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Let’s learn a bit about hydraulics and pools

By Marcelle Dibrell

Hydraulics is an area of study that a lot of people — even pool service technicians — would prefer to avoid. It is probable this is because it reminds people of school, and we’d just as soon not get bogged down with the terminology involved not to mention a lot of math problems. But when it comes to pool service, knowledge of hydraulics is important: information about flow rate, pressure and resistance is essential to choosing the proper equipment for adequate water circulation. 

These days, determining water flow at pools is pretty simple. Attach a flow meter to the pump output, divide the reading by the pool volume, and you have calculated the turnover rate. 

But to properly size a system, you also need to know the desired turnover rate and the resistance to flow.

Resistance to flow is measured in feet of head, which is a combination of static and dynamic head. Static head doesn’t change, and is determined by factors such as the vertical distance above the pool to where the pump is located. But static head is only a small aspect of the total head in the pool’s circulation system. Most of the head loss is due to dynamic head, which is determined by friction in the plumbing: drag in the pipes, resistance in each piece of equipment, and every twist and turn the water has to make. Other components, such as the main drain and skimmer, also offer resistance to flow.

For a pump to work at all, the pressure head of the pump must exceed the total resistance head of the system. 

To determine total dynamic head, you could measure off the entire plumbing system, adding lengths of pipe and each of the components, to calculate the head loss. Indeed, this is how it is done in a new system. But for existing systems, it’s a lot easier to measure the head loss while the system is operating with a vacuum gauge and pressure gauge. 

Place the vacuum gauge on the suction side to measure the vacuum. The units may be given in inches of mercury, which must be converted to feet of head. One inch of mercury is equal to 1.13 feet of head. Place the pressure gauge on the discharge side of the pump, which will give the pressure in pounds per square inch, psi. One psi is equal to 2.31 feet of head.

Take these two measurements, convert them to feet of head, and add them together to get the total dynamic head of the system. 

For example, suppose the vacuum gauge reads 6 inches of mercury, while the pressure gauge reads 15 psi.

The 6-inch vacuum measurement converts to 6.78 feet of head (6 x 1.13 = 6.78). The 15-psi measurement converts to 34.65 feet of head (15 x 2.31 = 34.65). Therefore, the total head calculated is: 41.43 feet of head (34.65 + 6.78 = 41.43).

Next, we need to figure out the design flow rate. This is determined by the volume of the pool and the turnover rate. For residential pools, the turnover rate is recommended to be between 8 to 12 hours, which needs to be converted to gallons per minute. 

For example, suppose we have a 20,000-gallon pool and we want an 8-hour turnover. We would calculate a minimum flow rate of about 42 GPM. 

20,000 gallons ÷ 8 = 2,500

gallons per hour.

2,500 gallons per hour ÷ 60 min per hour = 41.6 gallons per min

Note: The design flow rate is a little higher than the minimum flow rate because most people want a pump slightly larger than absolutely necessary to account for a dirty filter, which will slow the flow rate. It’s usually recommended to get a pump about 25 percent larger than the absolute minimum. 

Therefore, we can multiply our minimum flow rate by 1.25 to get the design flow rate.

41.6 gpm x 1.25 = 52 gpm

The last item to consider is the circulation system’s pipe size. Each size pipe has a maximum velocity of water that it can accommodate. Most pools and spas are plumbed with Schedule 40 PVC pipe with 1½- to 2-inch diameters being the most common sizes. For these sizes, the APSP recommends a maximum flow rate of about 44 gpm and 78 gpm, respectively.

Also, it is important to check the maximum flow rate of the filter, assuring the pump flow rate rating does not exceed the maximum flow rating of the filter.

The last step to picking out the proper pump is to consult manufacturer-provided pump performance curves, which are graphs with the vertical axis given in total dynamic head, and the horizontal axis in flow rate. 

If the head and flow operating point of your system is located on or below a pump’s performance curve, then that pump will be sufficient to do the job. 

Consulting a pump performance curve chart will help you choose the correct size pump. And a better understanding of the pump motor, especially the newer, more energy efficient models, will help make the choice easier.

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