NfPA 13 pipe schedules require 15 psi residual pressure at the highest sprinkler. If a flow test shows 15 psi at the base above the riser, and the highest sprinkler is 9 feet above the riser, how is the correct pressure at the highest sprinkler determined?

To determine the correct pressure at the highest sprinkler, it is essential to understand how hydraulic pressure changes with elevation. As water is distributed vertically, pressure decreases by approximately 0.433 psi for each foot of elevation gain.

In this scenario, the flow test provides a residual pressure of 15 psi at the base above the riser. Since the highest sprinkler is located 9 feet above this point, the pressure at the highest sprinkler will be calculated based on the loss of pressure due to the elevation gain.

By multiplying the elevation (9 feet) by the pressure loss per foot (0.433 psi), you arrive at the total pressure loss: ( 0.433 \times 9 ). This result gives the amount of pressure that needs to be subtracted from the initial 15 psi measured at the base to find out the pressure at the highest sprinkler.

Therefore, to find the pressure at the highest sprinkler, one would add the calculated pressure loss back to the initial reading. This method accurately reflects the decrease in pressure due to the elevation and confirms that the pressure at the highest sprinkler should take into account both the initial pressure at the base and the vertical height that contributes to pressure loss. Thus, the comprehensive approach ensures that the sprinkler system