Engineering of Water Systems

Standby and Backup Systems: Engine Rooms, Part 3

By Ed Butts, PE, CPI

The April edition of The Water Works included part two of this three-part series on engine room design. This month, we will conclude this series with the third and final part.

Fuel Storage Methods and Capacity-2

Determining the required storage capacity requires consideration of the factors initially outlined plus knowledge of the specific fuel consumption (BSFC) of the engine or engines in use.

As a rule of thumb, you can assume the following as an estimated fuel consumption. Although it should be noted this assumes a standard BSFC value as an estimated value, different engines possess different BSFC values that can vary between 0.33 to 0.80. For estimation purposes, assume BSFC will equal 0.45 lbm/HP/hour.

Therefore, the approximate fuel consumption for each naturally aspirated fully loaded engine’s fuel type is equal to 1 gallon per hour per 0.0563 BHP for diesel and 1 gallon per hour per 0.050 BHP for gasoline. This means that a 100 HP-rated diesel engine will consume approximately 0.0563 HP/gph × 100 HP = 5.63 gallons per hour of fuel at full (100%) load.

As previously stated, other considerations of the stored volume must include additional factors such as the needed volume for other uses, required operating duration, and need for fuel rotation.

Needed volume for other uses must be determined on a case-by-case basis. In situations involving fire pumps, the fuel storage must be dedicated to the fire pump and is not permitted to be shared with other uses.

On a more practical basis, however, when an engine is used as a backup for a water supply system, the fuel can sit unused for months or even years. Using the fuel for other purposes such as city vehicles, equipment, and tractors is often beneficial to the process as it aids in turning over and rotating the fuel, ensuring the fuel does not wax or become contaminated. When used in this manner, though, it is vital that a proper inventory of the fuel extracted and used is noted and replaced as soon as possible to retain a full capacity tank for potential emergencies.

Portable fuel tank capacities are often limited by axle or tire capacity as well as allowable roadway capacity. Typically, when combined with the engine, pump, trailer, and accessories, the maximum volume of fuel that can be transported is limited to around 1000 gallons (7500 pounds).

Finally, the expected duration of continuous engine operation is obviously the greatest variable, but even this variable includes other factors. For a stationary engine or generator set that operates a backup or primary pumping unit or generator plant, this variable depends heavily on individual elements including
importance of site, redundancy of alternate sites, regulatory requirements, automatic or manual operation, length of expected outage (for utility power), and load (individual horsepower and number of units) variances during operation.

Generally, for optimum sizing and protection, these various elements must be weighed together in a risk-to-benefit analysis to determine the optimum tank sizing.

Typically, as a safety factor, it is not warranted or wise to consider possible variances in the load or number of operating units unless the system includes specific safeguards and options to permit this function. Therefore, for safety, the operating load is generally assumed to be at 100% continuous output.

In the case of a prolonged power outage from a storm, flood, or other natural disaster—although utilities are often given the highest priority for restoring—access limitations, competition for power restoration, and severe damage to the structure can impede power restoration for up to a month in some cases.

In a practical setting, this degree of fuel storage is obviously unrealistic, so system designers must usually consider as an alternative the maximum duration in days before the consumed fuel supply can be replenished along with alternative methods for refueling.

In severe cases, this may mean procurement of available fuel from other assets such as tractors, backhoes, dozers, or service vehicles. This is, once again, where competition for available fuel can become an issue, but potable pumping plants are typically given higher priority.

Through experience, I have learned that in worse-case situations, providing an onsite fuel storage volume equivalent to no less than three days of continuous operation at full load along with inclusion of a 5% sump and 5% expansion allowance volume is usually adequate. This volume will generally provide adequate storage for three days of continuous operation or up to five to six days for a partially loaded or intermittently operating pump or generator, while providing a balance between required fuel rotation and enough time for tank refueling versus unwarranted exceedance to limit greater cost and possible fuel stagnation issues.

For estimating purposes and fuel tank sizing, Tables 1a and 1b from Part 1 of this series (January WWJ, January 2026) illustrate the approximate hourly fuel consumption for diesel and natural gas fueled engines of various horsepower and power ratings, respectively.

Example 1: Find the estimated fuel volume, in gallons, for a 225 BHP diesel pump/engine combo for three days.

Solution: 0.0563 HP/gph × 225 BHP = 12.67 gph × 72 hours (3 days) = 912 × 1.10 = 1003 gallons

Engine Room Design Considerations and Examples

There are many other installation parameters and site conditions that must be considered to have a successful engine or generator set installation. The following is a partial checklist of areas to consider during design.

