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The Basics of Swimming Pool Pumps

A pump powered by an electric motor, is the essential piece of mechanical equipment that creates the hydraulic flow and pressure needed to circulate water through a pool or spa’s plumbing system and related equipment. The pump is nothing less than the literal heart of the pool/spa system to which it is linked. Without exception, pools and spas have pumps. Other equipment, including filters, heaters and automatic sanitizers, will work only when the pump moves water through the system. To assist working with these devices, we have assembled the following list of terms and definitions. Please note, All deftnitions are provided as they relate to pool and spa systems; they maybe defined differently in other industry or science manuals.

Cavitation: The vacuum created when the discharge capacity of the pump exceeds water replacement in the suction line. Cavitation occurs when the pump is oversized, when the diameter of the influent line is too small or when there is an excessiveiy long influent line. Cavitation causes bubbling and vibrations that can damage the pump if they occur over an extended period.

Centrifugal force: The outward force created by anything in circular motion. In centrifugal pumps, this is the force created when water is propelled outward by the circular motion of the impeller.

Centrifugal pump: The only type of pump used in a pool or spa circulation system. Uses centrifugal force created by the rotation of the impeller to power water through the circulation system.

Circulation system (recirculation system): Powered by the pump, this is the system of pipes and equipment through which pool and spa water circulates. Water cycles from the main drain and skimmer drain via the influent plumbing line, through the pump, filter, heater and automatic sanitation equipment, and back to the pool or spa through the effluent plumbing line via the return inlets.

Diffuser: A housing or varied part that fits around the impeller. The diffuser strips water from the rotating impeller and forces it out the discharge line of the pump, reducing the velocity of water through the pump while increasing the static pressure. In some pump models, the diffuser and the volute are designed as a single part.

Effluent line: The plumbing line leading from the equipment to the pool or spa.

Electric motor: The power source of a centrifugal pump that converts electrical energy into mechanical energy.

Flow meter: A gauge installed in the circulation line that measures the flow rate in gallons per minute (gpm).

Flow rate: The volume of water flowing past a given point during a specified time; measured in gallons per minute (gpm) or gallons per hour (gph).

Head: A measure of the amount of pressure or resistance in a hydraulic system. Resistance may include the vertical distance and the amount of friction in a system and is measured in pounds per square inch (psi) or inches of mercury.

Impeller: A circular pump part with vanes that rotates to provide centrifugal force for propelling water in the circulation system. The impeller spins on the end of the electric motor shaft and is the only moving part of a centrifugal pump. Closed, semi-open or open impellers are used:

Closed impeller: A pump impeller with vanes contained on both sides and an opening in    the center (the eye) Water enters the eye and is centrifugally forced out the end of the closed impeller.

Semi-open impeller: A pump impeller with vanes exposed on one side; open vanes face    the volute wall, forming chambers through which the water flows.

Open impeller: A pump impeller with exposed vanes like a water wheel. (Open impellers     are rarely used in pumps manufactured for use in the pool and spa industry.)

Influent line: The plumbing line that leads from the pool or spa to the equipment. Also known as the suction line.

Pressure differential: The difference in water pressure between two points in a hydraulic system.

Pressure gauge: A gauge that measures the amount of pressure built up inside a closed system.

Prime (priming the pump): Initiating or restarting the water flow in a circulating system, usually accomplished by filling the pump volute by hand, turning the pump on and allowing the pump to disperse all air from the influent line.

Pump capacity: The volume ot water a pump can circulate through a given resistance (total dynamic head) during a period of time; expressed as gallons per minute (gpm) or gallons per hour (gph).

Pump curve: The curve on a bar graph that shows the performance characteristics of pumps. Variables include size, flow rate and resistance factors.

Self-priming centrifugal pump: A centrifugal pump that automatically removes air from the suction line and replenishes itself with water from within the pump, permitting uninterrupted water flow. In comparison, a flooded suction centrifugal pump would require manual priming if air interrupted the flow of water through the pump.

Strainer basket: A plastic mesh basket positioned in the strainer pot that traps debris in the water before it reaches the impeller.

Strainer pot: A housing on the influent side of the pump that serves as a priming chamber and a receptacle for the strainer basket.

Total dynamic head: The sum of all the resistance in a pool amd spa circulation system during operation.

Turnover rate: The time required to circulate a volume of a given pool or spa.

Two-speed centrifugal pump: A centrifugal pump that has a two-speed motor that turns the shaft and impeller at two revolutions-per-minute (rpm) levels, providing an optional pump Capacity.

