Pumps and Motors
As a pool or spa owner, you’ll be happy to know that you pump requires little maintenance so long as it’s been installed and operated properly. Nevertheless, the more you know about the pump, the more you’ll appreciate it when it’s functioning smoothly. In the first few seconds after a pump is turned on, the shaft accelerates from zero to thousands of rotations per minute. The incredible torque it takes to accomplish this accounts for most of the wear and tear a pump experiences. The pump shaft is connected to the pump’s impeller, which subsequently spins, creating a centrifugal force that moves water through the circulation system.
Most pumps are tightly sealed and self-lubricating. They’re powered by electricity and should be connected to a ground fault circuit interrupter (GFCI) to prevent accidental electrocution. What differentiates one pump from the next? Let’s take a look.
The more you know about pump design and operation, the more you’ll appreciate the difficult task it performs of moving water steadily through your pool and spa filtration system. A basic understanding of pump mechanics will also help you troubleshoot potential problems down the road and maybe even avoid costly service repairs. Here, then, is a quick look at the inner workings of a typical pool and spa pump.
Pump motors are available in a wide range of horsepower ratings and with different starter systems and casing materials. All, however, follow the same mechanical principles: A large number of copper wire coils are wound and woven together to form windings. The windings take up most of the space inside the cylindrical casing of the motor. A shaft runs through the hollow center of the windings, held in place by bearings at each end of the motor. This shaft carries the armature – an iron and copper electromagnet. When the motor is switched on, an electric current passes through the windings and creates a strong electromagnetic force that turns the armature and shaft at a high speed. The high-speed revolutions of the shaft drive up the pump.
Starting at the motor, however, is like starting you car – it requires much more energy to start the motor than it does to run it a full speed. To provide this extra boost of electricity, motors have a starting mechanism. Two types are commonly used: switched and switchless.
Switched motors provide and extra boost of electricity when operation of the motor begins. Many switched motors have two compartments; one contains the switch, terminal board, and other sensitive components and the other the windings and bearings. Some experts believe that the two-compartment design separates the “work room” form the “control room” and protects the controls from the environmental conditions such as heat and moisture. Switched motors provide a high initial starting torque, which is important if the pump’s impeller needs to overcome trapped dust and debris.
Switchless motors store enough of an electrical current in a capacitor to start the motor. They use a permanent split capacitor start-and-run design and a continuous duty run capacitor. This design eliminates the use of an internal starting switch and governor, thereby offering greater dependability, according to some experts.
Most modern swimming pool pumps are self-priming, which means that the pump is always full of water and does not need to be primed with water before starting. Pumps found on older pools may be “regular” pumps (not self-priming). If such a pump loses its prime, the motor can overheat during operation, damaging both the motor and the pump.
A leaf strainer is a standard feature of most pump units sold today. It catches leaves, hair, lint, and other large particles of debris before they can enter and clog the pump. Many strainers have a clear, see-through plastic top so you can easily tell whether the basket needs emptying. The strainer should be cleaned regularly. When clogged, it will greatly reduce the flow of water to the filter.
The pump body, or volute, holds the components of the pump, including the impeller. When the impeller spins, water or air is thrown out from the center by its vanes, lowering pressure in the center and creating the pumping action. The depth of the vanes on the impeller determines how much water a pump can move. If you install an impeller with deeper vanes than those of the pumps original vanes, you must also increase the horsepower of the pump.
In some cases, the impeller works in conjunction with a diffuser. The diffuser is a round, flat plate with raised radiating fins. It faces the impeller, with a narrow gap between two sets of fins or vanes. The diffuser help the impeller create a vacuum by limiting open space immediately around the impeller. It also promotes water flow through the system. Over a period of time, this diffuser plate can be gouged or worn by tiny bits of gravel or sand that pass through the strainer. Some pumps feature a stainless-steel wear plate to reduce damage to the diffuser.
Occasionally, small stones, grass, and other debris can plug the pumps impeller, impeding the flow of water. If that’s the care, disassemble the pump and use a semi-rigid wire to clean the impeller.
