ABSTRACT
A plastic piping system can be viewed as a dynamic balance between the complex forces exerted on a fluid and the vessel that holds it together. The “vessel” needs to be well thought out in advance to insure a successful installation. The vessel of course is the piping system, which can both affect the fluid being transported and be affected by the media. Besides chemical resistance, there are other factors such as: Relationship between pressure and temperature Shock pressurePressure rating, both internal and collapse Pressure loss through pipes, fittings, and valves Dimensions, sizes, weightsCreep, impact resistance, and other physical propertiesAmbient conditionsRow control through valves A brief word about differences in U.S. and European standards for pipe dimensions is in order. In the United States the adoption schedule 40, 80, 120, etc. outer diameter (OD) and wall thickness of steel pipes resulted in pipe of different sizes having different pressure ratings. It has also resulted in different pressure ratings for the same diameter pipe of different materials. A similar philosophy was followed with the introduction of SDR (standard dimensional ratio) pipe sizes. The “OD” was fixed, as was the thickness. The pressure rating for all dimensions of the same material is constant, but varies for each plastic material used.In Europe under the auspices of ISO (International Organization for Standardization) a different approach was followed. ISO 161 was developed to fix the ODs of plastic pipe and working pressure (vs. wall thickness). The nominal pressures designated were in kilograms force per square centimeter, that is,
The preferred series was: 1 kgf/cm2 = 14.22 psi
2.5 kgf/cm2 = 35.5 psi 4 kgf/cm2 = 56.90 psi 6 kgf/cm2 = 85.30 psi
10 kgf/cm2 = 142.20 psi 16 kgf/cm2 = 227.50 psi
At a later date with the introduction of the SI system into ISO standards the bar was adopted for use instead of the kgf/cm2. A bar equals 14.5 psi. Since the difference was slight, the bar has been essentially substituted for kgf/cm2 for general use with no adjustment for the difference.Subsequently ISO 4065-1978 (E) was developed, which proposed a preferred pipe Series S where wall thicknesses are designated. The value S when compared with the U.S. SDR system is: SDR = 2S + 1
Pressure rating in both the United States and Europe is derived using the ISO formula, which is derived from the Barlow formula (P = 2St/D):
whereS = hydrostatic design stress (psi)P = pressure rating (psi)D = pipe outside diameter (in.) t = minimal wall thickness (in.)D/t = R (or SDR)
Circumferential stress is the largest stress present in any pressurized piping system. It is this factor that determines the pressure that a section of pipe can withstand. The relationship of stress, pressure, and pipe dimensions is described by the ISO equation already shown. B. Long-Term Stress
To determine the long-term strength of thermoplastic pipe, lengths of pipe are capped at both ends (Fig. 1) and subjected to various internal pressures, to produce circumferential stresses that will produce failure in from 10 to 10,000 h. The test is run according to ASTM D 1598, “Standard Test for Time to Failure of Plastic Pipe Under Long Term Hydrostatic Pressure.”The resulting failure points are used in a statistical analysis (outlined in ASTM D 2837) to determine the characteristics regression curve that represents the stress/time-to-failure relationship for the particular thermoplastic pipe compound. The curve is represented by the equation: log T = a + b log S
where a and b are constants describing the slope and intercept of the curve, and T and S are time-to-failure and stress, respectively.The regression curve may be plotted on log-log paper as shown in Fig. 2 and extrapolated from 10,000 to 100,000 h (11.4 yr). The stress at 100,000 h is known as the hydrostatic design basis (HDB) for that particular thermoplastic
compound. From this HDB, the hydrostatic design stress (HDS) is determined by applying the service factor multiplier. C. Service Factor
The Hydrostatic Stress Committee of the Plastic Pipe Institute (PPI) has determined that a service (design) factor of one-half the hydrostatic design basis would provide an adequate safety margin for use with water to ensure useful plastic pipe service for a long period if time. While not stated in the standard, it is generally understood that “long period of time” is a minimum of 50 yr [1].Some engineers in the United States have felt that a safety factor of 2.5 would be better accommodate surge pressures found in water systems and have accordingly used a service factor of 0.4 instead of 0.5. D. ANSI vs. ISO
The outside dimensions are fixed in both the United States (ANSI) and Europe (ISO). Wall thickness is fixed in the United States, while pressure rating is fixed in Europe. Table 1 gives a comparison of actual ODs for PVC pipe sizes from l/fc in. to 12 in.Table 2 gives a comparison showing pressure ratings OD, wall thickness, and weight per foot in pounds for selected sizes and systems.