Velocity: The speed of water as it flows between two points; expressed in feet per second.

Velocity head: The amount of energy required to accelerate water to a given velocity; expressed in feet of head.

Volute: A housing that contains the impeller and diffuser. (The diffuser is sometimes designed within the volute, instead of being a separate part.) Water is propelled through the volute’s continuously enlarging spiral chamber, increasing the effect of the centrifugal force initiated by the spinning impeller.

Pumps by the numbers
Behold the pump
: It seems such a simple little piece of pool equipment, but its looks certainly are deceiving. In fact, the pump is unquestionably the most important single piece of equipment in the circulation system, no matter the size or type of pool. Looks are similarly misleading when it comes to selecting the right pump for the job: A surprisingly long list of factors must be taken into account in order to choose the pump best suited to a given pool side task. ‘Right size’ is not an absolute but rather depends on a number of variables that will change from pool to pool. In that light, pump selection becomes an act of balancing pool size and the pool’s hydraulic system against customer satisfaction, including a pump’s economic efficiency and quiet performance.

To select the best pump for the job, you will need to determine the pool volume and the number of gallons it will hold; the required flow rate with respect to turnover time; and the system’s total dynamic head (the friction loss due to piping, fittings and equipment).
Once these variables are determined by simple mathematics, you will be ready to select the pump that has all the right stuff for any type of pool. Moreover, even if you are replacing a pump in an existing system, it’s best to go through these simple calculations to make sure you’re installing the right pump for the job.

Pool volume:
To calculate volume, you'll need to use some basic geometry to estimate the number of cubic feet in the pool. Don’t worry about the math— the equations are simple, and all you need to do is plug in the numbers and multiply. First, you need the basic dimensions of the pool; if you don’t have the specs handy you’ll need to measure it. Don’t be too concerned about absolute accuracy because close estimates will be adequate for these equations. (Although if you must estimate, be sure not to underestimate.) For simplicity, make your measurements in feet and tenths of feet to keep your calculations in easy decimal form.

Naturally, the shape of the pool will determine which equation to use to calculate the volume in total cubic feet. To find the number of cubic feet in a square or rectangular pool, multiply the length by the width, then multiply by the average depth. This tells you the number of cubic feet in the pool. To calculate the volume of an oval swimming pool, multiply the length by the width, then multiply by the constant, pi (3.14).That number in turn is multiplied by the average depth of the pool.

If the pool is circular, you can calculate the number of cubic feet by multiplying the radius (one-half the diameter) of the pool by itself (that is, the radius squared) and multiplying that by pi (3.14). Again, you multiply the total by the average depth to find the volume.

When working on an irregular-shaped or free-form pool, estimate the volume by using the equation for an oval or rectangle as needed. If you really want to get technical, you can measure the pool with a planimeter, but that is not necessary here: All you need is a close estimate.

Pool capacity: Once you’ve calculated the volume of the pool in cubic feet, you will want to calculate the number of gallons of water that the pool will hold, There are 7.5 gallons in 1 cubic foot of water, so simply multiply the number of cubic feet in the pool by 7,5.

Flow rate: Now that your calculator is warmed up, it’s time to determine the flow rate needed to circulate the number of gallons of water contained in the pool by the preferred turnover time, The recommended turnover time varies between regions of the country and individual preferences, but eight hours can Be considered an adequate turnover time. Simply divide the number of gallons of water in the pool by the turnover time, eight hours, to find the flow rate in gallons per hour, Then convert this figure into gallons per minute, by dividing by 60.

Total dynamic head: Total dynamic head (TDH) is the sum of the resistance experienced by water flowing through the circulation system. Resistance is applied throughout the system — when water Flows through elbows, fittings, valves, equipment and even through straight lengths of pipe. The TDH will affect flow rate and therefore is an important factor in selecting the right pump. All components in the circulation system, from the skimmer and main drain to the return inlets, must be factored into TDH calculations. Referring to the circulation system blueprints, list all components and dimensions, including the total length and size of PVC pipe and copper pipe, the skimmer and drains, 45 degree elbows, 90 degree elbows, gate valves, backwash valves, and the size and type of filter and heater.

The next step calls for consulting head-loss charts provided by pump manufactures to find the appropriate value for each component. Most manufactures have developed their own methods for interpreting a pool’s needs, so contact them to obtain charts that detail their preferred techniques. Once your list of values is complete, add them together; their sum is the total dynamic head.

