A Little Giant Injector is a non-lifting, single-feed steam injector that forces cold feedwater into a live boiler against its own working pressure using nothing but a high-velocity steam jet. The combining cone is the heart of it — that's where steam condenses into the incoming water stream and converts thermal energy into kinetic head. The point is to feed a boiler without a mechanical pump, so it works on locomotives, traction engines, and stationary plants where simplicity and reliability matter. A properly set 9 mm Little Giant will deliver around 50 to 80 gallons per hour at 150 psig boiler pressure.
Little Giant Injector Interactive Calculator
Vary injector size and boiler pressure to estimate the Little Giant feedwater delivery range and steam jet speed.
Equation Used
This calculator uses the article baseline for a 9 mm Little Giant at 150 psig and scales delivery by cone area and the square root of pressure. It is a practical estimate for clean, correctly set, gravity-fed injectors rather than a replacement for shop testing.
- Uses the article baseline of a properly set 9 mm Little Giant delivering about 50 to 80 gph at 150 psig.
- Flow is scaled approximately with injector area and the square root of boiler pressure.
- Assumes a clean, gravity-fed, non-lifting injector with cold feedwater and correct cone condition.
How the Little Giant Injector Actually Works
The Little Giant works on the Giffard principle — high-pressure steam from the boiler accelerates through a converging steam cone, reaching velocities well above 1,000 m/s as it expands. That jet enters a combining cone where it meets the incoming feedwater. The steam condenses almost instantly, dumping its latent heat into the water and accelerating the combined stream. The water then enters a diverging delivery cone, where velocity converts back to pressure — enough pressure to lift the boiler check valve and force water in against 150 psig or more. No moving parts during operation. Just three precisely machined cones and a starting handle.
Why non-lifting? The Little Giant is gravity-fed — the saddle tank or tender water sits above the injector body, so the feedwater arrives with a small positive head. That makes the steam cone geometry simpler and the injector more tolerant of warm feedwater. A lifting injector has to first generate vacuum to suck water up before it can feed, which limits how hot the supply water can be before the injector breaks. With the Little Giant, supply water up to about 50 °C is workable, but past that it gets unreliable.
Get the cone tolerances wrong and the whole thing stops working. The combining cone throat must be machined to within roughly 0.05 mm of spec — too large and the steam jet doesn't fully condense, leaving steam in the delivery side and the injector blows back out the overflow. Too small and you choke the water flow and the injector won't pick up at all. Common failure modes you'll see on a heritage workshop bench: scale build-up inside the cones (the injector starts but won't hold), a worn or dirty water valve seat (steady drip from the overflow even when picked up), and a leaking boiler check valve (injector picks up, then blows back when you shut the steam). The fix is usually to pull the cones, gauge them against new ones, and lap the seats.
Key Components
- Steam Cone: Converging nozzle that accelerates boiler steam to supersonic velocity. Throat diameter on a 9 mm Little Giant runs about 3.2 mm — undersize by 0.1 mm and capacity drops noticeably, oversize and steam consumption jumps without delivery gain.
- Combining Cone: Where the steam jet condenses into the feedwater stream and transfers momentum. The mixing throat must be concentric with the steam cone within 0.05 mm or you get unstable picking-up and chronic overflow drip.
- Delivery Cone: Diverging passage that converts the high-velocity mixed stream back into pressure head. Length-to-throat ratio is typically 8:1 to 10:1 — shorter and you lose efficiency, longer and friction eats the gains.
- Overflow Valve: Hinged flap or ball that opens to atmosphere during start-up and closes once the injector picks up. A worn seat here is the single most common cause of a Little Giant that won't hold once the steam handle is opened fully.
- Water Regulating Valve: Manual cone valve that meters incoming feedwater. On a Gresham & Craven Little Giant it's the small handle on the inlet side — typically opened ¾ turn during pickup, then trimmed to suit boiler pressure.
- Boiler Check Valve (Clack): Non-return valve between the injector and the boiler. Holds boiler pressure back when the injector is shut down. A leaking clack causes blow-back into the injector body within seconds of shutoff.
Where the Little Giant Injector Is Used
The Little Giant pattern was made by Gresham & Craven, Penberthy, Nathan, and a dozen other firms from the 1880s onward. You'll find them today on heritage locomotives, traction engines, miniature live steam, and any small stationary boiler where a feedwater pump would be overkill. The selling points are no moving parts in operation, no power supply needed, and the heating effect — every gallon delivered enters the boiler at around 80 °C, which means less thermal shock to the firebox crown and better fuel economy.
- Heritage Railways: Gresham & Craven No. 9 Little Giant fitted as the secondary injector on Bluebell Railway's SECR P-class 0-6-0T locomotives, working at 160 psig boiler pressure.
