The Little Giant is a fixed-cone, single-feed locomotive injector made by Gresham & Craven of Manchester, used to force feedwater into a steam boiler against its own working pressure. It works by expanding live steam through a steam cone to supersonic velocity, mixing that jet with cold feedwater in a combining cone, and recovering the kinetic energy as pressure in a delivery cone — no moving pumps, no eccentrics, no drive off the wheels. The purpose is reliable feed when the locomotive is standing or coasting and a crosshead pump cannot deliver. A No. 9 Little Giant lifts roughly 1,000 gallons per hour against 180 psig.
Little Giant Locomotive Injector Interactive Calculator
Vary the steam cone throat and boiler pressure to estimate the Little Giant injector feed capacity and see the cone flow path animate.
Equation Used
The calculator uses the article's No. 9 Little Giant reference point as the calibration: a 6.1 mm steam-cone throat feeds about 1000 gal/hr against 180 psig. Flow is scaled by throat area and by the square root of pressure ratio.
- Calibrated to the article's No. 9 Little Giant reference: 6.1 mm throat, 180 psig, about 1000 gal/hr.
- Capacity scales with steam cone throat area and the square root of boiler pressure.
- Injector cones are clean, delivery clack is sealing, and feedwater is cold enough to condense the steam jet.
- This is an educational sizing estimate, not a replacement for manufacturer tables.
Inside the Little Giant Locomotive Injector
A Little Giant injector is three precision cones in a brass body, plus a steam cock, a water valve, an overflow, and a delivery clack. You crack the water valve, then open the steam cock. Steam at boiler pressure rushes through the steam cone — a converging-diverging nozzle — and accelerates to roughly twice the speed of sound, dropping its pressure below atmospheric in the process. That low pressure draws cold feedwater into the combining cone where the two streams collide. The steam condenses on the cold water in microseconds, collapsing in volume by a factor of about 1,600, and that condensation is what does the real work. The combined stream leaves the combining cone as a coherent slug of warm water travelling fast enough that, when it decelerates through the diverging delivery cone, it recovers a stagnation pressure higher than the boiler. The clack lifts, water enters the boiler, and the overflow shuts.
Why fixed cones? Gresham & Craven settled on fixed geometry because a locomotive fireman has neither the time nor the light to fiddle with adjustment screws on a moving footplate. The trade-off is a narrow operating window. Feedwater above about 38 °C will not condense the steam fast enough and the injector breaks off — you hear it as a sudden roar from the overflow. Back-pressure beyond the rated boiler pressure does the same thing. So does a partially blocked strainer, because the combining cone needs full water flow to seal the steam jet.
Tolerances are tight. The throat of the steam cone on a No. 9 is 6.1 mm — not 6.0, not 6.2. Wear that throat by 0.1 mm through scale erosion and the injector loses its lift, picks up at higher water temperature, and refuses to restart hot. Common failure modes are scale build-up on the combining-cone face, a leaking steam cock that warms the water gallery and prevents starting, and a worn delivery clack that lets boiler water back-flow and steam-lock the body.
Key Components
- Steam cone: The converging-diverging nozzle that accelerates boiler steam to about Mach 2 and drops static pressure below atmospheric to lift the feedwater. Throat diameter sets the injector's class — 6.1 mm for a No. 9, 7.0 mm for a No. 10. Erosion of more than 0.1 mm at the throat takes the injector out of spec.
- Combining cone: Where steam and cold water meet and the steam condenses. The annular gap between steam-cone exit and combining-cone entry is set at the works to roughly 1.5 mm and must not be altered. A scaled or scored combining cone is the most common reason a Little Giant will not pick up.
- Delivery cone: A diverging passage that converts the high-velocity warm-water jet back into pressure higher than boiler pressure. Internal finish must stay smooth — Ra better than 0.8 µm — or turbulent loss kills the pressure recovery and the overflow streams continuously.
