Friction Loss Calculations For Rigid PVC Drip Laterals
If the lateral is composed of a single pipe size, then the friction loss can be calculated as if the maximum flow went the entire length with the answer multiplied by a factor determined by the number of outlets from the lateral (Table 1).



























Example for a single size lateral loss:

For a lateral 300' long, 1/2CL 315 PVC pipe with 30 outlets and each outlet discharging 6 g.p.h. (no elevation changes).
30 outlets x 6 g.p.h. = 180 g.p.h.
loss per 100 feet of pipe @ 180 g.p.h. flow= 1.65 p.s.i.
1.65 p.s.i. x 3 = 4.95 p.s.i. loss for the 300' lateral
The factor for 30 outlets is .350
4.95p.s.i X .350=1.73 p.s.i loss in the lateral
If the emitter at each outlet requires 20 p.s.i. to discharge the required 6 gallons per hour (and there are no elevation pressure gains or losses to consider), the pressure setting would be for 21.73 p.s.i. at the entrance to the lateral. Probably a 25 p.s.i. fix regulator would be used.

Now let's look at a mix and match example for lateral calculations. (Remember, all these emitters would have to meet the water requirements for the various plants in the same amount of irrigation time and operate at the same pressure and watering frequency.)

  
  
  
  



















CL 315 PVC PIPE ,300' length
Number of outlets on tube=23
Total g.p.h. flow =
11 6-g.p.h. emitters = 66 g.p.h.
2 .5-g.p.h. emitters = 1 g.p.h.
10 2-g.p.h. emitters = 20 g.p.h.
                                 87 g.p.h.
In place of 87 g.p.h., we would use 90 from the table which shows a loss of .46 p.s.i./100'.
.46 x 3 = 1.38 p.s.i. loss for 300' lateral
In place of 23, the 20 outlet factor of .354 would be used from Table 1.
1.38 p.s.i. x .354 = .488 or only about 1/2p.s.i. loss along the entire lateral. If the lateral was underground with 12", risers up to the surface for the emitters, an additional loss because of elevation would need to be considered.
.488 friction loss
.433 elevation loss
20.000 p.s.i. need for emitter
20.921 or 21 p.s.i. needed at tube entrance



Number of outlets
Factor
Number of outlets
Factor
Number of outlets
Factor
                 1
           1
               5
         .380
        30     
       .350
                 2
       .512
               10
            .365
           40
         .349
                  3
         . 434
               15
         .357
              50
         .348
                 4
        .405
               20
            .350
          100
           .347






Table 1
Flows in  GPH
  Loss in PSI
Flows in  GPH
  Loss in PSI
Flows in  GPH
  Loss in PSI
              15
          .017
          70
             .29
         200
             2
              20
          .028
           80
             .37
         220
             2.39
              25
           .043
            90
               .46
          240
              2.82
             30
            .06
            100
              .56
          260
               3.26
Table II
Friction loss chart for 1/2" class 315 pipe (loss in PSI per 100' of pipe at flows shown)
               35
            .08
               120
            .78
             280
           3.73
               40
            .10
                130
            ..90
              300
            4.26
               45
             .13
               150
             1.18
              320
             4.78
              50
             .15
              170
             1.48
              340
             5.34
              60              
             .22
              180
              1.65
              360
              5.97
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