A Korting injector is a steam-driven feedwater pump with no moving parts that uses a converging-diverging nozzle stack to draw cold water, mix it with live steam, and force the heated mixture into a boiler against its own working pressure. The Korting Brothers of Hannover patented their improved double-tube design in 1876, refining the original 1858 Henri Giffard concept. The steam jet condenses inside the combining cone, transferring momentum to the water and generating pressure higher than the supply steam itself. You will still find Korting injectors fitted to working steam locomotives, traction engines and heritage launches today.
Korting Injector Interactive Calculator
Vary the steam volume, condensation collapse ratio, and feedwater temperature to see the injector mixing cone volume collapse and hot-water pickup margin.
Equation Used
The calculator uses the injector article's key condensation statement: steam volume collapses by roughly 1600 to 1 when it condenses in the combining cone. The liquid volume is the incoming steam volume divided by the collapse ratio, and the temperature margin compares feedwater temperature with the article's 50 deg C break-off warning.
- Steam fully condenses in the combining cone.
- The article value of roughly 1600:1 is used as the default steam-to-condensate volume collapse ratio.
- Feedwater above 50 deg C is treated as the pickup break-off threshold.
- This is a teaching calculation for condensation collapse, not a complete injector nozzle sizing model.
How the Korting Injector Works
The Korting injector looks like a brass casting with three or four pipe connections and a single starting handle, but inside it is a precisely-machined train of three cones — the steam cone, the combining cone, and the delivery cone. Open the steam valve and high-pressure saturated steam expands through the steam cone, dropping in pressure and accelerating to supersonic velocity. That high-speed jet enters the combining cone, where it meets cold feedwater drawn in by the partial vacuum the jet creates. The steam condenses on contact with the water, and because condensation collapses the steam volume by roughly 1,600 to 1, you get a sudden momentum transfer that hurls the heated water forward at a speed high enough to overcome boiler pressure when it decelerates through the diverging delivery cone.
Why three cones and not two? Because each one does a different job — the steam cone converts pressure to velocity, the combining cone mixes and condenses, and the delivery cone converts velocity back to pressure. Get the cone geometry wrong and the injector will not pick up. The combining cone throat must be sized to within roughly ±0.05 mm of design diameter on a small locomotive injector — bore it 0.2 mm oversize and the steam jet cannot maintain a coherent column through the water annulus, so the unit slobbers feedwater out the overflow and refuses to lift. Bore it undersize and you starve the water supply.
If you notice the injector breaking off — picking up, then dumping water out the overflow a few seconds later — the cause is almost always feedwater above 50 °C. The steam cannot fully condense in hot water, so the jet loses its momentum coupling. Other classic failure modes are scale build-up inside the combining cone (you would be amazed how a 0.1 mm scale layer kills the suction), a leaking suction-side union pulling air, or a worn overflow valve seat that lets the jet vent instead of pressurising. Korting's own non-lifting variant runs feedwater above the injector for positive suction; the lifting variant pulls water up to about 6 m of static head from a tender or hotwell.
Key Components
- Steam Cone: Converging nozzle that accelerates saturated steam from boiler pressure down to atmospheric or below, reaching velocities around 900–1,200 m/s. Throat diameter is typically 3–8 mm on locomotive sizes and must be held to ±0.05 mm to keep the jet coherent.
- Combining Cone: The mixing chamber where the steam jet condenses into the cold feedwater annulus. Its throat is the smallest diameter in the injector and sets the delivery rate — a No. 9 Korting locomotive injector uses roughly a 9 mm combining cone throat for around 90 gallons per hour at 150 psig.
- Delivery Cone: Diverging nozzle that decelerates the high-velocity heated water, converting kinetic energy into pressure higher than the steam supply. Outlet is connected through a clack (non-return) valve into the boiler.
- Overflow Valve: Spring-loaded or weighted check valve below the combining cone that vents excess water and air during starting, then seals when the jet pressurises. A leaking overflow seat is the single most common reason a Korting injector will not pick up after a rebuild.
- Steam Valve and Starting Handle: Operator control that admits steam to the steam cone. On Korting's automatic restart pattern the handle also lifts a feedwater check, so a single lever motion sequences water then steam in the correct order.
- Suction and Delivery Clacks: Non-return valves on the feedwater inlet and boiler delivery line. The delivery clack must seat against full boiler pressure when the injector shuts off, otherwise hot boiler water back-feeds through the injector and steams off the overflow.
Industries That Rely on the Korting Injector
Korting injectors earned their place because they have no moving parts in the working stream, need no separate drive, and recover the steam's heat back into the boiler — feedwater enters at maybe 15 °C and arrives in the boiler at 70–85 °C. That makes them ideal for any steam plant that cannot afford a mechanical feed pump or a stopped engine. They are still standard equipment on most preserved steam locomotives and traction engines, and a backup feed device on many small heritage boilers.
- Heritage Railway: Both live-steam injectors on a preserved LMS Stanier Black 5 4-6-0 — typically a Gresham & Craven exhaust injector on the driver's side and a Korting-pattern live-steam injector on the fireman's side, feeding the boiler at 225 psig.
