Friction loss in irrigation pipes, part 1/2 (go to part 2)

Friction loss is one of the most important problems in irrigation systems.  There are hidden myths about pressure, friction, and pipes.  Most of your prospective clients will not understand these concepts, and will be attracted to low bids from your competitors, who prey on  misunderstanding.

Friction loss is the reduction of water pressure which occurs as water travels over distance and through any kind of restriction.  The colloquial expression "bottleneck" is a good metaphor for understanding friction loss in irrigation pipes.  When traffic on Interstate 595 slows to a crawl, it is often because of an accident or a closed lane.  A brief restriction in the number of lanes can severely slow down the movement of cars.  While the same concept applies to irrigation systems, here's a big caution:  don't mix up the concepts of speed and pressure!   Carrying this phony metaphor a little farther, the narrower the road, the few cars that can be carried.  Also, increasing the number of cars on a narrow road will almost surely worsen the flow.  Now here's the whole truth.

What makes irrigation flow quite, quite different from cars on the highway, or the movement of gas, is that water cannot be compressed, or is essentially non-compressible within the range of pressures encountered in pipes.  Water in a pipe consists generally of a continuous column.  Assuming that the pipe is filled, the moment the water starts flowing from the pump, water comes out of the sprinkler heads.  When an irrigation system is pressurized, there is some resulting pressure throughout all interconnected, open parts of the system.  The moment water stops moving from the source, water must stop moving out of the heads.  Just as water is not compressible, it cannot be stretched!

What makes water move in pipes is not the momentum of the water, but the difference in pressure between the source, the pump or municipal water line, and the surrounding atmosphere.  (We usually express all pressure measurements relative to atmospheric, so when we say the pressure is "zero," that means actually that the pressure is the same as the atmosphere.)  Pressure is also needed to make the water rise high out of the sprinkler heads.  When there isn't enough pressure, little water comes out of the sprinkler.  As water moves, there is a constant reduction of pressure throughout the system.  The smaller the pipe, the more quickly is pressure reduced.  What corrupts irrigation systems and makes them work badly is that there is too great a pressure loss, too soon.  Have you ever seen where sprinklers at the end of the line just trickle?

Ignoring the effects of water depth and topography, which are not usually important in level areas such as Florida:

1.  When water is not moving, there will be no difference in pressure throughout a system.

2.  When water does move, from a pressurized source, such as a pump, pressure will always reduce the farther the distance from the source.

3.  Pressure reduction will be related to the smallness of the pipe.  The smaller the cross-sectional area of the pipe, the greater the pressure loss over distance.

To measure pressure loss in an 0.5-inch Schedule 40 pipe, students measured flow, in gallons per 30 seconds, and pressure 0, 50, and 100 feet from a water source.  Flow was varied by placing 4, 3, 2, 1, or 0 sprinkler heads at the end of the line.  One measurement was also performed using a 1-inch Schedule 40 pipe.  Here is the data:

(go to part 2)