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Quick question, plumbing pros: What is the most important component of any fire sprinkler system? Here’s a hint, it’s also the most plentiful compound on Earth. That’s right: water! Water is the fire protection world's hero due to its plentiful supply, relatively low cost, massive heat of vaporization, and ease of transport.
Most of us intuitively understand that water can be applied directly to most fires to cool the fuel source and reduce the endothermic pyrolysis rate (that is, slow the burning process). Additionally, the steam created during the vaporization process not only pulls heat from the fire, but it can also displace the oxygen from the fire source.
Unfortunately, sometimes engineers and installers are so concerned about the hydraulically remote area that they become complacent about the water supply. This inattention can have deadly results. Let’s chat about a couple of theoretical case studies and how to avoid them.
Case Study No. 1: Hydrant Test Without a Home
The engineer’s plan shows only the following flow test and site information for a public water supply to a building without a water storage tank:
• 50 psi static; and
• 42 psi residual flowing 880 gallons/minute.
What’s the problem? The opportunity is that the test location and the distance and pipe characteristics (diameter, length and fittings) between the test hydrant and the lead-in are not provided. Additionally, the static (test) hydrant's elevation is unknown relative to the building’s floor elevation.
It is impossible to determine the length or the diameter of the underground piping installed between the flow test location and the building. Both the elevation differential and the friction loss in the underground piping are necessary considerations when calculating a building’s sprinkler system. The information provided by the engineer is essentially worthless.
What’s the fix? Hydrant (flow) tests should always be conducted in accordance with national standards, such as NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants. One of the most critical requirements is to physically locate the hydrant test relative to the building.
NFPA 291 states that a sketch of the site should be provided, with the line sizes and distance to the next cross-connected line, and the tested hydrants' locations. This information should be recreated on the installation drawings.
Case Study No. 2: The Case of the Missing Water
The engineer’s plan shows the following information for a public water supply to a building with a fire pump:
• 50 psi static;
• 37 psi residual flowing 880 gpm;
• Site plan indicating 500-foot equivalent of 6-inch ductile iron pipe with an 8-inch double-check backflow preventer installed between the test hydrant and the pump suction flange;
• Test hydrant elevation = 750 feet AMSL;
• Building floor elevation = 770 feet AMSL;
• Maximum sprinkler plus hose demand = 1150 gpm at 37 psi; and
• Fire pump rated at 1,250 gpm at 75 psi.
What’s the problem? For this scenario, the necessary information is provided in accordance with NFPA 13 requirements. The engineer (correctly) identified that the water supply is not adequate to support the sprinkler demand and included a fire pump in the design. The opportunity is that the water pressure available to the fire pump suction is insufficient when flowing at the flow demand.
Consider the two primary sources of pressure loss: friction and elevation. If the friction loss in the pipe between the test and the building plus the pressure loss due to elevation increase exceeds the available pressure, the water will stop flowing. In our hypothetical example, the pump will be starved for water and probably won’t operate. Let’s look more closely at the friction and elevation factors.
Using the Hazen-Williams equation, the friction loss in the underground piping alone is approximately 20 psi:
Q = flow demand, in gpm
L = equivalent pipe length, in feet
C = friction factor
d = internal pipe diameter, in inches
A typical 8-inch double-check backflow preventer has a 4-psi friction loss across the device at 1,150 gpm.
The pressure loss due to elevation is nearly 8.7 psi:
We must also convert the test pressure to the demand flow rate using the hydrant flow equation from NFPA 291, Equation 126.96.36.199:
QR = flow predicted at desired residual pressure, or Qdesired
QF = residual flow measured during test, or Qresidual
hR = pressure drop to desired residual pressure, or PStatic – Pdesired
hf = pressure drop measured during test, or PStatic – Presidual
Substituting and rearranging the equation to solve for Pdesired is a fun algebraic exercise. Here is the rearranged equation:
The resulting net pressure available at the fire pump suction is:
Whoops. Although a fire pump is designed to draft from a negative suction pressure, any such condition in the water supply will likely result in a “no-flow” condition to the fire pump suction and a potentially catastrophic situation for the building's occupants.
What’s the fix? There are several potential solutions to this situation. First, is another water main available as a source? Tapping the water supply at two locations may provide more volume and pressure than a single source. Be careful; a simultaneous flow test at both locations is necessary to confirm that the public water supply can support the total demand. Most likely, both tapping locations are affected by a large demand elsewhere in the system, including the other tap itself.
A second solution is to upsize the underground main diameter from 6 inches to 8 inches or larger. Just one pipe size larger would reduce the friction loss from 20.5 psi to only 4.7 psi. That change alone may be sufficient to yield an adequate pump suction.
A third solution is to install a water storage tank on-site to serve as a water source. In this situation, the water pressure is sufficient to (relatively slowly) fill the water storage tank, allowing a fire pump to draft from that water supply to support the fire sprinkler system. A hydraulic analysis is still needed to verify the tank fill rates meet NFPA requirements (maximum eight hours fill time, per NFPA 22).
Fire protection professionals must consider the entire sprinkler system, including the public water supply system from the test hydrant to the building, when designing a fire sprinkler system. Complying with NFPA requirements for water supply documentation is an essential first step in that analysis.