Tail-end pressure loss in a silt-laden pomegranate drip system
Farm: 1.5 ha of 4-year-old pomegranate at 3 m × 3 m in Solapur district, one 16 mm lateral per row with non-pressure-compensating 4 lph drippers at 50 cm. Source: a 5 HP submersible in a borewell. A February pump test gave 9 m³/h at 32 m head; in May the well yields only about 3 m³/h, the pump starts snoring and sometimes trips. System: 20 m³/h sand filter, 120-mesh screen filter, venturi, 63 mm submain and four valves of about 0.375 ha each. The design requirement at the filter outlet is 13 m³/h at 2.5 kg/cm² with all valves open, but the farmer normally runs one valve at a time. Readings at 08:00: pump discharge 2.3 kg/cm²; before sand filter 2.3; after sand filter 1.5; after screen 1.3; tail-end lateral 0.7. The filter is backwashed every 6 hours and the backwash water stays muddy for the first 3 minutes. Tail-end emitters deliver about 2.6 lph instead of 4 lph. The farmer injects 19:19:19 through the venturi upstream of the sand filter and then backwashes. He has asked for a price for a 10 HP pump.
1. Using the gauge readings, diagnose the main causes of low tail-end pressure and rank the corrective actions. Should the farmer buy the 10 HP pump first?
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Pressure budget: pump gauge 2.3 kg/cm² = 23 m head. Sand filter drop 0.8 kg = 8 m; screen 0.2 kg = 2 m; tail lateral 0.7 kg = 7 m. A clean sand filter should show about 0.2–0.4 kg; 0.8 kg means the bed is blinded, channelled or the backwash is inadequate. A 20 m³/h sand filter needs roughly 2.5–3 times its design flow for backwash, i.e. 50–60 m³/h, but the pump can give only 9 m³/h, so backwashing merely rinses the surface. The muddy first 3 minutes supports heavy silt loading. The 0.2 kg screen drop is acceptable. A tail pressure of 0.7 kg is too low for 4 lph non-PC drippers at 50 cm; expect about 1.0–1.2 kg at the lateral inlet, with 0.2–0.3 kg friction loss. Rank: (1) stop injecting fertilizer before the filter, open and inspect the sand bed, replace agglomerated media; (2) provide adequate backwash water through a storage tank and backwash pump, or install a smaller filter matched to the available flow; (3) check pump wear and install a low-level cutoff. A 10 HP pump is not the first fix: the well yield is only 3 m³/h in May, so a bigger pump will draw down faster and trip; it also does not provide 50–60 m³/h backwash. Measure the pump curve and static/drawdown first; replace with a matched 5 HP unit only if the existing pump is worn.
2. The farmer injects fertilizer upstream of the sand filter. Explain why this is wrong, state the correct injection point, and give the operating sequence.
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Injecting upstream of the sand filter is wrong for three reasons: (1) fertilizer phosphorus and sulfate can precipitate with iron, calcium and silt in the media, cementing the bed and creating channels; (2) salts and organic additives feed biofilm, increasing pressure drop and emitter clogging; (3) concentrated fertilizer or acid corrodes the filter body and backwash valve. Correct point: downstream of both filters, before a static mixer, with a check valve and a vacuum breaker or reduced-pressure backflow preventer so fertilizer cannot siphon back to the well or pump. A 120-mesh screen after the injection point protects the laterals. Sequence: irrigate with clean water for 10–15 minutes to wet the soil and confirm pressure; inject for the middle 60–70% of the set; flush with clean water for at least 15–20 minutes, or until the EC at the tail returns to the source EC. Never backwash with fertilizer in the line. Record EC, pH and the amount injected, and calibrate the venturi with a bucket and stopwatch. Keep the injection point labelled and locked so it cannot be confused with the backwash line.
3. Compare a 20 m³ storage tank plus booster with a 10 HP pump for the May water shortage. Which do you recommend, and what controls are needed?
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Recommend a storage tank plus booster. The May well yield is 3 m³/h; a 20 m³ tank fills in about 7 hours. One valve at a time needs 3.25 m³/h (13/4), and a 1.5–2 hour shift uses 5–6.5 m³, so 20 m³ supports three or four shifts without stressing the well. A 5 HP booster at 6–9 m³/h and 3.5 kg/cm² can supply the irrigation demand; backwash needs a separate 3 HP pump or a smaller filter sized to the available backwash flow. Add a float switch, low-level cutoff and dry-run relay on the submersible, and a non-return valve between the tank and booster. A 10 HP pump would cost more to run, cycle frequently, and still fail in May because the aquifer cannot supply it; it also does not solve the filter backwash deficit. Compare costs: a 10 HP pump plus installation may be ₹45,000–50,000, while a 20 m³ tank, booster and controls may be ₹60,000–70,000, but only the latter solves both the yield and backwash problems. Keep the existing 5 HP pump if its curve is healthy.