Strength and Operating Pressures

Operating pressure is the pressure at which the pipe and system are normally expected to operate, and is used interchangeably with working pressure, pipe class pressure, and system pressure.

The formula for calculating operating pressure (OP) is: OP = [2S/(DR-1)]-Ps, where:

1. S (Hydrostatic Stress) = HDB (Hydrostatic Design Basis) divided by F (Safety Factor). There are two recognized safety factors that can be used for the calculation:
   1. For distribution lines, a factor of 2.5 is used. A distribution line's general definition is one that has multiple service branches or taps and is of a diameter up to 12". This is consistent with how AWWA C900 is defined and properties calculated. (Fig. 1)
   2. For transmission lines, a factor of 2 is used. Transmission lines have a diameter of 14" and greater and have few if any service demands.(Fig. 2)

   Fig. 1 - Pressure vs. Expanded Dimension Ratio Relationship (6"-12" CIOD Dimension)

   Fig. 2 - Pressure vs. Expanded Dimension Ratio Relationship (12" and larger CIOD Dimension)

2. DR (Dimension Ratio) = OD (Outside Diameter) divided by the Minimum Wall Thickness. Units of measure in both cases are inches, and the result has no unit attached to it.
3. Ps (Surge Pressure) = the added short term pressure increase caused by the sudden change in velocity in a system. This is commonly referred to as water hammer and can be caused by the opening and /or closing of a valve, pumps starting and stopping, or changes in flow demand, to name some examples. This phenomenon occurs on lines designated as distribution lines and is typically only applied to distribution lines. Surge Pressure has been arrived at through calculation and testing. Figure 3 is based upon data as represented in the Uni-Bell PVC handbook. It is expressed in psi and varies as a result of the DR. Surge pressure is usually based on a velocity change of 2-ft/ sec. Higher or lower changes can be calculated and applied if the service requires. As the DR increases, the Ps decreases. Because of the elastic properties of PVC, the ability to handle pressure surge is significantly greater than other materials.Fig. 3 - Surge Pressure in PVC Pipe.
   

   

   Fig. 3 - Surge Pressure in PVC Pipe

HDB (Hydrostatic Design Basis) is the main indicator of pressure performance capability. It is a laboratory defined value arrived at through multiple tests and mathematical projections. HDB is measured in pounds per square inch, and is the value for the circumferential hoop stress in PVC pipe.

The value is derived by pressurizing sections of PVC pipe with water under lab conditions, and testing the pipe to failure. Test pressure is varied so that failure occurs over a variety of time intervals. Results of these tests are plotted on a logarithmic scale, and a line is projected after a sufficient accumulation of data points to a time interval of 100,000 hours, which equates to long-term strength over 50 years.

The HDB for the Duraliner™ starting stock has been tested in the PVC industry numerous times, and is 4000 psi. This is the value of all C900/C905 AWWA PVC water pipe. AWWA C909 standard establishes a different and higher HDB for PVCO or MOPVC pipe (molecularly oriented).

A process developed in Europe in the 1970's expands PVC in the extrusion facility in two different ways:

1. By pulling the extruded pipe over a mandrel.
2. By expanding the 20' length of pipe in a water bath using heat and pressure.

The expansion is roughly 100% or 2:1. A 3" starting stock is expanded to 6" OD; a 4" is expanded to 8", and so on. A company called Uponor is the US manufacturer. Lab tests of this expanded product have yielded an HDB of 7100 psi.

The increase in HDB through expansion is attributed to the phenomenon of molecular reorientation. As the PVC expands, the molecules of PVC change their orientation from random and longitudinal to a circumferential mode. This realignment produces increased strength in the radial or circumferential direction. The circumference or "hoop" is the primary indicator of pressure capability.

The HDB for the Duraliner™ expanded product is presently being determined through testing at Hauser Laboratories in Boulder, Colorado. It is expected to be between 4000 psi and 7100 psi, most likely in the higher end of the range. Hauser is the nation's preeminent independent testing laboratory for PVC pipe. The testing of Duraliner™ indicates that during expansion, molecular reorientation occurs. Duraliner™ is typically expanded in the 10% to 50% range and not the 100% as in AWWA C909. Figure 1 shows the band of range of HDB values between the starting stock and fully-oriented C909. A surge pressure component using 2 ft/sec as the velocity change is included in this example. Fig. 2 is the same band representation for 12" and larger diameters.