It’s just a simple matter of mathematics
By David Dickman
Service enough swimming pools, and eventually you’re going to do some work on the circulation system. And at the heart of that system is the pump – the component that drives the water so it can be filtered, heated, mixed with air bubbles, treated with chemicals or just moved around so bathers can enjoy a relaxing dip.
Depending on the size of the vessel and what you want to do to the water, every pool or spa (that’s not a wading pool) will be fitted out with various pieces of equipment placed in line with the plumbing. These will usually include a filter and may include a heater, an automatic chemical feeder, an air blower and even an automatic pool cleaner. The entire network forms the pool’s recirculation system or “plumbing loop.”
To choose the proper pump for any pool, you’re going to have to familiarize yourself with the recirculation system to find out just how big a pump you really need. You’ll want to know how much water you want to move, how far and how fast you want to move it how much resistance you’re going to encounter along the way.
Pump manufacturers sum up all of these factors into two basic measurements and rate their pumps accordingly. The measurements are “flow rate” and “head.”
The term “flow rate” means pretty much what it sounds like – the amount of water that can be moved in a given period of time. It is usually measured in gallons per minute or gpm. The term “head” is best defined as “resistance to flow” – that is, the characteristics of the circulation system that tend to impede the flow of water.
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); whether it is caused by the standing weight of the water (static head) or by the movement of water through the system (dynamic head); and whether the resistance is caused by simple friction due to pipe sizing (friction head).
The vertical distance that the water has to travel obviously affects the performance of a pump. Manufacturers measure vertical lift in “feet of water.”
Static head accounts for just a small portion of the total head in a recirculation system. The majority of head loss is caused by dynamic head. As it flows, the water encounters friction or drag from the- pipe and resistance from each piece of equipment and each turn it has to make.
The amount of friction head is determined by the diameter of the pipe and the speed of the water flowing. At a flow rate of 50 gpm, for example, there would be 10.24 feet of friction head developed for every 100 feet of 1 1/2-inch pipe. Using 2-inch pipe, only 4.3 feet of friction head would develop for every 100 feet. And with 3-inch pipe, only .65 feet of friction head would develop for every 100 feet.
Pipe fittings – including gate valves, 90-degree elbows, 45-degree elbows, Tee connectors and check valves – also contribute to friction head. Each twist or turn that the water has to take while traveling through the recirculation system is treated as if the water had to travel that much farther through straight pipe.
A 1 1/2-inch, 90-degree elbow, for example, is the equivalent of 7.5 feet of straight 1 1/2-inch pipe. A 2-inch, 90-degree elbow equals 8.6 feet of straight 2-inch pipe. And a 3-inch, 90-degree elbow equals 11.1 feet of straight 3-inch pipe.
In addition, such components as the main drain and skimmer have a resistance that contributes to dynamic head. At 50 gpm, a 1 1/2-inch main drain adds 2 feet of head to the system.
The filter and heater also contribute to the total dynamic head in a recirculation system. According to standards set by the National Sanitation Foundation, a clean filter can have a drop of no more than 3 pounds per square inch (psi) from input pressure to output pressure. Each 1 psi is equal to 2.31 feet of head. So a new, clean NSF- approved filter will add no more than 6.9 feet of head to the system. As the filter gets dirty, of course, the pressure drop will increase and there will be a greater head loss.
The heater can contribute anywhere from 4 to more than 20 feet of dynamic head, depending on size and flow rate. One manufacturer’s 250,000-Btu heater, for example, adds 5 feet of head at 40 gpm, 8 feet of head at 60 gpm and 11 feet of head at 80 gpm. Contact the heater manufacturer to find out the head loss for a particular model at a given flow rate.
Pump manufacturers have prepared engineering data for calculating dynamic head in recirculating systems of various sizes and at various flow rates. This data may include various “head loss” or “friction loss” charts and is often part of the manufacturers’ catalogues, easily available at your local wholesale distributor.
