While a diesel fire pump can provide an extremely reliable source of water in the event of a fire, the system must be properly assessed to ensure the water will be available for use. The following issues should be considered.
Examine the factory-certified test report for the fire pump to see where the power curve peaks and begins to drop. This information can help determine the correct size fire pump driver.
Fuel Supply
If a diesel fire pump is a part of your facility’s fire protection system, it’s important to understand that the fuel supply for the engine must be maintained and tested periodically. This is because commercially-available diesel fuel can degrade over time, leading to water accumulation in the tank and clogging of the engine’s filters. These problems can result in the failure of the fire pump to operate as designed, causing loss of life or property. The 2016 Edition of NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, includes updates to requirements that are intended to minimize this degradation.
The first new requirement is that the fuel tank must be sized to accommodate the driver’s fuel consumption rate at its rated horsepower for eight hours run time, plus 5 percent for expansion and sump. NFPA 20 appendix A provides a formula for this calculation. The engineer of record on a project may use this information to size the fuel tank for a fire pump.
This is a major change from previous requirements that allowed the engineer of record to rely on a manufacturer’s data sheet for this calculation. This new method is more scientific and should be used for all fire pump projects going forward.
Another addition to NFPA 20 is a requirement that the fire pump fuel tank be tested annually for water and foreign material. It is also required that the tank be cleaned and its fuel filters replaced, if necessary. While this may seem like an unwelcome burden on facility managers, it is a necessary requirement to ensure that the diesel fire pump is available for its intended purpose in an emergency.
Another issue to consider when sizing the fuel tank for a fire pump is that it must be reserved exclusively for this purpose. It should not be used as a storage tank for an emergency generator. This is because electric motors and engines that are intended to drive generators have different operating conditions than those required to drive a fire pump.
Ventilation
As fire protection engineers, we use fire pumps as a means to boost pressures required for the automatic sprinklers and standpipes on our projects. Municipal water supplies often cannot deliver the required pressures that these systems demand. As such, we must design the system using either diesel or electric-driven fire pumps. While many people believe that diesel engine fire pumps are superior to their electric counterparts, there are a number of issues associated with them which make these systems difficult to manage in certain environments.
The first issue is that diesel fire pumps generate significant heat during operation, and this heat must be vented out of the pump room. This is required per NFPA 20. Additionally, the exhaust from a diesel-driven fire pump is noxious and could be dangerous to occupants of the building, which should be considered. Finally, the sound from a diesel fire pump should also be taken into consideration, especially in noise sensitive applications.
Additionally, a diesel fire pump requires its fuel tank to be located in the fire pump room. This tank is required to be sized at one gallon per engine horsepower, plus five percent for expansion and five percent for the sump. In addition, the tank must be able to hold eight hours of fuel.
In addition, a diesel engine driven fire pump will have a fan and louver installed and wired to open when the motor is running. This is required per NFPA 20, and it helps to keep the temperature in the fire pump room below 120f while also helping to ventilate combustion exhaust.
While this may seem like a minor issue, it is critical to ensure that the fire pump is not operated in an environment which is unable to properly accommodate these requirements. It is also important to note that the fire pump must be located in a two-hour fire-rated room and that the fire pump room should be protected from flood waters, explosions and other factors which may prevent its operation.
Another issue that is commonly encountered when dealing with a diesel fire pump is the sizing of the emergency transformer needed to serve it. The engineer must consider the maximum locked rotor current of the fire pump and make sure that the selected transformer is not too small to meet this requirement, while also ensuring that all loads are served with a minimum voltage drop permitted by NFPA 70.
Over-Pressurization
The over-pressurization of a fire pump can lead to damage to system components. This issue is caused by the pump operating at a high-speed, producing a significant amount of pressure. The solution is to ensure that the fire pump is properly sized and installed, ensuring that the maximum churn pressure plus the system rated suction pressure will not exceed the system piping rating.
In addition to ensuring that the pump is sized correctly, it is important to have proper maintenance procedures in place. This includes regular inspection of the battery, oil level, cooling system, hoses and engine in addition to a weekly run test. A failure to follow the required maintenance schedule can result in a number of issues, including over-pressurization of the pump.
When installing a diesel fire pump, it is essential to have a dedicated fuel supply tank that is inspected regularly to ensure that it can meet the demand for power from the engine. Also, it is critical to have the necessary ventilation requirements in place to prevent the accumulation of exhaust fumes.
Diesel fire pumps are used in many applications, from warehouses to aircraft hangars, due to their ability to increase water pressure to an extremely high level. However, the footprint of these pumps is larger than electric fire pumps, making them inappropriate for facilities with limited space.
The fire pump driver, whether it is an electric motor, a diesel engine or steam turbine, must be properly sized to meet the full horsepower demands of the fire pump. The best way to do this is by reviewing the factory test curve and comparing it to the NFPA 20 standard.
While it is possible to use a relief valve in the pump discharge piping to reduce routine pressure fluctuations, this is not allowed by NFPA 20, as it will over pressurize the system at an inopportune time. In order to avoid this, a main pressure relief valve must be provided when 121% of the net rated shutoff (churn) pressure plus the system maximum static suction pressure is exceeded.
A diesel-driven fire pump system can be a cost-effective, reliable alternative to an electric-powered system in areas where normal electrical service is not available. Having a reliable fire pump is essential to protect your building and the people who live, work and play there.
Maintenance
Fire pumps are designed to increase the pressure (measured in psi or bar) of water when the available water supply is not sufficient to protect buildings. Proper design, installation and acceptance testing ensure that a fire pump will function as intended when needed. But, after acceptance testing is complete and “the keys” are handed over to building owners, a system requires routine inspections and tests to remain ready for service. These inspections and tests can only be completed by an experienced fire protection professional, who specializes in UL/FM Diesel Engine driven fire pumps.
To make sure a diesel fire pump will continue to operate as designed, it must have a fuel supply that is capable of keeping up with the motor’s horsepower demands. This means the fuel tank must be sized correctly, the diesel driver must be properly matched to the engine, and the diesel driver should have a factory test curve that meets or exceeds the requirements of NFPA 25.
When selecting a diesel fuel tank for a fire pump application, the size of the tank should be based on the estimated annual consumption rate of the engine, and the maximum continuous running horsepower of the fire pump. This information can be found in the fire pump manufacturer’s sizing data sheet or a sizing calculator that is available online.
Another important consideration when installing a diesel fire pump is the location of the fuel tank. It should be located away from combustible materials and in an area that is free of vibration and noise. Additionally, the fuel tank must be located with a proper ventilation system that is capable of removing combustion by-products and preventing the accumulation of vapors in the storage tank.
Finally, it’s also important to ensure that the fire pump alarm panel is able to receive signals from the control panel and communicate them with the fire alarm system. This will help to prevent a condition where the fire alarm panel is not notified of an emergency, leaving your business vulnerable to costly damages and losses.
Electrical services to the fire pump are required to be installed in a separate conduit or encased in concrete no less than 2 inches thick, and they must be provided with an alternate source of power per NFPA 20. Additionally, piping connecting the fire pump to nozzles, bypasses and test headers must be of a size equal to or greater than the diameter of the fire pump.