Modulating (float) valves are not tight-closing, in fact, our valves are designed to pass some flow when the ball is in the full up or “closed” position, so as not to stop main drain flow entirely.  The optimum operating condition for the float controlled valve is between 60% and 70% open.  The resistance to flow between these two opening positions establishes the minimum depth required for the center of the main drain line so that the desired flow can be maintained.  If the depth and size of the line are incorrect the proper flow rate will not be attainable.  See table #1 for values of depth versus flow.  Use the charts given to select a valve and minimum depth needed to achieve the flow rate for the pool turnover you require.  Note that the depth from the chart is not all that is required.  To this depth you must add an estimate of the losses in the main drain line before the valve.  The valve tables are limited to a maximum water velocity of six feet per second.  This velocity will generally give line losses which are much smaller than the losses through the modulating valve. 

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