Feeling the Pressure

Connecting pressure, flow and overall efficiency

Published online: Jun 27, 2017 Articles, Irrigation
Viewed 690 time(s)

This article appears in Potato Grower's July 2017 Smart Irrigation Month issue.

Pressure regulation can be a topic of dispute in some regions. Some growers will tell you it’s the best way to save water and energy, while others will claim it’s nothing but a waste of time and money unless you have sloped fields.

The truth is, every irrigation system experiences pressure fluctuations, and these result in unwanted flow deviations and over- and under-watering. Proper use of pressure regulators helps maintain the overall efficiency of an irrigation system. In addition, they can help save water and reduce premature wear on system components.

 

How does a pressure regulator work?

The basic function of a pressure regulator is to limit excessive and varying inlet pressure to a constant outlet pressure. This is done to maintain the desired performance of an irrigation system.

Pressure regulators accomplish this by automatically modulating their areas of opening. By changing the area of opening as upstream pressures fluctuate, pressure loss through the regulator changes proportionally. This allows them to maintain the downstream pressure at a relative constant.

 

Why do I need pressure regulators?

No matter what type of irrigation system you have, correct system pressure is essential for optimum distribution uniformity and proper flow through emitters. Crops need the right amount of water at the right time, and pressure regulators ensure this need is met. Sprinklers are made to operate within a specific range of flows and pressures. They’ll maintain their application pattern and adequate droplet size as long as they can operate under normal conditions. If these conditions are altered, their application pattern will be affected, and you could easily run into uniformity issues, over- or under-watering, and irregular crop growth.

 

What causes pressure deviations?

Every irrigation system will experience some sort of pressure fluctuation. Pressures fluctuate for several reasons, including elevation changes within the irrigated area and pressure loss through pipes and fittings. In solid-set, nursery, residential and commercial landscape systems, pressures can fluctuate when zones cycle on or off. Pressure issues are also common on large projects using multiple wells to provide water. In mechanical-move systems, pressures are also affected by activation of end guns and corner arms and changes in water supply.

 

Do I need a pressure regulator with flat land?

Yes! Your system pressure can change due to the various causes listed prior, as well as a host of other reasons. Elevation is just one of the many things that cause pressure fluctuations.

No matter what degree of elevation change you have, controlling flow and pressure is essential for systems operating at lower pressures. For example, if a system operating at 30 pounds per square inch (psi) experiences a 6-psi pressure variation, which will result in a 10 percent flow variation. That flow variation can adversely impact sprinkler performance and ultimately affect yields.

 

Pressure Variations

Design Pressure

1 psi

2 psi

3 psi

5 psi

6 psi

8.3%

16.7%

25.0%

41.7%

10 psi

5.0%

10.0%

15.0%

25.0%

15 psi

3.3%

6.7%

10.0%

16.7%

20 psi

2.5%

5.0%

7.5%

12.5%

% Flow Variation

Pressure regulators are recommended if there is a 10 percent pressure and/or a 5 percent flow variation.
The lower a system’s design pressure, the more critical it is to accurately control its pressure.

 

How does pressure affect flow rate?

The operating pressure of an irrigation system always affects the flow rate. Higher pressures increase flow along any pipe. As that flow increases, water velocity increases as well. This results in pressure decreasing downstream due to friction loss. When water is rushing through a pipe at high velocities, the interior walls of the pipe create friction against it, which causes pressure loss.

 

How long does a pressure regulator last?

High-quality pressure regulators are built using high-impact, engineering grade thermoplastics. However, like sprinklers, they do not last forever. Irrigators are encouraged to check their pressure regulators at least every three years.

Factors that can shorten a pressure regulator’s lifespan include:

  • Unflushed chemicals in the pipeline
  • Suspended abrasive materials in the water
  • Long operating hours
  • Water hammer

 

How can I tell whether a pressure regulator is bad?

Issues such as dry spots near the head, over-watering in a doughnut-shaped pattern, uneven crop growth and streaking in the field, and misting are all common signs of pressure regulator malfunctions. Using yield maps and aerial imaging may prove helpful in identifying issues.

 

What is the difference between a pressure regulator and a pressure-regulating limit valve?

Pressure-regulating limit valves are generally used where there is a shut-off valve downstream. When this shut-off valve is closed, the limit valve’s T-stem flow-passage closes and seals, thus limiting the outlet pressure to only 10 to 15 psi above its normal regulating pressure. This helps protect downstream components from potential damage due to high static upstream water pressure.

With a standard regulator, when the downstream shut-off valve is closed, the T-stem is unable to completely seal against the harder seat.

The high inlet pressure eventually equalizes across the regulator and up to the valve.  Upon opening the shut-off valve, a high-pressure surge could possibly damage downstream meters, sprinklers or other plumbing components.

Growers who would like to incorporate pressure regulators into their irrigation systems or replace older models are encouraged to contact their local irrigation dealer.