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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

Determining Total Head via Field Measurement Method

Once this is calculated, the operator can use the pump curve chart to determine the proper pump size. Most pump manufacturers provide pump curve charts for their equipment. If the operator wants to determine if the existing pump is sized properly, a pump chart can be obtained to check the design formulas.

Encountering Cavitation

One of the major issues encountered in public pool renovation is bringing the pool circulation system up to new codes. It is critical to determine if the existing pool piping is capable of handling the flow rate of a newer pump. At a velocity of seven feet per second (the industry standard), a maximum flow through 1 ½-inch pipe is 43 gpm and 2-inch pipe is 72 gpm. If a new pump is installed without consulting the pump curve and maximum flow rate, the operator may encounter issues regarding cavitation. In simple form, cavitation occurs when water is stretched, causing separation of the hydrogen and oxygen molecules to form vapor bubbles. Symptoms of cavitation are the following: the pump sounds like it is pumping rocks, the vacuum gauge has a high reading, and the pressure gauge has a low reading. To remedy these issues, the operator should first check to see if the suction line is clogged with debris. The main drain may be covered with leaves, or the skimmer basket may be full of leaves. If a new pump has been installed, the pump may be attempting to pull more water through the piping than the pipe is capable of handling.

Preventing Entrapment

Any study of swimming pool hydraulics must include a discussion of entrapment. Care must be taken to ensure the size, type and location of main drains. Covers must be in place and fastened to the body of the drain. Pool professionals must understand the ramifications of improperly installed suction components.

The major types of entrapment are bodily and mechanical. The aquatic manager must protect swimmers from harm and should ensure all precautions are taken to prevent an entrapment occurrence. Any renovation of the pool shell should include the installation of at least a two-main drain system.

Sizing For The Proper Filter

The hydraulics of the swimming pool pump should be understood in order to size the filter properly. Most swimming pool codes mandate the filter media rate depending on the type of filter media used--sand, Diatomaceous Earth, or cartridge. The filter media rate is based on gpm per square foot of filter surface area. If the filter is not sized to the pool pump dynamics, several problems will develop. The number one issue is diminishing water clarity due to insufficient filtration. If a low flow rate is experienced, the filter media will not be able to cleanse the pool water of debris or trap particulate matter into the media, and the filter vessel will maintain a low-pressure reading. Conversely, if the flow rate is too high, it may damage Diatomaceous Earth grids, rip cartridge filter fiber, and create voids in sand filters; this is called channeling.

A Hydraulic Fact To Consider

If a return line carries 30 gpm, a return fitting with a ¾-inch eyeball will create a loss of 7.4 feet of head. By increasing the size of the eyeball to 1inch, it will reduce the head pressure to 2.3 feet. Why is this important? If the pool operator is experiencing a high-pressure reading, increasing the eyeball size to 1inch will create less friction on the return side, and reduce the strain on the pressure side of the pump.

The aquatic facility manager has a responsibility to maintain a safe, clean and operational-free swimming pool environment. One of the key requirements is an understanding of hydraulics and circulation. Examining friction losses and sizing the proper pump to the pool will lead to an efficient system, which will lower electrical requirements and thus reduce the energy costs for the facility.

Connie Gibson Centrella is Program Director for the online Aquatic Engineering Program at Keiser University eCampus. She was recently honored with the Evelyn C. Keiser Teaching Excellence Award “Instructor of Distinction.” Ms. Centrella is an industry veteran with over 40 years experience in the pool and spa industry. She is a former pool builder with extensive knowledge in pool construction and equipment installation as well as manufacturing. She can be reached via e-mail at

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