More things that affect the hydraulics of a swimming pool and spa

When we use the phrase ‘restriction to flow’, we are talking about items in a pool and spa system that restrict flow, thus causing back pressure or head loss.

Here are the most common items that restrict flow:  filter, heater, pipe and fittings, return fittings (eyeballs), salt cell, backwash valve, 3/2-way valves, skimmer and main drain, along with any water feature at the end of a plumbing run.

Remember, the head loss for a particular item will change as the flow/GPM changes.   So if the head loss at 50GPM for a filter is 4.1 ft of head (1.8PSI), the head loss at 75GPM would be around 8.25 ft of head (3.6PSI).  Head loss factors or pressure drop across these items should be available from the manufacture.

We have included a PVC pipe head loss and velocity chart, fittings equivalent feet in pipe chart and a pressure to feet of head chart for reference.  Let’s take a look at the PVC pipe head loss and velocity chart.  Note on the left hand side it shows the GPM.  Across the top is the pipe size.  For every pipe size, there is a velocity and feet in head loss column.  Note this chart is for 100 ft of pipe, so make adjustments as necessary for lengths less or more.

At 50GPM for 1.5” pipe, the velocity is 7.88 ft per second and the ft. of head is 13.5.  Note that the factors reduce as the pipe size increases.  Also note that if you keep the same pipe size, but increase the flow, the numbers increase.  For all pipe sizes, there is a point that there is no more data.  This is a point at where the loss is substantial and the pipe should be increased.

The pressure to feet of head chart shows the relationship between pressure and feet of head.   One PSI = 2.31 feet of head and 1 inch of mercury (vacuum) = 1.13 feet of head.

Next up, we’ll talk more about industry standards.  See you then.  We look forward to you jumping into the conversation.  Get involved, stay connected!