- Overflow valve: A weighted or spring-loaded valve that vents excess water and unstable steam-water mixture during start-up, then seats once the delivery jet establishes. A dribbling overflow in service tells you the injector has broken off — usually feedwater too hot or boiler pressure dropped below about 40 psig.
- Delivery clack: A non-return valve between the injector and the boiler. Holds boiler pressure back when the injector is shut and lifts when delivery pressure exceeds boiler pressure. A leaking clack lets hot boiler water creep back into the body and steam-locks the cones on the next start.
- Steam cock and water valve: The fireman's two controls. Water cracked first, steam opened second, and both shut in reverse on stopping. A worn steam-cock plug that weeps even a small amount will warm the water gallery above the 38 °C lift threshold and the injector becomes a one-shot device until it cools.
Industries That Rely on the Little Giant Locomotive Injector
The Little Giant became the standard live-steam injector across British practice from the 1880s onward and you still find them on operating heritage locomotives, traction engines, and stationary plant worldwide. Sizes run from the tiny No. 4 on miniature railway locos up to the No. 11 on heavy industrials. The same casting pattern, slightly modernised, is still made today for restoration work.
- Heritage main-line locomotives: LMS Stanier Black Five 45305 carries a pair of Gresham & Craven Class 10 Little Giants as live-steam injectors in addition to its exhaust steam injector, used for standing feed and during light running.
- Industrial saddle tanks: Hunslet Austerity 0-6-0ST locomotives built for the War Department and now running on lines like the Keighley & Worth Valley use a single No. 9 Little Giant on the fireman's side.
- Traction engines and road rollers: Aveling & Porter and Burrell traction engines on the rally circuit are routinely fitted with a No. 6 or No. 7 Little Giant as the second feed alongside the crankshaft pump.
- Miniature and narrow-gauge railways: Ravenglass & Eskdale Railway 15 inch gauge locos and similar 7¼ inch gauge live-steam locomotives run No. 4 and No. 5 Little Giants made to the original drawings.
- Stationary boiler houses: Heritage Lancashire and Cornish boilers at sites such as Kew Bridge Steam Museum use a Little Giant as a backup feed when the main feed pump is down for maintenance.
- Steam launches: Thames and Windermere heritage launches running vertical fire-tube boilers commonly carry a No. 4 Little Giant for in-harbour standing feed when the engine-driven pump is stopped.
The Formula Behind the Little Giant Locomotive Injector
You size or check a Little Giant by predicting its volumetric water delivery from the steam-cone throat diameter and the boiler pressure. The formula matters because the injector has a narrow operating window — push boiler pressure to the low end of its rated range and delivery falls off sharply, run at the high end and the cone wear rate climbs. The sweet spot for any given size sits roughly two-thirds up its rated pressure band, where the steam jet has enough energy to entrain the design water flow without eroding the combining cone. Use this to confirm a recommissioned injector matches the boiler's evaporation rate, with margin.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Qw | Feedwater delivery rate | kg/s | lb/h |
| Cd | Discharge coefficient of the steam cone (typ. 0.78–0.85 for a clean Little Giant) | dimensionless | dimensionless |
| At | Steam-cone throat area | m² | in² |
| ρw | Feedwater density at tank temperature | kg/m³ | lb/ft³ |
| Pb | Boiler gauge pressure at the steam cock | Pa | psig |
| Patm | Atmospheric pressure | Pa | psia |
| Rwsr | Water-to-steam mass ratio (typ. 9–12 for a Little Giant on saturated steam) | dimensionless | dimensionless |
Worked Example: Little Giant Locomotive Injector in a heritage colliery winding-house boiler
You are predicting feedwater delivery from a No. 9 Little Giant being refitted to a recommissioned 1908 Daniel Adamson Cornish boiler at a heritage colliery winding-engine house in Wakefield West Yorkshire, where the boiler runs at 120 psig working pressure and feedwater from the hotwell sits at 32 °C. Steam-cone throat is the standard 6.1 mm. You need to confirm the injector can match the 850 lb/h evaporation rate of the boiler with margin, across the boiler's normal pressure swing from 100 to 140 psig.
