The Leffel double-runner turbine is a vertical-shaft mixed-flow water turbine that stacks two runners on a single shaft inside one casing. The defining component is the pair of cast-iron runners — a smaller upper runner and a larger lower runner — sized so the operator can admit water to one, the other, or both depending on available flow. James Leffel patented this layout in 1862 to keep efficiency high across wildly variable creek and millrace conditions. Thousands powered 19th-century gristmills, sawmills, and early electric plants on heads of 5 to 50 ft.
Leffel Double-runner Turbine Interactive Calculator
Vary seasonal flow and turbine efficiency to compare Leffel double-runner output against a single-runner turbine.
Equation Used
This calculator applies the article comparison that shaft output is proportional to available flow times efficiency. At the same head and flow, the Leffel double-runner advantage comes from holding higher efficiency during low-flow seasons.
- Same head and available flow for both turbines.
- Output is shown as percent of peak ideal hydraulic power.
- Efficiency includes runner, gate, leakage, and draft-tube effects.
Inside the Leffel Double-runner Turbine
Water enters the casing through a ring of wicket gates, hits the runner buckets at a near-tangential angle, drives the shaft, and exits axially through a draft tube into the tailrace. What makes the Leffel different from a single-runner Francis or a Jonval is the stacked twin-runner geometry — two distinct runners keyed to one vertical shaft, each with its own gate ring. The smaller upper runner handles low-flow conditions efficiently. The larger lower runner takes over when the creek is running high. Open both gate rings and you get full output. This is the whole reason the design exists: a single-runner turbine sized for spring flood runs at 30-40% efficiency in late summer, while a Leffel sized for the same site holds 70-75% efficiency across the seasonal range by switching runners.
The runner buckets are cast iron, profiled to receive water at the gate exit angle without shock loss. If the gate-to-runner angle is off by more than about 3°, you'll see cavitation pitting on the bucket backs within a season — the classic symptom is a hissing rattle under load and a measurable drop in shaft horsepower at the same head and flow. Wicket gate clearance to the runner crown matters too. The factory spec on most surviving Leffels is 1/16 inch (1.6 mm) radial clearance. Open that up to 1/8 inch through wear and leakage past the gates eats 8-12% of your output before the water ever does useful work.
The draft tube below the lower runner is not optional. It recovers velocity head by decelerating the exit water, and a Leffel without a properly flared draft tube loses roughly 15% of available head. If your measured power is well below predicted and the gates and runners look clean, check the draft tube first — collapsed liners, debris, or a tailwater level above the runner crown all kill output fast.
Key Components
- Upper runner (small): Cast-iron mixed-flow runner sized for low-flow operation. Diameter typically 60-70% of the lower runner. Handles roughly one-third of the casing's rated flow and lets the turbine stay efficient through dry months when the creek drops to 25-35% of peak.
- Lower runner (large): Primary runner that carries full-flow operation. On a typical 30-inch Leffel the lower runner is around 30 in (760 mm) diameter with 18-24 buckets. Bucket profile must match the gate exit angle within 3° to avoid shock-loss cavitation.
- Wicket gates (two rings): Independent rings of pivoting vanes — one ring per runner. Operator adjusts each ring with a hand wheel or governor link. Radial clearance to the runner crown should hold at 1/16 in (1.6 mm); wear past 1/8 in causes serious leakage loss.
- Vertical main shaft: Single forged shaft keying both runners. Sized for the combined torque of both runners running together — typically 2.5-3.5 in (63-89 mm) diameter on mid-size units. Misalignment over 0.005 in TIR at the upper bearing causes packing wear and eventual journal scoring.
- Draft tube: Flared cast-iron or riveted steel tube below the lower runner. Decelerates exit water to recover velocity head. Length typically 4-6× runner diameter, with a flare ratio around 1.4:1. Submergence of the runner crown below tailwater must stay positive to avoid air entrainment.
- Casing and penstock connection: Cast-iron scroll or cylindrical casing receiving water from the penstock. Designed for the full operating head plus a 1.5× surge factor. Bolted flange to the penstock with a soft gasket — a leaking flange wastes head you paid to develop.
Real-World Applications of the Leffel Double-runner Turbine
The Leffel double-runner found its market in any site with seasonal flow variation and a head between 5 and 50 ft. That covered most of the mills, small factories, and rural electric plants of the late 19th and early 20th century in North America. Many original units are still in service or being restored for heritage micro-hydro work today.
- Heritage micro-hydro: Restored 1880s Leffel running the generator at the Wolcott Mill historic site near Ray, Michigan, producing roughly 8 kW for interpretive demonstration.
- Gristmill restoration: Cast-iron Leffel rebuilt and reinstalled at Burwell-Morgan Mill in Millwood, Virginia, driving the original buhr stones through bevel gearing.
