Calculating Friction Loss In Branching PVC Laterals
Friction loss per pipe section along critical path
Critical path (longest path for water to travel in the circuit) =Pipe section A, B, C, D & E.
Totaling friction losses backward from E to the control head
Section E = 12 G.P.H. along 30' = .0 1 7 x .3 = .0051 PSI
Section D = 36 G.P.H. along 40' = .08 x .4 = .032 PSI
Section C = 36 G.P.H. along 40' = .032 PSI
Section B = 56 G.P.H. along 60' = .22 x .6 = .132 PSI
Section A = 88 G.P.H. along 60' = .46 x .6 = .276 PSI
Total pipe loss= .4771 PSI
+25% for fitting loss=.1192 PSI
=.5963 PSI
Conservative Short Cut
All lateral pipe = 330'
90 G.P.H. = a loss of .46 per 100 feet
.46 x 3.3 (for 330') = 1.518 PSI x .354 (outlet factor for 20) = .5373
+ 25% for fitting= .1343
=.6716 PSI
The actual losses in each of these examples is somewhat lower than calculated using the various charts and tables. In most cases these quick reference charts will tell you whether you need a regulator with a setting above the range of the economically priced fixed pressure regulators.
If you have chosen emitters that require 20 -p.s.i. for the flow you desire,and the lateral loss is calculated at less than 5 p.s.i., you would simply choose between a 20 p.s.i. or 25 p.s.i. outlet pressure regulator. For losses under 1 p.s.i. use the lower outlet pressure regulator (20 PSI Model).
Complete these two circuiting example exercises and compare your calculations to the
answer page that follows the problems.
1. What is the pressure required to overcome flow losses in the following circuit and provide 20 psi to the emitters?
Circuit length 400' .
Emitter flow rate 2 gph each
Number of Emitters 100
Lateral material ½" CL 315 PVC
Total flow of circuit =_____________________gph
Total flow on the chart produces____________________ psi loss per 100' of pipe. This loss times ___________ gives me the loss for the total circuit length.
The number of outlets is _____________multiplier on Table I of______________
Total loss times this multiplier gives the circuit actual circuit loss of___________psi
The actual circuit loss of__________ plus the emitter pressure requirement of _______psi create a pressure requirement of _________psi at the pipe entrance.
2. Circuit data:12mm tubing, emitters are 6 gph multi-outlet emitters on 10' spacing.
The critical circuit length for the circuit is made up of the lengths of what lines? (List the letter designation of these lines from the P.O.C. out to the last line on the critical path
1.___________
If we take the total flow of the circuit out to the end of the first line you listed we would calculate a total circuit flow of
2.________ gph over a tube length of
3._________feet.
What flow rate would you use(conservatively)on 12mm tubing table ?
4._______ gph
What is the loss for that first line of tubing at full circuit flow?
5.__________
What is the flow through the next line in your critical path?
6.______ gph.
Even though this line is less than 100' long, use the 100' chart and the gph line above the actual listed in answer.
What is the loss for that line?
7. __________ psi.
The last line in the critical path has a flow of 30 gph. Using the 100' lateral length chart again and the 35 gph line, what loss do we calculate?
8. _________ psi.
Totaling our loss calculations for each of the critical path lines , what is our circuit loss?_________psi
Answers To Circuiting Exercises
1. What is the pressure required to overcome flow losses in the following
circuit and provide 20 psi to the emitters?
Circuit length 400'
Emitter flow rate 2 gph each
Number of Emitters 100
Lateral Material ½" CL .315 PVC
Total flow of circuit = 200 gph
Total flow on the chart produces 2.0 psi loss per 100' of pipe. This loss times 4 gives me the loss for the total circuit length.
The number of outlets is 100 which has a multiplier on table I of .347.
Total loss time this multiplier gives the circuit actual loss of 2.776psi
The actual circuit loss of 2.776 plus the emitter pressure requirement of 20 psi create a pressure requirement of 22.776 psi at the pipe entrance.
2. Answers
1. A, C, E.
2. 138
3. 100
4. 145
5. 1.607
6. 48 gph
7. .309
8. .142
9. 1.607 + .309 + .142 =
2.058 psi