Given
- dt = 6.1 mm
- Pb,nom = 120 psig
- Pb,low = 100 psig
- Pb,high = 140 psig
- Tw = 32 °C
- ρw = 995 kg/m³
- Cd = 0.82 —
- Rwsr = 10 —
Solution
Step 1 — convert the throat diameter to area in SI:
Step 2 — convert nominal boiler pressure 120 psig to absolute Pascals:
Step 3 — compute the steam mass flow at the nominal point, then scale by the water-to-steam ratio for delivery:
That is the textbook number. In practice a No. 9 Little Giant on this boiler delivers more like 1,000 lb/h once you account for real condensation efficiency and the published Gresham & Craven rating curve — the formula gives the upper bound, and field experience puts the realistic delivery at roughly 40 % of that figure for a single-stage fixed-cone injector. So call the realistic nominal delivery 1,030 lb/h.
Step 4 — repeat at the low-end boiler pressure of 100 psig:
At 100 psig the injector is sitting near the bottom of its useful band — it still picks up cleanly, but you'll notice the delivery clack chattering slightly and the overflow weeping a few drops per second. This is normal and not a fault.
Step 5 — and at the high-end 140 psig:
At 140 psig delivery rises only modestly because the fixed-cone geometry is already near its design point — but the steam jet velocity is higher and combining-cone wear rate climbs roughly with the cube of jet velocity. A No. 9 run continuously at 140 psig will need recone work in maybe 8 years rather than 15.
Result
The No. 9 Little Giant delivers a realistic 1,030 lb/h at the nominal 120 psig — comfortably above the boiler's 850 lb/h evaporation rate with about 21 % margin, which is exactly what you want for a single live-steam injector backing up the crosshead pump. Across the 100–140 psig swing delivery moves from roughly 950 to 1,100 lb/h, so the sweet spot for this installation is the middle of the band where cone wear stays low and overflow behaviour is clean. If you measure delivery materially below 950 lb/h at any pressure, the three things to check first are: (1) feedwater hotter than 38 °C at the water valve, often caused by a steam-cock gland weeping into the gallery; (2) a partially scaled combining cone, which you can spot as a rough spitting overflow during pickup; or (3) a delivery clack not seating fully, which lets boiler pressure backflow into the body and shows up as steam blowing back through the overflow when the injector is shut.
When to Use a Little Giant Locomotive Injector and When Not To
The Little Giant is the workhorse fixed-cone injector but it isn't the only feed option on a steam plant. Compare it against an exhaust-steam injector (which uses spent steam from the cylinders for economy) and a mechanical crosshead pump (driven off the locomotive's own motion).
| Property | Little Giant fixed-cone injector | Exhaust-steam injector (e.g. Davies & Metcalfe) | Crosshead-driven feed pump |
|---|---|---|---|
| Maximum feedwater temperature for reliable pickup | ≈38 °C | ≈55 °C (uses exhaust heat to its advantage) | Limited only by pump cavitation, ≈80 °C |
| Operates with locomotive standing | Yes | No — needs cylinder exhaust | No — needs wheels turning |
| Delivery rate per unit (typical loco size) | 900–1,200 lb/h (No. 9) | 2,500–3,500 lb/h | 1,000–2,000 lb/h depending on cylinder size |
| Steam consumption as % of delivered water heat | ~12–14 % | ~3–5 % (uses exhaust) | ~0 % (mechanical drive) |
| Sensitivity to back-pressure beyond rated | Breaks off immediately | Breaks off, harder to restart | Will keep pumping until relief valve lifts |
| Overhaul interval (continuous service) | 10–15 years cone life | 5–8 years (more complex internals) | 2–4 years (packing, valves, rod wear) |
| Capital cost relative | 1.0× (baseline) | 1.8–2.2× | 2.5–3.0× including drive linkage |
| Footplate complexity | Two valves, one overflow | Three valves, automatic changeover | No footplate controls, but bypass valve required |
Frequently Asked Questions About Little Giant Locomotive Injector
Heat soak. When you shut the injector, the brass body sits in contact with the boiler-side delivery passage and the cones warm to well above 38 °C within a minute or two. On a hot restart the incoming feedwater hits the warm combining cone, the steam jet cannot condense fast enough, and the whole flow blows out of the overflow.
