Polyvinyl chloride (PVC) pipe is the standard material for most residential and light commercial irrigation systems due to its durability, low cost, and ease of installation. Selecting the correct type is necessary for an efficient and long-lasting system. Choosing the right pipe involves understanding differences in wall thickness, how diameter affects water volume, and the appropriate pressure rating. This guide clarifies the technical specifications needed to select the ideal PVC pipe for your irrigation layout.
The Key Differences Between Schedule and Pressure-Rated PVC
The PVC pipe used in irrigation falls into two main categories: Schedule pipe (typically Schedule 40) and Pressure-Rated (or Class) pipe, defined by a Standard Dimension Ratio (SDR). Schedule pipe, governed by the ASTM D1785 standard, maintains a constant outside diameter (OD) and a constant wall thickness for a given Schedule number, regardless of the pipe’s size. Consequently, because the wall thickness remains constant, larger diameter Schedule pipes have a lower pressure rating than smaller ones.
Pressure-Rated pipe, conforming to the ASTM D2241 standard, uses the SDR to ensure a consistent pressure rating across all pipe sizes. The SDR is the ratio of the pipe’s outside diameter to its wall thickness. To maintain a constant pressure rating, the wall thickness of SDR pipe must increase proportionally with the diameter. This makes SDR pipe (e.g., SDR 26 or SDR 21) a practical choice for long mainlines requiring consistent pressure. Schedule 40 is often preferred for smaller, high-stress connections because its thicker walls offer better physical protection.
How Pipe Diameter Affects Water Flow
Pipe diameter is the primary factor determining the volume of water (measured in gallons per minute, or GPM) an irrigation system can efficiently deliver. PVC pipe is sized by its Nominal Pipe Size (NPS), which approximates the inner diameter, though the physical inside diameter dictates the flow capacity.
Friction loss is the resistance water encounters against the inner walls of the pipe. This loss increases exponentially if the flow rate is too high for the pipe size, causing a pressure drop that reduces sprinkler performance. A key design principle is maintaining water velocity below five feet per second to minimize friction loss. For example, a system requiring 12 GPM needs at least a 1-inch pipe, while 30 GPM necessitates a 1.5-inch pipe to avoid excessive pressure drops. Proper sizing ensures sprinklers operate at their intended pressure for uniform coverage.
Selecting the Correct Pressure Rating
Every pressure-rated PVC pipe has a stamped rating, measured in Pounds per Square Inch (PSI), which indicates the maximum sustained internal pressure the pipe can handle at a standard temperature of 73°F. The system’s working pressure should never exceed this rated pressure to ensure long-term reliability. A lower SDR number (e.g., SDR 21) indicates thicker walls relative to the diameter, resulting in a higher pressure rating than a pipe with a higher SDR number (e.g., SDR 26).
Irrigation systems must account for water hammer, a sudden pressure surge that occurs when a valve closes quickly. This transient pressure can momentarily exceed the system’s static pressure, necessitating a safety margin. For most residential systems, a minimum pressure rating of 160 PSI is adequate to handle typical operating pressures and account for these surges. Note that PVC’s pressure rating decreases as the water temperature increases above 73°F.
Choosing Pipe Based on Installation Method
The installation environment significantly influences the choice between Schedule and SDR pipe, particularly for mainlines and lateral lines. Buried mainlines, which are constantly under pressure, commonly use Schedule 40 or Class 200 (SDR 21) pipe. These thicker-walled pipes offer better resistance to external soil pressure and impact damage due to their strength and consistent pressure handling.
Lateral lines run from the valve to the sprinkler heads and are only pressurized when the zone is running. They may use thinner-walled pipe if pressure demands are lower, though flexibility and ease of routing are also considerations. For any pipe installed above ground, such as risers or exposed sections, a thicker-walled option like Schedule 40 is recommended. Thin-walled Class pipe degrades quickly when exposed to UV radiation, becoming brittle and prone to failure. The final decision integrates flow rate (to determine diameter), system pressure (to define PSI rating), and installation location (to select Schedule or SDR type).