RamPump KB

Section 17 Β· Design & Build

Design & sizing a ram system

Four site numbers β€” source flow, fall, lift and demand β€” determine everything: whether a ram is viable, how big it must be, and how much water you'll get. Here's how the pieces fit together.

On this page

  1. The geometry
  2. Rules of thumb
  3. Sizing procedure
  4. Worked designs
  5. Common mistakes
  6. Can it do the job?

The geometry

H β€” fall (drive head) h β€” lift (delivery head) L = 3–7 Γ— H Q β€” drive flow q β€” delivered flow Q βˆ’ q β€” wasted (the fuel)
The four numbers that define a ram site: fall H, lift h, drive-pipe length L, and source flow Q.
SymbolNameDefinition
HFall / drive headVertical drop from the source water surface to the ram's waste valve
hLift / delivery headVertical rise from the ram to the delivery tank's water surface
LDrive pipe lengthPipe length from intake to ram β€” sets the beat's timing and power
QDrive flowTotal water flowing through the ram (delivered + wasted)
qDelivered flowUseful water arriving at the storage tank

Rules of thumb

ParameterGuidanceWhy
Fall Hβ‰₯ 1 m comfortable; ~0.5 m bare minimumPower ∝ H β€” every metre of fall multiplies output
Drive pipe L3–7 Γ— H; rigid, constant diameter, as straight as possibleValve timing must beat the pressure wave's return (see Physics)
Drive velocity1–2 m/s at valve closureSets the spike: Ξ”p = ρcΞ”v
Drive diameterD = √(4Q/(Ο€v)) β€” see table belowMatch velocity band without excessive friction
Lift ratio h/H2–6 sweet spot; workable to ~10High ratios drop both output and efficiency
Delivery pipeβ‰ˆ half the drive diameter; pressure-ratedCarries a small steady flow; must hold static head + pulses
Friction budgetDrive: hf ≀ 10% of H Β· Delivery: ≀ ~15% of hDarcy–Weisbach check β€” see Physics
Air chamberVolume β‰ˆ 20–50 Γ— per-beat delivery; snifter hole β‰ˆ 1 mmCushions the spike and smooths delivery
IntakeSubmerged β‰₯ 2–3 pipe diameters; screened; settling spaceAir or grit in the drive pipe stops or grinds the beat
Waste waterChannelled clear of the foundation; reusable belowThe bulk of the flow β€” 70–95% of it β€” must go somewhere forever
Drive pipe diameter for a 1.5 m/s design velocity (round up to the nearest nominal size)
Drive flow QComputed diameterNominal pipe
0.5 L/s21 mm25 mm (1β€³)
1 L/s29 mm32 mm (1ΒΌβ€³)
2 L/s41 mm50 mm (2β€³)
4 L/s58 mm63 mm (2Β½β€³)
8 L/s82 mm90 mm (3Β½β€³)
15 L/s113 mm125 mm (5β€³)

Sizing procedure

  1. Measure the site

    Source flow (bucket & stopwatch, or a weir), fall from water surface to proposed ram location, and lift to the delivery point. Methods in Installation.

  2. Decide the drive flow Q

    How much water can you spare permanently? A ram needs a reliable surplus β€” in a dry month, not just after rain. Leave the stream's environmental flow intact.

  3. Check the output

    q = Ξ·Β·QΒ·H/h with Ξ· β‰ˆ 0.6. Compare against daily demand. If short: more fall (raise the intake), more flow, a bigger ram, or accept a smaller supply.

  4. Size the drive pipe

    Diameter from the table (1–2 m/s), length 3–7 Γ— H, route as straight as the terrain allows. Rigidity is non-negotiable.

  5. Design the intake

    A small forebay with a screen and settling depth; keep the pipe mouth submerged but off the bottom, away from vortices.

  6. Size the vessel & delivery line

    Vessel 20–50 Γ— per-beat volume (or fit a snifter); delivery pipe rated above the full static head, diameter for low friction on long runs.

  7. Plan the civil works

    Anchor pad, waste channel, frost protection, and access for the tuning you'll do in week one β€” then never again, if it's built well.

Worked designs

🏑 Homestead garden

Site: creek with 2 L/s to spare; falls 1.5 m over a rocky step; garden tank 9 m above the ram site.

Check ratio: 9/1.5 = 6:1 β€” top of the sweet spot. Output: q = 0.6 Γ— 2 Γ— (1.5/9) = 0.2 L/s β‰ˆ 17 mΒ³/day. Drive pipe: 50 mm (2β€³) Γ— ~7.5 m (5 Γ— 1.5).

Verdict: plenty for a garden, stock troughs and a house. A 2β€³ DIY ram handles it.

🏘 Hillside village

Site: stream with 15 L/s to spare; fall 3 m via a small diversion weir; storage tank 15 m above (5:1 ratio).

Output: q = 0.6 Γ— 15 Γ— (3/15) = 1.8 L/s β‰ˆ 155 mΒ³/day. Drive pipe: 125 mm (5β€³) steel Γ— ~18 m. Vessel: ~70–180 L with snifter β€” 20–50 Γ— the ~3.6 L delivered per beat at a slow ~30/min beat.

Verdict: ~3,000 people at 50 L/day each β€” one clacking iron heart for a whole village.

Common mistakes

πŸ›‘
The classics
  • Flexible drive pipe (hose, thin-wall HDPE) β€” absorbs the hammer; the pump barely beats.
  • Drive pipe β‰  3–7 Γ— H β€” too short: valve mistimed; too long: sluggish beats, stalls.
  • Air pockets from a sag or high point in the drive line β€” cushions the column, kills the beat.
  • Intake too near the surface or the bottom β€” sucks air, then silt.
  • Undersized source in the dry season β€” design for August, not April.
  • Thin-wall PVC air chamber β€” a burst waiting to happen; use pressure-rated parts only.
  • No waste-water plan β€” the spill undermines the pad and floods the neighbour's path.
  • Chasing extreme lift ratios β€” 15:1 "works" on YouTube; in the field it trickles and frustrates.

Can it do the job?

βœ… A ram shines when…

  • There's a permanent fall of β‰₯ 1 m and water to spare
  • The delivery point is 2–10Γ— higher than the fall
  • Power is absent, expensive or unreliable
  • Water is needed continuously (tanks, troughs, drip irrigation)
  • Nobody wants another thing to maintain

β›” Look elsewhere when…

  • The land is flat β€” no fall, no ram (and a static pond has zero energy)
  • The source flow is barely more than the demand
  • You need water on demand at pressure, not steady accumulation
  • The source runs dry seasonally
  • You must lift more than ~10Γ— the available fall

Run your own numbers in the sizing calculator, then follow the build in Build Your Own and the civil works in Installation.