What is the best location for back flow devices when a fire pump is utilized?
Automatic sprinkler and standpipe systems may utilize a fire pump for many reasons. This may be to provide additional pressure to the municipal system, provide pressure for a system which draws from an underground tank or to meet the necessary gpm for the system design. The water and pressure demands of sprinkler or standpipe systems truly establish the need for a fire pump.
Fire pumps are appropriately viewed as a critical component to a fire protection system, but it should be realized that the fire pump is supplemental to the entire system. In realty, the fire pump can not create a water supply. If the water supply is not there, the pump will not produce the desired results.
Water is supplied to a fire pump by two main methods. The first is from the municipal water system available in many of our communities. In addition to providing for domestic water needs, these systems provide our fire protection water. The second type of water supply is a private tank or private fire protection system used exclusively for fire protection.
Regardless of the use, the most current (2007) edition of NFPA 20 – Installation of Stationary Fire Pumps for Fire Protection, is the guiding standard for the installation of the system. NFPA has also issued a new handbook to accompany the 2007 edition, a significant enhancement over previous editions.
When discussing fire pumps, the side of the fire pump where the water enters the pump assembly is known as the suction side and the discharge side of the pump is known as the system side.
The NFPA 20 technical committee on fire pumps has had a long standing recommendation that back flow devices not be installed on systems which utilize fire pumps. If they are installed, they should only be installed on the system side of the pump. Many fire pumps need as much pressure and volume as possible in order to supply the system. Additionally, check valves and back flow devices in the suction piping are generally discouraged based on turbulent flow or even friction loss.
Section 18.104.22.168 of NFPA 20 States:
The following devices shall be permitted in the suction piping where the following requirements are met
(1) Check valves and back flow prevention devices and assemblies shall be permitted where required by other NFPA standards or the authority having jurisdiction
For many of us, this is an odd configuration to see. As it relates to fire protection, the back flow device is located just after the fire protection lead enters the building or just after a connection to the municipal system. Building and fire departments should consider discussing the location of the device on systems which utilize a fire pump with the local water authority. Preliminary discussions can aid the installing contractor in the design phase of the project.
Based on the above information, lets look at a simple example. A sprinkler system is installed which is utilizing a large orifice sprinkler in a big box warehouse. Each sprinkler will demand nearly 150 gpm per sprinkler. The total design area could require nearly 1700 gpm.
Some factors to consider are:
- The incoming water supply is supplied by a 10” fire protection lead.
- The water flow test produced plenty of GPM, but a low pressure reading.
- The back flow device (AMES Colt C300, 10”) has a friction loss of 4 psi friction loss at the above listed gpm.
If required by the AHJ, the device can be installed on the suction side of the pump, although a good fire protection practice is to install the device on the system side of the pump. The designer will have to account for the friction loss and work with the pump manufacture to size the system accordingly. The back flow device can most likely be downsized to an 8”. The designer of the system will still need to account for the friction loss, although it is easier to accommodate on the system side.
We recently encountered a system that would not work unless the back flow device was installed on the system side. The available pressure was so low, that the 7 pound loss from the back flow device would not permit the system design to work. The designer relocated the device to the system side and all parties where satisfied with the outcome.
During the design phase the fire pump make take pressures up to 175 psi. The back flow device should be evaluated to ensure its operation is not adversely affected by these higher pressures.
If the fire pump is installed in the suction piping, NFPA 20 section 5.26 provides requirements for the device. The important concept is the back-flow device must allow the flow of the fire pump at 150% of its rated capacity. If during field acceptance tests this can not be provided, revisions would be necessary to the various devices.
If the plan review and approval process is evaluated for a community, the designer will most likely work with multiple inspectors. The fire marshal may be concerned over the total system design, the plumbing inspector is concerned over proper back flow of the system, the plan reviewer is looking for code compliance and the mechanical inspector may be involved in some areas. With multiple interests, the community and contractor must work together in order to protect the water supply as well as the fire protection system.
Locating the device on the system side of the fire pump allows for there to be an unobstructed flow into the fire pump. Pump manufactures can then provide a more even flow into the fire pump ensuring the proper design of the pump assembly.