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The 2008 edition of NFPA 25 includes a new provision: Where a 10 percent reduction in the full flow pressure is observed from previous results, the cause of the reduction is to be identified and corrected. However, facility executives should look into any reduction in the residual pressure observed during a sprinkler system flow test. A decrease in residual pressure may indicate a deteriorated water supply, obstruction, closed valve or other system impairment. At a minimum, the investigation into the cause of the reduced residual pressure should include an impact analysis to determine the effect on the sprinkler system hydraulic demand.
In the design of a sprinkler system, the difference between the hydraulic demand and the available water supply is known as a design margin or buffer. The model codes governing the design of automatic sprinkler systems are silent regarding the minimum design buffer, leaving the size of the margin to the discretion of the designer. The buffer should account for foreseeable variations in water system strength.
The pressure buffer of a sprinkler system can vary depending on how the designer anticipates nominal consumption, future demand, infrastructure improvements or seasonal pressure changes. Minor changes in the main drain test residual pressure can swing the demand/supply relationship into a deficit, resulting in an inadequate water supply. When conducting a main drain test, a drop in residual pressure indicates diminished water supply strength. Facility executives should investigate any drop in main drain test residual pressure, even before the drop reaches the 10 percent threshold. That’s because this prescriptive requirement does not account for systems that have smaller design cushions that can be overcome by slight decreases in residual pressure. The main drain test can be used to detect a weakening water supply, but to really find out the impact on the sprinkler system, facility executives should conduct a hydrant flow test.
Investigate Reductions in Main Drain Residual Pressures