When designing an HDPE irrigation system, some understanding of the characteristic of HDPE materials is helpful. Over the past forty years, ASTM in the United States and International Standards Organization (ISO) in Europe have developed relations for determining the pressure ratings for thermoplastic piping systems.

The ISO equation was developed to correlate the hoop stress time-to-failure for various plastics. The equation below is used for HDPE pipe to determine the correct wall thickness of the pipe for a given temperature.

S = p(OD-t)/2t where : S = hoop stress, psi

p = internal pressure, psi

t = minimum pipe wall thickness, inches

OD = outside pipe diameter, inches

Long-term strength

The long-term strength of plastic pipe are determined by capping both ends of a section of pipe and applying internal pressures that will cause failure. Stress-rupture testing is conducted in accordance with ASTM D 1598 requirements, Time to Failure of Plastic Pipe Under Constant Internal Pressure.  Data points are recorded by D 1598 testing are plotted on a log-log plot of stress versus time-to-failure.  When the data obtained forms a straight line, a best least-squares straight line is determined mathematically and extrapolated to the 100,000 hour intercept to forecast the long-term hydrostatic strength (LTHS).  This extrapolation procedure is outlined in ASTM D 2837, Obtaining Hydrostatic Design Basis for Thermoplastic Materials.  The hydrostatic design basis (HDB) provides the range of the materials LTHS in a defined stress category.

Service Factor

The Plastic Pipe Institute has established the Hydrostatic Stress Committee to determine the service factor for various uses of thermoplastic pipe.  A factor of one-half the Hydrostatic Design Basis has been established as an adequate safety factor for water applications.  This factor is based on an expected life of 50 years.

According to the standards for plastic pipe, equation for determining the pressure rating of HDPE pipe for a water applications is given in this equation:

P =(2S/DR-1) x F

Where: S = Long term Hydrostatic Design Basis, 1600 psi

DR = Dimension Ratio of diameter/wall thickness

P = Design Pressure

F = Design Factor, .5 for water

Example:

What is the pressure rating for DR 11 HDPE pipe for water service?

Given: S = 1,600 psi

DR = 11

F = 0.5

Solve for P

P = 2 x 1600/ ( 11-1) x .5

P = [(2*1600)/(11-1)}*.5

P = 160 psi

For high density polyethylene  pipe, the 160 psi rating is the working pressure (wp). The working pressure includes the ability of the HDPE pipe to handle up to 1.5 times the working pressure (240 psi) in surge on a continuing basis.

As the temperature changes, the pressure rating changes. The table below provides pressure ratings at various temperatures for PE 3408 HDPE pipe.