If you ever have the opportunity to attend the lecture series on hydraulics given by Steve Barnes, Director of Science and Compliance for AquaStar Pool Products, you will be providing yourself with a real education.

A 39-year pool and spa industry veteran, Barnes is chairman of the VGBA-2017 Drain Cover Standard Writing Committee, and he also chairs the Pool & Hot Tub Alliance’s American National Standards Policy Committee, which oversees the PHTA standard development process. He is part of AquaStar’s New Product Development Team and is responsible for designing and operating AquaStar’s ISO 17025 certified testing laboratory.

This is a man who not only understands the physics of hydraulics but can explain it such that a novice can understand it, which is a rare gift and the mark of a true teacher.

This year, the * Service Industry News* team had the opportunity to attend Barnes’ two-part “Hydraulics for Existing Pools“ lecture at the Western Pool and Spa Show. Over the course of just a few hours, Barnes managed to hit on most of the main aspects of hydraulics relevant to the pool and spa service industry, and not only was it educational — it was totally interesting.

While we can say from personal experience that it is best to attend this series in person, highlights of Barnes’ hydraulics lectures may be found at * www.poolmanuniversity. com.* Go to the site and create a free account to access this and dozens of other informative lectures on both the basics and the science behind pool care.

In this special issue of * Service Industry News,* we will focus on hydraulics, a topic about which every self-respecting service professional should have a bit more than passing knowledge.

Pool and spa care is largely about water quality maintenance, and a huge aspect of that involves the circulation of water. And the circulation of water is governed by hydraulics.

Whether you are new to the pool and spa industry or a 30-year veteran professional, hydraulics can be a complicated topic.

Hydraulics is the branch of science that deals with the movement of liquids through pipes and channels, especially in terms of mechanical force or control.

In a pool, that happens through the skimmers and/or underwater suction. And that requires a pump to get the water moving. The objective is circulation, or moving the water throughout the pool such that two things are accomplished: filtration and the mixing of chemicals.

The discussion begins with water. Water is a molecule composed of two hydrogen atoms and one atom of oxygen. It is a particle that taken as a group has volume, no fixed shape, and whose state is determined by its temperature and pressure. As we know, water can exist as a solid, liquid, or gas. In a functioning swimming pool, we are primarily concerned with its state as a liquid.

Because the objective is circulation, the question is how to get the water moving. So, what moves water?

The answer to that question is pressure. In a pool, there are two key pressures that get the water moving: atmospheric pressure and pump pressure. Atmospheric pressure is the weight of the atmosphere that pushes down on the water, keeping it together and on the planet, ultimately driven by gravity.

In a swimming pool, atmospheric pressure pushes the water to the pump.

It’s a common misconception that the pump pulls the water through the suction outlets. But one of the main keys to understanding hydraulics is the fact that water is pushed, not pulled.

Within the pump, the water is pushed by a rotating impeller. That is, the centrifugal force of the impeller pushes the water. The mechanical force is converted to pressure, and that pressure pushes the water through the pipes back to the pool.

There are three pressure units to be concerned with.

They are: 1. Pounds per square inch. 2. Total Dynamic Head, (resistance to water flow) measured in feet of water.

3. Inches of mercury, where mercury is referred by its Latin abbreviation Hg.

These units can be converted among one another.

For example, a column of water that is one inch square and 2.31 feet tall will weigh 1 pound.

Inches of mercury is a unit of atmospheric pressure similar to the concept of the weight of a column of water. The name comes from the use of mercury barometers, which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 1.13 feet of water in a column that is one inch square.

1 PSI = 2.31 feet of water. 1 inch Hg = 1.13 feet of water.

In a swimming pool, the suction side-pressure is measured in inches of mercury, while the return side pressure is measured in PSI, so it is necessary to convert between the units.

And the gauges we use can measure the difference between atmospheric pressure and the pressure at the suction side or the return side.

Let’s turn the discussion briefly to flow.

