PUMPS A


An energy audit on this turf farm revealed a significant head loss due to biofilm buildup in the layflat pipe.


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Blockage here


Natural surface elevation


Clear pipe Blocked pipe


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while back, I got an unusual phone call from an irrigator. “Rob,” he said, “I think there’s a trolley in my pipeline. Can you locate it?”


When he said “trolley,” he wasn’t referring to the old-style San Francisco cable cars; he was talking about wheeled carts that were used to carry equipment for reinstating cement lining during mild steel cement-lined pipeline construction in the old days. It may sound extreme, but it’s not the first time I had heard of a trolley being left


in a large pipeline. Legend has it that it happened during the rehabilitation of a large government irrigation area in South Australia in 1980s. However, it may just have been a believable excuse for unaccounted friction losses in a brand-new 3-foot trunk main.


I agreed to help this client out. The case involved an 18-inch diameter PVC rising main delivering recycled water from a sewage treatment works pumping station to a reservoir 6 miles away.


The problem was that, after a year in operation, there was about a 10% reduction in pumping output. The client assured me that he had tested the pumps and they were OK. Therefore, it just had to be a trolley in the pipe.


I approached this problem much as I had during my time in SA Water, where I had experience during the 1980s locating isolated partial blockages in deteriorated 1930s vintage cast iron cement-lined water supply pipelines.


Recording hydraulic gradients


The first step was to record accurate pressure readings along the pipeline at multiple locations (at least 10) that had been surveyed to provide accurate elevation information. The sum of the pressure reading plus the elevation at each point gave the hydraulic head (termed the peizometric height) at each point. Plotting the hydraulic heads with chainage gives a multiple point hydraulic gradient, or HG, much like in the graph shown in figure 1.


Locating a pipe blockage Dam


Given that the HG was fairly straight, there was clearly no blockage along the way, which would otherwise be evident by a sudden change in slope of the HG at the position of the red arrow.


2 3 Main length (miles) Copyright Tallemenco Pty Ltd 2021


Figure 1. The hydraulic gradient (HG) blue line from the friction tests indicated a consistent gradient, indicating there was no trolley in the pipe. If there was something large in the pipe, the HG would be like the red line, with the blockage in the vicinity of the red arrow. All photos and graphics by Robert Welke.


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So, we determined that the head loss must be due to a general friction buildup in the pipeline. To confirm this theory, we decided to “pig” the pipeline. This involves using the pumps to force two foam cylinders along


the pipe from the pump end and exiting into the reservoir. The cylinders, or pigs, are about 2 inches larger than the pipe inside diameter, making them a tight interference fit, and 2 feet, 6 inches long.


irrigationtoday.org Spring 2021 | Irrigation TODAY 13


Pump


Natural surface


HG clear pipe


HG blocked pipe


Feet above water source


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