Two practical checks — first, feel the body of the injector after a 5-minute shutdown. If you cannot keep your hand on it, the delivery clack is leaking and back-feeding hot boiler water into the body. Second, crack the water valve only for 10–15 seconds before opening steam, to flush cold water through and cool the cones. Most heritage railways fit a small drain cock on the body specifically to bleed off the hot pocket before a restart.
Match the injector's realistic delivery to the boiler's continuous evaporation rate with at least 20 % margin, but ideally not more than 40 %. Undersized and you cannot keep up with hard steaming. Oversized and the injector spends most of its life cycling on and off, which wears the steam-cock plug far faster than continuous service.
For a boiler evaporating 600–750 lb/h continuous use a No. 8 (delivers ~800 lb/h realistic). For 800–1,000 lb/h step up to a No. 9 (~1,030 lb/h). If you sit between sizes, take the larger one and accept the extra cycling — undersizing is the worse failure mode because it forces you to run the injector continuously at full output, which accelerates combining-cone erosion.
The catalogue number, always. The throat-area formula computes the theoretical maximum mass flow through the steam cone treating it as a clean choked nozzle — it ignores condensation kinetics, the water-side pressure drop, and the real-world discharge coefficient under two-phase flow. A fixed-cone injector achieves roughly 35–45 % of that theoretical figure as actual delivered water.
Use the formula for two purposes only — confirming a worn injector still falls within the expected ratio of theoretical to rated, and comparing two unknown injectors on the same basis. For sizing decisions and acceptance testing, the manufacturer's rating curve is the truth.
Almost always the delivery cone is the culprit, not the overflow itself. If the diverging passage of the delivery cone is scored, pitted, or has a roughened internal finish from cavitation damage, it cannot recover the jet velocity into stagnation pressure efficiently. The combined stream then doesn't reach boiler pressure plus the clack lift margin, so part of it spills back to overflow.
Pull the delivery cone and inspect with a borescope. Surface finish needs to be Ra 0.8 µm or better. Light scoring can be polished out with crocus cloth on a wooden mandrel; deeper damage means a new cone. A second possibility is a delivery pipe that has dropped below the injector body — water column weight in the down-leg adds to the back-pressure and can push you over the recovery threshold.
Yes — a Little Giant is a lifting injector and will draw water from a tank up to roughly 4.5 metres (15 ft) below its centreline at sea level, provided the suction pipework is genuinely airtight and the strainer is clean. The lift comes from the partial vacuum created in the combining cone during start-up, not from atmospheric pressure pushing on the tank.
In practice keep the lift to 2.5 m or less for reliable hot-day starting. Above that, any small air leak at a union or a partially closed strainer will prevent pickup. A common mistake on heritage rebuilds is using PTFE tape on the suction-side joints — under vacuum it can creep and let air past. Use a proper jointing compound rated for steam plant on every suction joint, and test the suction line by capping the inlet and pulling vacuum with a hand pump before commissioning.
Almost certainly the steam-cone-to-combining-cone gap. That annular gap is set at the works to roughly 1.5 mm and the cones are matched as a set. If you fitted a generic replacement combining cone without checking the gap, even 0.3 mm error will measurably reduce delivery because it shifts where the steam jet impinges relative to the water annulus.
Strip the injector and measure the gap with feeler gauges through the water inlet port. If it's wrong, the fix is a thin shim under the combining cone seat or, in stubborn cases, a light face-off of the combining cone shoulder on a lathe. Always re-measure after reassembly because gasket compression changes the gap by 0.1–0.2 mm depending on the joint material.
References & Further Reading
- Wikipedia contributors. Injector. Wikipedia
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