Flow is described as the number of gallons that pass a fixed point within a minute. In a pool or spa, that fixed point is the pump or filter.

Flow is related to the velocity of water, and the velocity of the water is affected by the size of the pipe through which it is traveling. To maintain the same flow (or gallons per minute) through different sized pipes, water must travel at different velocities. Water travels slower through larger pipes and faster through smaller pipes.

In swimming pool design, for safety reasons, pipes are sized such that the water velocity doesn’t exceed a certain speed. According to the APSP 4th edition Service Technician Manual, to avoid suction entrapment, it is recommended that the water velocity in residential pool or spa piping not exceed 8 feet per second, and 6 feet per second in public pools or spa pipes.

Because of this requirement, different sized pipes will have different maximum allowed flow rates. Most pools are plumbed with Schedule 40 PVC pipes and fittings, with 1 ½ and 2-inch pipes being the most common sizes for residential pools and spas. One can use manufacturer pipe sizing tables similar to the brief accompanying table to determine the maximum flow rate for a given sized pipe. Conversely, pool builders should size pipes based on the desired flow rate, which is determined by the required turnover rate.

Onecanalsocalculatethesevaluesby using the following velocity formula.

Velocity (feet per second) = flow rate (gallons per minute) x 0.32 (constant) ÷ inside area of the pipe.

Note that the inside diameter of the pipe does not have an inside diameter that corresponds exactly to the stated diameter. Use manufacturer’s specifications for the true inside diameter.

Understanding flow is critical to understanding hydraulics.

Flow is easily measured with a flow meter, but many residential pools don’t have a flow meter, so it can be determined by measuring the total dynamic head and looking up the flow in gallons per minute on a pump curve chart.

In a typical configuration, where the pump is located above the pool, the following method will help service techs determine the flow.

First, determine the total dynamic head.

You can calculate the total dynamic head in the pool plumbing by placing a vacuum gauge on the suction side of the pool pump and a pressure gauge at the pressure side. If the pool filter is clean and has a pressure gauge you can use that.

Assuming the pump is variable speed, once the gauges are installed, run the pump at various rpm speeds and record the vacuum and pressure readings for each.

Using the vacuum gauge, read the inches of mercury on the suction side. Read the pounds per square inch on the pressure side. Convert each to feet of head, and add the two together.

Consider the following numbers recorded at 3,000 rpm: If the vacuum gauge reads 10 inches Hg, simply multiply by 1.13 to determine the feet of head on the suction side.

10 x 1.13 = 11.3 feet of head on suction side.

If the pressure side reads 20 psi, convert to feet of head by multiplying by 2.31.

20 x 2.31 = 46.2 feet of head on the pressure side.

Next, add the two values together: 11.3 + 46.2 = 57.5 feet of head. Finally, consult a pump curve to determine the operational flow rate, tracing the curve for 3,000 rpm (See accompanying graphic). On the Y axis, locate 57 feet of head. Follow the 3,000 rpm curve where 57 feet intersects with the curve, and locate the corresponding gpm for that amount of head. In this case, the flow is about 95 gpm.

See accompanying curve for the Pentair IntelliFlo on page 10. Note that the values of total dynamic head change at different speeds, so be sure to rerun the calculations for the different rpm curves to determine flow at various speeds.

** Understanding Cavitation**

Cavitation is not good. Cavitation is the rapid formation and collapse of vapor bubbles within a liquid. Cavitation is a kind of boiling, but rather than forming vapor bubbles as a result of increased temperature, the bubbles are formed as the result of decreased pressure.

The phase diagram of water depicts whether it is in its solid, liquid, or gaseous state as a function of temperature and pressure.

The vertical axis of the graph shows pressure, while its horizontal axis shows temperature. The curved line within is the vapor pressure curve. It indicates at which pressures and temperatures water vaporizes to its gaseous form or condenses to liquid. At normal atmospheric pressure, water melts from solid to liquid at 0 degrees Celsius and evaporates to gas at 100 degrees Celsius.

