8.1: Water Supply and Fire Pumps

Introduction to Water Supply Systems

Water supply systems represent the foundational backbone of any water-based fire suppression system, including automatic sprinklers, standpipes, and foam systems. For a fire suppression system to operate effectively, the water supply must be capable of delivering the required volume of water at a sufficient residual pressure. A water supply that offers high static pressure but drops precipitously under flow is hydraulically inadequate. Certified Fire Protection Specialist (CFPS) candidates must understand how to evaluate municipal water systems, private water storage tanks, and fire pump installations to ensure compliance with NFPA standards and design demands.

Municipal Water Distribution Networks

Municipal water systems are the most common source of water for fire protection systems. These systems are classified based on their configuration and piping layout:

• Grid Systems: A network of piping that forms loops, allowing water to flow from multiple directions to any single point. This layout is highly advantageous because it minimizes friction loss and ensures reliability if a portion of the main is shut down for maintenance.
• Loop Systems: Piping loops that encircle a specific area. Like grid systems, loops provide two-way flow, which dramatically reduces pressure drops under high flow demand.
• Dead-End Mains: A single pipe that terminates without connecting back to a loop or grid. Dead-end mains suffer from high friction loss, lower available flow rates, and are prone to sediment accumulation and freezing.

In municipal water supply design, piping size is critical. NFPA 24 recommends a minimum pipe diameter of 6 inches (150 mm) for fire mains, with 8 inches (200 mm) or larger preferred in commercial and industrial zones to limit friction losses. The flow capacity of these pipes is heavily influenced by the internal roughness of the pipe material, represented by the Hazen-Williams C-factor. A higher C-factor indicates a smoother pipe interior:

• New plastic (CPVC) or ductile iron pipes typically have a C-factor of 140 to 150.
• Older cast-iron mains can have their C-factor drop to 100 or lower due to tuberculation (the build-up of rust and mineral deposits). This drop in C-factor significantly increases friction loss and reduces the available flow rate for fire fighting.

Private Water Storage Tanks

When municipal supplies are unavailable, unreliable, or lack sufficient pressure or volume, private water storage tanks must be utilized. NFPA 22, Standard for Water Tanks for Private Fire Protection, governs the design and installation of these tanks. The three primary types of water storage tanks are:

1. Gravity Tanks: Elevated tanks that rely on gravity to generate pressure. The static pressure at the base of the tank is directly proportional to its elevation above that point. The pressure is calculated using the hydrostatic formula: $\text{Pressure (psi)} = 0.433 \times \text{Height (ft)}$ For example, an elevated tank with a water level 120 feet above the discharge outlet will produce a static pressure of approximately $52\text{ psi}$ ($120 \times 0.433 = 51.96\text{ psi}$). While highly reliable because they do not depend on mechanical pumps, gravity tanks are expensive to construct and maintain.
2. Suction Tanks: Located at or near ground level, suction tanks store water that is drafted by a fire pump. Suction tanks must be equipped with an anti-vortex plate at the suction outlet. The anti-vortex plate prevents air from being drawn into the pump suction piping during high-flow conditions, which could cause pump cavitation and complete loss of suction.
3. Pressure Tanks: Closed vessels containing both water and compressed air. Typically, these tanks are filled with 2/3 water and 1/3 compressed air, with a minimum air pressure maintained at $75\text{ psi}$ (or higher depending on design requirements). Pressure tanks provide immediate pressure to the system, but their capacity is limited, making them suitable only for light hazard occupancies or as a secondary supply.

Fire Pumps and Drivers (NFPA 20)

When the available water supply pressure is insufficient to meet the hydraulic demands of a sprinkler or standpipe system, a fire pump must be installed in accordance with NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection. The primary pump types include:

• Horizontal Split-Case Pumps: The most common type of fire pump. They are centrifugal pumps with a casing that splits horizontally, allowing easy access to the impeller and internal components for maintenance. However, they require a positive suction pressure and cannot draft water from a source below the pump's impeller.
• Vertical Shaft Turbine Pumps: Used when the water source is located below the pump impellers, such as a well, wet pit, or pond. The impellers are submerged in the water, eliminating the need for priming.
• Vertical Inline Pumps: Compact centrifugal pumps mounted directly in the piping. They save floor space but are generally limited in capacity (typically up to 1,500 gpm).

Fire pumps are driven by either an electric motor or a diesel engine:

• Electric Drivers: Highly reliable, simple to operate, and require minimal maintenance. However, they are vulnerable to utility power failures. To mitigate this risk, NFPA 20 requires a secondary source of power (such as an emergency generator) and an automatic transfer switch (ATS) unless the utility connection is deemed highly reliable.
• Diesel Drivers: Independent of the municipal electrical grid. However, they require weekly testing, dedicated fuel tanks (sized for 1 gallon of fuel per horsepower plus a 10% reserve), starting batteries, and combustion air ventilation.