After making these calculations, the rest is simple: Just choose a pump model that provides the flow rate you need, taking into consideration the head loss in the system. As an aid, pump manufactures have developed pump curves that show the performance characteristics for pumps on the market. These curves are plotted on a grid to indicate the size of pump needed to produce the calculated flow rate with respect to the loss of a system. To use the curve, just locate the required flow rate and head along the borders of the chart and extend lines to a point where the two values intersect. The proximity of the point with the pump curves plotted on the graph will indicate which pump you should choose. If your specs fall between the curves of two pump models, choose the larger model rather than the smaller one.

While you will need a pump big enough to do the job, be careful not to exceed the maximum flow rate recommended for the plumbing, filter and heater. If water is pumped through the system at too high a velocity, it may damage the equipment, cause cavitation that may damage the pump, or strip the metal from copper pipes to produce copper stains on pool walls. Pool piping must be of the correct size to handle the needed flow rate without causing the water to flow too fast. Water velocity should not exceed 10 feet per second for discharge piping or 8 feet per second for suction piping when PVC piping is used. In the case of copper piping, the velocity should in no case exceed 8 feet per second, according to the National Spa & Pool Institute Residential Pool Standard. Now you’ve narrowed the selection process down to choosing your preferred brand of pump. You’re probably already familiar with most of the pumps on the market from the pools you service on your route, so the decision won't be all that tough.

A self-priming centrifugal pump provides the power to circulate water from the pool through the influent (vacuum) line; into and through the filter, heater and automatic sanitation equipment; and back to the pool through the effluent (discharge) line. That may sound simple, but there’s a lot more to it than that.first of all, the pump is powered by an attached electric motor, which converts electrical energy into mechanical energy that spins the motor shaft. As the shaft spins, the pump impeller fitted on the end of the shaft spins at high speeds.As water is propelled (pushed) by the pump through the effluent line, water is displaced (pulled) by atmospheric pressure from the influent side of the pump through the eye of the impeller and against the impeller blades. The spinning impeller then propels the water outward in a circular motion that creates centrifugal force. As though that weren’t enough, the action of the impeller on the water creates energy in the form of water velocity. Simply stated, the faster the impeller spins, the greater the velocity of the water flow.

The velocity of the water is converted to pressure when it passes through the volute, a continuously enlarging spiral chamber, before exiting the pump through the effluent line. (in some pump models the diffuser and volute are designed as a single part.) Parts of this system break down from time to time. The text that Follows offers a guide to getting the pump up and running again.

To repair a disabled pump, you’ll need to know the workings of a centrifugal pump inside and out, Are you prepared? First, you’ll need to know a bit about motors and their potential problems and Learn to troubleshoot them. For more information, consult P/SN’s Nov. 28, 1988, issue, pp. 82-83; or P/SN’s special publication, The Basics of Motor Maintenance.From that point on, the information below will serve as your guide. But please remember: Before working on a pump or motor, be sure to disconnect the electric power at the fuse or breaker box. In addition, place a sign on the fuse/breaker box to alert others that the electricity is turned off specifically for equipment repair, to ensure that the electricity is not turned on by another person. You've looked over your swimming pool equipment and diagnosed the problem as a leaking pump.

What to do?   Before you take our your toolkit, spend some time with this refresher course on basic pump teardown and repair. Murray Hunt, who for two decades has been repairing and reviving ailing pumps in Southern California, provides step-by-step guidance in working on a bit of equipment familiar to most technicians: the Purex Series AH bronze pump, manufactured by Hydrotech Chemical Corp., Marietta Ga. Once you have assembled the proper tools, Hunt says, the procedure for curing what ails the pump is pretty straightforward provided you exercise due care in handling pump parts and take some necessary precautions before working on the motor. First, shut off the electricity at the fuse box, suggests Hunt. If that’s not possible, turn off the power at the timer. At all locations, leave a note that the power is off to permit completion of repairs. "You absolutely don’t want anyone to turn the current back on while you are working," Hunt says. Also, double check the motor’s electrical connections with a voltage meter or multimeter to ensure that the electricity is indeed off.

Once you remove the motor’s terminal cover and disengage the electrical wires and conduit from the motor housing, you’re ready to tackle the task at hand. A final caution: pump brass is a durable, but soft material, Unless you are absolutely confident you have the jeweler’s touch needed to do the job as gently as is necessary, you would be well advised to use a mallet or soft hammer to avoid bending the seal flange or impeller.

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