• Consider foundation type needed such as crushed rock, concrete, dirt, separate concrete inertia pad.
• The weight of the support pad should be no less than 150% of the unit wet weight (fuel included). Thus, a 10,000-pound engine or generator set would require a minimum 15,000-pound pad.
• The fire resistance rating of interior walls, floors, and ceilings of engine rooms needs to be a minimum of one hour. In the event the room is located on the top floor of a structure, then it is permissible for the ceiling to be noncombustible or protected with an automatic fire suppression system.
• Engines using Class I liquid fuels (gasoline) must be located in rooms with exterior exposure and accessibility for fire-fighting operations.
• For fuels other than Class I fuels, aggregate capacity of fuel tanks not installed within a dedicated room cannot exceed 660 gallons. Tanks with an aggregate capacity between 660 and 1320 gallons must be installed outdoors or within a dedicated room with a minimum one-hour fire rating. Tanks with an aggregate capacity exceeding 1320 gallons must be installed outdoors or within dedicated rooms of minimum three-hour fire rating.
• Secure foundation to engine with good mounting. Add vibration dampening (spring type, rubber, etc.).
• Is noise attenuation caused by radiator fan, mechanical, exhaust, or air intake sources required?
• Include all combustion and cooling air requirements in intake louver (free open area) calculations.
• Include exhaust backpressure in engine design and wrap muffler and exhaust piping for 1200°F, minimum.
• Verify whether an emission, fuel storage, or operating permits are required.
• Is a means required for unloading, setting, and future service or removal of engine or genset?
• Use engine/genset derating for high altitudes and excessive ambient temperatures.
• Any hazardous waste or onsite (limited) storage volume considerations for fuel, antifreeze, engine oil?
• Design facility to meet all local and applicable building, fire, life safety, and electrical codes.
• Any special exposures such as coastal conditions, dust, presence of chemicals or gases?
• Specify properly sized starting system for engine and generator unit (starter, batteries and rapid charger).
• Provide adequate clear and unimpeded space around the engine or genset for installation and future maintenance (air filter removal, oil drainage and changes, general inspection, tighten belts).
• Provide 4 to 5 feet of clear space in front of electrical panel/circuit breakers to comply with electrical codes.
• Provide flexible connections on all systems that attach the engine/genset to a rigid structure (foundation vibration isolators, exhaust for muffler and piping, direct feed air intake, radiator flanges to air exhaust duct work, flexible connections on power and control wiring conduits).
• Use a diesel day tank system for remotely located fuel tanks (add for pumps, suction and return piping).
• Comply with all local codes for fuel storage tank requirements (double wall, clearances, fire codes).
• Even if outside air is brought in and routed over the engine or genset for combustion and cooling, a supplementary method of room cooling must often be provided. An excess exhaust ventilation fan compensates for the thermal expansion of incoming air and creates an indraft to confine heat and odor to the engine room. This can be a wall- or roof-mounted exhaust fan sized to provide at least one room volumetric air exchange every 5 to 10 minutes. Thus, an engine room with dimensions
  of 15 feet wide by 20 feet in length and 10 feet in height (3000 ft3) will require an exhaust fan
  capable of 250 to 500 CFM. Don’t forget to include the exhaust capacity of the fan in the exhaust louver design.
• Ventilation air exhaust systems should be designed to maintain a slight positive pressure in the engine room, depending on the specific application. Positive room pressures from forced air ventilation should not exceed 0.20 inches and negative pressures should never exceed –0.50-inch of water column, respectively.
• If the engine room temperature exceeds 104°F, the generator must be derated per the manufacturer’s derate schedule and cooler outside air must be ducted directly to the generator air intake.
• If natural gas or liquid propane (LPG) fueled engines are planned, ensure the supply piping is of adequate size for the intended pipe run and engine size to avoid excessive friction loss. In addition, provisions must be provided to prevent over-pressurization of a 2 psi natural gas service in the event of pressure regulator failure. Always observe all codes related to the installation and
  piping requirements for all fuel types and all assembled piping and fittings must be adequately pressure tested before use to verify that no leaks are present.
• Engine room temperature should not exceed 120°F during continuous operation. Depending on the background ambient air temperature, this will equal a temperature rise between 15° to 30°F.
• Engines and weatherproof housing located on the roof of a structure or outdoors needs to be a minimum of 5 feet from wall openings and a minimum of 5 feet from structures with combustible walls. Reduced clearances are acceptable if all portions of the structure that are closer than 5 feet from engine enclosure have a fire resistance rating of minimum one hour. Reduced clearances are also acceptable if it can be demonstrated that fire within the housing will not ignite combustible
  structures and the underlying reasoning and methodology is reviewed and accepted by the AHJ.
• Engines with an output of 10 HP or more must be capable of being shut down at the engine and from a remote location in the event of an emergency.

Although all local and state fire and building codes should be followed when designing an engine room and fuel storage facility, Figure 1 illustrates a typical fuel storage system that can be used for stationary engines or generator sets. Figure 2 shows a horizontal exhaust and muffler installation, and Figures 3a and 3b illustrate various methods to bolt down and secure engine or generator plants.

