Irrigation systems distribute water efficiently from a source to plants. The choice of piping material is a foundational decision that impacts the system’s longevity and efficiency. Different parts of an irrigation layout require pipes with varying properties, as demands on a high-pressure main line differ from those on a low-flow drip emitter line. Materials must be selected based on physical characteristics, ability to handle water pressure, and the specific application, such as underground conveyance or direct plant watering.
Primary Materials for Irrigation Systems
Modern irrigation systems primarily use plastic piping for main and lateral lines, dominated by Polyvinyl Chloride (PVC) and Polyethylene (PE). PVC is a rigid, strong material typically sold in straight lengths. Its stiffness makes it well-suited for direct burial applications where it must maintain a straight line and withstand soil pressure.
PVC sections are joined using specialized solvent cement and primer, which chemically welds the pieces together for a permanent, watertight seal. This rigidity and solvent-weld connection make PVC popular for constantly pressurized main lines requiring high structural stability. However, this lack of flexibility means fittings must be used whenever the line turns or changes elevation.
Polyethylene (PE), often called poly pipe or HDPE, offers high flexibility. It is sold in large, coiled rolls, allowing for fewer joints over long runs and easier installation around curves or uneven terrain. This flexibility provides greater resistance to freeze damage and shifting soil conditions compared to rigid PVC.
PE pipes are typically connected using barbed insert fittings or specialized compression fittings, which do not require glues or solvents. While it may have a lower maximum pressure tolerance than thick-walled PVC, modern HDPE is sufficient for most irrigation pressures. Its superior resistance to UV exposure makes it a better choice for above-ground applications.
Pipe Sizing and Pressure Ratings
Pipe diameter significantly affects system performance, as the internal size dictates the water volume carried and directly affects pressure loss due to friction. Friction between the water and pipe walls reduces pressure, and this loss is amplified in smaller diameter pipes. Selecting a pipe diameter too small for the required flow rate results in a substantial pressure drop, causing sprinklers to underperform.
The pipe’s pressure rating is determined by its wall thickness relative to its diameter, using two main systems: Schedule and Class/Standard Dimension Ratio (SDR). The Schedule system, common for rigid PVC like Schedule 40, indicates a fixed wall thickness across all pipe sizes. Because the wall thickness remains constant, the pressure rating of Schedule-rated pipe decreases as the pipe’s diameter increases.
The Class or SDR rating system is applied to thinner-walled PVC and polyethylene pipes and maintains a consistent pressure rating across all diameters. SDR is a ratio of the pipe’s outside diameter to its wall thickness; a lower SDR number signifies a thicker wall and a higher pressure rating. For example, a Class 200 pipe is rated for 200 pounds per square inch (psi) regardless of diameter, because the wall thickness adjusts to maintain that rating. Main lines, which are always under pressure, require a higher pressure rating, making Schedule 40 PVC or high-pressure SDR pipe common selections.
Specialized Tubing for Low-Flow Drip Systems
Micro-irrigation systems rely on flexible, small-diameter tubing, typically made from low-density polyethylene (LDPE). This tubing is designed to operate under low pressures, usually 20 to 40 psi, often requiring a pressure regulator to step down pressure from the main line. Common sizes include half-inch (1/2″) tubing, used as a lateral line, and quarter-inch (1/4″) micro-tubing, which connects the lateral line to individual emitters.
This flexible tubing delivers water directly to the plant root zone through small emission devices. LDPE is valued for its resistance to ultraviolet (UV) light degradation, achieved by incorporating carbon black, allowing the tubing to be laid above ground. Connections use small, barbed fittings pushed into the pipe, or specialized punch tools to create holes for drip emitters directly into the lateral line. The maximum flow rate for half-inch drip tubing is typically around 220 gallons per hour, which limits the number of emitters that can be placed on a single line.