adjustments and working through unplanned interruptions tends to be much easier with a dedicated combustion management system. As noted above, field personnel are taught to make typical adjustments to the system. If problems arise that they can’t work through themselves, many problems can be worked through over the phone since the field personnel don’t need to understand programming or need a special software to make changes.
Another factor related to the above points that must be considered in the controls design is which code the system needs to be designed for. Heater control codes include NFPA and API, along with other regional codes used internationally. Some codes, like NFPA, are much more stringent on the type of PLC that can be used – calling for “safety rated” PLC’s. Other requirements that can come about because of code requirements include redundant components (or even redundant PLC systems) and SIL 3 rating requirements. These requirements can be extremely expensive to comply with, making a dedicated combustion control system seem more attractive.
While the above items point out the advantages to using a combustion management unit over a PLC, there are still cases where PLC’s are preferred or required. If you need to control more than just the heater, you will likely need to use a PLC. For example, many plants want a PLC to control an entire hot oil system – including not only the heater, but the exchangers, pumps, and control loops throughout the system. This type of control would require many different inputs and outputs of varying types, which is not ideal for a dedicated combustion management system.
Still have questions? Feel free to contact us and we can try to help you get what you need.