Water Works

“How can I get rid of these brown spots?” is a question you have probably asked yourself. To find the answers, it will help to understand something about basic irrigation hydraulics — the movement of water through a piping system.

HOW HYDRAULICS WORK

Because water is a fluid, it fills up the space it occupies. It also follows the path of least resistance. When pressure is generated behind water, it pushes it along to satisfy a demand (such as that for a sprinkler or emitter), or until it reaches a stopping point. If no pressure is present, water flow simply is a function of gravity.

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The density of water creates a certain weight, or “head.” Pump experts discuss pressure in terms of head, which is measured in feet. For instance, each vertical foot (head) of a column of water creates 0.433 psi (pounds per square inch) of pressure in terms of force. A calculation of 100 feet of head results in 43.3 psi in terms of water pressure. This same rate is applicable to uphill or downhill runs in a piping system.

Many towns use water towers as the water supply for residences and businesses. The tower must be sufficiently high above the points of demand (faucets and the like) to provide enough pressure for the water to flow properly.

Irrigation systems have either a city-supplied water meter or a pump for the source. Depending on the type of water, you may need to install a backflow preventer in front of the valves on the irrigation system. Valves determine which sprinklers operate at one time, and controllers turn on the valves for certain lengths of time.

During the time of day when no sprinklers are operating, the water pressure is called “static.” This term means non-moving, since the water is at rest in the pipes. The pressure will generally be the same throughout the system. Once any demand occurs (such as a valve opening to turn on sprinklers), the water begins to move through the valves and pipes. Pressure tends to drop, and is known as operating or dynamic pressure.

The reason that the pressure falls during the dynamic stage is that water molecules rub against one another, causing friction. Pressure drop is often called “friction loss.” Each element of the irrigation system has a calculable loss associated with it, depending on the amount of water flowing at the time. For instance, a 1-inch electric valve supplying 20 gpm (gallons per minute, demanded by the sprinklers) loses about 3.0 psi; while a 2-inch valve flowing 80 gpm loses only 2.5 psi. These figures are available in manufacturers' catalogs.

Likewise, water pressure is lost through each other element of the system. The water meter, backflow preventer, main line (constant-pressure) pipe and lateral (periodic-pressure) piping all include losses in pressure that you must calculate. Pipe manufacturers' charts list losses per 100 feet, and you can find friction losses for the other items from suppliers or manufacturers, as well.

It is crucial that you know pressure at the water source prior to design or installation. Sometimes you can measure this with a gauge, and other times you must call the city to find out the static pressure. Most basic irrigation systems will have between 20 and 25 psi of friction loss, from the water meter and backflow preventer through the piping system and valves. Generally, 55 to 65 psi static is acceptable water pressure at the source; 40 is low and you must take steps to keep frictional losses to a minimum.

Velocity goes hand-in-hand with water pressure. The faster water travels through pipe, the higher the friction loss. Also, pvc pipe manufacturers have established 5.5 fps (feet per second) as the maximum safe velocity for optimum irrigation system performance. Beyond this, you risk breakage due to surges. Water will tend to “snake” through the pipe rather than traveling straight at high velocities, causing the pipe to want to move in the earth. Think of your garden hose when you turn it on full blast, and you watch it snake around. The same situation exists in underground piping, and sudden failure may be the result.

In relatively small systems such as those for a residential back yard, it may work well to connect into the hose bib, plumb some valves, and run all ¾-inch pvc pipe. The key is to know how many sprinklers to place on each valve, based upon the water pressure available.

Larger systems such as parks or schools require more planning. It is crucial for optimum performance to provide the design pressure at the base of the sprinkler farthest away from the water source. The calculations mentioned above must be made by the designer, as well as taking elevation change into account, before the system is installed. “Looping” the main line is a good way to keep water moving in two directions, decreasing friction loss and keeping the piping network cleaner.

IDENTIFYING PROBLEMS

With today's automatic systems, operation of the irrigation valves often happens at night when water pressures are higher and wind and evaporation are less of a factor. Most owners or managers don't watch the irrigation system on a regular basis. The thing to observe is the landscape itself, to see whether or not the performance of the plants and turf is acceptable.

Having brown spots in the turf is indicative of an irrigation problem. There are other symptoms of a poorly-irrigated landscape that also may be attributed to badly-designed (or installed) hydraulics: dying or wilting plants, “donuts” around sprinklers and wet and dry spots are classic examples. Before you can troubleshoot and fix problems, you must first notice them by evaluating the conditions in the field.

TROUBLESHOOTING

Once you have determined that there is a problem, the next step is to find the cause — or troubleshoot. This involves actually watching the irrigation system operate. It is important to simulate the conditions when the automatic cycle occurs if possible: the same time, numbers of valves in operation and water pressure. Often, if you test a single valve during the daytime, sprinklers will appear to be receiving proper pressure; but in its normal schedule with other circuits running, the performance is decreased.

There can be several symptoms of poor performance due to hydraulic conditions:

  • fogging heads;
  • short distance of coverage;
  • unequal distribution;
  • end heads get no water; and
  • puddling of water.

The causes of these identified problems can be:

  • too many heads on a valve;
  • too great a distance from the valve or pressure regulator;
  • undersized pipe;
  • pressure regulation not adjusted properly (psi too high or low); and
  • individual nozzles not adjusted correctly.

Site conditions play a part in this evaluation. You must factor into your identification process sloping terrain, soil types, shade and sun and building exposure.

Once you have identified the cause of your symptom, it's time to plan a course of action.

WHAT TO DO ABOUT PROBLEMS

Some solutions are simple and inexpensive. Some are not. Once you have decided that site conditions are not the main cause of the problem, plan your attack.

If fogging or puddling is the problem, it may be that adjusting the flow control on the valve is the answer because the pressure is too high. Adjusting individual sprinkler nozzles may also do the trick. Sometimes changing nozzles to newer, pressure-compensating types will work for your situation.

More often than not, sprinklers or emitters are underperforming due to low system pressure. Here are some possible solutions:

  • Loop the main line pipe from the water source. This cuts the loss in half.

  • Add more valves to the system. If too many heads are on a circuit, or if distance is a problem, split them in two. You will have to add wiring, and you'll need to have additional clock stations available.

  • Place valves in the center of the lateral piping section. If end heads are not getting water, move the valves to the middle. Extend wiring.

  • “Donuts” are caused by low pressure at the base of the head. You should increase overall system pressure or substitute sprinklers with a lower operating pressure.

  • Dry spots may be caused by low pressure, poor spacing or deficient scheduling. Increasing the time may work, but often creates wet spots when attempting to correct dry areas. Sprinklers may be moved, but this is tedious and expensive. An overall increase in pressure may prove to be your best answer.

  • If undersized main line pipe is your problem, you may need to add sections of new pipe. Connect just downstream from the water source (or where the problem exists), and run a parallel main line that ties in again at the other end. This will provide a type of loop — two paths for the water to travel, decreasing pressure drop.

  • Add a booster pump. If you are looking for more than a 10-psi increase, a mechanical means of boosting the pressure is your answer. Electrical power is required, and most pumps need 230- or 460-volt, 3-phase juice.

  • In emitter systems, you may replace older types with pressure-compensating drippers. This will equalize water flow to each plant, increasing the ability to reach to the extreme ends of the lateral piping.

HYDRAULICS HOMEWORK

It pays to know the basics of hydraulics. Before you tear into a repair or decide to renovate entirely, study the characteristics of water movement. It may save you time and money.

Allen N. George is design associate for Coates Irrigation Consultants, Inc. (Scottsdale, Ariz.).

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