Figure 4 illustrates a typical engine or genset support pad with corner bolt down provisions. Note that the pad is at least as wide and long as the generator itself and is resting upon a base of compacted rock well below the floor level. Also note that the pad is independent and not part of the floor slab and is isolated by a perimeter mastic joint to prevent contact and cracking of the floor slab from vibration. It also contains 150% of the mass (weight) of the generator to absorb possible harmonic vibration, more than the minimum value of 125%.

Finally, Figure 5 illustrates a vertical turbine pump installation driven by a diesel engine. The four primary design features of note with this 250 HP, water jacket-cooled, turbocharged diesel engine installation include the wrapped muffler, concrete engine support block, dual battery racks, and driveline shield.

Figure 1. Typical diesel fuel supply system schematic.

Figure 2. Horizontal exhaust schematic.

The muffler is attached to the engine exhaust using a flexible adapter for isolating vibration and the wrapped and insulated muffler assembly is independently supported from the concrete engine support block, which was constructed with 150% of the engine mass and extends 24 inches below grade. The battery racks include two sets of lead-acid batteries for starting redundancy and each set is supported on a steel support bracket and not allowed to touch or rest on the concrete floor, which would drain and destroy the batteries through a chemical process (electrolysis). The driveline is protected by an outer steel shroud, split in the middle to allow regular greasing and servicing of the driveline. The air inlet louver is both DC-powered and open to admit combustion and cooling air upon engine starting.

Since the engine is water jacket-cooled, there is no radiator or need of an engine-driven fan for cooling. In this case, a small percentage of the pumped water is delivered to the engine for cooling and then discharged to a storm sewer, although cold water is always available from the water system to prevent engine overheat.

Also note that the engine is placed to provide adequate perimeter space for service and maintenance and to provide free air circulation around the engine for convection heat radiating (cooling).

Figure 3a. Engine or genset mounted base construction.

Common Engine Rules of Thumb

As with most powered machines, engines are also capable of temporarily operating under an overload or within a wider tolerance or service band for short periods of time without incurring serious and irreputable damage.

Figure 3b. Generator set mounted on pad isolation.

The following rules of thumb are commonly used estimates and performance indicators used by engine system designers and operators that can be applied for estimating and preliminary design purposes. However, final design and maintenance elements should always be obtained from the engine manufacturer’s data sheets and other applicable sources to ensure the proper variables have been considered:

Fuels:

Gasoline: 18,900 BTU/lb. at 6.60 lbs./gallon
Diesel: 18,400 BTU/lb. at 7.20 lbs./gallon
LPG: 19,900 BTU/lb. at 91,500 BTU/gallon (2500 BTU per 1 ft³ of LPG)
Natural Gas: 1000 BTU per 1 ft³ of gas

Relative Engine Performance:

Gasoline: Baseline 100% of rating peak efficiency at ~75% of maximum HP
LPG produces 96% of gasoline rating peak efficiency at ~75% of maximum HP
Diesel produces 90% of gasoline rating peak efficiency at 1500-2000 RPM
Natural Gas produces 80% of gasoline rating peak efficiency at ~75% of maximum. HP

Engine Power Ratings: Based on an equivalent power rating of 100 HP, maximum load and minimum load

Desired: Add 15% for overload, but run at no less than 70% underload: <115 HP >70 HP
Penalty in efficiency and life for >115% overload or less than 50% underload: <115 HP >50 HP
Severe Loss of Performance for turbocharged engines below 60% of rating: <115-125 HP >60 HP

Typical Fuel Consumption per Type of Engine and Fuel

Gasoline: 0.075-0.10 gallons (0.50-0.60 lbs.) per BHP per hour
Diesel: 0.05-0.07 gallons (0.35-0.50 lbs.) per BHP per hour (for naturally aspirated)
LPG/Natural Gas: 8,000-10,000 BTU/BHP/hour

Figure 4. Typical generator pad.

In most instances, the minimum continuous operating period is two to three hours, but local codes and specific design criteria may often extend the design operating period to as much as 48 hours.

Another shortcut method to determine the minimum fuel storage volume needed for 12 to 18 hours of continuous engine operation, common for many fire and backup potable water service pumps, is to use 1 gallon per engine brake horsepower plus 10% volume for fuel expansion and to provide a sump.

Figure 5. Engine-driven vertical turbine pump example.

For a 225 BHP example, this would equal 225 BHP × 1 gallon = 225 gallons + 22.50 gallons = 247.5 gallons (~250 gallons).

Once again, these values should be used for estimating purposes only. Precise sizing of a fuel storage vessel must include the variables listed above as well as comply with all local regulations and codes applicable to the required or expected duration of engine operation.

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This concludes this series on standby and backup engine and generator systems. In the edition of The Water Works in the October 2026 issue, we will review methods of cybersecurity for SCADA and other control systems.

Until then, keep them pumping!

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He was honored by the National Ground Water Association with the 2025 Technology Award. He can be reached at epbpe@juno.com.