Designing pool filtration systems is really an engineering task. Engineering is essentially the application of science to solve practical problems. To do that you’ve got to understand the science. Hydraulics is the science of fluids (in our case, water) flowing through pipes and channels. In this book we will focus on flow through pipes. There are a few essential concepts that you should master in order to design effective pool systems. Gaining an intuitive understanding should be your goal. These are:
- Flow Rate
- Velocity and Pipe Size
- Head Loss (major and minor)
- Total Dynamic Head
Much of the vocabulary of engineering coincides with common speech but has very specific meaning. So my explanations here should be understood in lay terms.
Volume is simply a measure of quantity by space occupied. Whether still or flowing or falling, for our purposes volume is the amount of space that liquid water occupies (at a given temperature). Nothing more, nothing less. Water is usually measured in gallons in the U.S. (CFMs in large systems) or liters where the metric standard prevails.
Pressure is the force that water exerts against its container. Water in a glass pushes out against the glass. Water in a container, whether a pool, a bath tub, the Ocean all exert pressure. That pressure is produced by the weight of the water, and the pressure increases with depth. The kind of pressure produced by a static body of water is called static pressure and it is a function of gravity and depth. Now imagine that there is a crack in the glass, now water leaks through. The reason is that crack is a tiny opening where there is no glass to push back against the water. So, the water pushes through. It begins to move. That is how water moves through pipes as well. It flows from the direction of higher pressure toward the point of lower pressure, but more on that later. Water pressure is commonly measured in pounds per square inch (PSI) but there are many other units of measurement more common to specific disciplines or places in the world.
Flow rate measured in gallons per minute (GPM) is applied to flowing water. It describes the volume of water traveling past a given point in one minute. Important to note, it is not strictly a measure of velocity but there is usually an element of velocity implicit in any consideration of flow rate as there is a close relationship between flow rate and velocity. Consider this analogy. You are standing by a busy highway observing all the cars zooming by. Now, set your stopwatch for one minute and begin counting each car that passes in that time. Say you count sixty cars, the “flow rate” of cars on that highway is one car per second (average cars/60 seconds). But you don’t know how fast they were going or their “velocity”. You might intuit that they would need to travel at high speed for such a high number. But, maybe the highway had 10 lanes. In that case the cars could have been traveling at a moderate pace and easily achieve the 60 cars / minute. So there is another important variable to consider before we understand the complete picture. That is what I will introduce next.
Velocity & Pipe Size
Velocity is the measure of distance traveled in a given time. In our field the standard unit of measurement is feet per second (FPS). Water’s velocity through pipes is referred to as line velocity. Understanding line velocity is very important in the design of hydraulic systems because it is a pretty reliable shorthand for energy efficiency. Velocity translates to the speed of the cars in our analogy above. All other things being equal, line velocity is determined by the size or the diameter of the pipe. A larger pipe produces a lower velocity for a given flow rate than a smaller pipe will. Think of how a larger highway eg 10 lanes contrasts with a 2 lane road conveying 60 cars in a minute. 6 cars per lane rather than 30 can travel more slowly and therefore conserve more energy. Pool pipes are usually made of polyvinyl chloride (PVC) in two major wall thickness standards. Schedule 40 (SCH 40) or schedule 80 (SCH 80). Schedule 40 is more common except when SCH 80’s added strength is need. The sizes are listed as diameters in inches. So a 2″ SCH 40 PVC pipe has a nominal 2″ diameter. The actual size varies slightly, so we use the actual diameter in calculations. A 2″ SCH 80 PVC pipe has a smaller inside pipe diameter owing to the thicker walls.
Friction is the resistance to flow caused by water “sticking” to the inside of the pipe. If the water is not moving there is no problem with the sticky water. But, when it begins to flow, friction steadily increases to rob the system of it’s kinetic (moving) energy. It is possible for friction to become so great that it can bring circulation to a virtual dead stop. Obviously, that would be a very poorly designed system. Friction, then you might now guess is behind the inefficiency of high velocity. Correct. Higher velocity = greater friction = lower efficiency. Friction is the major cause of efficiency loss in systems with long pipe runs. The typical pool system however, loses more from other sources of loss which we will cover later. Friction along with those other sources of energy loss is collectively expressed as head loss measured in feet. Here is how it works:
Head is a kind of shorthand for pressure. The pump in our pool system creates positive head or it creates pressure at its outlet. That pressure is measured in feet of head. That is the equivalent of water pressure under that number of feet deep. Example: A pressure gauge attached to a pump that produces 100 feet of head pressure would read the same if it were applied to the bottom of a 100 foot deep reservoir. So, head loss is the progressive loss of pressure in the system as water travels farther from the pump. A pressure gauge at the pump’s discharge outlet would register a higher reading than one attached 50 feet downstream in a circulating system. As noted before, the reason for this loss of pressure is friction along with other sources. These sources are traditionally divided into “major loss” and “minor loss”. Major loss refers to head loss resulting from friction in the pipes. Turbulence from fittings such as elbows, and valves etc are known as minor loss. Component loss results from flowing through components such as heaters, filters, chlorinators etc. As pressure is what drives flow in a circulating or dynamic system, head loss is a key factor to consider in design.
Total Dynamic Head
This is the head required to circulate water through the system at a given flow rate. In other words, TDH measured in feet is the depth of water that a reservoir would need to provide to drive the system at that given flow rate. Finding the TDH of the system you design is a very important exercise. Armed with this knowledge, you can select the appropriate pump for the job and be certain that you have built the best engineered system possible within the constraints you’re given.
Several types of pumps exist. Each type has its strengths and weaknesses. Centrifugal pumps are the type used in pool systems as well as most water features. Centrifugal pumps work by accepting water into a centrifuge through an inlet and accelerating water in a circular pattern with a spinning impeller powered by a motor. The impeller then directs the water out through a discharge port connected to the piping system. An advantage of centrifugal pumps is that they produce a constant steady stream of water rather than pulses as with some other types. They also produce high flow rates. Each pump performs based on its design and size. Pump manufacturers provide performance information in the form of pump curves. These curves are a graphic chart that tracks the pumps performance output in GPM for a given TDH. Now you see why TDH is necessary. Pumps are designed to function within certain ranges of TDH. We need to know it so we can size the pump correctly.
Now that you have the basic engineering concepts down, next I’ll introduce you to the various components that go into a well designed residential pool. Stay tuned!