Page 232 - Buried Pipe Design

206 Chapter Four

or
P (D t)
(4.15)
2t
where P internal pressure, lb/in , and tensile strength, lb/in .
2
2
The hydrostatic design stress HDS, or in the equation, is the
hydrostatic design basis (HDB) times a service factor. HDB for PVC is
2
4000 lb/in for water-pipe compounds. The service factor is defined in
the appendix of ASTM D 2241 and recommended by the Plastic Pipe
Institute as equal to 0.5. (The inverse of the service factor is the safety
factor, in this case 2.) Thus, the long-term hydrostatic design stress for
PVC 1120 pressure-rated pipe meeting ASTM D 2241 is 2000 lb/in 2
(HDB 0.5 or HDB/2).
Equation (4.15) can be rearranged algebraically to reveal the term
SDR (standard dimension ratio) or

D average outside diameter

t minimum wall thickness

This term is widely used in the thermoplastic pipe industry.
Equation (4.15) can therefore be rearranged as follows:
D t D
2 P but SDR
t t
Therefore,

2 P (SDR 1)

The term standard dimension ratio refers to a preferred series of
numbers. Also note that the pressure rating of a given SDR is the same
no matter what the size; that is, 2-in and 12-in SDR 26 have the same
pressure rating.
In review, the four basic ideas that are important to the designer of
thermoplastic pressure pipe are as follows:

1. The hydrostatic design basis for a given PVC pipe extrusion com-
pound is established through long-term hydrostatic pressure test-
ing for pipe extruded from that compound.
2. The hydrostatic design stress is the stress in the pipe wall at which
plastic pipe will perform indefinitely.
3. The service factor (0.5) times (or the safety factor, 2 to 1, divided into)
the hydrostatic-design basis, equals the hydrostatic design stress.
4. Plastic pipe does not lose strength with time.