The designer knows he has an emitter spaced every twenty feet along the tube run and his total loss reads 2.164 p.s.i. in the right hand column on the chart.He knows his lateral needs 2.164 p.s.i. to overcome tube flow and barb losses, plus whatever design or catalog pressure is required for the emitter. If the emitter needed 20 p.s.i. to discharge 6 g.p.h., the entrance to the lateral requires:
20.0 p.s.i. for the emitter
+2.164 p.s.i. to overcome flow and barb loss
22.164 p.s.i. Total
If the site was not level and the line of trees rose 5 feet along its run, the elevation losses would also need to be taken into account.
5 feet elevation rise x .433 p.s.i. per foot of elevation change
2.165 p.s.i. loss to elevation rise
Now the calculation would be
20 p.s.i. for emitter
+ 2.164 p.s.i. to overcome flow and barb loss
+ 2.165 p.s.i. to overcome elevation rise
24.329 p.s.i. total required at tube entrance
If the trees were on a slight downward slope and the last tree was five feet lower than the source, the 2.165 p.s.i. would be a gain instead of a loss. In that case the elevation gain would about equal and cancel out the flow loss. Twenty p.s.i. would be the total needed to supply the tube and emitters.
Before going on to our next step in the design procedure, let's look at some more circuit calculation methods.In both emitter connection diagrams, number two and three from the previous example, there are three outlets off the main tube for each tree.In this type of spacing situation where you have a run of tubing, several emitters in one place, then more tubing to the next group, you can use an average for spacing calculations.
In connection diagram number two, your averaging method for spacing would be:
15 trees x 3 emitters = 45 total emitters
Total tube length = 300 feet
300' lateral = 6.6 foot average spacing
Again, the right hand chart on the table is used. The 6.6 foot spacing falls between the 51 and 10, spacing columns. Using again the 95 g.p.h. line, we would use the more conservative higher loss column for 5 foot spacing. The resulting loss is listed as 2.193 p.s.i. for the full length of the lateral.
Emitter connection example number three would be calculated in the same fashion. However, the additional length of tubing required for each loop around a tree could well add up to another 150 feet of lateral tubing and be beyond our 300' limit. In that situation, one of the other methods would be used for emitter connections.
The method for calculating circuit friction loss in rigid PVC pipe is somewhat different from the one used for .580", tubing. For one thing, emitters mounted on PVC pipe fittings have threaded connections and no barbed inlets are partially blocking the tube.
The rigid pipe friction loss calculation for drip circuits is for laterals made of Class 315 PVC.