Water feature hydraulics

by Sally Bouorm | June 1, 2010 12:53 pm

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By Phil Bach

Imagine the perfect water feature; now, ask a colleague or customer to do the same and have them describe it. What are the odds the descriptions will match? Slim to none.

This is because the esthetic beauty of a water feature is truly in the eyes—and ears—of the beholder. Esthetically speaking, the perfect water feature may resemble cascading rapids or a glass-like sheet of water. It may have a large vertical drop or it may be flat and smooth, appearing almost motionless. It is, put simply, a matter of taste.

The technical beauty of a water feature, however, is much easier to describe; it is a matter of proper hydraulics. Water in any swimming pool or water feature is always in motion—or at least it should be, so it can be properly filtered, sanitized and, in some cases, heated. Controlling the water’s motion so each customer sees and hears the effects they imagined is what water feature hydraulics are all about.

Three primary considerations

To create a successful water feature, three things must be kept in mind. First, everything water encounters has an impact on its flow. This includes filters, straight and curved plumbing and other equipment, fittings, nozzles and edges water is intended to flow over. This even includes the swimmers themselves.

Second, close attention must be paid to what the client wants (and doesn’t want) to see. For example, in the case of a vanishing edge, one must carefully consider where the water will go after it leaves the pool. If the overflow will always be hidden from view, the design of the catch basin can be utilitarian. However, that same overflow could be a water feature itself when viewed from a different vantage point.

Third, variable-speed pump technology has dramatically changed water feature design parameters. Now, a single pump can provide the flow required for an infinite range of applications. When using a single- or two-speed pump, sizing and selection is critical; however, a well-designed variable-speed pump can meet many different needs (with the correct programming, of course).

Fundamentals of hydraulics

Before discussing how to achieve a particular look and the impact of new pump technology, it is necessary to understand some important elements of hydraulics.

As mentioned earlier, every fitting, elbow, length of pipe and plumbed in-line piece of equipment (e.g. heaters, chlorinators, filters, etc.) adds resistance to the system. Each of these items also has a flow requirement. For example, flow for a single spa jet or arcing laminar nozzle may be 38 litres per minute (lpm) (10 gallons per minute [gpm]), while a heater may perform best at 151, 189 or 379 lpm (40, 50 or 100 gpm), depending on the type and size. Keep in mind, the more features being added (e.g. arcing laminars) the resistance and flow requirements are additive. The key is to make sure the pump is selected (and, if relevant, programmed) to efficiently draw water through everything included in the project (e.g. dual suction ports, fittings, pipe length and size and the nozzles used to create the desired effect).

One factor that dramatically affects resistance is velocity. Water flowing at an excessive speed will dramatically increase resistance; this is also known as friction loss. If a given horsepower is necessary, make sure the size of the lines provide the most efficient and safe flows—1.8 metres per second (mps) (6 feet per second [fps]) on the suction side in particular.

The Association of Pool and Spa Professionals (APSP) recommends the velocity in pool plumbing should not exceed 3 mps (10 fps) on discharge plumbing and 2.4 mps (8 fps) on suction plumbing. The Hydraulic Institute recommends a maximum velocity of 2 mps (7 fps) for optimum hydraulic efficiency. Velocity in copper pipe should never exceed 1.5 mps (5 fps). One of the best ways to reduce velocity is to use bigger pipe. While larger plumbing is initially more expensive, it will reduce energy costs over time.

One must also remember that water vaporizes as it turns through elbow fittings and then returns to fluid as the pipe straightens. Therefore, it is good practice to leave four to five times the pipe diameter in straight pipe between any curve and the pump strainer housing. For example, when using 51-mm (2-in.) pipe, make sure there is 203 to 254 mm (8 to 10 in.) of straight pipe going to the pump.

Smooth glass or tumbling rapids?

With a solid understanding of flow, resistance, velocity and the plumbing choices that affect these factors, one can begin planning the specific looks and sounds that make every water feature unique. The following example will focus on vanishing edges.

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Like any water feature, when planning a vanishing edge design always start with the flow requirement. If the design calls for sending 6 mm (0.25 in.) of water over a square edge, plan for 26.5 to 38 lpm (7 to 10 gpm) per lineal edge foot. By aiming for the higher end of this range, more leeway is given if the edge is not perfectly level. If the client wants 25 mm (1 in.) of water to flow over the edge, 136 lpm (36 gpm) per foot may be needed.

The shape and surface material of the weir (i.e. what the water flows over) also has an impact on flow requirement. The smoother and more level the edge, the less flow is needed. An edge built to a 1.6-mm (0.0625-in.) tolerance will need 3.175 mm (0.125 in.) of water to consistently cover it, while an edge built to a 6-mm (0.25-in.) tolerance will require 12.7 mm (0.5 in.) of water. A weir with a rounded edge will need at least 25 mm (1 in.) of water.

