Fluid In Motion

The importance of hydraulics in swimming pool/spa design and application has been misunderstood by some aquatic professionals who believe that understanding hydraulics is only necessary when first designing an aquatics facility. Hydraulics plays an important role in the everyday operation of aquatic facilities. Knowing how the pool circulation system operates on a daily basis can curb excessive and sometimes unnecessary repair costs. The aquatic manager should have the ability to recognize changes in flow rate, analyze fluctuations in pressure and vacuum gauge readings, and be capable of making minor adjustments to the system to enhance the circulation and filtration process.

Early Pioneers Of Hydraulics

Hydraulics was first studied by Archimedes (287-212 B.C.). The Greek mathematician, physicist and engineer invented the first pump, called a screw pump. The Archimedes screw was operated by hand and raised water efficiently.

Subsequently, Blaise Pascal, the French scientist and philosopher, studied atmospheric pressure and principles of hydrostatics.

This bit of history is important, as swimming pool hydraulic applications still rely on these principles. The swimming pool pump is the “heart” of the operation. The ability to size the pump to the pool in order to achieve proper circulation is necessary. In the August article on mathematics, I reviewed how to calculate gallons per minute. The pool volume, or gallons, (length x width x average depth x 7.5 gallons/cubic foot), divided by the turnover rate in minutes, determines gallons per minute (gpm). All swimming pool hydraulic calculations are based on gpm. Most state codes now require public pools to install a flowmeter on the return side of the filtration system. The flowmeter is a device that measures the amount of water flow in gpm. A quick look at the flowmeter enables an operator to diagnose if the pump is performing within the turnover requirement; a drop in flow indicates a dirty filter or some other factor that may be impeding the water flow. A word of caution–it is important for the operator to install the correct size flowmeter, based on the size of pipe as well as pipe clearance.

Determining The Correct Pump Size

In a pool or spa filtration system, the movement of water–from the pool or spa, through the pump, filter, heater and back to the pool—is a concern.

Pump sizing is based on achieving a flow rate in gpm. However, the distance from the pool to the pump and whether the pump is above or below water level are major factors when sizing a pump. In a pump-sizing chart, there are two axes–the horizontal axis gives gpm, and the vertical gives Total Dynamic Head (TDH). To read a pump curve, determine the required gpm, then calculate the TDH. Where the two numbers meet on the grid will identify the proper pump size.

Calculating Total Dynamic Head

In layman’s terms, TDH is defined as resistance to flow. The entire loop around the pool containing the total length of piping, friction losses of each fitting, valve, filter, heater and chemical feeder are combined to compute TDH. The term “head” is further modified by whether the resistance is encountered on the suction side of the pump (suction head) or the discharge side (discharge head), and whether it is caused by the standing weight of the water (static head) or by the movement of water through the system (dynamic head).

There is an easy way to determine TDH when the aquatic operator does not have access to the original engineering plans for a facility. In most existing public pool facilities, there are two gauges installed on the system–pressure and vacuum. The pressure gauge measures the friction losses on the discharge side. Gauge-reading is in pounds per square inch (psi). The vacuum gauge–located on the suction side of the system–measures the friction losses on that side and is expressed in inches of mercury (Hg). To determine Total Head, both gauge readings must be converted to a common factor, such as feet of head. To change Hg to feet of head, multiply it by 1.13. PSI is multiplied by 2.31 to convert feet of head. In the chart below, the readings are converted to achieve the total feet of head.

Pressure gauge (psi) reading x 2.31 + Vacuum gauge (Hg) reading x 1.13 = Total feet of head

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