Consulting-Specifying Engineer - November 2008 - (Page S5)

on-peakperformance <<<<<<<<<<<< in non-emergency situations. If the tank is sized for 72 hours of full load operation, it could take 22 years to get one fuel turn on the tank, assuming a 60% typical load level, weekly no-load exercising, and an average of 4 hours of outage per year. With such slow fuel turnover, contamination can become an issue. The two most common fuel contaminants are water and biomass. Water enters the tank as humidity through the tank’s normal vent and condenses during the daily thermal cycle. Initially, moisture binders in the fuel capture and contain the moisture. However, as these binders become overloaded, the water drops to the bottom of the tank and begins accumulating. At some point, the moisture may be sucked into the diesel engine, potentially resulting in loss of power and lubrication, along with corrosion. Water also creates an environment that can support biomass growth at the water-fuel interface. When such microbes are pulled into the engine, they can clog the fuel filter, resulting in the engine losing power and shutting down. To minimize these effects, fuel tanks should have a well defined low point and the water should be removed monthly. In addition to fuel contamination, fuel breakdown seems to be more prevalent with today’s low-sulfur fuels. The additional refining processes needed to remove the sulfur may be removing some of the fuel’s stability elements as well. The end result is increased fuel varnishing and gum formation. As diesel gets older, a fine sediment and gum forms, brought about by the reaction of diesel components with oxygen. Additives can help treat common fuel breakdown issues when integrated into a fuelfiltering preventive maintenance program, but, at some point, the fuel simply may need to be replaced. For example, BP recommends that fuel tanks should be emptied and cleaned at least once every 10 years and more frequently if there is a major fuel contamination. Storing large amounts of diesel on-site also requires comprehensive handling and maintenance plans to ensure the system can deliver fuel reliably, while preventing any possibility for hazardous spills. Meeting both of these demands can boost system complexity, and result in the need for added monitoring and control capabilities. In an effort to minimize the negative aspects of on-site diesel fuel, systems that use simple sub-base (generator mounted) fuel tanks and limit on-site fuel to a more easily maintained quantity are generally preferred. Natural gas alternatives Spark ignited generators offer a number of advantages, including an extended run-time when they are paired with a reliable supply of natural gas. Through four Florida hurricanes in 2004 and the Northeast grid failure of 2003, the natural gas infrastructure was unaffected. Other benefits can include reduced permitting requirements, lower preventive maintenance costs, less risk of environmental contamination, and cleaner engine emissions. Spark ignited (natural gas and liquid propane fueled) generators can compete with diesel models on cost in the automotive engine classification. While, historically, this has meant a maxiDiesel versus gas mum power out100 put of 100 kW, Natural gas 80 various manufac60 turers have incorporated turbo40 Diesel charging and 20 optimized revolu0 tions-per-minute 0 100 200 300 400 500 600 technologies to kW boost this power range up to 125 kW, and even 150 kW, within the last few years. Figure 1 Typical diesel vs natural Above this output level, though, spark-ignited gen- gas profile for a bi-fuel generator. Source: Generac Power Systems erators still pose capital cost premiums of 1.75 to 2.5 times the cost of comparable diesel generators. But manufacturers are developing new technologies to improve power outputs and extend the use of automotive-based engines to create new opportunities for natural gas installations. Optimizing engine revolutions-per-minute ratings is one of the key technology shifts now broadening opportunities for spark-ignited engines. Fifty years ago, most generators operated at speeds below 900 rpm, but within the past 30 years, the diesel standby generator market has moved from 1,200 to 1,800 rpm as engine outputs have increased. The prime power natural gas driven generator market has also migrated from 900 to 1,200 rpm, with some recent offerings at 1,800 rpm. Percent fuel november2008 5

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Consulting-Specifying Engineer - November 2008 Contents Viewpoint News M/E Roundtable Comissioning Lighting Controls Prevent Plugholing: Smoke Control Done Right Commercial Geothermal Heating and Cooling System Design Codes &�Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - November 2008

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