- Traction & Road Engines: Penberthy-pattern Little Giant on Aveling & Porter 4 NHP road rollers running at heritage rallies in Kent and the Great Dorset Steam Fair.
- Live Steam Miniature: Reeves2000 Little Giant clones on 5-inch gauge LBSC-designed locomotives such as the Tich and the Maisie, running at 80 to 90 psig.
- Stationary Boilers: Penberthy LL-series Little Giants used as backup feedwater on small Cleaver-Brooks package boilers in laundries and small industrial plants.
- Steam Launches: Nathan-pattern Little Giant fitted to the heritage launch Branksome on Coniston Water as backup to the engine-driven feed pump.
- Agricultural Steam: Original 1905 Gresham Little Giants still running on the Burrell showman's engines preserved at the Bressingham Steam Museum.
The Formula Behind the Little Giant Injector
What every operator wants to know is how much water the injector will actually shift per hour at a given boiler pressure and feedwater temperature. The classic Pollock-Macnair sizing relation links the combining-cone throat area, steam pressure, and feedwater temperature to delivery rate. At the low end of the typical pressure range — say 60 psig on a small launch boiler — a 9 mm Little Giant moves around 30 gph and barely warms the water. At the nominal 150 psig you're hitting the design sweet spot, around 60 to 70 gph with strong picking-up. Push past 200 psig and capacity climbs further but the injector becomes fussy about feedwater temperature, breaking off if the supply tank warms past about 35 °C.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Qdel | Feedwater delivery rate | kg/s | gph |
| K | Empirical injector coefficient (typically 0.55 to 0.70 for a well-machined Little Giant) | dimensionless | dimensionless |
| Ac | Combining cone throat area | m² | in² |
| Ps | Boiler steam pressure (absolute) | Pa | psia |
| ρw | Feedwater density | kg/m³ | lb/ft³ |
| f(Tw) | Feedwater temperature derate factor (1.0 at 10 °C, falling to 0.6 at 45 °C) | dimensionless | dimensionless |
Worked Example: Little Giant Injector in a heritage colliery winding-engine standby injector
Predicting feedwater delivery from a Gresham & Craven No. 9 Little Giant Injector being refitted as the standby feed on a recommissioned 1908 Cornish boiler at the Pleasley Colliery heritage site in Derbyshire, where the boiler runs at 130 psig saturated and supplies a small horizontal Robey winder driving a demonstration headgear. The injector draws from a hotwell at 22 °C and must deliver at least 55 gph to keep up with the winder's steam consumption during a typical 90-minute demonstration cycle.
Given
- Ac = 12.6 mm² (4.0 mm throat dia)
- Ps = 130 psig (144.7 psia)
- ρw = 997.8 kg/m³
- Tw = 22 °C
- K = 0.62 dimensionless
- f(Tw) = 0.92 dimensionless
Solution
Step 1 — convert boiler pressure to SI absolute and combining-cone area to m²:
Step 2 — compute nominal delivery at 130 psig and 22 °C feedwater:
That puts the injector squarely in its sweet spot — strong, stable picking-up with a clean shut overflow and no drip. Step 3 — at the low end of the operating range, drop boiler pressure to 80 psig (94.7 psia, 653,000 Pa) which represents the boiler still warming up at the start of a demonstration:
That's right at the demonstration cycle's minimum — the injector still picks up but you'll notice the delivery feels softer and the water valve needs cracking open further to hold it. Step 4 — at the high end, the boiler hits its 150 psig safety setting (164.7 psia, 1,135,500 Pa):
At 150 psig the injector is hammering — strong note, sharp pickup, and you'll need to nip the water valve back about an eighth of a turn to avoid overflow drip. Beyond about 170 psig with this 22 °C hotwell feedwater you start to see breaking-off, because the rising steam temperature in the combining cone outpaces the water's ability to absorb the latent heat.
Result
Nominal delivery is approximately 65 gph at 130 psig with 22 °C hotwell feedwater — comfortably above the 55 gph requirement for the demonstration cycle. Across the operating range you'll see roughly 53 gph at the 80 psig warming-up point, 65 gph at nominal, and 70 gph at the 150 psig safety setting, with the sweet spot for stable running sitting between 120 and 150 psig. If you measure delivery 20% below this prediction, three causes dominate: (1) a worn combining-cone throat oversize by more than 0.1 mm — gauge it against a new cone and you'll often find a half-millimetre of erosion on a 50-year-old set; (2) a partly fouled feedwater strainer cutting supply head, which shows up as a thin, hissing pickup that won't hold; and (3) air leakage at the union between the water valve body and the injector — a hairline gasket weep there pulls air into the combining cone and kills the vacuum stage of pickup, even though the joint looks tight from the outside.
