What is Pipe SDR

What is pipe SDR, and what kind of information can you glean from this parameter for high-density polyethylene (HDPE) pipe? This article explores the core concepts of SDR and its real-world HDPE piping applications.

What is the Difference Between SDR and DR? 

Dimensional Ratio (DR) is a critical parameter in pipe design and engineering, particularly in determining the pipe’s pressure class. DR, by definition, is the ratio of the nominal outside diameter (OD) to the nominal wall thickness of a pipe for select ratios. Understanding this ratio is essential for ensuring a piping system’s integrity, safety, and performance across various industries. As the diameters change, the pressure rating remains unchanged for the same material, dimension ratio, and application. 

On the other hand, the Standard Dimensional Ratio (SDR) reflects specified ASTM International number series. SDRs include 9, 11, 13.5, 17, 21, 26, and 32.5 and have an approximate 20% difference in minimum wall thickness. These numbers correspond to specific pressure ratings and are approximately a 20% increase from one to the next. One advantage of this uniform step-up is that it accounts for the derating factors associated with some HDPE fabricated segmented pipe fittings. This derating factor is typically 0.8, meaning that derated fittings must be made with a pipe wall one SDR thicker for the overall pipe system to maintain the same pressure rating.

What is Pipe SDR: Core Concepts

SDR provides a straightforward method for evaluating the design capabilities of a pipe.

This mathematical relationship allows engineers and designers to quickly understand a pipe’s strength, flexibility, and pressure capacity (1). Essentially, the SDR number indicates the pipe’s ability to withstand internal pressures; higher SDR values mean thinner walls and lower pressure ratings, while lower SDR values mean thicker walls and higher pressure resistance (2). 

One of the primary benefits of using SDR is its ability to simplify calculations related to pressure and cross-sectional area. SDR ensures that pipes are designed with consistent wall thicknesses in relation to their diameter, allowing engineers to accurately predict performance under various conditions (3, 4). This parameter becomes even more critical when designing systems requiring consistent safety margins across different pipe sections (5). In practice, if the SDR remains the same, the pressure rating of a pipe will not change, regardless of its diameter (3). This feature simplifies the design process by enabling a single SDR across an entire piping system, leading to more straightforward calculations and reduced design complexity. 

SDR and Real-World Applications

SDR plays a vital role in many industries, particularly those that rely on high-pressure piping systems. For instance, in the water and gas distribution sectors, SDR is commonly used to classify the strength and durability of polyethylene pipes (6). The pipes’ ability to withstand internal pressure is vital in such applications, and understanding the SDR of a pipe helps ensure that the correct material is used for the appropriate pressure class (7).

During the pipe’s installation, whether by traditional trenching or trenchless methods like pipe bursting, SDR helps engineers select the correct pipe that can endure installation stress while meeting the required performance standards. For example, in water main installations, an SDR of 21 may be chosen for pipes that need to withstand moderate pressures, depending on the scenario. However, an SDR of 17 or even 11 may be used for areas where higher pressures are encountered. SDR determines the pipe’s capacity to handle installation forces and operational pressures (8). The difference in SDR values results in varying wall thicknesses, affecting the pipe’s ability to sustain pressure loads (9). Additionally, SDR values help to calculate the internal pressure capacity, tensile strength, and compressive strength of pipes, particularly during burial and trenching operations.

Another feature of pipe SDR is that the HDPE piping industry has set standards corresponding to a 20% stepwise increase in pressure for each decrease in SDR (see Table 1). This feature simplifies pipe design, allowing engineers to use standard derating factors when necessary and quickly identify the required SDR to match the system’s overall pressure rating. 

SDR	SDR Difference (%)	Pressure Class (psi)	Pressure Difference (%)
41	79	50	79
32.5	80	63	79
26	81	80	80
21	81	100	80
17	79	125	78
13.5	81	160	80
11	82	200	80
9		250

As previously stated, the SDR must be adjusted to account for derating factors in a system that uses a fabricated segmented elbow; for example, typically around 0.8. Therefore, a pipe system designed with an SDR of 21 might require an SDR of 17 pipes for the elbows to maintain the same system pressure (see Table 2). This ensures that the system, including its junctions and bends, can withstand the intended operational pressures without failure. 

SDR is a convenient method for calculating the cross-sectional area of HDPE pipe using the following equation. The cross-sectional area is necessary for determining welding pressure, allowable pulling force on the pipe, and other factors, such as pipe weight per foot.

Cross-Sectional Area = π * D² * [ 1/SDR – 1/SDR² ]

SDR and Pushing the Limits of Design

SDR enables engineers to expand the boundaries of piping system designs by balancing wall thickness and diameter for optimal performance under various pressures. By adjusting SDR, manufacturers can create pipes that meet high-pressure requirements while ensuring durability and resistance to cracking. For example, advanced manufacturing techniques and materials allow for the production of larger HDPE pipes at lower SDRs—providing engineers with a broader range of options.

AGRU America has adopted SDR as a key parameter to demonstrate its pipe production capabilities. The company can manufacture HDPE pipes in nearly any DR within the range of possible sizes to meet specific project requirements. Its innovations in materials and manufacturing enable it to produce large-diameter HDPE pressure pipes that withstand extreme pressures while maintaining optimal flow efficiency at a wide SDR range and even support DIPS sizing.

Summary

Understanding the SDR is essential for anyone designing, manufacturing, and installing piping systems. SDR provides a clear and consistent method for evaluating a pipe’s pressure capacity based on its outside diameter and wall thickness, making it an essential industry parameter. As discussed in this article, SDR simplifies the design process by maintaining a consistent relationship between wall thickness and diameter across various pressure classes. Furthermore, innovations in materials and manufacturing techniques continue to push the boundaries of what can be achieved with SDR, allowing companies like AGRU to produce improved pipes for various applications.