- Traction Engines and Showmans: Single Korting No. 7 injector fitted to a Burrell showman's road locomotive, drawing from a saddle tank and feeding the boiler at 180 psig during road runs at heritage steam fairs like the Great Dorset Steam Fair.
- Heritage Marine: Backup feed on a Clyde puffer's Scotch boiler at the Scottish Maritime Museum, alongside a Weir reciprocating feed pump — the injector takes over if the pump is stopped in dock.
- Small Industrial Steam: Primary feed on a vertical cross-tube boiler at a heritage rope-walk in Chatham Historic Dockyard, supplying a small horizontal mill engine driving the rope-laying line shaft.
- Stationary Mill Engines: Korting non-lifting injector mounted below the hotwell on a Lancashire boiler at Queen Street Mill in Burnley, feeding 100 psig saturated steam supply to a Roberts horizontal tandem mill engine.
- Steam Launches: Lifting Korting injector on a Stuart-boilered 22 ft launch on Lake Windermere, drawing lake water through a strainer when the engine-driven feed pump is taken out of circuit for inspection.
The Formula Behind the Korting Injector
The headline calculation for sizing or checking a Korting injector is the steady-state delivery rate based on the combining cone throat area, the steam supply pressure, and the feedwater temperature. At the low end of typical heritage operating range — say 60 psig boiler pressure on a small vertical boiler — the injector is sluggish and may struggle to lift water more than 2 m. At the nominal range of 120–180 psig the injector hits its design sweet spot, delivering rated flow with feedwater arriving around 80 °C. Push above 220 psig with the same cone set and you actually lose delivery rate, because the steam jet velocity outruns the condensation rate and the unit becomes unstable.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Qfw | Feedwater delivery rate | m³/h | gallons/hour |
| Cd | Discharge coefficient of the combining cone (typically 0.78–0.85 for a clean Korting cone set) | dimensionless | dimensionless |
| Acc | Combining cone throat cross-sectional area | m² | in² |
| ρw | Density of feedwater at delivery temperature | kg/m³ | lb/ft³ |
| ΔPeff | Effective pressure differential available across the cone train (steam supply pressure minus boiler back-pressure plus condensation gain) | Pa | psi |
Worked Example: Korting Injector in a heritage steam crane boiler
You are sizing a replacement Korting No. 8 lifting injector on a recommissioned 1908 vertical cross-tube boiler aboard a heritage steam crane being returned to demonstration lifting at the Great Yarmouth Time and Tide Museum, where the boiler runs at 140 psig saturated steam, the feedwater is drawn from a dockside tank with 1.8 m of static lift, and the original injector body has been bored out and refitted with a new cone set having a combining cone throat of 8.0 mm.
Given
- Psteam = 140 psig
- Pboiler = 140 psig
- dcc = 8.0 mm
- Cd = 0.82 dimensionless
- Tfw,in = 12 °C
- Static lift = 1.8 m
Solution
Step 1 — compute the combining cone throat area from the 8.0 mm bore:
Step 2 — at nominal 140 psig steam supply, the effective pressure differential after accounting for the condensation momentum gain (a Korting cone set typically delivers around 1.4× the supply pressure as effective ΔP because condensation collapses the steam volume) works out to roughly 1,350 kPa. With feedwater density ρw ≈ 985 kg/m³ at 80 °C delivery temperature:
That is roughly 174 gallons/hour, which is exactly what a No. 8 Korting is rated for at this pressure — comfortable feed for a vertical boiler evaporating around 150 lb/hr.
Step 3 — at the low end of usable range, drop the steam pressure to 60 psig (a cold boiler being warmed up). ΔPeff falls to about 580 kPa:
That is around 114 gallons/hour, but more importantly the 1.8 m static lift gets marginal — below about 50 psig the jet cannot generate enough vacuum to lift water that height, and the injector will simply hiss and dribble at the overflow. Step 4 — at the high end, push to 200 psig:
In practice you will not see 0.95 m³/h. Above roughly 180 psig with this cone set, the steam jet velocity climbs past the rate at which the feedwater can absorb the heat, and the injector starts breaking off intermittently — you get pulsing delivery and water out the overflow. The sweet spot is 120–170 psig.
Result
Nominal delivery is 0. 79 m³/h or about 174 gallons/hour at 140 psig — enough to keep the vertical boiler topped up at full firing rate with margin to spare. Across the range you go from 114 gph at warm-up pressure to a theoretical 209 gph at 200 psig that you will never actually see in service, with the genuine usable sweet spot sitting between 120 and 170 psig. If you measure substantially less than predicted on the test run, the three most likely culprits are: (1) a leaking suction-side union upstream of the combining cone pulling air into the water annulus, which shows up as a hissing intermittent pickup; (2) a combining cone throat machined oversize — even 0.2 mm over 8.0 mm drops delivery by 5–8% and can stop the injector lifting altogether; or (3) feedwater warmer than 25 °C in the dockside tank on a hot day, which prevents full steam condensation and causes the unit to break off after a few seconds of running.
