ARMAC DevNotes · pages: Overview · Design & Calculations · Throttle Pressure-Drop Tables · Build & Commissioning · Calibration
Givens
| Symbol | Meaning | Value |
|---|---|---|
| Q_main | Throttled delivery (sprinkler baseline) | 120 L/min = 7200 L/H = 2.000 L/s |
| Q_open | Delivery wide open (tap out of circuit) | ~220 L/min |
| Pump | 10 HP High-Flow 3" set | shut-off 530 kPa (5.3 bar); flat to ~400 L/min knee |
| P_high | Pump / high side at 120 L/min | ~5.1 bar |
| P_low | Sprinkler / low side at 120 L/min | ~1.5 bar |
| DeltaP_tap | Drop across the manual tap | ~3.6 bar |
| C | Target concentration | 0.014 (1.4%) |
| Q_side | Branch flow through injector | ~2000 L/H (needle-trimmed) |
| r | Dosatron dial | ~5% (range 1-10%) |
| Dosatron | Huiji DN009 | 50-2500 L/H, 0.2-6 bar, 1-10%, G3/4" |
1. Why full-flow injection is impossible
The injector passes at most 2500 L/H; the main is 7200 L/H -- 2.88x larger. So the whole main can never pass through it. Only a small branch is dosed and returned; the 120 L/min to the sprinklers is preserved.
2. Mass balance (governs everything)
Delivered concentration C = Q_gel / Q_total, where Q_total is the flow reaching the sprinklers during dosing. At the 120 L/min baseline:
Q_gel = C x Q_main = 0.014 x 7200 = 100.8 L/H = 1.68 L/min
During dosing the open branch adds flow, so Q_total rises to ~130-150 L/min and the true gel target is ~110-125 L/H. The exact figure is set by drum-drawdown calibration against the main flow sensor (see Build & Commissioning), not guessed.
3. Injector capacity -- one unit is ample
Q_gel,max = r_max x Q_side,max = 0.10 x 2500 = 250 L/H -> margin ~2-2.5x
4. Operating point
Run the branch at ~2000 L/H (needle-trimmed), dial ~5%:
r = Q_gel / Q_side ~= 100.8 / 2000 ~= 5%
Both sit mid-range in the 50-2500 L/H window and the 1-10% dial. Needle valve = flow trim; dial = concentration trim.
Sprinkler floor is structural. The branch is a parallel path around the tap, so opening it adds a second inflow to the low-side node, raising low-side pressure and therefore sprinkler flow (Q = K*sqrt(P)). During dosing:
Q_total in [120, 120 + Q_branch] ~= 130-150 L/min -> always >= 120
5. Pressure -- driven by the throttle (from the real curve)
The 3" curve is flat: shut-off 5.30 bar, h(120) ~= 5.1, h(220) ~= 4.9 bar. Anchor the sprinkler square law at wide open (sprinklers ~= pump head at 220 L/min):
P_low(120) = 4.9 x (120/220)^2 ~= 1.5 bar DeltaP_tap = 5.1 - 1.5 ~= 3.6 bar (~7x the injector's ~0.5 bar need)
Because the flow reduction is a field measurement, the full DeltaP and P_low lookup is tabulated over Qopen, Qdel = 100-300 L/min in Throttle Pressure-Drop Tables.
Pressure around the branch:
High tap ~5.1 bar - needle valve sheds ~3.0 bar (surplus) <- adjustable Dosatron inlet ~2.0 bar in [0.2, 6] OK (4 bar headroom) - Dosatron loss ~0.5 bar Dosatron outlet ~1.6 bar - check valve ~0.1 bar Low tap ~1.5 bar
The Dosatron inlet self-parks near 2 bar regardless of the exact DeltaP, so the design is insensitive to the precise throttle drop. The flat pump curve also means the branch's extra ~17-33 L/min draw barely shifts the operating point.
6. Pipe sizing
Branch ~= 2000 L/H = 33 L/min = 0.556 L/s -> 25 mm branch pipe (v ~= 1.13 m/s), reducing to G3/4" at the injector. The high-side leg must be rated for ~5.1 bar.