Back flow Devices Need to be Tested at Full Flow
Backflow devices are intended to prevent that nasty sprinkler water from going into your drinking water. These devices need some tender loving care in order to operate efficiently and test must be conducted to ensure they can open to meet the system demand. In the 2002 edition of National Fire Protection Association (NFPA) 13, Standard for the Installation of Automatic Fire Sprinkler Systems, a testing requirement was added for the forward flow testing of back-flow devices.
In fire protection systems, the back flow device is installed below the system control valves and can be the cause of reduced flow for the system. Reduced flow can occur if the system is not properly designed to include the device or tested to ensure the system flow is not reduced. The device is regulated by the local plumbing code for the protection of the water system.
NFPA 13 Section 24.2.5 states (2007 edition):
The backflow prevention assembly shall be forward flow tested to ensure proper operation. The minimum flow rate shall be the system demand, including hose stream allowance where applicable.
The user is also referred to 18.104.22.168 for clarification on the installation of these devices. Although the requirement was published starting in 2002 (the most current standard is 2007) many sprinkler contractors and inspectors are unaware of the requirement.
Unfortunately there is no one size fits all answer for this code requirement. It is important to understand that sprinkler system may have multiple design capacities. When sizing for the forward flow test, the highest demand (GPM) with the hose allowance should be utilized. This information can be obtained from the cover sheet of the hydraulic calculation or the hydraulic design placard.
When plans are submitted and reviewed by the local building or fire department, the reviewer should verify the details provided by the designer. Details should include maximum system flow (based on the hydraulic calculations) and details on the test equipment configuration including size and number of outlets.
There are several common options available and which include (but not limited to):
- A bypass may be installed around the check valve installed in the piping between the system and Fire Department Connection (FDC). The bypass would include a supervised valve in the normally closed position. During testing the contractor would flow out the outlet of the fire department connection through the appropriate hose and nozzle configuration
- Install various outlets on the system side of the back-flow device. The outlets would look similar to a hose or 2.5” standpipe connection.
A good rule of thumb is that a 2.5” connection can accommodate approximately 250 gpm. Shop drawings for sprinkler systems can easily identify this requirement in the riser detail.
Acceptance Test Procedures
During the acceptance testing of a new system, the contractor will install various hose lines to the outlets and include appropriate orifices on the end of the line. The lines must be secured from movement and the lines should discharge to an area that can accommodate the flow of water and not damage the area.
The valves will be slowly opened, a pitot gauge utilized to ensure the design flow is met. Assuming the system flows at the design rate the device passes. When the desired rate is not achieved, the contractor shall inspect the system for defects and develop a strategy to remedy the situation. The contractor shall indicate the flow and test conducted on the completion documents. During initial acceptance tests the forward flow of the back flow should be conducted after the velocity flush test of the under ground piping (would reduce damage to the device).
We have noticed that this requirement is not easily known by the industry and can include some added cost to the system design. The forward flow test ensures that the backflow devices can operate at the system design.
The safety considerations during the test can not be understated. Contractors, inspectors, and witnesses should ensure proper procedures are identified. The lines shall be properly secured from movement. This may include utilizing devices such as a hose monster or a secure bracket that can hold multiple lines. Flow of 300 gpm at 70 psi can create significant discharge issues if not properly addressed. Utilizing additional staff who are able to shut down the line quickly is a great practice.