Step 1: Determine Your System's Design Pressure
The foundation of your choice is the Maximum Operating Pressure (MOP) your system requires. This is not just the pump's output pressure; it's the continuous internal pressure the pipe must withstand, often including allowances for surge (pressure spikes) and a safety margin .
Key Question: What is the maximum internal pressure, in bars or psi, that the pipe will face during normal operation?
Step 2: Know Your HDPE Material Grade (PE Grade)
HDPE pipes are made from different material grades, with PE100 being the standard for modern water supply systems . The material grade defines the pipe's long-term strength, known as the Minimum Required Strength (MRS) .
PE100: MRS = 10 MPa (Megapascals). This is the most common and recommended grade for potable water due to its high strength and economy .
PE80: MRS = 8 MPa. An older, lower-strength grade that requires a thicker wall (lower SDR) for the same pressure .
PE4710: The North American equivalent of PE100, often with slightly higher design stress ratings .
For a new project, selecting PE100 is the standard practice.
Step 3: Apply the Formula or Use a Reference Table
The relationship between SDR, material grade, and pressure rating is defined by a standard engineering formula derived from the hoop stress in the pipe wall .
Where:
MOP = Maximum Operating Pressure (in bars)
MRS = Minimum Required Strength (10 MPa for PE100)
C = Overall service (design) coefficient (a safety factor, typically 1.25 for water)
SDR = Standard Dimension Ratio
This formula has been used to create standard tables that directly show the pressure rating for common SDR values. This is the most practical tool for selection.
How to use the table:
Calculate your required pressure rating (e.g., 10 bar).
Find the SDR value that meets or exceeds this pressure. For 10 bar, you would select SDR 17 (rated for 10 bar) .
It is always good practice to select an SDR with a slightly higher rating to provide a safety buffer for pressure surges.
Step 4: Consider Project-Specific Factors
While the pressure rating is the primary driver, other factors can influence your final SDR choice.
Installation Method: For trenchless methods like horizontal directional drilling (HDD), a pipe with a thicker wall (lower SDR, e.g., SDR 11 or 17) is often preferred to withstand the installation stresses .
External Loads: For deep burials or installations under roads, a lower SDR (thicker wall) provides greater ring stiffness to resist external soil and traffic loads without collapsing .
Temperature: The pressure ratings in the table are valid for water at 20°C. For higher temperatures (up to a max of 40-45°C), the pressure capacity is reduced (derated), and you may need a lower SDR to compensate .
Future-Proofing: Some engineers may opt for a thicker wall (e.g., SDR 11 instead of SDR 17) to allow for potential future system upgrades or to provide an extra margin of safety in critical areas
