APPENDIX A. PRESSURE CLASS SELECTION PER AWWA C906

Selecting the right pressure class for high density polyethylene pipe (HDPE) in accordance with AWWA C906 can be done in two easy steps. AWWA C906 takes into account the continuous pumping and transient (surge) pressures that occur in municipal water pipes.
  
 Step 1. Compare the pipeline working pressure with the pipe's pressure class.
AWWA C906 defines working pressure as "the maximum anticipated, sustained operating pressure applied to the pipe exclusive of transient pressures." The maximum working pressure for a pipe must be less than or equal to the pipe's pressure class. Table A-1 gives pressure class for standard dimension ratio's (DR) HDPE pipe made from PE3608 material.

 

TABLE A-1: MAXIMUM ALLOWABLE PRESSURE FOR HDPE PIPE (PE3608) AT 80°F1 (per AWWA C906)  
 Pipe
DR
Pressure
 Class/Maximum
Working Pressure
(psi)
Maximum Total
Pressure2 Allowed
 During Recurring
Surge (psi)
Maximum Total
Pressure2 Allowed
During Occasional
Surge (psi)
Maximum
Test Pressure
Allowed per AWWA
Manual M55 (psi)
 7.3
 254
380
510
380
 9
 200
 300
 400
 300
 11
 160
 240
 320
 240
 13.5
 128
 185
 250
 185
 17
 100
 150
 200
 150
 21
 80
 120
 160
 120
1 Average annual water temperature above 80°F require derating. See Table 4.
2 Total pressure equals the combined pumping (working) pressure plus surge pressure. Recurring or frequently occurring surges are inherent to the design and operation of the system. Occasional surges are caused by emergency operations such as fire flows.
 
 
Step 2. Compare the peak pipeline pressure during surge with the pipe’s allowable maximum total pressure.
Peak pressure during a surge is equal to the sum of the pumping pressure and the transient surge pressure. Transient surge pressure depends on the instantaneous change in flow velocity. Maximum transient pressure due to the change is given in Table A-2. Peak pressure may be obtained by adding the surge pressure at the design velocity from Table A-2 to the pumping pressure. Peak pressure is compared with the maximum total pressure allowed during surge found in Table A-1. The maximum total pressure allowed equals 1.5 times the pipe’s pressure class for recurring surge and 2.0 times the pipe’s pressure class for occasional surge.
Note: The surge pressure occurring in HDPE pipe is significantly lower than surge pressures occurring in cast or ductile iron pipe and is lower than that in PVC pipe of the same DR. For example, a 4 fps instantaneous velocity change in HDPE DR17 pipe results in a 45.0 psi surge whereas for DI pipe the surge is 200 psi and for PVC DR18 pipe the surge is 69.6 psi. When HDPE pipe is connected to DI pipe the surge pressure is dampened by the HDPE pipe.
 
 
Table A-2. Surge Pressure at 80°F for Sudden Velocity Change, psi (Per AWWA M-55)
Pipe ​Surge Pressure, psi ​ ​ ​ ​ ​ ​ ​
DR
1 fps
2 fps
3 fps
4 fps
5 fps
6 fps
7 fps
8 fps
 7.3
18.4
36.8
 55.2
 73.6
 92.0
 110.4
 128.8
 147.2
 9
 16.2
 32.4
 48.5
 64.7
 80.9
 97.1
 113.2
 129.4
 11
 14.4
 28.7
 43.1
 57.5
 71.9
 86.2
 100.6
 115.0
 13.5
 12.8
 25.6
 38.4
 51.2
 63.9
 76.7
 89.5
 102.3
 17
 11.3
 22.5
 33.8
 45.0
 56.3
 67.5
 78.8
 90.0
 21
 10.0
 20.1
 30.1
 40.1
 50.2
 60.2
 70.2
 80.3
 
  
 Working Pressure and Surge Pressure Example: 
An engineer is designing a water system that operates at 85 psi and has some runs in it where the flow velocity is 4 fps. In addition, his/her state requires a 150 psi test for the pipeline. What DR pipe does the engineer use?
 
Step 1. Compare the pumping pressure (85 psi) with the available pressure classes in Table A-1. DR17 has a PC of 100 psi>85 psi. The test pressure of DR17 is also 150 psi, which meets the specified test pressure.
 
Step 2. The anticipated peak pressure in the pipeline is found by adding the pumping pressure of 85 psi to the surge pressure of 45.0 psi (given in Table A-2 for a 4 fps velocity). The sum equals 130.2 psi and is less than the maximum total pressure allowed for recurring surge for DR17 pipe of 150 psi. DR17 pipe is ok. A similar comparison can be made for peak pressure during fire flow where velocity may reach 8 fps. In this case add 90.0 psi (from Table A-2) to 85 psi to obtain a peak pressure during occasional surge of 175 psi. Compare with the maximum total pressure allowed for occasional surge for DR17 of 200 psi. DR17 pipe is ok.