4.4 Review of Friction Loss

In our real sprinkler system the pressure will be the same throughout in a static situation.  In a static situation there will be no water flow. When water is actually flowing through the pipeline the water will drag along the sides of the pipe walls and this dragging or friction will consume energy intended to push the water through the pipe.  This energy loss due to friction is called friction loss and is described in terms of pressure loss, psi. 

This friction loss will become a part of the issue and pressures will vary from the initial pressure at the pump to those pressures at the nozzles based on losses that accumulate along the way.

Situation - In a typical irrigation system there is a main line that distributes water to laterals that run down a growing bed.  The laterals would have sprinklers (or drip emitters) on them.  Sketch (on a sheet of paper now) an irrigation system with a mainline from the pump and four perpendicular laterals off the main line, each having 5 sprinklers.  In a perfect world without friction one would expect 1 gpm to flow to each of the sprinklers so how much water must go to each lateral?  (5 sprinklers x 1 gpm = 5 gpm) And, further, how much water flows in each segment of pipeline between laterals and between sprinklers?  The water flowing in each segment has to be the water that is going to the sprinkler nozzles downstream, correct?  In our example above, the 20 gpm must divide up to reach the 20 sprinklers.  Start at the end of a lateral and write the flow in gpm that must pass through that pipe segment to reach sprinklers beyond it.  The segment between the pump and first lateral must carry the 20 gpm, correct.

Friction loss comes into consideration when water is flowing (a dynamic situation).  If all pipelines above are the same size (same diameter), then the Area term is constant/not changing in the equation Q (flow) = Velocity (V) x Area (A), where A is the cross-sectional area of the pipe.  To simplify things all pipe sizes will be the same.  Water velocity will depend on the water flow in each individual segments of the pipeline.

As learned earlier, friction loss is greater at a higher water velocity.  Looking at our sketch made earlier, the mainline has a flowrate of 20 gpm right after the pump but in each lateral the flow rate is much lower and before the last sprinkler in each lateral the flowrate is very low (1 gpm).  Remember all pipes are the same size so A is constant or fixed.  From this description one can see that the water velocity will vary a lot, being highest just after the pump (Q = V x A, velocity varies directly with flow rate) and lowest near end of laterals.  Friction loss will vary in the same manner, being highest just after the pump and lowest at the end of each lateral.

In the dynamic irrigation system, the friction loss will cause the real pressure and flow rate values at each sprinkler nozzle to differ from the others on the same lateral.  There will be a slight drop in pressure from one nozzle to the next and with the slightly lower pressure there will be slightly less water pushed through the nozzle.  Real flow through each nozzle will differ from one nozzle to the other.  The diameter of throw of the nozzle and the amount of water reaching the ground will be slightly less. 

Pipeline sizes are determined by a rule of thumb that allows for some non-uniformity of application but within some limitations, based in part on the economics of buying larger pipe sizes.  The goal is to keep the pressure variations low in a system so that water application uniformity is high.  A lateral line is allowed to vary 10% above or below the average lateral pressure.  So for an average pressure of 50 psi, the initial pressure might be 55 psi and the end pressure might be 45 psi on the lateral.  This 20 percent change results in a 10 percent difference in water discharge between the first and end nozzles.

The sprinkler nozzle, in this case, will give us the water distribution we want in the pressure range of 45-55 psi and flow range of 0.95 to 1.05 gpm.  This is the real situation of a pressure and flow rate combination in a dynamic situation and it is important to understand that nozzles have these properties and give different water distribution patterns for different pressures.  The amount of water that goes through the nozzle is dependent on the amount of pressure pushing the water.