The volume of flow in a sprinkler system is measured in gallons per minute (gpm) in the US, and in liters per second (L/s) metrically. The velocity of flow is measured in linear feet per second (ft/s), meters per hour (m³/h), or liters per minute (l/m).
Flow in pipe is always accompanied by friction loss in the direction of the water flow.
The amount of friction loss (pressure loss) is due to four conditions:
- The velocity (speed) of the flow.
- Diameter of the pipe.
- Length of the pipe.
- Roughness of the pipe.
Velocity is simply the speed at what water moves through the pipe. As water travels through the sprinkler pipe the pressure gradually decreases. The faster the water moves through a sprinkler pipe, the higher the friction loss. Industry standards have have established 5 ft/s (1,5m/s) as an acceptable maximum velocity. Damaging surge pressures and rapidly increasing friction loss are two problems encountered when velocities exceed 5 feet per second (ft/s) – (1,5 m/s). To help determine what flow is safe to design with, there are pipe flow charts for various types of pipe in numerous sizes. Usually 1/2″ – 6″.
What all this means is that it is critical to be aware of some of these basic principles when designing or changing aspects of your sprinkler system. The most common problem I encounter when working on an existing sprinkler system is that the designer and/or the installer has not used these design principles. As an example, the last system I worked on had a 3/4″ water meter feeding the system, the pipe size was 3/4″ PVC Schedule 40, and the water pressure was 75 psi. The installer had put 10 sprinkler heads on one circuit and the homeowners were unhappy with how the heads were working and the coverage the heads were providing. The problem here was the heads were trying to use 20 gallons per minute and the sprinkler system could only safely deliver around 12 gallons per minute. To solve this design problem often times is tricky and potentially expensive for the homeowner.
I am including a table below so you can get an idea on what a pressure loss chart looks like. This table should be helpful if you are designing or installing your own sprinkler system. Try and keep your flow rates in the shaded areas of the chart. Notice that for 1/2″ PVC pipe I wouldn’t want to try and force 10 gpm through the sprinkler pipe, but would be fine using 10 gpm through a 1″ sprinkler pipe. The table is for PVC Schedule 40 pipe which is the most common pipe currently being used in sprinkler systems. The psi loss on the table is for 100 feet of pipe (psi/100 ft).