- Small-scale electric generation: Off-grid homestead installations on Vermont and West Virginia tributaries where flow varies 4:1 between spring and August — the double-runner layout outperforms a single Francis on annual energy capture.
- Sawmill heritage operation: Hammond's Sawmill in Greater Sudbury, Ontario, where a refurbished Leffel drives the headsaw arbor on demonstration days.
- Industrial museum exhibits: Static and live-running Leffel turbines at the Hagley Museum on the Brandywine in Wilmington, Delaware, showing the evolution from breast wheels to enclosed turbines.
- Textile mill restoration: Original Leffel installations preserved in New England woollen mill restorations along the Blackstone River corridor in Massachusetts and Rhode Island.
The Formula Behind the Leffel Double-runner Turbine
The shaft power output of a Leffel — or any reaction turbine — comes from the standard hydraulic power equation modified by the turbine's overall efficiency. What matters for a double-runner specifically is that the efficiency η is not a single number. At the low end of your typical flow range, running upper runner only, η holds around 0.70-0.75. At nominal flow, both runners admitting partial gate, η peaks at 0.78-0.82. Push past rated flow with both runners wide open and η falls back to 0.65-0.70 because the bucket geometry stops matching the inflow angle. The sweet spot is wherever the gate setting puts the inflow angle within 3° of the bucket entry angle.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Pshaft | Shaft power delivered by the turbine | W | hp |
| ρ | Water density (≈ 1000 kg/m³) | kg/m³ | lb/ft³ |
| g | Gravitational acceleration (9.81 m/s²) | m/s² | ft/s² |
| Q | Volumetric flow rate through the turbine | m³/s | ft³/s (cfs) |
| H | Net head across the turbine (gross head minus penstock losses) | m | ft |
| η | Overall turbine efficiency at the operating point | dimensionless | dimensionless |
Worked Example: Leffel Double-runner Turbine in a heritage paper-mill turbine rebuild
You are sizing a refurbished 28-inch Leffel double-runner for a heritage paper-mill restoration on the Otonabee River near Lakefield, Ontario. The site has a measured net head of 14 ft (4.27 m) and seasonal flow that swings between 12 cfs in late summer and 48 cfs at spring freshet, with a nominal design flow of 30 cfs (0.85 m³/s). The rebuilt turbine will drive a small synchronous generator through a flat-belt drive for interpretive operation.
Given
- H = 4.27 m
- Qnom = 0.85 m³/s
- Qlow = 0.34 m³/s
- Qhigh = 1.36 m³/s
- ρ = 1000 kg/m³
- g = 9.81 m/s²
Solution
Step 1 — at nominal flow of 0.85 m³/s, both runners admitting partial gate, efficiency holds near 0.80. Compute the hydraulic input power first:
Step 2 — apply the nominal efficiency to get shaft power at the design point:
Step 3 — at the low end of the typical operating range (0.34 m³/s, late August), only the upper runner is gated open. Efficiency drops to about 0.72 because you've lost the larger runner's contribution and the gate is operating near the bottom of its useful range:
That's barely a third of nominal output, but it's still enough to keep a small generator excited and the mill lit. A single-runner Francis sized for the 30 cfs design point would deliver maybe 4-5 kW at this flow because it would be running far off its best efficiency point — the double-runner's whole reason for existing.
Step 4 — at the high end of the typical operating range (1.36 m³/s, spring freshet), both runners are wide open. Efficiency falls back to 0.68 because the bucket inflow angle no longer matches the gate exit angle at this overload condition:
You'd feel a noticeable drone change in the casing at this flow and the draft tube will be flowing partly aerated — that's the audible cue you've pushed past the efficient operating window even though raw output is still climbing.
Result
Nominal shaft output is approximately 28. 5 kW (38 hp) at 30 cfs and 14 ft net head. In practice that means a steady, even drone from the casing, a tailrace running smooth and clear, and a generator holding rated output without governor hunting. Across the seasonal range you'll see roughly 10 kW in late summer on upper runner only and up to 38 kW at freshet with both runners wide open — the sweet spot sits at the nominal design point where the gate angle matches the bucket entry angle within 3°. If your measured output runs 15-20% below predicted at nominal flow, suspect three things in this order: penstock head loss higher than estimated (clogged trash rack or partly closed isolation gate dropping net head below 14 ft), wicket gate leakage from worn pivot bushings letting water bypass the runner, or a partly collapsed draft tube liner killing velocity head recovery downstream of the lower runner.
