Simple Techniques to Optimize Boiler Efficiency
From an operations and engineering standpoint, running a steam plant with limited manpower can be difficult and expensive, particularly at larger facilities where there are multiple boilers, steam headers, deaerators and water treatment units. When the equipment runs well, there is usually adequate time to monitor operations and pursue cost-reducing actions. But what happens when things are not going well?
In these situations, knowing the extent of problems and how to prioritize for responsive action is necessary. Most seasoned personnel know the first priority is safety, preventing injury to personnel and equipment. The second priority is keeping the facility running -- the plant needs the steam or things can get real expensive with lots of losses. The third priority usually is to keep costs at a minimum. Simple tools and techniques can be used to help you get a handle on identifying cost issues and how to improve them.
- How is the steam plant performing?
- How often and when do the boilers trip? What are the real causes?
- Even if units generally are operating effectively (not shutting down and starting up very often), what is the relative cost of steam for each unit?
- On a fundamental level, how are valves, damper drives and control loops performing?
- What is the failure status of transmitters, valves or motors?
When there are problems and little time to properly investigate them, often experts are called in to help. These experts are experienced at listening, observing and investigating. Once they understand your operation, they are able to offer suggestions for improvement. However, the time delay, cost and expenses to bring them can be high. If plant personnel can quickly access this same type of information, then relative priorities can be defined and actions committed by either operating more efficiently or by preventing failures.
Optimize Design and Operation. It almost goes without saying, but a well designed and maintained operation is key to minimizing problems and costs. At the center of this complex maze of equipment is the control, safety and monitoring system. Control systems range from the simple to the complex, and processors have several options. Regardless of what control system is used, the basic foundation to optimal operation is having access to accurate and timely information, controls equipped with the right strategies for operating conditions, a reliable safety system and final elements that respond properly. Without a good foundation, more advanced levels are not possible.
Employ Diagnostic Tools. Having excellent diagnostic tools is a valuable resource. Field devices are getting smarter and more accurate, and some provide the ability to monitor key metrics of performance. For example, poorly tuned loops or sticking valves often cause valves to oscillate more, resulting in greater valve stem travel. This not only results in poor control, but the valve seals and parts wear out sooner. Having tools to monitor process performance as well as process equipment such as valves is valuable to avoid and fix problems.
Unfortunately, if the control system is not capable or set up to identify problems, or if the plant personnel does not have time to correct these problems, the tools are of little value. One difficulty today faced by industry is that where these systems are in place, little documentation about benefits is available. For example, one processor was having problems controlling drum level in a boiler, resulting in a higher incidence of trips and severe effects on operations for many months. No diagnostic tools were available, and a visit to site and observation found no problems with control logic. But, control tuning was unusual, with ten times greater than expected reset. Further investigation found that the positioner yoke, which adjusts feedwater valve position, had worn a pocket that required a much greater change in valve position signal to change flow. This situation cost the customer thousands to get a consultant in and find the problem - a serious expense. More costly, though, were the thousands spent on fuel, loss of control and negative effect on plant production. Adequate control information would have greatly reduced these losses.
Provide Access to the Right Information. With more process information at one's fingertips and computers having dialup modems or direct Internet access, the ability to quickly identify and solve day-to-day problems is allowing greater and better control and process optimization.
For example, a telephone call for help was received from one facility where the boilers were not responding as fast as required to load changes. The result was that the steam header was either sagging too low, affecting operations, or pressure went too high, causing boiler trips. Control system access was obtained via modem and the firing rate demand controls were tuned. Problem solved. This facility was located in another country, but because the control system allowed remote access and management, corrective action could be taken in just a few hours. No expensive cost for travel or several days delay resulted.
Improve Measurement Accuracy. Fuel costs comprise a majority of the operating costs associated with steam generation. Fuel flow rates for gas, oil or waste fuels are measured directly by field instrumentation. The accuracy of calculated fuel costs is impacted by transmitter accuracy, drift, affects of ambient conditions, and whether measurements are based on volumetric or mass flows.
Instruments often are selected based on cost, but how well they are able to remain accurate over time can have major cost impacts. In one plant, the controls for several boilers were retrofitted while retaining 10- to 20-year-old pneumatic and electronic instrumentation. During startup, load tests showed the values used from years earlier could not be reproduced. Without detailed calculations, duplicate instruments or periodic recalibration, there was no way to tell whether airflow, fuel flow or steam flow measurements were correct. For this plant, the expense of replacing instruments while trying to operate the plant was costly and delayed bringing a new production facility online.
Upgrade Existing Controls. In plants where steam is not the primary product, it is often difficult to justify improvements. Defining failure costs is one way to develop justification, but will the new system increase efficiency? For example, if one proposed to operate excess air at reduced levels, savings and increased efficiency only can be achieved if present operations really are excessive. In most cases, if the old controls are working adequately, replacing them will not increase boiler efficiency.
What happens in many plants, however, is that because the older controls or mechanical systems do not respond well, they are operated conservatively to avoid trips. In one plant where no problems were noted, the air damper had been set wide open at all loads to prevent accidental trips or low oxygen. Fuel costs were extremely high compared to what they could be using controls that could operate reliably. If the equipment is operated less efficiently than it could be, upgrading the controls can improve performance, increase efficiency and reduce fuel costs.
Modeling and Economic Allocation. Advanced methods of improving efficiency by allocation of units for least cost operation exist and are operated successfully quite often. However, the cost of maintenance of such systems and the need for extensive modeling to ensure that measurements are reliable often is significant. If they are not maintained properly, measurements may drift, and new operating philosophies may evolve that result in these systems becoming unreliable.
In an existing boiler room, the plant engineer is likely to have a good idea of the steam production based on the process loads. So, in most plants, may be more practical to perform continuous online performance monitoring and provide operators cost information. Simpler systems that totalize fuel and steam and monitor their ratio usually suffice.
There are simple ways to reduce cost and improve efficiency resulting from new technologies for instruments, enhanced diagnostics, controls, and with application of the Internet. How much can be saved depends on buy in by operations, engineering and what information is made available; how devices and features are selected; and how well systems are implemented with proper training.
Certainly, more advanced methods such as modeling and optimization are possible, but incremental benefits with online methods and greater utilization of more sophisticated controls systems offer a layer of cost savings all too often overlooked.