One other critical component of the pump is the mechanical seal that stops water from leaking out around the shafts between the pump and the electric motor. This mechanical seal is commonly a spring-loaded, rubber-cased unit, which is routinely replaced whenever the pump is professionally disassembled for other repairs. The seal itself uses ceramic and carbon parts. Ceramic and carbon seal are delicate and can be easily cracked if tapped with a screwdriver or other metal object. It’s a good idea to know where the seal is and how to identify damage to it. If you determine that it needs to be replaced, call in a pool serviceman; the disassembly of a pump and repair of worn or damaged seals is a job best left to a professional.
Sizing a Pump Correctly
When sizing a pool pump, bigger isn’t always better. A pump that’s too large could damage the filter and waste energy, while a pump that’s too small may not turn over the water as often as is necessary. To work efficiently, a pump must be sized correctly for your pool and spa.
To figure out what size pump you need, you need to know the pool volume in gallons, the flow rate needed to turn over the water (circulate all of the water once through the filtration system) in a certain amount of time, and the system’s total dynamic head (the flow restriction caused by piping, fittings, and equipment). The builder usually does these calculations when the pool is installed, but its a good idea to check them again whenever you need to replace a pump just to make sure the calculations are right.
First, calculate the pool capacity using basic geometry. There are 7.5 gallons of water in 1 cubic foot, so after you’ve estimated the number of cubic feet in your pool, multiply that number by 7.5 to find the number of gallons the pool can contain.
Next, determine the flow rate needed to circulate that many gallons of water in the specified amount of time. The recommended turnover rate varies by region and amount different kinds of pools, but the widely accepted norm is eight hours. Thus simply divide the number of gallon of water by the turnover time of eight hours to find out how many gallons per hour the pump needs to circulate. You can convert that figure into gallons per minute by dividing by 60. (To convert gallons to liters, multiple the number of gallons by 3.79.)
Finally, total dynamic head (TDH) should be factored into the equation. TDH is the sum of the resistance or friction water encounters as it flows through the circulation system. Pipes, fittings, valves, and equipment cause resistance. For example, the longer the pump runs and the more turns it makes, the greater resistance it places on water flowing through it. All components – from the skimmer to main drain – must be accounted for when figuring TDH. If possible, work from a blueprint of the pool’s circulation system and list the number of 45-degree elbows, valves, pipes (including their length and size), filters, heaters, and any other equipment attaches to the circulation system. Then use the charts provided by pump manufacturers to determine the appropriate head-loss value (the amount of pressure, expressed in feet)that each component imposes on the water-circulation system. The total of all the components added together equals the TDH for your pool.
Now you’re ready to select a pump that will meet your needs. Most manufacturers provide graphic charts that make it easy to select the proper pump. The chart shows a curve plotted on a grid that indicated the size of pump needed to produce the desired flow rate given the calculated TDH. Simply plot on the graph, and choose the corresponding pump size. If the plotted point fall between two pumps, choose the large one.
In all other cases, resist the urge to oversize the pump. Your goal is to select the smallest pump possible for the job. If the pump is too large and the flow rate is too great, the result could be damaged equipment. Of the pump is too small, you won’t get adequate filtration.
Also, make sure that the piping can handle the flow rate you want without causing the water to flow too fast. In general, water velocity should not exceed 10 feet per second for discharge piping or 8 feet per second for suction piping, a pool professional can help you determine whether all components are sized properly to work together.
Pool pumps and motors should be installed in a cool, clean, dry location so that dust, leaves, and other debris can’t clog the motor’s ventilation passages. Don’t install a pool pump anywhere near the laundry room. Lint from dryers will be sucked up by the motor fan and clog the air intake. The motor should be covered and slightly elevated so that water puddles will not be sucked into the motor by the cooling fan. Ideally, the motor and pump unit should be enclosed in a waterproof structure with louvered sides to provide ventilation and protection from the rain. Some manufacturers will not guarantee motor unless they have been protected in this way.
A pump and motor can fail for a variety of reasons. Keep in mind that pump and motor repair can be complicated; don’t assume that you should undertake these repairs yourself. In fact, the warranty on the pump may become void if you tamper with the equipment yourself rather than having the work done by an authorized repair professional. If you believe you’re qualified to make these repairs, proceed with caution. Consult the manufacturer’s literature, which often offers its own troubleshooting guide, and be sure to turf off the electrical power before getting to work.