PUMPS


with the manufacturer’s rating of 46 feet for the same flow — and with no kinks in the hose! That’s a whopping 85% increase in head loss. Not surprising considering that this layflat was transporting algae- contaminated river water and lay in the hot sun all summer, breeding those little critters on the pipe inside wall.


Two foam cylinders emerge from the pipeline during the “pigging” process. The pipeline flow capacity was improved 10% as a result.


The instant improvement in the pipeline friction from pigging was nothing short of amazing. The system head loss had been almost totally restored to original performance, resulting in about a 10% flow improvement from the pump station. So, instead of finding a trolley, we determined that a biofilm was responsible for pipe friction buildup.


Pipeline energy efficiency


Pipeline performance can always be viewed from an energy efficiency perspective. Figure 2 shows the biofilm affected (red line) and restored (black line) system curves for the client’s pipeline, before and after pigging.


The increase in system head due to biofilm caused the pumps not only to operate at a higher head, but because of the lower


Effect of biofilm buildup 350 300 250 200 150 100 50 1 Pump 2 Pump


Energy cost with layflat hose


0 1000 2000 3000 Flow (gpm)


Figure 2. The system curve (red line) indicates a biofilm buildup. The black line (broken) shows the system curve after pigging. The biofilm raised pumping costs by 15% in the first year.


14 Irrigation TODAY | Spring 2021 4000 5000 6000


A case in point was observed in an energy efficiency audit conducted in 2014 on a turf farm in New South Wales. A 600-foot-long 3-inch layflat pipe delivering water to a soft hose boom had a head loss of 85 feet compared


System curve before pigging


Layflat hose can contribute up to 46% of total pumping energy consumption on an irrigated farm.


pumped flow rate, some of the pumping was forced into peak electricity tariff. The reduced performance pipeline ultimately accounted for about 15% additional pumping energy costs.


Not everyone has an 18-inch pipeline


Of course, not everyone has an 18-inch pipeline in their irrigation system. So how does that relate to the standard irrigator?


A new 18-inch PVC pipe has a Hazen- Williams friction value of about C=150. When reduced to C=135 (10%) through biofilm buildup, the pipe will have the equivalent of a wall roughness of 6 thousanths of an inch. The same roughness in a 3-inch pipe represents a Hazen-Williams C value of 127. That’s a 16% reduction in flow — or a 32% friction loss increase for the same flow!


System curve after pigging


To maintain the original flow rate in the 3-inch pipe, 32% more pumping energy is required. And that’s just in the first year.


Calculated in terms of energy consumption, the layflat hose was responsible for 46% of total pumping energy costs through a combination of its small diameter plus biofilm buildup.


Here’s the solution


So, what’s the solution? Move to a larger diameter hose. A 3½-inch hose has a new pipe head loss of only 20 feet/600 feet at the same flow, but when that deteriorates due to biofilm, head loss may rise to only about 30 feet/200 feet instead of 85 feet/600 feet, kinks and fittings excluded.


PIPELINE PERFORMANCE CAN ALWAYS BE VIEWED FROM AN ENERGY


EFFICIENCY PERSPECTIVE.


The pump impeller would need to be trimmed or a VFD fitted to potentiate the energy savings. In some cases, the pump may have to be changed out for a lower head pump.


Everyone has a trolley in their pipeline, and it only gets bigger with time. You can’t get rid of it, but you can control its effects, either through energy-efficient pipeline design in the first place or by “pigging” the pipe to get rid of it!


As for the trolley in my client’s pipeline, the legend lives on. He and I still joke about it whenever we can’t explain a pipeline head loss.


Robert Welke, Dip. Mech. Eng., CIAg, CIAL, is the founder and director of Tallemenco Pty Ltd, trading as WATER PUMPING INSTITUTE, a pumping and hydraulics training body.


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Head (feet)


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