Chemical makers want to increase the sustainability of their operations but always must keep costs in mind. Fortunately, enhancing the sustainability of a plant and significantly improving the triple bottom line (people, planet, profit) of a company doesn’t have to blow the budget. Some relatively inexpensive measures can keep a plant running at peak efficiency, providing year after year cost reductions and holding its carbon footprint in check. Here, we’ll look at several steps: loop tuning, boiler tuning, addressing steam leaks, using variable frequency drives (VFDs) and lighting upgrades. These steps can yield decent savings on their own, but combined as a program, could provide a substantial boost.
A well-tuned loop enables a controller to get a desired output based on a set point in a repeatable and accurate manner. Having valves oscillate greatly to find that desired output costs a plant power, time and money. Good efficient control of a loop yields energy savings, minimizes raw material losses and contributes to more-consistent product quality. In contrast, a loop that’s not tuned properly not only undermines performance but also may pose a safety issue.
Valves constantly wear — the more a valve travels, the more wear it likely will suffer. To put this into perspective, a valve may travel five to ten times more than necessary to have the same output in a tuned loop. The greater a loop is out of tune, the greater the valve travels. This extra movement does two things. It prematurely wears out the valve, causing more frequent removal from service and rebuilding, which can be quite costly. Operating the valve also requires more power, which puts an unnecessary added burden on your pneumatic system. Multiply that extra power by the number of valves in the system and the impact can be enormous.
Many different methods can tune loops. Quite a few very good software packages optimize loop tuning; these are relatively low cost compared to doing manual Ziegler-Nichols tuning on each loop, which can be very time consuming. However, some people contend the automated tuning packages are meant for newer valves and don’t take into account the wear in the linkages. Regardless of whether you use an automated or manual approach, the payback period for tuning a loop sometimes is only a matter of hours. Loop tuning may be the most important element of improving energy efficiency.
While performing these loop tunings, also look for leaking instrument air. Stopping such leaks can provide additional savings. Compressed air is a considerable expense at most plants, typically around $100/scfm. So, a careful investigation of the overall compressed-air system often can lead to significant savings. (For more on optimizing compressed-air systems, see: “Properly Assess Compressed Air Demand” and “Energy Savings are Often Disguised as Problems.”)
An inefficient, poorly maintained boiler wastes money and makes a plant’s carbon footprint larger than necessary. Keeping a boiler tuned reduces greenhouse gases and saves money in the long run. Cutting the fuel consumption of a boiler directly results in less air pollution. Tuning up a boiler allows precise measurement of the required air/fuel ratio. A good maintenance program is imperative for an efficient boiler.
There are four main objectives in making a boiler more efficient: reducing the stack losses, cutting the amount of excess air used, improving heat transfer and recovering waste heat. Here’s a high-spotting of what to do:
• Stack losses. Measure and monitor flue-gas temperature. The system can absorb more heat by decreasing that temperature. Reducing air leaks and keeping your stacks and airways free of soot are essential for achieving optimal stack performance.
• Excess air. Test and monitor oxygen levels and reduce or eliminate air leaks in the boiler. To precisely control the airflow, either install or adjust air dampers and put in or calibrate oxygen controls.
• Heat transfer. Keep boiler tubes clean, free of scale and fouling.
• Heat recovery. Collect the steam condensate from traps and add it to the feed water to improve heat recovery. In addition, employing stack economizers to pre-heat boiler feed water can pay extra dividends.
Unlike tuning a simple control loop, you must meet a variety of objectives to bring a boiler to optimal efficiency. This is especially the case for boilers that are multi-fuel, e.g., that burn oil and coal or other solids. These boilers are subject to strict emissions limits and may require further testing under new U.S. Environmental Protection Agency guidelines. To maintain these limits, the boiler may need tuning more often than the specified 24-mo cycle.
Good communication between plant operations and the boiler operator is key to obtaining further efficiencies. Alerting the boiler operator to process changes or upsets will help that person better deal with load changes. However, communications can be very difficult, especially in a continuously changing production environment.