Page 246 - Buried Pipe Design

220 Chapter Four

Example Problem 4.2 Select a PVC pipe to meet the following design conditions:

Pipe diameter 8 in
Woking pressure (WP) 160 psi
Design life 50 years
Sustained operating temperature 60 F
Recurring surge:
Anticipated cycles per day 55
Factor of safety on number of cycles 2.0
Anticipated recurring surge pressure (P RS ) 30 psi
Occasional surge
Anticipated occasional instantaneous change in flow velocity 7 fps
Pipe dimension ratio: Try DR 18, PC 235 psi (see Table 1, AWWA C900).

Analysis:
Step 1: Check temperature considerations
Operating temperature does not exceed 73 F
From Table 4.6, the percentage of allowable pressure class is 100%
or F T 1.0
Therefore, pressure-rating reduction is not necessary
Step 2: Check working pressure versus pressure class
Working pressure: 160 psi
Allowable pressure PC F T 235 psi > 160 psi working pressure
Result: DR 18 OK for working pressure

Step 3: Check occasional surge pressure capacity, WP P OS
Anticipated occasional surge pressure, P OS
From Table 4.2, wave speed for PVC pipe a 1311 ft/s
Use Eq. (4.7) to obtain P OS
a 1311
P os 0.43 V 0.43 7 122 psi
g
32.17
Anticipated occasional surge pressure
WP P OS 160 psi 122 psi 282 psi
According to AWWA C900, the short-term pressure rating is 1.60
larger or
WP P OS < 1.60 PC F T
Allowable occasional surge pressure capacity
1.60 PC F T 1.60 235 psi 1 376 psi
Checking allowable versus anticipated
376 psi > 282 psi anticipated

Step 4: Check recurring surge pressure capacity, WP P RS
Design pressures:
Maximum design pressure
P max WP P RS 160 psi 30 psi 190 psi