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Hydraulics — What pool techs need to know

By Marcelle Dibrell

Unless you are a pool builder and are able to start with brand new plumbing components, or unless you have the blueprints to have detailed knowledge of the underground plumbing configuration, much of the hydraulics of any given pool is theoretical. You don’t and can’t know much about what plumbing may exist underground or how much resistance to water flow is being created by old pumps, filters, heaters, and other water features.

So why bother learning about hydraulics?

Well, you might need to understand hydraulics to pass a contractor’s license.

But, perhaps more importantly, understanding hydraulics helps you to estimate proper replacement equipment, or when water circulation is poor at a particular pool. Understanding hydraulics can also help prevent adding plumbing components that could create a bad situation. And, you might be tasked with actually designing a fountain, pond, spa, or pool, in which case learning hydraulics is essential.

In this, the hydraulics issue of Service Industry News, we’ll look at some of the basic ideas.

The study of hydraulics begins with defining a few terms.

Head and flow rate: Head is the resistance of water to flow through plumbing and equipment. It is expressed in feet. The lower the head (the lower the resistance) the better. The flow rate is the amount of water that is moved for a given time increment.

To understand these concepts, consider this example. Say you have a source of water plumbed to a pump and motor with a 1-foot pipe pointing vertically up from the output of the pump. Now, turn on the pump and collect the water while timing the process. Say you collect 10 gallons of water in a minute. That pump would then be rated at 10 gallons per min (gpm) for 1 foot of head.

Now, say you increase the length of vertical pipe to a total of 7 feet and then measure the flow rate. Say you capture 5 gallons per minute. You capture less water in the same time period because the additional resistance means the pump can’t push the water as fast. This is called head loss, a misnomer, really, because it is actually flow loss due to head increase.

If you continue to increase the length of the pipe, you will eventually get to the point where you can’t get any water to flow at all. Let’s say that happens at 10 feet of head.

This very small pump can now be charted on a graph that shows the maximum head that the pump can overcome to obtain any flow. With a graph like this, you can determine the flow you can expect to obtain given the resistance of the system. Conversely, you can choose the flow rate you want to obtain and design the system such that it will not exceed that resistance.

Pump performance curves are useful because if you know the total amount of resistance in your pool area and you know the desired flow, you can choose the pump that will satisfy those needs by consulting a pump performance curve.

Total head is the head created by every aspect of resistance in the pool system. That includes suction head, which is the head created by restricting the intake or by requiring the pump to lift water from the suction side. It also includes the friction flowing through the various sized pipes as well as each and every fitting and valve. It further includes the resistance through all of the equipment and water features.

There are tables available that list the amount of head that is created for each length and diameter of pipe and for each type of joint. Furthermore, filter, heater, and additional equipment manufacturers provide the amount of head for each component in the swimming pool. Using these values, the total head created by the pool configuration can be calculated. However, as mentioned before, unless you have a blueprint for the entire system, you have no way of knowing what is going on underground, so this is not a practical method for estimating the total head it will be necessary to overcome.

The easiest method is to use the existing pump to measure the head. You can measure the vacuum on the suction side of the pump and the pressure on the discharge side. Install a vacuum gauge on the pipe entering the pump. It usually measures inches of mercury. Every inch of mercury is 1.13 feet of head. Plumb a pressure gauge on the pipe coming out of the pump. It measures pounds per square inch. Every psi of pressure is 2.31 feet of head. Multiply the pressure readings out accordingly and sum the results, and that gives the total dynamic head of the system.

To use a pump performance curve, determine the total dynamic head that your pump must overcome, and the flow rate that you will require for your desired turnover time. Find the intersection of those values on the curve. If the head and flow operating point of your system is located on or below a given pump’s performance curve, then the pump will be sufficient to do that job.

A word about fittings

The lower the head that is created in any installation job, the better. And the types of fittings that are chosen make a big difference in the head that results. To make it easier to assess how much head results from a fitting type, that pressure loss is converted to an equivalent length of straight pipe.

Unions and straight connectors act like straight pipes, so no special calculations are needed. However, going around corners does create head. And the steeper the corner, the more head created.

Consider the following examples: 1 ½ inch (38 mm) pipe, 90° elbow = 7.4 equivalent feet of head 1 ½ inch (38 mm) pipe, 45° elbow = 2.1 equivalent feet of head 2 inch, (51 mm) pipe, 90° elbow = 8.5 equivalent feet of head 2 inch, (51 mm) pipe, 45° elbow = 2.7 equivalent feet of head

It is interesting to see that more than three times as much head is created when using a 90-degree fitting than a 45-degree elbow.

There are times when there is a choice between using a one 90-degree fitting or two 45-degree elbows. Given the choice, using two 45-degree elbows results in less head loss.

Tables are available that provide equivalent feet of head for all types of fittings, valves, and unions so that similar choices can be made.

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