To know what flow rate you need for a given swimming pool, you must first decide on what turnover rate you want. The turnover rate is the length of time it takes for the recirculation system to filter an amount of water equal to the amount of water in the pool. This doesn’t mean that every gallon of water will actually be filtered, but it does mean that a sufficient amount will pass through the filtration system to provide good, clean water.
Generally, pools need one to two turnovers (or filter cycles) per day. And a turnover rate of4 to 6 hours is preferred. So to determine the flow rate that a pool requires, you must first calculate the gallons of water in the pool and then divide by the number of hours you want per turnover. Then divide that total by 60 (because there are 60 minutes in an hour) to determine the flow rate of the pump in gallons per minute.
To calculate the gallons of water in the pool, you first multiply the length times the width times the average depth, which gives you the volume in cubic feet. Then multiply this total by 7.5 (the approximate number of gallons in 1 cubic foot of water), and you have your total gallons.
For a l5-by-30-foot pool with an average depth of 5 feet, for example, you would multiply 15 (the width in feet) times 30 (the length in feet) times 5 (the average depth in feet) and came up with 2,250 (the volume in cubic feet). Multiplying that by 7.5 will give you a total of 16,875 gallons.
Let’s say you wanted a 6-hour turnover rate. 16,875 divided by 6 equals 2,812.5 gallons per hour. Dividing that by 60 will give you 46.9 gallons per minute. So you would be looking for a pump that will deliver about 50 gpm.
If you had wanted a 4-hour turnover rate, you would have to move 4,219 gallons per hour or 70 gpm. And for an 8-hour turnover rate, you would need to move 2,109 gallons per hour or 35 gpm.
Once you know what flow rate you’re looking for, the next step is to determine how much head loss there will be at that rate of flow.
Total dynamic head loss is a combination of suction head and discharge head. For determining suction head, the rule of thumb is that equipment installations located around 20 feet from the pool will have approximately 4 1/2 feet of suction head. Installations located between 20 and 30 feet from the pool will have about 5 1/2 feet of suction head.
To calculate discharge head, you would need to know exactly how many feet of straight pipe were used in the plumbing on the discharge side as well as the exact number and types of pipe fittings and other components. Then, from charts provided by pump manufacturers, you can calculate the amount of dynamic head loss for all of the components in the system.
But unless you are dealing with a new installation or your customer can provide you with the plumbing blueprints, you will have to use some other way of determining head loss. You can often estimate total dynamic head with a fair degree of accuracy by simply pacing off the plumbing run and doing some simple calculations.
First, pace off and count the number of feet of plumbing. Then, if the pool is plumbed with 1 1/2-inch pipe, double the number to allow for the fittings, then calculate the head loss at the desired flow rate, using manufacturer charts. Then, add in the head loss for the main drain, skimmer, filter and heater.
If the existing pump is working, you can also measure head loss by using a vacuum gauge and a pressure gauge.
Place the vacuum gauge on the suction side of the pump, and it will measure the vacuum in inches of mercury. Every inch of mercury is equal to 1.13 feet of head.
Place the pressure gauge on the discharge side of the pump, and it will measure the pressure in pounds per square inch (psi), Every 1 psi is equal to 2.31 feet of head.
Take the two measurements, convert them into feet of head and add them together, and you have the total dynamic head in the system. Pressure and vacuum conversion charts are easily available from pump manufacturers or through your local distributor.
On some older pools and many spas, the filter is located on the suction side of the pump. Although this arrangement will increase suction head loss and decrease pressure head loss, it will have no effect on the total dynamic head of the system. Any increase on one side will be offset by an equal decrease on the other, so you can use the same methods described above for calculating or estimating total dynamic head.
There is, of course, one other pump sizing choice available. And that is simply to replace the existing pump with one identical to it. This is the easiest choice, but not necessarily the best one. If the person who installed the pump the last time didn’t size it correctly, you will simply be duplicating the error.
Turnover rate is of little consideration in sizing a pump for a spa. Rather, flow rate will be determined primarily by the requirements of the hydrotherapy jets – both their number and their recommended flow – as well as the size of the filter. Head loss is calculated the same way as on a pool.
