Diesel-Driven Fire Pump Systems
Calculations and Installation Issues Engineers, Designers, Plan Reviewers, and Inspectors Often Miss
John August Denhardt, PE, FSFPE
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Diesel-driven fire pump systems are economical, reliable, and a proven method when an increase in water pressure is required for fire protection systems where reliable normal electrical power is not available. In accordance with NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, 2022 edition, 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 is more fuel efficient and produces fewer emissions than ever before.
For a diesel-driven fire pump system to operate as intended, several design and installation calculations or considerations must be completed during the design and shop drawing. These include:
- Operation ambient temperature of the diesel driver,
- Elevation above sea level of the diesel driver,
- Combustion ventilation requirement,
- Room ventilation requirements based on the type of driver cooling,
- Floor drain sizing,
- Fuel tank sizing,
- Battery sizing and voltage,
- Battery cable sizing,
- Controller incoming voltage,
- Block heater incoming voltage, and
- Exhaust piping sizing.
Operation ambient temperature
The maximum operating ambient temperature at the engine air inlet connection is limited to 120°F. The minimum room ambient temperature is 70°F if the diesel engine is not equipped with an engine block heater or 40°F if not equipped. However, every diesel engine manufactured for the U.S. market is equipped with an engine block heater. The upper limit of 120°F is specified by NFPA 20 and the listing of the diesel engine. The temperature limitation of the diesel engine is determined by considering the engine’s full load operating conditions at the location’s highest expected outside temperature. This requirement is not an easy requirement to satisfy in warmer locations without a large ventilation system or an air conditioning system. For operating air temperatures above 77°F, NFPA 20 requires a derating of the output of the diesel engine (horsepower). In the real world, how many existing fire pump rooms (enclosures) are over 77°F when the diesel engine is operating under full load? I’m sure the engineer of record, the designer, or the plan reviewer ensured the diesel engine’s power was derated.
NFPA 20 requires a derating of the output of the diesel engine (horsepower) for all elevations above 300 feet above sea level. This derating accounts for the lower oxygen level in the combustion air as lless oxygen results in less power.
As part of the listing, the diesel engine manufacturers specify the minimum quantity of combustion air needed for the diesel engine to operate properly at full load operating conditions. The combustion air must always be available with the room at normal operating conditions. Think about the fire pump operating test you witnessed. Was the door to the outside chocked open? A public input was submitted for the 2025 edition of NFPA 20, which will require all operational tests to be conducted with the room at normal conditions—3:00 a.m. Sunday when no one is around.
The fire pump room (enclosure) must have sufficient ventilation to ensure the diesel engine is operated at the design temperature and radiant heat does not raise the temperature in the room. If the diesel engine is cooled with air through a radiator, ventilation must be supplied with sufficient air to allow proper cooling with the diesel engine under operating under full load conditions. Think of a generator installation. The amount of air flow required is huge. In addition, the room temperature must be kept over the minimum under all air flow conditions even when the incoming air temperature is very cold.
A floor drain is required by NFPA 20 and usually is provided. However, is it sized correctly? The drain must handle normal drips and packing glands, but with a diesel engine that uses a heat exchanger, the heat exchanger will need to discharge sufficient water to keep the engine cool under full operating load. The diesel engine manufacturer can supply the required water flow rate.
Fuel tank sizing
The method to determine the capacity of the fuel storage tank for the 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 8 hours run time plus 5% for sump plus 5% for expansion.
A basic example:
A 50 Hp driver:
1 pt x 50 Hp x 8 hours x 1.05 x1.05 = 441 pints, which equals 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 plus 5% for expansion
Using the example above of a 50 Hp driver:
1 gallon x 50 Hp x 1.05 x 1.05 = 55.125 gallons
So, the capacity of the fuel tank has been essentially the same until the 2022 edition of NFPA 20. During the 2022 revision cycle, a public input was submitted to change the formula to:
Driver’s fuel consumption rate at rated Hp multiplied by 8 hours run time plus 5% for sump plus 5% for expansion
This public input was resolved by the Technical Committee. The Technical Committee provided a substation for their 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 per gallon. A modern Ford Expedition can easily obtain 20 miles per gallon. This improvement in fuel efficiency has 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 in the world 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 diesel fuel quality is an issue. Specific monitoring, testing, and correction language for 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 which requested the Technical Committee reconsider this approach. The submitter provided rationale for the reconsideration like the points made above. The Technical Committee did change their direction. The resulting new formula is as follows:
Driver’s fuel consumption rate at rated Hp multiplied by 12 hours run time plus 5% for sump plus 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. A Technical Interim Amendment (TIA) was processed by NFPA 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 condition. By definition, the fuel supply rate minus the fuel rate equals the fuel consumption rate.
Almost all modern gasoline and diesel engines use the above method in delivering fuel to the engine. The fuel that is returned to the tank is filtered and, in some cases, preheated. This method helps ensure the fuel in the tank is kept cleaner and when preheated, will aid in the combination process. 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 above. If, for some reason, the fuel consumption rate is not known, the original formula is allowed to be utilized. However, in almost every situation I have reviewed, the new formula will produce a minimum capacity fuel tank smaller than utilizing the original formula.
It should be noted, with the original formula, diesel drivers are designed to operate at least 5 hours and 20 minutes since the low fuel alarm is designed to operate at the two-thirds level of a full fuel tank (8 hours x 2/3). With the new formal, a 12-hour time was chosen by the Technical Committee in lieu of the recommended 8 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, utilizing the new formula, under maximum load, the driver will have sufficient fuel to operate 8 hours (12 hours x 2/3). For comparison purposes, the longest time duration I have ever designed for was 4 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 an 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, but for the 2028 edition of NFPA 20 it is a possibility. In any case, if the engineer of record or owner of the project feels 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 so much full, 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.
Battery sizing and voltage
The diesel engine manufacturer will have several options for battery sizing in terms of capacity and voltage. These available options should be reviewed to ensure the best option is chosen for the specific installation. While larger capacity batteries are more expensive, they might offer a better value for the client.
Battery cable sizing
The battery cables need to be of sufficient length and diameter to ensure proper operating conditions. Where are the batteries going to be located? Based on the length and load, is the proper wire size being provided?
Controller incoming operating voltage
Controllers are available with several incoming voltage choices. Has the electrical correlation been made between the design and installation team?
Block heater operating voltage
Diesel engine block heaters are available with several incoming voltage choices. Has the electrical correlation been made between the design and installation team. Higher voltage heaters will draw less amps and might be a better choice for the installation.
Exhaust pipe sizing
The backpressure applied to a diesel engine will negatively affect performance of the engine. The length, number of fittings, and the type of silencer (muffler) will affect the back pressure. Increasing the exhaust piping size might be required to allow the backpressure to be in the acceptable range. The engine manufacturer will provide the maximum backpressure allowed for a specific diesel engine. Silencers with greater noise reduction will cause more backpressure.
The diesel engine manufacturer will provide the discharge exhaust temperature under full operating load. Modern diesel engines with emission controls will typically operate at a higher temperature. These temperatures need to be reviewed to ensure personal injury due to contact and radiant heat loads need to be considered. Properly rated insulation can assist with the exhaust temperature concerns.
John August Denhardt, PE, FSFPE is with the American Fire Sprinkler Association