While we are all familiar with the fact that water vaporizes when it is exposed to sufficient heat, it is also true that if the pressure is reduced by enough at a constant temperature, the water converts to its gaseous form.

This is the reason for cavitation. Bubbles of water vapor form when the pressure in a certain location decreases below the vapor pressure.

So, how is it possible that the pressure becomes very low in certain areas of the liquid?

Looking at an impeller or propeller, cavitation bubbles form on the fastmoving blades moving through the water because there is a higher pressure on the side facing the flow and a lower pressure on the opposite side.

In a piped system, cavitation can occur where the pressure is lowest, such as at narrowed pipe sections, where the water’s velocity is faster than at wider pipe sections, but the static pressure is lower. Cavitation bubbles then form and occupy a significantly greater volume than the water does in its liquid state. The bubbles collapse as soon as they reach a region where the vapor pressure is high enough. Each collapse is a spectacular event as the liquid water rushes to fill the displaced volume. It implodes, emitting pressure shocks. Pressure shocks from many collapses results in an audible noise characteristic of cavitation.

Cavitation is bad because through its concentrated impact, even highstrength material can be damaged if it is close to the implosion of cavitation bubbles. The implosion of the cavitation bubbles releases energy in a very short time concentrated in a very small spot. The material impacted begins to develop micro cracks, and later, material can begin to break off.

2. Cavitation damage of an impeller. From left to right: Impeller without cavitation; impeller after 6 months of cavitation; Impeller with 1 year of cavitation; impeller with two years of cavitation.

The accompanying image shows an impeller that has been exposed to increasing durations of constant cavitation. One can see that after 2 years of exposure, the impeller blades have been completely eroded, making it impossible for the pump to function.

Cavitation is caused by anything that causes local pressure to decrease below the vapor pressure. Causes can be changes from larger to smaller pipe sections, a clogged skimmer or pump basket, closing a valve too fast, blocking suction or a drain, running a variable-speed pump faster than the system can handle, and so on.

It is essential to mitigate cavitation to prevent long-term destruction of materials.

** The Pump Affinity Law**

“Slow down, you move too fast,” begins The 59th Street Bridge Song (Feeling Groovy) by Simon and Garfunkel. That’s the basic premise of the Pump Affinity Law and what the thinking is with respect to pool and spa pumps: Slow them down.

The pump affinity law is a law of physics, also known as the pump law, and it applies to all pumps. It is the reason why variable-speed pumps are now required.

The affinity law applies to centrifugal pumps, which have a rotating impeller that pushes water into the pipes of the swimming pool.

The affinity law affects three elements of every pump.

Flow (gpm). Pressure. Power. All three of these are proportional to the pump’s speed, but it is the power element that the U.S. Department of Energy is concerned about.

When you slow down the pump’s speed, all three of these elements are reduced, but especially the amount of power used.

The pump affinity law can be used to determine flow (in gallons per minute) based on a known speed (in revolutions per minute). The equation is simple:

Rearrangement of the equation provides the unknown flow:

Alternately, it can be used to determine the necessary speed to set the pump (in revolutions per minute) based on a desired flow (in gallons per minute):

So, (assuming 100-percent efficient pumps) if you know what you want your turnover flow rate to be, you can calculate what the pump speed should be based on the flow rate it provided at a different pump speed.

It is a one-to-one relationship, so if setting the pump at full speed provides the maximum flow, setting the pump at half that speed cuts the flow in half.

The second element of the pump affinity law is related to pressure, and that equation is just a bit more complicated. That part of the affinity law is that pressure (PSI) is proportional to the square of the pump speed: What that means is that if the speed is cut in half, the pressure will be quartered. For example, if setting the pump at its full speed results in 40 psi, reducing the speed by half will result in 10 psi.

The third element of the pump affinity law is the aspect the Department of Energy cares about.

