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.
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 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 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.
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.
It's almost impossible to match plant demand and compressor capacity exactly because demand varies continuously from minimum to maximum. As a result, compressors often spend more time operating in an unloaded or partially loaded mode than at design capacity. In addition to the energy losses and costs, this seriously affects the compressor's reliability and useful life. The solution is to store compressed air in a properly sized receiver to permit the compressor to be shutdown when air is not needed. This drastically reduces cost and extends the compressor's useful life.
The second major loss associated with compressors is leakage. Most plant air systems are riddled with leaks. Obviously, this is a complete waste of the energy required to compress the air or gas. It is estimated leaks increase compression cost by 10% to 25% in typical plants.
Where they're used, leaking or damaged steam traps are a major source of energy loss. As an example, the energy loss through a single 3/4-inch steam trap is equivalent to $75,000 to $150,000 per year. The root causes of steam trap problems are improper selection and contamination. Little effort is expended in the selection process. Undersized or inadequate traps quickly fail. Contamination in the form of solids in the steam generation process finds its way to the bleed valve that purges condensate from the system. As a result, the valve can't seat properly and continuous blow-down occurs. Proper selection and a viable preventive maintenance program could eliminate most of the losses associated with steam traps.
Few of the heat exchangers found in domestic plants are sized properly for their application or insulated adequately. The major energy loss occurs on the condensate side of the exchanger. If the amount of condensate is too great and simply flashed to atmosphere, it wastes most of the energy needed to generate the steam in the first place. Because excess condensate doesn't affect the process directly, these losses are hidden and allowed to continue indefinitely.
The piping associated with chilled water, steam or hot liquidsmaterials essential to many production processesmay not be insulated properly. Some estimates claim energy loss associated with improper insulation adds between 10% and 35% to the recurring energy cost in a typical process plant. Correcting insulation problems is relatively straightforward. Infrared surveys quickly isolate areas within the plant where excessive heat loss occurs. Systematic correction of these problem areas eliminates the losses and increases the plant's overall efficiency.
There are many other forms of energy loss. These few examples should start you thinking about unnecessary energy costs that batter your plant's profitability measures. Most losses are preventable. All that's necessary to eliminate them is some attention to detail. Allocate a part of your improvement efforts to the basics and acknowledge the contribution that effective energy use will have on your plant's bottom line.
Contributing Editor Keith Mobley can be reached at [email protected].