Choosing the Little Giant Injector: Pros and Cons
The Little Giant isn't the only way to feed a heritage boiler, and it isn't always the right one. Compare it on the dimensions that matter — capacity range, feedwater temperature tolerance, mechanical complexity, and what happens when something goes wrong.
| Property | Little Giant Injector (non-lifting) | Lifting Injector (Gresham Class B) | Crank-Driven Feed Pump |
|---|---|---|---|
| Max feedwater inlet temperature | ~50 °C | ~30 °C | >90 °C |
| Typical delivery range (9 mm size) | 30–80 gph | 20–60 gph | 10–200 gph (gear-dependent) |
| Moving parts during operation | None | None | Piston, valves, eccentric |
| Operates at zero engine speed | Yes | Yes | No |
| Capital cost (heritage rebuild) | £400–£700 | £500–£900 | £1,200–£3,000 |
| Sensitivity to scale fouling | High — cones must be pulled and lapped | High | Low — large clearances |
| Service life of cones / parts | 20–40 years | 20–40 years | Pump rebuild every 5–10 years |
| Best application fit | Locomotives, traction engines, small stationary | Tank-below-injector installations | Marine and large stationary boilers |
Frequently Asked Questions About Little Giant Injector
Almost always feedwater that's too warm. The Little Giant relies on the combining cone fully condensing the steam jet — once supply water passes about 45 to 50 °C the water can't absorb the latent heat fast enough, condensation goes incomplete, and you get steam through to the delivery side which blows the overflow open.
Quick diagnostic — stick a thermometer in the tender or hotwell. If it reads above 40 °C, drain off some warm water and top up with cold. On a locomotive in service this is why the second injector is often fed from a separate cold tank rather than the main tender after a long run.
If cone geometry checks out, the next suspects are the steam supply pipework and the boiler check valve. A partially blocked steam pipe or a half-seized stop valve drops pressure at the steam cone inlet without showing on the boiler gauge — measure pressure at the injector's steam inlet directly with a test gauge and compare to boiler reading. More than 5 psi drop and you've got a restriction.
For the check valve, listen for a faint hiss or feel the delivery pipe between the injector and the clack — if it goes hot when the injector should be shut, the clack is leaking back and starving the delivery side of pressure differential.
Match the injector to the boiler's evaporation rate, not the locomotive's size. A typical 5-inch gauge boiler evaporates 15 to 25 lb/hr at working pressure. A 9 mm Little Giant delivers around 30 lb/hr at 80 psig — that's a comfortable 1.3 to 2× margin, which is what you want for a single injector.
An 11 mm is overkill on most 5-inch locomotives — you'll struggle to throttle it down without it dripping or breaking off, because the water valve becomes hypersensitive at the bottom of its range. Stick with 9 mm unless you're running a large 5-inch Pacific or a 7¼-inch gauge engine.
Not failing — but the overflow valve seat is worn or carrying debris. Once a Little Giant has picked up, the overflow should be bone dry. A continuous drip means the overflow valve isn't seating fully. On Gresham-pattern injectors the overflow is usually a small ball or a hinged brass flap on a phosphor-bronze seat.
Strip it, inspect the seat under a loupe, and lap with fine paste on a flat surface. A scored seat from grit ingestion is the usual culprit — fit a 100-mesh strainer on the feedwater inlet if you don't have one. Living with the drip is fine for a demonstration day but it represents perhaps 5 to 10% delivery loss and a slow waste of feedwater.
Below about 40 psig the steam jet velocity drops to where the kinetic head developed in the delivery cone barely exceeds the boiler pressure plus the check valve cracking pressure. The injector becomes erratic — it'll pick up on a quiet bench but break off the moment you put any feed demand on it.
For low-pressure work below 40 psig, fit an exhaust steam injector or a mechanical feed pump instead. The Little Giant's design sweet spot is 80 to 200 psig — that's where the cone geometry was originally optimised by Gresham in the 1870s and it's where you'll get reliable picking-up every time.
That's the steam supply collapsing through the combining cone faster than the water valve closes — momentary cavitation as the delivery side depressurises and water flashes back through the cone. It's not normally damaging in the short term but repeated rough shutdowns hammer the cone faces and accelerate erosion of the combining cone throat.
The correct shutoff sequence is water valve first (smartly closed), then steam valve. That collapses the jet cleanly and lets the overflow drop without flash-back. If yours rattles even with correct sequence, the steam valve is leaking through and needs reseating.
References & Further Reading
- Wikipedia contributors. Injector. Wikipedia
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