The average 1 1/2-inch hydrotherapy jet requires a flow rate of 15 gpm to function properly. Trying to use the same pump for a pool and spa combination can prove difficult. The pool may need 50 gpm, but the spa, if it has 6 jets, may require 90 gpm to function properly.
In the case of a pool/spa combination, it may be easier to use a separate pump for the spa or a two-speed pump that can deliver water at two different flow rates.
Once you have determined the flow rate and head requirements, the next step in choosing a pump is to read a pump curve to find a model that will suit your needs.
A pump curve – also known as a performance curve – is a graph that pump manufacturers use to describe the performance of their products.
The graph compares flow rate (sometimes called “capacity”) in gallons per minute (gpm) to feet of head and pounds per square inch (psi). A sample pump curve appears on page 1 and gives specifications for a 1/2-, 3/4-, 1- and 1 1/2-horsepower pump in a manufacturer’s line.
Let’s say that you are looking for a pump that will deliver 30 gpm at 43 feet of head. To find a pump that fits those specifications, you would need to locate the point on the graph where the line indicating 30 gpm crosses the line indicating 43 feet of head. To find that point, you find the vertical line closest to 30 gpm and follow it up the graph until it crosses the line closest to 43 feet of head. Then you must locate the pump with the curve that passes closest to that point.
If the point falls between two curves, you should select the curve that passes over the point rather than under it. On this particular pump chart, the curve for the 1-horsepower pump would meet your specifications.
A pump curve is a fairly accurate way to find a pump that meets the specific needs of a particular pool. You will probably find many pumps that meet your specific needs made by different manufacturers, and with varying horsepowers. Once you have found a group of pumps that meet the task at hand, you can make your buying decision based on such factors as durability, cost, ease of service, company reputation and warranty.
You may be tempted to replace an existing pump with one of greater size or horsepower. Prospective customers are often sold equipment based on the claim that “bigger” is always “better.”
The truth of the matter is that with today’s efficiencies and higher performanse characteristics of both motors and wet ends, you may actually be able to use a smaller pump to accomplish the same task.
When choosing a pump, you must be sure that it will meet the task at hand. At the same time, you must be careful not to exceed the recommended flow rate for the plumbing, filter and heater.
Choosing a pump that is too small will result in poor filtration and surface skimming, excessively long filter cycles and in the case of a spa, poor action from the hydrotherapy jets.
You must also remember that as dirt is removed by the filter, the pressure goes up, increasing dynamic head and slowing flow rate. Therefore, you must allow for that increase in pressure when choosing a pump.
Choosing a pump that is too large can result in damage to the plumbing and equipment. It can also result in cavitation, which can seriously damage the pump itself.
Cavitation is the formation of bubbles in the water, very near the impeller, that occurs when the water is intensely vibrated. As the bubbles pop, shock waves are created within the pump that not only make noise but also burst with enough force to damage the impeller and other pump parts. Pressures caused by cavitation have been calculated to be in the range of 30,000 psi!
Cavitation occurs when the discharge capacity of the pump exceeds the supply of water available. The vacuum created within the pump is enough to literally suck the oxygen out of the water, causing bubbles to form. The condition can occur when you install a pump that is too large for the suction side of the recirculation system or when there is an excessively long suction line.
An oversized pump can also create excessive flow, which can cause erosion of the system’s piping. According to NSF and IAPMO standards, the flow rate in 1 1/2-inch pipe should never exceed 8 feet per second for copper and 10 feet per second for PVC.
A flow rate of 50 gpm in 1 1/2-inch pipe is equal to 7.9 feet per second. A flow rate of 60 gpm in 1 1/2-inch pipe is equal to 9.3 feet per second.
Likewise, overdriving a filter can damage filter grids, cartridge elements, manifolds and fittings and can also make a sand or D.E. filter inefficient. Excessive flow through a heater can erode the heat exchanger and heat sink.
So when it comes time to replace a pool pump, the key factor should be performance rather than size. Determine the flow requirements of the system, then consult the pump curves provided by one or more manufacturers to choose a pump that meets your performance needs.
That way, you’ll be replacing the pump truly “by the numbers.”