Calculating HDPE Pipe Pressure Rating

August 23, 2022

High-density polyethylene (HDPE) is a tough, durable plastic that can maintain its structural integrity under heavy loads. However, it’s important to make sure you’re using pipes that are properly rated for both the magnitude and the duration of the load it will need to tolerate. At WL Plastics, we produce pipes that are built to last in heavy-duty commercial and industrial applications. They comply with ASTM and AWWA standards, and each product reliably meets Hydrostatic Design Stress (HDS) ratings to handle internal pressure levels in environments of up to 140° F for long periods of work. For example, our PE4710 polyethylene pipe compounded can withstand internal pressures of 335 psi at 73° F per AWWA standards.

Let’s learn more about internal pressure ratings and choosing the right pipes for your project.

Internal Pressure Rating

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The internal pressure rating of a pipe is determined by the equation:

PR = [2HDS(fE)(fT)] / (DR-1)

This formula calculates the pressure rating by (1) determining the product of double the material’s hydrostatic design stress at 73°F, multiplied by the unique operating temperature multiplier and environmental design factor, and then (2) dividing it by the pipe’s dimension ratio minus one. The pipe’s dimension ratio is the ratio of the pipe’s outer diameter to the thickness of the thinnest point in the wall.

Once you know the pressure rating of a pipe, you can determine if it’s the right fit for the anticipated pressure conditions or if you need a pipe with different dimensions. While the material properties of HDPE remain consistent, manufacturers can increase the PR of a pipe by increasing the thickness of the pipe walls.

For example, a PE4710 pipe with a DR of 11 that was transporting 125° F brine water would have a PR of 140 psi. To find the individual temperature de-rating factors, see our WL118 Pressure Ratings document on our website. 

Liquid Flows

With liquids, it’s important to consider the motion of the fluid inside the pipe. Water hammer from velocity changes can create a pressure surge, and this can frequently occur in systems with pumps or system draws. 

Occasional pressure surges [P(OS)] are calculated at 2.00 x PR, and recurring pressure surges [P(RS)] are calculated at 1.50 x PR.

The liquid flow velocity (V) itself, which can change and cause pressure surges, is determined by this calculation:

V = (1.283Q) / ([pi]D[i]^2), where Q is the quantity of flow in gallons per minute and D is the average inside diameter of the pipe

The elastic modulus of the pipe material determines how well the pipe can withstand water hammer and other pressure surges. Manufacturers calculate the pressure wave velocity (a) as:

a = 4660 / (the square root of 1 + (kD[i]/Et), where k is the fluid bulk modulus in psi, D[i] is the average inside diameter of the pipe, and E is the instantaneous dynamic elastic modulus of the pipe material in psi, 150,000 psi for PE4710 HDPE pipe.

See our WL118 Pressure Ratings online resource for a more in-depth look at liquid flow.

External Pressure/Vacuum Resistance

If the pipe is exposed to external pressure (such as by being buried underground), by being in a vacuum, or both, this can flatten or crush the pipe. Properties of the pipe, such as its wall thickness, the material’s elastic modulus, and its roundness, will determine how much external pressure it can withstand. 

Manufacturers use this calculation to determine a pipe’s flattening resistance limit (P[CR]) in psi:

P[CR] = ((2EF[O])/(1-mu^2)) x (1/(DR-1))^3

Contact WL Plastics Today for Pipes You Can Rely On

Choosing the right dimensions and thickness for any PE4710 pipe is essential for guaranteeing long-term performance in field conditions. Fluid pressure, changes in velocity, and adverse environmental conditions each affect how pipes can handle loads. Contact our team at WL Plastics to find the right pipe for any project or read more about ascertaining the right pressure ratings for your PE4710 pipes.