95 m laterals with 35% flow variation in a tomato block
Client: 1.2 ha tomato, beds 1.5 m centre-to-centre, in-row spacing 0.3 m. The quotation specifies 16 mm laterals 95 m long with 2 lph drippers at 0.3 m, 20 laterals per valve, a 63 mm PVC submain 120 m long, a 50 mm main 80 m, and a 15 m³/h pump at 3.5 kg/cm². Manufacturer chart: for this 16 mm lateral at 2 lph and 0.3 m spacing, the maximum length is 70 m for ±10% flow variation and 85 m for ±20% at 1.2 kg/cm² inlet; at 95 m the chart shows about ±35%. Field: the block is 120 m × 100 m; the submain runs along the 120 m side; laterals run 95 m down the slope, which is less than 0.3%. The farmer has 8 hours of power supply and wants to apply 6 mm/day. Dealer: says 'just raise the pressure to 2 kg/cm² at the lateral inlet and the 95 m laterals will be fine'. The pump curve shows 15 m³/h at 3.5 kg/cm² and 12 m³/h at 4.2 kg/cm².
1. Check the lateral length against the manufacturer chart. Explain the consequence for the crop and recommend the cheapest layout fix.
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At 95 m the chart predicts about ±35% flow variation, far outside the usual ±10% design target. The last plants receive about 35% less than the first, so the crop is uneven and the field cannot be managed as one block. The dealer's pressure fix is not enough: raising inlet pressure from 1.2 to 2.0 kg increases flow everywhere, but friction loss rises roughly with Q^1.75, and the 15 m³/h pump cannot hold 2 kg at the lateral inlet while also supplying 20 laterals. The cheapest hydraulic fix is to keep 16 mm pipe but split each 95 m run into two 47.5 m laterals fed from a mid-submain. At 47.5 m the chart gives well under ±10%. The extra cost is about 120 m of 63 mm PVC and 20 tees, much less than the yield loss from 35% unevenness. A second option is 20 mm laterals at 95 m, but the pipe cost is higher. Reducing dripper size to 1.2 lph would allow a longer run but doubles the runtime beyond the 8-hour supply.
2. Recompute the valve grouping, block flow and runtime for the split-lateral design. Can the farmer apply 6 mm/day within the 8-hour supply?
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With 47.5 m laterals, each lateral flow = 47.5/0.3 × 2 = 317 lph = 0.317 m³/h. To keep the same 0.285 ha block, use 40 laterals per valve: 40 × 0.317 = 12.7 m³/h, matching the pump. The application rate is unchanged at 4.17 mm/h. Gross depth for 6 mm net at 90% efficiency = 6.67 mm, so runtime = 6.67/4.17 = 1.6 h per block. Number of blocks: 1.2 ha / 0.285 ha = 4.2, so 5 valves or 5 shifts. Total pumping = 5 × 1.6 = 8.0 h, exactly the available power window with no margin. Either reduce the target to 5.5 mm/day (7.3 h), split into 6 blocks and shorten each shift, or add a small storage tank to decouple pumping from the 8-hour supply. Do not plan on 8.0 h; leave at least 30 minutes for flushing and breakdowns. Check that the 63 mm submain at 12.7 m³/h is within its velocity limit: area = 0.00312 m², velocity = 1.13 m/s, which is acceptable for PVC but should not be exceeded.
3. Evaluate the dealer's advice to raise the pressure. What commissioning checks would you insist on?
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Raising pressure is the wrong lever. At 2 kg/cm² at the lateral inlet, a non-PC dripper discharges more, so block flow rises above 12.7 m³/h; friction loss in the submain and lateral rises, and the pump may move to the right of its curve, where 15 m³/h at 3.5 kg is already near its limit. The result can be higher energy use, burst fittings and no improvement in uniformity. The correct sequence is: fix lateral length first, then measure pressure at the head and tail of the worst lateral, and only then adjust the pump or install a pressure regulator. Keep the lateral inlet at 1.0–1.2 kg/cm² and use a pressure gauge at the tail during commissioning. Insist on: a 1-hour pressure test at 1.5× working pressure, flushing of all laterals until clear, a flow test at the head and tail of each block, a pressure check at the farthest emitter, and a demo of filter backwash. Also install an air release valve at the high point and a flush valve at each lateral end. Record the readings in the commissioning report.