If an insufficient flow rate is used, the water will not effectively break the plane and could wrap under the edge, rather than flowing smoothly over. This may be desirable if designing a wet wall, but not if the client wants a waterfall on the backside.

When designing a vanishing-edge water feature it is also important to consider where the falling water will land, as this is critical in designing the catch basin. A very slow trickle from a thin film of water will drop nearly straight down, while greater flow will propel the water further. If 6 mm (0.25 in.) of water is flowing over a wall that is 1-m (3-ft) high on the back end, the water will meet the catch basin roughly 0.3 m (1 ft) from the wall. By the same example, 25 mm (1 in.) of water will meet the catch basin approximately 0.7 m (2.5 ft) from the wall. Remember, however, if six swimmers cannonball into the pool, more water will flow over the edge. As such, the catch basin should be designed to accommodate a sudden surge of water, too.

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When sizing the catch basin, the area of the main pool is the critical measurement. Some people assume the spillover length is the key factor but they are wrong. A tiny pool with perimeter overflow will require a smaller catch basin than a large pool with mostly standard walls and a simple 0.7-m (5-ft) spillover. The catch basin needs to be large enough to hold all the water that would spill into it when the circulation is off, in addition to whatever would spill into it during the pool’s heaviest use. A good rule of thumb is to design a large enough catch basin to hold the number of litres (gallons) in 51 mm (2 in.) of the pool’s water.

For example, a 4.9- x 9.8-m (16- x 32-ft) pool has a surface area of 48 m² (512 sf). Multiplying the surface area by 28.4 L (7.5 gal), the volume of water in a cubic metre (foot), will determine there are 14,498 L (3,830 gal) in the top 305 mm (12 in.) of water. By dividing this result by six, it can be determined there are 2,415 L (638 gal) of water in the top 51 mm (2 in.) of the pool. Therefore, the catch basin for this pool must be able to hold at least 2,415 L of water.

Obviously, the larger the pool’s surface, the more litres (gallons) must be accommodated, no matter how many linear feet of spillover.

Other bodies of water, such as an elevated spa, should also be considered. If the spa’s check valve fails, spa water will flow into the catch basin; as such, it is important to factor in the spa’s water volume when designing the catch basin. Also, to avoid vortexing (i.e. pumping air instead of water), the catch basin must always contain at least 0.3 m (1 ft) of water.

In total, the catch basin must be large enough to hold the water coming over the edge, including bather surge; the volume of any raised spas or other bodies of water flowing into the pool; and the vortex-prevention reserve.

Changing the game

The advent of variable-speed pump technology has changed the way water features are designed. In terms of saving time, using a variable-speed pump takes the guesswork out of pump sizing and, to a certain extent, line sizing. A well-designed variable-speed pump will provide the right amount of flow on windy days or calm ones; at the beginning, middle and end of the filter cycle; and whether or not the system has large-diameter pipe.

When using a standard pump, one can only hope the conditions, which were assumed when sizing the pump, hold true most of the time. Many designers have seen laminars and negative edges look perfect one week and wimpy the next, almost as if there isn’t enough water. This is completely understandable if the pump is always running at the same speed.

A pool system naturally builds up head pressure as the filter does its job between cleanings. Normal pumps have no choice but to continue trying to flood the edge at their set speed. Some variable-speed pumps, however, have the capability to measure flow automatically, respond to the naturally increasing system resistance and increase speed to maintain the required flow for the water feature.

The first time a variable-speed pump is used there may be a short learning curve with respect to programming the proper settings; however, most are quite easy to figure out.

Another advantage, with respect to water features, is the ability to vary the effect. For example, a pool owner may want to show off the full cascading rapids of a waterfall when guests first arrive for a party, but dial down the look and sound later in the evening. Similarly, a family may want a louder, more boisterous effect for a children’s party than an elegant poolside soirée. With a variable-speed pump and well-designed automation controls, it is possible to program these effects to automatically turn on at specific times. It is also possible to push a button on a remote control and make flow adjustments as easily as using a dimmer switch for a light.

In essence, the possibilities for water features are truly endless—so long as the hydraulic design is technically sound and the design takes advantage of the latest technology.

 

Phil Bach is the senior sales manager for Pentair Water Pool and Spa in eastern and central Canada. He has been working in the pool industry since 1978, starting as a pool service technician. Bach joined SwimQuip in 1988 and stayed with the company as it became Sta-Rite and then Pentair Water Pool and Spa.

Source URL: https://www.poolspamarketing.com/trade/water-feature-hydraulics/

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