Fire Pump Performance Curves

A fire pump's performance is characterized by its flow-pressure relationship, plotted on a graph known as the pump curve. According to NFPA 20, a fire pump must meet three critical performance points:

1. Shutoff or Churn Pressure (0% Flow): The pressure developed by the pump when operating at rated speed with the discharge valve completely closed. The shutoff pressure must not exceed 140% of the pump's rated head pressure. This limits the maximum pressure that can be exerted on the system piping.
2. Rated Capacity (100% Flow): The design point of the pump. The pump must deliver its rated flow (e.g., 1,000 gpm) at 100% of its rated pressure (e.g., 100 psi).
3. Peak Capacity (150% Flow): The pump must be capable of operating at 150% of its rated flow capacity while maintaining a net pressure of not less than 65% of its rated pressure. For example, a pump rated for 1,000 gpm at 100 psi must deliver at least 1,500 gpm at a minimum pressure of 65 psi.

It is crucial to distinguish between net pressure (the pressure added by the pump itself) and gross pressure (the total pressure at the discharge flange, which includes the incoming suction pressure). During acceptance testing, suction pressure must be subtracted from discharge pressure to verify the pump's net curve.

Water Flow Testing and Pitot Tube Operations

Water flow testing is performed to determine the capacity and pressure characteristics of a water supply system. The test involves measuring static pressure (pressure when no water is flowing), residual pressure (pressure remaining in the system when water is flowing), and the flow rate of the discharged water.

The procedure requires at least two hydrants:

1. Test Hydrant (Residual Hydrant): A pressure gauge is attached to this hydrant to record the Static Pressure before any water flows. Once the flow hydrant is opened, the pressure on this gauge drops to the Residual Pressure.
2. Flow Hydrant: One or more hydrants located downstream from the test hydrant. Water is discharged from these hydrants, and a pitot tube is used to measure the velocity pressure (pitot pressure) of the escaping water stream.

A pitot tube is held in the center of the water stream, approximately one-half the diameter of the outlet away from the hydrant nozzle face. The velocity pressure measured by the pitot tube is converted to flow (gpm) using the physical discharge formula: $Q = 29.84 \cdot C \cdot d^2 \cdot \sqrt{P}$ Where:

• $Q$ = Flow rate in gallons per minute (gpm).
• $C$ = Hydrant discharge coefficient based on the outlet shape:
  • $C = 0.90$ for smooth, well-rounded outlets.
  • $C = 0.80$ for sharp-edged outlets.
  • $C = 0.70$ for outlets protruding into the hydrant barrel.
• $d$ = Inside diameter of the hydrant outlet in inches (typically 2.5 inches or 4.5 inches).
• $P$ = Velocity pressure measured by the pitot tube in psi.

Worked Example 1: Calculating Hydrant Flow

During a flow test, water is discharged from a 2.5-inch hydrant nozzle ($d = 2.5$). The hydrant outlet has a smooth, rounded edge ($C = 0.90$). The pitot pressure gauge reads $16\text{ psi}$ ($P = 16$). $Q = 29.84 \times 0.90 \times (2.5)^2 \times \sqrt{16}$ $Q = 29.84 \times 0.90 \times 6.25 \times 4$ $Q = 167.85 \times 4 = 671.4\text{ gpm}$

Hydraulic Calculations and Flow at 20 psi Residual Pressure

To evaluate the total flow capacity available for fire department pumper trucks, flow test data must be extrapolated to a standard residual pressure of $20\text{ psi}$. This is the minimum pressure required to prevent municipal water main collapse and to prevent pump cavitation in fire apparatus. The extrapolation is performed using the Hazen-Williams relation: $Q_R = Q_F \times \left(\frac{S - 20}{S - R}\right)^{0.54}$ Where:

• $Q_R$ = Available flow at 20 psi residual pressure (gpm).
• $Q_F$ = Total flow measured during the test (gpm).
• $S$ = Static pressure measured at the test hydrant (psi).
• $R$ = Residual pressure measured at the test hydrant (psi).

Worked Example 2: Calculating Flow at 20 psi

• Static Pressure ($S$) = $85\text{ psi}$
• Residual Pressure ($R$) = $65\text{ psi}$
• Flow Hydrant Flow ($Q_F$) = $900\text{ gpm}$ (calculated using the pitot formula) $Q_R = 900 \times \left(\frac{85 - 20}{85 - 65}\right)^{0.54}$ $Q_R = 900 \times \left(\frac{65}{20}\right)^{0.54}$ $Q_R = 900 \times (3.25)^{0.54}$ Using a calculator, $(3.25)^{0.54} \approx 1.888$. $Q_R = 900 \times 1.888 \approx 1,699\text{ gpm}$ This indicates that while the hydrant flowed $900\text{ gpm}$ during the test, the municipal water main can safely deliver up to $1,699\text{ gpm}$ before the system pressure drops to the $20\text{ psi}$ safety limit.