First, what is a backflow preventer, and why do we need it? Backflow preventers are required to protect the non-potable fire sprinkler system water from backing up into the public water supply. They are an assembly of two internal check valves between two control valves. This test is the somewhat forgotten Sprinkler Acceptance and Annual Test. The backflow is checked for backflow integrity but also needs to be tested for forward flow integrity. Forward flow testing was made a code requirement in the (2002) NFPA 25 Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems.
Why is this important?
The backflow assembly, whether a double check or reduced pressure zone type, can over time, develop a memory in the closed position of the two internally loaded check valves if not exercised regularly. If the required flow from the sprinkler system is hampered by a hanging check valve that is not opening cleanly or partially, the flow can be reduced, which could be detrimental to the designed flow of the fire sprinkler system. Backflow preventers must be sized correctly for the designed fire sprinkler system maximum fire flow demand plus the added outside fire department hose stream allowances.
How is this design accomplished?
Several methods of physical design can be utilized for forward flow testing. Although, much is dependent on the situation, size of the system, and availability to flow water safely outside of the building with fire hoses.
- Provide a bypass around the check valve used for the fire department connection line with a supervised closed control valve routed to flow water out of the fire department connection. (This can only be used for FDCs, in which the clapper in the FDC body can be easily removed and reinstalled.)
- Provide the correct amount of 2.5” hose valve connections on the riser after the backflow preventer allowing one hose valve for every 250 GPM of fire flow demand plus hose stream allowance. (This can only be used when there is an exterior access door from the fire riser valve room where the water flow can be safely discharged.)
- Size the fire sprinkler system main drain to handle the required flow to the outside of the building. This option does require the designer to hydraulically calculate the flow using design software to determine the correct size of the main drain line. Some smaller systems could be designed and verified from a 2” main drain or upsizing to a 2.5” main drain.
- If there is a fire pump for the fire sprinkler suppression system, the easiest solution to obtain the forward flow test is to use the fire pump test header if the backflow preventer is installed on the suction or downstream side of the fire pump.
- If the building has a standpipe, the fire department hose valves can be used to provide the flow requirements of the forward flow test. This requires great coordination and owner approval to run the fire hoses safely down through stairwells outside of the building through the means of egress for the building and safely outside.
- Install a dedicated forward flow test header from the riser that is specifically valved and sized with enough outlets with hose valves extended outside to a manifold or forward flow header.
Ahern has extensive experience in designing, installing, and maintaining fire protection systems. When partnering with Ahern, you receive a team of code experts looking out for your safety, schedule, and budget. Contact Ahern for customized solutions specific to your facility.