Page 124 - Pipeline Rules of Thumb Handbook

Pipe Design 111

Polypipe design data

Internal pressure

Polyethylene pipe for industrial, municipal, and mining SDR = Standard dimension ratio, (D/t)
applications is manufactured to specific dimensions as pre- D = Outside diameter, inches
scribed by applicable American Society for Testing and t = Minimum wall thickness, inches
Materials (ASTM) standards. Piping outside diameters meet F = Design safety factor (0.5 for water @ 73°F)
the IPS system. Wall thicknesses are based on the Standard
Dimension Ratio (SDR) system, a specific ratio of the outside Use of other factors determined from Figures 1, 2 and 3
diameter to the minimum specified wall thickness. Selected will provide for a more accurate performance characteristic
SDR numbers are from the ANSI Preferred Number Series. for systems operating at other temperatures for shorter
Use of the SDR number in the ISO equation, recognized as periods of time.
an equation depicting the relationship of pipe dimensions’— The ISO equation now becomes:
both wall and o.d.—internal pressure carrying capabilities and
tensile stress, in conjunction with a suitable safety factor (F),
will give the design engineer confidence that the pipe will not P = S 2 ¥ F ¥ F ¥ F ¥ F 3
1
2
fail prematurely due to internal pressurization. SDR -1
A pressure differential is required to move a material
through a pipeline. For atmospheric systems (gravity flow), F 1 = Operational life factor
gravitational forces provide the impetus for movement of F 2 = Temperature correction
heavier-than-air mass. Internal pressure requirements must F 3 = Environmental service factor (Figure 3)
be recognized in the design stage in order to provide desired
operational life. In some cases a gravity flow system must be Other design factors may be necessary in the design of
treated comparable to the design considerations of a pressure systems where abrasion, corrosion or chemical attack may be
flow system. a factor.
Polyethylene pipe under pressure is time/temperature/
pressure dependent. As the temperature increases and the Example: A horizontal process water line, 2,000ft. in length,
pressure remains constant the creep rupture strength 125°F water, 60psig pressure, with a desired life of five
decreases. Conversely, as the temperature decreases the years, is proposed. Determine the standard pipe SDR for the
strength increases. Varying these parameters, in conjunction application.
with the SDR, can provide the desired service within the
capabilities of polyethylene pipe.
¥
Calculations for determining the pressure rating of Poly 60 = 2 1600 ¥ 05 109 ¥ 0 620
¥
.
.
.
pipe is based on the ISO equation as follows: SDR - 1
S 2
P = ¥ F 1081
1
SDR -1 SDR -=
60
where
P = Internal pressure, psi SDR = 18 + 1 = 19
S = Long term hydrostatic strength, psi (1,600psi for Therefore use SDR = 17.
Poly pipe) Note: Using SDR 17 is conservative.

Figure 1
Internal Pressure Ratings (psi) (PE Pipe at 73°F for 50 years).
SDR 7.3 9 11 13.5 15.5 17 21 26 32.5
Pressure Rating 250 200 160 125 110 100 80 64 50