Fire Pumps: Diesel vs. Electric
by , from the 2/2004 issue of Sprinkler Age, page 22
The conventional wisdom within the fire protection industry is that an electric motor driven fire pump is almost always cheaper than a diesel engine driven pump. Therefore, electric motor driven fire pumps are utilized except when reliable electric power is unavailable or when a backup fire pump is required to provide increased reliability.
However, a recent White Paper prepared by the Schirmer Engineering Corporation revealed that, in many situations, properly installed and maintained diesel engine driven fire pumps can be more reliable - and much more cost efficient - than electric motor driven pumps.
The Schirmer study reviewed the advantages and disadvantages of both electric motor driven and diesel engine driven fire pumps, and when each is the appropriate choice.
Reliability of Fire Pump Drivers
Generally, electric motor drivers for fire pumps are highly reliable. They operate efficiently and, in and of themselves, have an extremely low rate of failure. However, the electric driver - and therefore the pump and sprinkler system - will immediately be affected by an interruption in power supply.
In comparison, diesel engine driven fire pumps are not affected by power outages, and are designed to be more reliable than automotive, truck or standby generator set engines. Most of the reasons that engines fail to start are minimized or eliminated in a fire pump installation. The fire pump diesel engine is not subjected to cold temperature. Backup batteries are a part of every fire pump installation, the batteries are monitored and the engine is tested weekly for 30 minutes. The fuel level is supervised.
Reliability of Electric Power Sources
NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, requires the power supply to an electric motor driven fire pump to be reliable, but does not define reliable. Many things including natural disasters, transformer or substation failure, and utility grid maintenance affect the reliability of electric power. In August 2003, much of the northeastern grid, covering parts of New York, New Jersey, New England, Pennsylvania, Ohio, Michigan, and several Canadian provinces was blacked-out up to several days. Hurricane Isabel caused massive power outages the following month, up and down the east coast. Since 1999, the utility company serving Chicago, Ill., has experienced various outages, some lasting three to four days. Downtown Chicago recently experienced outages involving multiple substations.
The loss of power and the start of a fire cannot be viewed as entirely independent events, as there is a higher likelihood of a fire occurring during a power outage. Storms, lightning strikes, tornados, hurricanes and terrorism may cause both a power outage and a fire. Also, the reaction to a power outage - such as using candles for light - may result in a fire.
In all of the above scenarios, an electric motor driven fire pump with power supplied by a single utility has the greatest risk of failure.
Providing a backup power source can increase the reliability of electric motor driven pumps. Typical backup sources include a second substation or an onsite generator. However, while backup power from a second substation improves reliability, substations are not totally independent. Events that cause the loss of power at one substation may cause loss of power at other substations, as with the northeast outage. Storms, lightning strikes, tornados, hurricanes, generator and grid outages are examples that may cause power outages at multiple substations.
Properly installed and maintained emergency generators also increase the reliability of an electric motor driven fire pump. However, there are significant differences between NFPA 20 diesel engine fire pump drivers and NFPA 110 emergency generator drivers. Specifically, the difference in engine design, installation and testing requirements make a diesel engine driven fire pump more reliable than an emergency generator.
The Schirmer study analyzes various power source combinations and compares diesel engine versus electric motor pump drivers in terms of failure probability, reliability, and expected loss. The study concludes that a highly reliable - greater than 99.75 percent reliable - fire pump can be provided by either:
1. Using a properly installed and maintained diesel engine.
2. Connecting an electric pump to both a public utility and an emergency generator.
For high value facilities, dual fire pumps provide significant reduction in the expected loss, with dual diesel engine driven fire pumps providing the lowest expected loss overall.
In view of the impact of power supply on system reliability, a comparison of diesel versus electric driven fire pumps must consider the cost of electric power. When the site's transformer is installed and owned by the utility company, (typically a larger transformer is required to supply an electric motor driven fire pump), the additional cost of the transformer is built into the rates charged by the utility company. However, if the owner is required to pay for the transformer, it is likely that a diesel engine fire pump will cost less than the total cost of the electric motor driven fire pump including transformer.
The Schirmer study compares the installation, maintenance and operating costs for electric versus diesel drivers in five sample installations. The significant variables affecting costs for electric motor driven fire pumps include:
- The length of the electric feeder from the transformer to the fire pump.
- The use of multiple power sources.
- System operation during periods of Peak Demand Rates, Non-Peak Demand Rates, and No Demand Charges.
According to the Schirmer study, under all scenarios and with any combination of electrical demand rates, diesel drivers were significantly less expensive to install, operate and maintain than electric drivers or electric drivers with generator backup.
"It is not unusual to see overall cost savings of 50 percent and more with diesel engine driven fire pumps, even when talking about electric motors with reduced voltage starters," comments Mark Evans, global marketing manager for Clarke Fire Protection, the world's leading manufacturer of diesels for driving fire pumps.
"When comparing installed, operating and maintenance costs among electric motor-driven, electric motor-driven with generator backup and diesel engine-driven fire pumps, diesel is the clear winner," he concludes.
In spite of the conventional wisdom, there is solid evidence that diesel engine driven fire pumps are often more economical than their electric motor driven counterparts. Diesel also scores high in reliability.
1. Diesel engine driven fire pumps are more reliable than electric motor driven fire pumps with power from a single utility, or two substations. The reliability of a diesel engine driven fire pump is only slightly less than an electric motor driven fire pump with power available from both a utility and an emergency generator.
2. In general, when the transformer is supplied by the utility and the feeder is not too long, electric motor driven pumps with a single utility power source and utility furnished transformer can be installed for less than diesel engine driven pumps. The cost differential is less when a reduced voltage starter is required for the electric motor driven fire pump. The other significant variable in the total installed cost is the length of the electric feed from the transformer to the electric motor. This can quickly make the electric motor driven fire pump more expensive than the diesel driven fire pump.
3. Diesel engine driven pumps provide a better value when considering installed cost and expected loss when the exposed value is moderate ($10 million to $100 million).
4. The installed cost of diesel engine driven fire pumps is less than electric motor driven fire pumps with a dual power source (i.e., an emergency generator or two substations), and provides nearly the equivalent reliability to a system with an emergency generator and greater reliability than two substations.
5. For high exposed value (>$100 million), dual pumps (two diesel engine driven or one diesel engine driven and one electric motor driven) with independent water supplies are needed to provide suitable reliability.
6. In areas that may be subjected to prolonged power outages, the reliability of an electric fire pump supplied from a utility without emergency power decreases and the expected loss increases accordingly.
7. Increased reliability should also be provided whenever there are significant life safety considerations. Many health care facilities, i.e. nursing homes and assisted care living facilities, have immobile patients who are difficult to evacuate. Assembly occupancies have high population densities where crowd control and evacuation issues necessitate increased fire pump reliability.
8. Lastly, the electricity demand charge has a significant impact on yearly maintenance and present value cost for electric fire pumps. In general, in areas with significant demand charges for electricity, present value cost for a diesel engine is less than for an electric motor driven fire pump when the pump's electrical demand exceeds electrical demand for other uses. Caution should be exercised when selecting a fire pump driver based upon No Demand Charges for electricity. Most utility companies will have some demand charge in their rate structure. It may also be difficult, in actual practice, to limit fire pump testing to Non-Peak Demand hours.
To request a copy of the complete Schirmer Engineering White Paper , call 614.764.1274 or send an email to email@example.com.
As in any construction industry decision, material cost plays a part but is not the only consideration in making the decision about what equipment should be installed to meet requirements of the project. This article was written from one manufacturer's point of view and helps identify other factors to keep in mind when selecting a fire pump.