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Balancing System Hydraulics: Pressure for Performance

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Perhaps the most overlooked problem with landscape irrigation system performance is pressure. A healthy balance of pressure is good for the performance and longevity of your irrigation system, and thus the aesthetics of your landscape. Keeping some fundamental hydraulics in mind, minimizing the impacts of too much or too little pressure in an irrigation system is simple, quick and effective.

Static and Working Pressure

Static pressure is the point when water is motionless. In a closed level system, pressure is the same at every point. If the system is not level, there will be slight differences in pressure caused by elevation differences.

Dynamic, or working, pressure is pressure at any point when water us flowing. This pressure is always less than static because the flow of water results in a loss of pressure due to friction in the pipe and through any pipe fittings. Pressure loss in a system equals static pressure minus working pressure. Now that the basics have been reviewed, the following troubleshooting guidelines can be considered.


Pressure Problems and Solutions

Inadequate pressure is a primary cause of poor turf performance at many sites. Signs are doughnut patterns, runoff and large water drops in the stream of the sprinklers. The results are poor uniformity and possibly sprinkler rotation problems for gear drive and impact heads, which result in increased dry areas. The best techniques to overcome inadequate pressure are:

      • Increase pressure.
      • Use low-pressure nozzles. They have barbs on them so that the diameter of throw can't change, but more water will be placed along the stream and closer to the sprinkler.
      • Use shorter laterals, particularly if there are differences between pressure at the first and last sprinkler on a zone. If pressure is too low along the entire lateral, add a valve and split the lateral into two separate stations.
      • Reduce the nozzle sizes on all heads on the zone by one nozzle size. Remember that you are reducing the amount of water being applied overall, but it can improve uniformity and you might not have to adjust your scheduling.
      • Consult a designer or manufacturer before making large retrofit changes.

Too much pressure causes misting and drift of your irrigation water and that can potentially damage impact heads from rapid rotation. Some valves have pressure-regulating features (not to be confused with flow control), and can be field adjusted.

If the site has no regulator, one can be installed behind the valve. To adjust the pressure regulator, place a pressure gauge on the most distant sprinkler. Adjust the regulator or flow-control valve so that the sprinkler pressure is at the middle of its pressure range. Pressures can vary, and this pressure will be inadequate during low pressure and too high during high pressure. A regulator may be required in these instances.



Pressure Regulation

Many systems rely exclusively on delivered pressure. When this fluctuates, so will the distance of throw and precipitation rates of the sprinklers. A 10 percent pressure difference between laterals can cause a 5 percent difference in precipitation rates. A 30 percent difference can cause a 14 percent difference in the precipitation rate. A 15 percent differential should be the maximum.

Once the ratio is set, the difference between sprinklers will be almost constant. Precipitation will increase if pressure increases because of the increase in flow through the nozzle. This differential is controlled by the size of the pipe.

Many sites have pressure regulators to set flow and pressure rates. Oftentimes these devices fail or are set at a pressure too low during installation. They might have to be set to a higher pressure or replaced altogether. Consider installing a regulator with a higher range, for increased pressure downline of the regulator.


Flow and Pressure Control

Using flow control for pressure control almost always results in uneven pressure and poor uniformity. Most valves come with a flow control knob or dial on top of the valve, which maintenance personnel tend to use as a pressure regulator when the pressure is too great.

Unfortunately, this device responds to changes in pressure and does not compensate for them. Always use a pressure regulator designed to regulate pressure. A low-pressure problem might also be attributable to partially closed valves. Check the valve stems to ensure that they are completely open.



Altering the System

Adding new heads or laterals to an existing valve will absolutely affect pressure in the zone. Although the intent might be to increase coverage or reduce hot spots, it actually increases problems during the peak stress months of summer. As with any design, additions to an existing system should consider both static and dynamic pressure, pipe size, sprinkler head type and flow, and distances between existing heads and laterals. Try changing nozzle sizes or heads to improve coverage.

Changing a sprinkler's nozzle size can be a help or a hindrance, depending on system pressure and the head's precipitation rate. Low-flow or flow-control nozzles are used to improve uniformity, and can be life savers on systems with small pipes or areas with elevation differences.

Properly designed flow nozzles deliver the same flow rate from each sprinkler, even though each sprinkler has a different pressure. A flexible orifice is used in the nozzle to deflect the stream of water. With high pressure, the orifice is small, at low pressure it's large. This helps to improve flow rate and precipitation.

Another method, matched precipitation, is a nozzle configuration used on different arcs of heads. As an example, a 45 degree head would not have the same nozzle size as a 90 degree because of the difference in coverage. If they did, the 45 degree would put out twice the water as the 90 degree, resulting in higher precipitation and poor uniformity. Always use matched nozzles when working with pressure problems.

Unfortunately, head-to-head coverage too often is a luxury, rather than a standard. Less than a 75 percent head-to-head throw will result in hot spots and low precipitation. If pressure is low, these problems are amplified. If pressure has dropped, you might have to move heads or change to a brand with better throw with lower pressure. It is always better to have excessive pressure than inadequate pressure, since pressure regulation costs less than trying to overcome a low-pressure problem.

The performance of your irrigation system relies on the integrity of the design, installation and maintenance. When any of these three components is compromised, so is the hydraulic performance. Maintaining the system's designed pressure results in water conservation, healthy plant material and overall system durability.

 
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