It says that power is proportional to the cube of the pump speed: So, if you set the pump at half the speed, the power used will be reduced to an eighth of what it was. For example, if setting the pump at its full speed results in expending 1,000 Watts, cutting the speed in half will reduce the power expended to 125 Watts.

Before we get too excited by this energy savings, its important to recognize that it will be necessary to compensate for reducing the speed by running the pump longer.

For example, if you can achieve your target number of turnovers at full speed in 8 hours, but then reduce your speed by half, you will have to run the pump for 16 hours to achieve the same number of turnovers.

But the savings is still huge: That 125 Watts becomes 250 Watts in 16 hours, which is still 75-percent less than it was when it was 1,000 Watts!

**Predicting a New Flow (GPM) using Pump Speed**

Let’s consider a real example. Say that you have determined your target flow rate. Now you want to know at what speed you should set the pump. To do this, you will need to know a different flow rate at some other speed. This is easy enough if your pump comes equipped with a flow meter, but if it is not, you will have to determine what the flow rate is based on using gauges to measure the Total Dynamic Head and consulting a manufacturer’s pump curve.

Once you know this, the math is simple.

If you have determined that when you set the speed at 3,450 rpm it results in a flow rate of 87 gpm, at what speed should you set the pump to obtain a flow rate of 44 gpm?

Answer: Unknown rpm = (3,450 rpm x 44 gpm) ÷ 87 gpm.

3,450 x 44 = 151,800. 151,800 ÷ 87 =1,745 1,745 rpm should get you where you want to be.

**Programming VS Pumps for Water Quality**

What, exactly, are we trying to achieve with our pumps?

We are looking for continuous skimming. We want the water to be filtered. We want our chemicals mixed.

We’ve seen that operating the pumps at lower speed significantly reduces the power. But while it is true that operating your pump at 1 RPM would use very little power, it would also take 17 years to actually turn over the water.

With variable-speed pumps, however, we have the luxury to set the pump at several different speeds.

In fact, we might like to really get that water moving at full speed for an hour, and then let it drop down to a slow cruise for several hours, before ramping it back up again for another hour. If we play our cards right with this approach, we could run the pump 24/7 and still achieve an energy savings compared to running the pump full speed for 8 hours.

To do this, set your cruising speed for effective skimming, assuming a dirty filter. Set your second speed for an hour and a half for a circulation boost. Your third speed can be set for the automatic cleaner operation, and the fourth speed could be set for any auxiliary features, such as spa jets or waterfalls.

Let’s see how this 24/7 approach works on a residential pool.

Consider a variable-speed pump that at maximum speed consumes 2,300 Watts. The national average cost of electricity is 17 cents per kilowatt hour. For this example, the pump uses 55.2 kW in 24 hours, which costs nearly $9.40 a day if it is run 24/7 at full speed. Obviously, no one would do this — it negates the point of a variable-speed pump.

What many homeowners do with variable-speed pumps is to run the pump instead for eight hours. That reduces the cost to about $3 a day, which is great savings, but it’s not the way a professional service tech, who wants the pump running constantly, would handle the problem.

For simplicity’s sake, let’s consider running the pump at half the speed for segments of the day. In that case, the power used would be an eighth of what it was, or 287 Watts.

So, we could run the pump at half speed for 6.5 hours, and then full speed for 1.5 hours, then again at half speed for another 6.5 hours, and again at full speed for 1.5 hours, and so on. This way, the pump would be operating constantly, but get a big circulation boost occasionally.

What would that cost?

2.3 kW x 4.5 hours = 10.35 kWh .287 kW x 19.5 hours = 5.6 kWh 10.35 kWh + 5.6 kWh = 15.95 kWh total used in 24 hours. 15.95 kWh used x 17 cents/ kWh (national average cost of electricity) = $2.71 per day So now the pump is being run constantly, which means that leaves and debris go straight to the filter rather than drifting to the floor, and the customer is still paying a little less than they would than if they ran it for only 8 hours a day. That’s less work for the service tech, and money in the customer’s pocket.

Sounds like a win-win.