Leffel Double-runner Turbine vs Alternatives
The Leffel double-runner competes with single-runner Francis turbines and Jonval-pattern turbines for low-to-medium head sites. The right choice depends on how variable your flow is, how much capital you can spend, and whether you are restoring a heritage machine or building new.
| Property | Leffel double-runner | Single-runner Francis | Jonval turbine |
|---|---|---|---|
| Head range | 5-50 ft (1.5-15 m) | 10-1000 ft (3-300 m) | 5-30 ft (1.5-9 m) |
| Efficiency at design flow | 78-82% | 88-94% | 70-75% |
| Efficiency at 25% design flow | 70-75% (upper runner only) | 30-45% | 40-50% |
| Mechanical complexity | High — two runners, two gate rings | Moderate — one runner, one gate ring | Low — fixed guide vanes |
| Typical lifespan (cast iron) | 80-120 years with rebuilds | 60-100 years | 100+ years (proven on 1850s installs) |
| Capital cost (modern equivalent) | High | Moderate | Low to moderate |
| Best application fit | Variable-flow micro-hydro, heritage mill restorations | Steady-flow utility-scale low/medium head | Very low head, simple installations |
| Part-gate operation quality | Excellent — switch runners | Poor — efficiency falls fast off BEP | Moderate — fixed geometry limits range |
Frequently Asked Questions About Leffel Double-runner Turbine
You're almost certainly looking at a phasing or balance issue between the two runners. Each runner was originally cast and balanced as a pair on the same shaft, with the bucket count chosen so the pressure pulses don't reinforce each other. If a previous restoration replaced one runner with a salvaged part from a different unit, the bucket counts may now share a common factor (say 18 and 24 instead of the original 17 and 24) and you get a strong beat frequency at the shaft RPM.
Check the bucket counts on both runners. If they share any common divisor above 1, you'll get reinforced pulses. The fix is either re-clocking the upper runner relative to the lower (rotate one on the shaft key) or accepting the vibration and running the runners independently rather than together.
Almost never on pure efficiency grounds. A modern Francis hits 92% at design point versus the Leffel's 80%. The Leffel wins on annual energy capture only when your flow varies more than about 3:1 across the year and you can't afford a variable-speed inverter setup to keep a Francis on its best efficiency point.
The other case is heritage restoration where the original Leffel exists and the goal is authenticity. Rebuilding the original casting is usually cheaper than fabricating a new Francis runner and casing for the same site, and historical interpretation value is part of the project deliverable.
A 15-point efficiency gap on a Leffel that's gated and loaded correctly almost always points to net head loss you haven't accounted for. The formula uses net head at the turbine inlet, not gross head from the headpond. On older installations the penstock has often accumulated tubercles, the trash rack has partially clogged, or an isolation valve is not fully open — all of which drop your real H below your assumed H.
Put a pressure gauge on the penstock just upstream of the casing flange and convert to feet of head. If the gauge reads 12 ft when you assumed 14 ft, that's a 14% drop in available power right there, and your 65% measured is now actually 76% of corrected available — much closer to spec.
Running the upper runner alone with the lower gates closed creates a low-pressure zone in the lower casing because there's no flow keeping the draft tube primed. The upper runner discharge has to drag water past a stationary lower runner, which spikes the pressure drop across the upper bucket exits and pushes them below vapour pressure.
The fix on most original Leffel installations was a small bypass — either crack the lower gates open about 5-10% to keep flow moving past the lower runner, or fit a vacuum-breaker valve at the top of the casing. If you've inherited a unit without either, expect upper-runner-only operation to chew the bucket backs unless you maintain a small bleed flow through the lower gates.
Three things kill a Leffel beyond economic repair. First, casing cracks running through the volute — these are weld-repairable in cast iron only if the crack hasn't passed through a bolt boss or a draft tube transition. Second, draft tube perforation from a century of submerged corrosion — you can fabricate a steel replacement, but if the flange is gone you're starting from scratch. Third, runner bucket erosion deeper than about 6 mm into the bucket face — the cast profile is critical to efficiency and you can't just weld up and grind back without a pattern.
What's almost always rebuildable: shaft, bearings, packing, gates, gate rings, and gate linkage. Budget 60-70% of project cost on the runners and casing if those are sound, 90%+ if they're not.
You're seeing a transient under-speed event. When you open the lower gates, flow through the casing jumps before the additional torque from the lower runner builds up. There's a brief window — usually 2-4 seconds on a small Leffel — where head across the upper runner drops because the lower gates stole flow capacity, but lower runner output hasn't ramped up yet. Shaft RPM dips, generator frequency drops, and you see the voltage sag.
The 19th-century fix was to open lower gates slowly via the hand wheel — taking 10+ seconds to fully transition. Modern installations use a governor with a programmed gate opening rate that matches the runner spin-up time, typically 5-8 seconds per full gate stroke for a mid-size Leffel.
References & Further Reading
- Wikipedia contributors. Water turbine. Wikipedia
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