Choosing the Korting Injector: Pros and Cons
The injector is one of three feedwater options on a heritage steam plant — the others being a mechanical reciprocating feed pump (Weir, Worthington) and a centrifugal electric pump on modernised installations. Each has a distinct operating envelope, and most well-found heritage boilers carry at least two different feed devices for redundancy.
| Property | Korting Injector | Weir Reciprocating Feed Pump | Centrifugal Electric Feed Pump |
|---|---|---|---|
| Moving parts in flow path | None | Piston, valves, packing | Impeller, shaft seal |
| Typical delivery rate (small boiler) | 100–300 gph | 200–800 gph continuous | 500–2000 gph |
| Maximum delivery pressure | Up to ~250 psig with correct cone set | Up to 600 psig | Stage-dependent, typically 150–300 psig |
| Feedwater temperature limit | 50 °C inlet max — hotter and it breaks off | Up to 90 °C | Up to 100 °C with NPSH margin |
| Heat recovery to feedwater | Excellent — adds 50–70 °C | None inherent | None inherent |
| Lift capability | Up to ~6 m suction lift | Limited by NPSH, typically 3–4 m | Poor — needs flooded suction |
| Failure modes | Cone scaling, overflow seat leak | Packing wear, valve seat wear | Seal failure, motor/drive failure |
| Cost (typical heritage rebuild) | £400–£900 for cone reset | £1,500–£4,000 overhaul | £600–£2,500 plus drive |
| Best application fit | Locomotives, traction engines, small marine | Stationary mill engines, ships | Modernised heritage installations |
Frequently Asked Questions About Korting Injector
This almost always points to feedwater that is heating up faster than you realise. The injector pulls from the tank, the boiler delivery clack leaks slightly back through the warm cone train when shut, and the tank temperature climbs through the morning. Once the inlet water passes about 50 °C the steam cannot fully condense in the combining cone, and the jet loses its momentum coupling.
Diagnostic check: put a thermometer in the suction line and watch it during a steaming session. If you see the inlet creep above 45 °C, lap in the delivery clack — a 0.05 mm scratch across the seat is enough to back-feed boiler heat into the tank.
The drawing dimension is the throat, but the cone profile matters as much as the throat. A Korting combining cone has a specific included angle — typically 6° to 8° on the converging section — and a sharp transition radius at the throat. If you bored a straight reamed hole rather than profiling the taper, the steam-water mixing zone shifts downstream and the discharge coefficient Cd drops from 0.82 to around 0.75.
The fix is to re-profile on a tracer lathe with the original Korting taper template, or buy a matched cone set from a heritage parts specialist. Do not be tempted to enlarge the throat to compensate for the lost coefficient — you will move the operating point off design entirely.
Decide on the geometry of the feedwater tank first. If the tank can sit higher than the injector body — flooded suction — fit the non-lifting pattern. It picks up faster, runs more reliably at low boiler pressures during warm-up, and tolerates slightly warmer feedwater because it does not depend on jet vacuum to lift water.
If the tank has to sit below the injector (typical on a locomotive tender or a steam crane drawing from a dockside tank), fit the lifting pattern, but be honest about the static lift. A Korting lifting injector handles 6 m at full boiler pressure and clean cones, but in practice you should design for 3–4 m lift to keep margin for cold mornings, scale build-up, and warm summer water.
Run this test cold. Crack the steam valve just enough to get a gentle hiss with no water turned on. Put your hand near the overflow — if you feel strong steam blowing from the overflow, the overflow valve is not seating. That is your problem and the cones are probably fine. Lap the overflow seat with fine paste until it holds.
If almost no steam vents at the overflow but the injector still will not lift water when you turn the suction on, the trouble is in the cone train — usually scale in the combining cone or a stripped steam cone tip. Pull the cones and inspect the throat with a bore gauge.
Bench testing usually exhausts the delivery to atmosphere or to a low-head test rig, so you are measuring the cone set against very little back-pressure. On the boiler it has to work against full boiler pressure plus the static head of water in the boiler plus any loss in the delivery clack. A 15% drop on a 140 psig boiler is normal and expected.
What is not normal is more than 20% drop. If you see that, suspect a delivery clack that is not opening fully — the spring is too stiff, or the lift is restricted by debris under the seat. Pull the clack and check that it lifts freely to its designed travel.
Yes, but the delivery rate will fall by roughly 35–40% and you will be operating very close to the lift-failure threshold. A No. 9 Korting rated 200 gph at 150 psig will deliver around 125 gph at 80 psig, and if your suction lift is more than about 2 m you will get intermittent pickup.
The cleaner solution for museum demonstration boilers running below design pressure is to fit a smaller-numbered cone set scaled to the reduced pressure. A No. 7 cone set at 80 psig is happier than a No. 9 at 80 psig — same body casting, just a different cone train inside.
References & Further Reading
- Wikipedia contributors. Injector. Wikipedia
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