12.3.1: System Design

Solid-set systems, especially permanent systems, are expensive to install and, therefore, should be carefully designed. Permanent solid-set systems can be tailored to specific fields conditions to minimize installation and operation costs. The size of mainlines, manifolds and laterals can be reduced in an incremental fashion to achieve pressure loss and flow guidelines while saving investment costs. For example, the pipe for the distal portion of the lateral may be smaller than at the inlet since flows decrease along the lateral. One lateral in the system will ultimately determine the maximum pressure required from the pump. Mainline, submain and lateral sizes for other portions of the system may be smaller to reduce investment cost. The size of nozzles along the lateral can be varied for solid-set systems which allows for enhanced uniformity with little investment.

Each lateral can be specifically designed for local conditions. Thus, some laterals may operate at different average pressures depending on the location in the network and elevation of the lateral. The discharge and application rate can be designed to apply the desired depth at the appropriate application rate to avoid runoff and erosion. The set time can be short to apply small depths each irrigation. The cost of solid-set systems depends on the number of laterals that are needed. Therefore, a common problem is that the distance between laterals is extended to reduce investment costs. This is critical because once the system is installed it is expensive to retrofit the system to operate appropriately.

The piping network in buried solid-set systems will probably be PVC. This has proven to be an economical pipe for construction and operation. However, the pipe cannot take large pressure surges. Therefore, special precautions should be taken to prevent pipeline damage due to water hammer or vacuum. Vacuum relief valves must be installed at the high locations in the field to allow air to enter when the system is shutdown. High-pressure surges can be dealt with in several ways. A high-pressure relief valve can be installed in areas where pressure reached peak values. Surge tanks can be installed, especially at the pump, to absorb some of the pressure surge ahead of the PVC pipeline. Special valves can also be used to throttle the flow at the pump until pressure develops in the mainline. This prevents the pressure surge that occurs when flowing water reaches the end of an enclosed pipeline. These valves can also be adjusted to maintain a constant downstream pressure. This is useful when a reduced number of laterals are operated. The pressure control of the valve keeps the operating pressure of the pipeline within an acceptable range. Depending on the characteristics of the pump, the pressure ahead of control valves may rise to high levels when a small flow rate is pumped; therefore, variable speed pumps or other controls may be needed.

Lateral spacing is not contingent on the length of mainline pipes for permanent solid-set systems; therefore, the lateral spacing should align with farming equipment operation. One of the major inconveniences with solid-set systems is that you must farm around the risers. If the lateral spacing is adjusted to match typical or critical equipment width, then farming practices are easier.

Laterals must be designed to prevent frost damage. The laterals should be drained when cold weather threatens. Drain valves can be used to drain the pipelines every irrigation, but this may not be desirable, especially if the laterals and mainlines are large containing substantial amounts of water. In this case, a long time is required to drain the pipes and drainage accumulates at lower elevations along the lateral or mainline. Thus, a good deal of water drains, and wet areas can develop that are difficult to accommodate. An alternative is to use compressed air to force water out of the system. The end of the mainline and laterals can be equipped with a manual ball valve. The valve is shut when irrigating. The valve is opened when the pipeline is drained, then compressed air is supplied into the mainline. Water in the mainline can be expelled first. Then valves for the laterals are sequentially opened to force water from the laterals. Valves may be needed on some risers to prevent compressed air from bleeding through sprinklers on rolling terrain. Pipelines must also be installed deep enough so that farming operations do not either crush or damage the pipe while tilling. Any control or power cabling should be installed consistent with local codes.

Obviously, solid-set systems must be carefully designed and installed. An experienced designer should be employed for sophisticated systems. Intricate software programs are available to customize designs to local needs.