Energy conservation gives a major boost to plant profitability

Taking the steps necessary to conserve energy will give a boost to plant profitability

By R. Keith Mobley, Contributing Editor

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I've never understood why corporations invest so much money in quick-fix improvement programssuch as total productive maintenance or just-in-time manufacturingand little or nothing on the basics that can greatly reduce operating costs and improve profitability. One clear example is energy utilization. Evaluating a typical domestic plant or facility will likely reveal that energy, if not the highest, is one of the larger recurring costs.

This explains why an early measurement of maintenance management styles was phrased in terms of cost per horsepower. Throughout the 1980s, maintenance effectiveness was stated as $18 per horsepower for breakdown; $11 per horsepower for preventive; and $8 per horsepower for predictive. The figures, provided by the Electric Power Research Institute, recognize that power consumptionprimarily electricitywas the major recurring cost throughout industry. Acceptance of the measurement was limited because few plants or facilities, if any, had clear knowledge of the total installed horsepower or actual power consumption. Think about it. Do you have any unambigous, objective idea of the total installed horsepower in your plant? Do you know how efficiently this horsepower is being used? If so, you're in the minority.

As a rule, little attention is given to energy use or energy conservation. There are exceptions, but in the few plants where the potential impact of energy use is recognized, the efforts to conserve or improve efficiency is less than ideal. For example, I was in a large, integrated facility several years ago when a concerted effort was made to reduce costs, including energy consumption. In this plant, I found a team of maintenance personnel removing one-half of the fluorescent lights in the offices and work areas. Management believed that this step would reduce total energy costs dramatically.

There were two flaws in the logic. First, the energy consumed by fluorescent lighting was less than one percent of the plant's total consumption. Second, the plant had a negotiated, flat-rate contract with its electric utility and would pay the same fee regardless of how much electricity it used. We don't understand, or even acknowledge, that inefficient energy use is a major cause of poor performance.

Correcting this problem requires a clear understanding of the impact energy utilization has on the plant. Generally, 80% of total energy consumption can be traced to 20% of plant systems or areas. The first step to finding that 20 percent is a detailed energy audit that identifies the users as well as major losses and how they can be minimized or eliminated. Gaining an accurate understanding of the site's energy use may require installing metering devices at each energy consumer, major cost center or major production system and monitoring actual use at regular intervals until a clear profile emerges. Once this determination is made, focus conservation efforts on areas or systems that account for most of the plant's energy bill.

Boiler plants

These are major energy users. The combustion efficiency in a boiler plant can be set and maintained at optimum levels quite easily. The major boiler losses include excessive combustion air, excessive surface losses and fouling of heat transfer surfaces.

Sustaining a fire requires consuming energy to heat combustion air to the flame temperature. Using excess combustion air is wasteful because it too must be heated, only to go straight up the stack without contributing anything to the combustion process itself. Using the boiler's oxygen trim control to hold excess air between two and three percent improves boiler efficiency by one to two percent.

Surface heat losses are generally a function of boiler loading. As the load is reduced, the relative surface heat loss increases. Typically, an annual boiler load factor is around 40%. At this level, the relative surface heat loss is 50%. To resolve this loss, a boiler should be operated at 80% or more of its maximum continuous rating. That suggests matching demand to boiler output. If the plant demand decreases, one or more boilers should be shut down so the online boilers can be operated at their highest sustainable rates.

Slag buildup on boiler tubes is another major loss. Because slag is almost a perfect insulator, its buildup must be controlled. In large, electric power generating plants, soot-blowers are used for this purpose. Most industrial boilers don't have this capability. Therefore, cleaning boiler tubes regularly must be an integral part of a preventive maintenance program.

Furnaces

Furnaces also use large amounts of energy. To minimize operating cost, a furnace must be well insulated and operated at maximum capacity, while waste heat in both flue gases and product is recovered.

Fans

Fans are used widely throughout most plants in applications ranging from small recirculating air movers to large induced draft fans in boilers and air-handling units. Typically, larger units operate at a fixed speed while throughput is adjusted with dampers that throttle air or gas flow. In most applications, the fan's full capacity seldom is needed, and the excess energy is wasted. For large fans, variable-speed drives will reduce the electricity consumption and operating costs, as well as extend the fan's useful life.

Pumps

The typical pump is sized to meet the system's maximum hydraulic requirement, even though this intensity of operation is rarely needed during normal operation. In addition, energy consumption is rarely considered during the procurement process. As a result, many pumping systems in the plant are major sources of energy loss. To remedy this situation, modify the procurement process to select new and replacement pumps on the basis of efficiency and energy consumption. In addition, modify engineering practices to better match the pump selection to the actual demand. In some cases, it may be smarter to use two smaller pumps instead of one large pump.

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