Golf Engineering Associates Technical Help Series

Pressure & Flow               

The Relationship Between Pressure & Flow

It's tough to visualize, but here we go:  higher pressure will cause greater flow through any given pipe size, but as the flow increases, the pressure will decrease downstream due to friction loss because water velocities increase as well. The following examples will help you visualize this relationship:

Example #1, The Garden Hose:  With a regular spray nozzle on a regular garden hose you will have high pressure when the nozzle is half-closed, creating a high pressure jet with higher velocity. Open that same nozzle all the way and observe much more flow, but far less pressure. Higher flow = more friction loss = less pressure.

Example #2, The Old Golf Course System:  On new GEA-designed systems there is always plenty of pressure because we limit our sprinklers to pairs. In other words, a station comes on and two sprinklers pop up. On older systems it is common for 4, 5 or even more sprinklers to come on at a time when activated. If you have 5 sprinklers on one lateral line you will have much higher flow and lower pressure. Higher flow = less pressure.

It's not an issue of "hey, we need more water so crank up the pressure", it's an issue of "we need more water, so increase the pipe size so we don't lose more pressure to friction loss". This is an advanced irrigation topic, so we offer the following definitions and conversion factors for further clarification.

PSI:
Pounds per square inch, the standard measurement of pressure in the United States.

Water Velocity:
The accepted standard for water velocity in piping systems is 5 feet per second or less. As flow increases in any given pipe size, the velocity of that water also increases. As velocity and/or flow increases in any given pipe size, the PSI loss also increases. The means of decreasing pressure loss for a given flow is to increase pipe size. (diameter)

Friction Loss:
The PSI loss which results from friction against the interior walls of pipes, directional fittings, valves or any other obstruction to the irrigation water. Once again, as flow increases so does friction loss. Friction loss is synonymous with PSI loss.

GPM, GPH, GPD:
Gallons Per Minute, the standard measure of flow; Gallons Per Hour, often used for low-volume flow such as drip irrigation; Gallons Per Day, a measure of overall water use on a daily basis.

Feet of Head:
Another term for water pressure. Pressure is directly affected by elevation change, and each 2.31 vertical feet of change upwards will decrease pressure by 1 psi in a holding tank. That is why such enormous pressures exist in the deep ocean; enough to crush a submarine as depth increases. Another way to look at it: each 1 foot elevation change equals .433 "feet of head".

Total Dynamic Head:
TDH is a measure of overall head (pressure) loss in a water system. When an irrigation designer or engineer calculates all of the friction (pressure) losses and outlet pressure required for an irrigation system, they will express the number as TDH. If an irrigation system has a maximum TDH of 250, that means that just over 108 PSI will be required to power the system.

Conversion Factors: