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Diesel-driven fire pump systems are economical, reliable and proven when an increase in water pressure is required for fire protection systems where reliable normal electrical power is unavailable. In accordance with the 2022 edition of NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, all diesel drivers must be listed for fire protection service.
In the United States and other countries, these drivers must comply with applicable emission standards. While the diesel engine has been around since the early 1900s, the modern diesel-driven fire pump system is more fuel-efficient and produces fewer emissions than ever before.
The method to determine the capacity of the fuel storage tank for diesel-driven fire pump systems has changed over the years. When I started in this business in the mid-1980s, the minimum fuel tank capacity was determined by the following formula: 1 pint of fuel per horsepower of the driver multiplied by eight hours run time, plus 5% for sump and 5% for expansion. A basic example for a 50-horsepower driver looks like this:
1 pint x 50 horsepower x 8 hours x 1.05 x 1.05 = 441 pints (55.125 gallons)
With the 1983 edition of NFPA 20, the formula was modified. However, this modification did not change the results. The change only made the formula easier to use: 1 gallon of fuel per horsepower, plus 5% for sump and 5% for expansion. Using the previous example of a 50-horsepower driver:
1 gallon x 50 horsepower x 1.05 x 1.05 = 55.125 gallons
So, the fuel tank’s capacity has been essentially the same until the 2022 edition of NFPA 20. During the 2022 revision cycle, public input was submitted to change the formula to the driver’s fuel consumption rate at rated horsepower multiplied by eight hours run time, plus 5% for sump and 5% for expansion.
The technical committee resolved this public input. It provided substantiation for its decision that, in my humble opinion, did not make sense. It is well-known that diesel fuel, when stored for long periods, will degrade unless proactive steps are employed to slow down the degradation.
Modern diesel drivers are much more fuel-efficient than previous drivers. Heck, look at my father’s vehicle in the 1970s: a 1974 Ford County Squire station wagon. In real-world driving, that vehicle was lucky to get 10 miles/gallon. A modern Ford Expedition can easily obtain 20 miles/gallon. This improvement in fuel efficiency resulted in the need for the capacity of diesel-driver fuel tanks to be reduced.
In the automotive world, manufacturers engineer the fuel tank to match the fuel efficiency and desired driving range. So, why would the technical committee not use the same approach?
NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, has long realized that diesel fuel quality is an issue. Specific monitoring, testing and correction language for the fuel quality is included in NFPA 25. In reality, many owners do not have these services completed.
During the 2022 revision cycle public comment period, a public comment was received requesting the technical committee reconsider this approach. The submitter provided a rationale for the reconsideration, including the previous points I stated. The technical committee did change its direction. The resulting new formula is as follows: Driver’s fuel consumption rate at rated horsepower multiplied by 12 hours run time, plus 5% for sump and 5% for expansion.
Let’s cover some of the confusing language. In fact, the technical committee had the incorrect language during the second-draft ballot process. The NFPA processed a Technical Interim Amendment to correct the language. One will see the following terms used in the industry:
Fuel Supply Rate. The amount of fuel supplied to an engine while the engine is in the operating condition.
Fuel Return Rate. The amount of fuel returned to the fuel tank while the engine is in the operating condition.
Fuel Consumption Rate. The amount of fuel the engine consumes in the operating condition. For fire pump driver applications, this value is the consumption rate at full-load conditions. The fuel supply rate minus the fuel rate equals the fuel consumption rate.
Almost all modern gasoline and diesel engines use the above method to deliver fuel to the engine. Fuel returned to the tank is filtered and, in some cases, preheated. This method helps ensure the fuel in the tank is kept cleaner and, where preheated, will aid in the combination process.
Providing Sufficient Fuel
When sizing a diesel fuel tank for fire pump service, the fuel consumption rate can be determined by reviewing the diesel driver manufacturer’s listing literature. The data sheet for the driver will list the fuel consumption rate. From there, apply the new formula as stated previously.
If, for some reason, the fuel consumption rate is not known, the original formula is allowed to be used. However, in almost every situation I have reviewed, the new formula will produce a minimum-capacity fuel tank smaller than using the original formula.
It should be noted that with the original formula, diesel drivers are designed to operate at least five hours and 20 minutes since the low fuel alarm is designed to operate at two-thirds the level of a full fuel tank (8 hours x 2/3). With the new formula, a 12-hour time was chosen by the technical committee in lieu of the recommended eight hours.
While I do not fully understand the technical committee’s justification for the 12-hour duration requirement, it is now in the standard. Thus, using the new formula under maximum load, the driver will have sufficient fuel to operate for eight hours (12 hours x 2/3). For comparison purposes, the longest time duration I have ever designed for was four hours.
In this specific case, the diesel fuel tank at the two-thirds level will double the water supply time duration. The fuel supply time duration seems like overkill to me, but the technical committee decided on this.
Maybe this requirement can be changed in a future edition. This requirement will not change for the 2025 edition of NFPA 20 as no public inputs were received on this topic. The 2028 edition of NFPA 20 is a possibility. In any case, if the engineer of record or owner of the projects believes a larger fuel tank is desired, they can specify one.
Correctly sizing the fuel tank will assist the owner in providing sufficient fuel for the required duration without having too much fuel, which could lead to stale and compromised fuel. A smaller-sized fuel tank will fit in a room easier, present a lower fire hazard, and cost less in terms of equipment and fuel.
John August Denhardt, P.E., ET, CWBSP, FSFPE, is vice president of engineering and technical services for the American Fire Sprinkler Association. He is a member of the NFPA 13 Sprinkler Discharge Committee and is a Fellow of the Society of Fire Protection Engineers. Denhardt is also a National Fire Protection Association Research Foundation trustee, sits on the University of Maryland Department of Fire Protection Engineering’s Board of Visitors, and is a member of the SFPE board.
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