Flow Rate and Water Velocity

In Tech Talk by Steve DonohoeLeave a Comment

Our last CPO class in 2011, a very good year at Superior Pool Products, Anaheim

We finished out the year of 2011 at PoolCorp’s Superior Pool Products building in Anaheim this past Thursday and Friday (December 8-9).  The CPO students were energetic and ready to learn about swimming  pool operation, and did quite well on the exam.  Thanks again, to PoolCorp and to the staff in Anaheim for all your hospitality and support throughout the year.  And a BIG “thanks” goes to Brian and Steve for helping us haul our classroom materials upstairs!


In our CPO class, we discuss the water Flow Rate as it relates to Turnover and Filter Sizing.  But what exactly IS Flow Rate?   Well, it’s the amount of water that is flowing through the pool’s hydraulic system at any given time.  Flow Rate is measured in gallons per minute (GPM).

Sometimes the terms “Flow Rate” and “Velocity” get mixed up and are used synonymously;  however, they are really quite different.

While the Flow Rate is the AMOUNT of water moving, the Velocity is the SPEED at which the water is moving.  The water’s velocity is measured in Feet Per Second (FPS).

On The Road Again

In our CPO class, we use the analogy of a truck with a 20 ton cargo going down the freeway at 60 miles per hour.  The amount of cargo moving down the freeway at any given time is 20 tons, but the velocity at which it is moving is 60 MPH.

While Flow Rate and Velocity are different, they are still very much related.  If you know the Diameter of the system’s pipe and the  Flow Rate, you can figure the Velocity by using the formula on the left.

If you know the Velocity and need to calculate the Flow Rate, use the formula on the right.  Or you can use the Flow Rate and Velocity calculator here.

As a general rule, the Association of Pool and Spa Professionals (APSP) recommends that the Velocity on the  suction  (vacuum) side of the pool plumbing system should not exceed 6 feet per second and should not exceed  8 feet per second on the discharge (pressure) side of the system.  The chart below shows you how much water (Flow Rate in GPM) will flow through a particular size pipe at a given Velocity (FPS).

Here’s a good web page describing Flow Rate and its relation to Velocity.

Elise and I want to wish all our readers and students a very blessed and Merry Christmas with friends and family, and we pray that the Lord will continue to bless all of us and our country in the coming New Year.

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