The Schaeffer and Budenberg injector is a steam-powered feedwater appliance that pumps cold water into a live boiler using only the boiler's own steam — no moving parts, no shaft drive. It solves the awkward problem of feeding water into a vessel that is already at higher pressure than the supply tank, by converting steam into a high-velocity jet that drags water along, condenses, and arrives at the check valve with enough momentum to overcome boiler pressure. The result is a compact, self-contained feed device used on locomotive, marine, and stationary boilers up to roughly 16 bar.
Schaeffer and Budenberg Injector Interactive Calculator
Vary steam-cone pressure drop, throat size, boiler pressure, and recovery margin to see jet velocity, steam flow, and delivery pressure.
Equation Used
This simplified injector estimate converts steam-cone pressure drop into jet velocity, then combines that velocity with throat area to estimate ideal steam flow. Delivery pressure is shown as the boiler pressure plus the article's typical 10-15% recovery margin needed to lift the check valve.
- Illustrative nozzle model using fixed steam density rho = 1.0 kg/m3.
- Pressure drop is treated as ideal conversion to jet kinetic energy.
- Recovery margin represents the article's 10-15% delivery pressure above boiler pressure.
- Heat transfer, choking, and condensation losses are not modeled.
Operating Principle of the Schaeffer and Budenberg Injector
The injector works on the Giffard principle, patented in 1858 and refined by Schaeffer and Budenberg of Magdeburg from the 1880s onwards. Steam from the boiler enters a converging steam cone where pressure energy converts to kinetic energy — a typical Schaeffer and Budenberg cone drops 10 bar of steam pressure into a jet moving at well over 1,000 m/s. That jet enters the combining cone, where it meets cold feedwater drawn in either by gravity (non-lifting type) or by the partial vacuum the jet creates (lifting type, capable of lifting roughly 2-3 m of suction head). The steam condenses into the water almost instantly and hands its momentum to the now-warmer water stream.
The combined stream enters the delivery cone, a diverging passage where velocity converts back into pressure. If the cones are sized right, the delivery pressure exceeds boiler pressure by 10-15%, the boiler check valve cracks open, and feedwater flows in. The overflow valve sits between the combining and delivery cones — it stays open during starting, then snaps shut once the jet establishes itself. If your injector keeps spitting from the overflow and refuses to pick up, nine times out of ten the cones are scaled, the strainer is partially blocked, or feedwater temperature is above 50-55°C, which is the practical upper limit for reliable condensation.
Tolerances on the cones matter. The throat diameter of the steam cone on a typical 8 mm Schaeffer and Budenberg unit must hold to within ±0.05 mm — bore it out by 0.1 mm and the injector will draw too much steam, drown the combining cone, and refuse to start above 7 bar. Score the combining cone with a wire brush instead of a proper reamer and you'll get permanent overflow because the jet entrains air through the surface roughness.
Key Components
- Steam Cone: A converging brass or gunmetal nozzle that accelerates boiler steam into a high-velocity jet. Throat diameters typically run 4-12 mm depending on injector class, with bore tolerance held to ±0.05 mm. Wear opens the throat over time, dropping delivery pressure.
- Combining Cone: Where the steam jet meets and entrains feedwater, condensing steam and transferring momentum. The annular gap between steam-cone exit and combining-cone entry is critical — typically 1.5-2.5 mm on a 50 gph unit. Too tight and the jet won't draw water, too wide and condensation completes before the delivery cone.
- Delivery Cone: A diverging passage that recovers velocity into pressure, raising the stream above boiler pressure. Length-to-throat ratio sits around 7:1 for stable recovery. A scored or pitted delivery cone bleeds pressure and prevents the check valve from lifting.
- Overflow Valve: A spring-loaded or clack valve at the junction of combining and delivery cones. Discharges to atmosphere during starting and during any flow instability. If it drips continuously in service, the cones are mismatched or the feedwater is too hot.
- Steam Stop and Water Regulating Valves: Manual valves on the inlet side. The water valve is set first, then steam is admitted progressively — Schaeffer and Budenberg recommend a 2-second open sequence to avoid hammering the cones.
- Feedwater Check Valve (Clack): Boiler-mounted non-return valve that admits delivered water and prevents boiler pressure from blowing back through the injector when it shuts off.
- Strainer: Inline mesh, typically 0.4 mm aperture, on the cold-water inlet. Even a partial blockage starves the combining cone and causes overflow. Inspect every 50 hours of steaming on a heritage installation.
Where the Schaeffer and Budenberg Injector Is Used
The injector turns up wherever a boiler needs feedwater but a mechanical pump is awkward, redundant, or forbidden by the boiler inspector as a sole feed. On a steam locomotive the injector is the legally required second means of feed. On a stationary mill boiler it sits as a backup to the duplex feed pump. Marine launches use them as primary feed when the engine isn't running, since a crosshead-driven pump only works while the engine turns. Schaeffer and Budenberg units specifically still appear on continental European heritage stock because the firm supplied DRG and Reichsbahn locomotives through the 1930s.
- Steam Locomotives: Class 52 Kriegslok 2-10-0 locomotives carried twin Schaeffer and Budenberg lifting injectors as primary and secondary feedwater appliances.
- Stationary Boilers: Lancashire boilers at heritage sites such as Ellenroad Engine House use period Schaeffer and Budenberg injectors as the boiler-inspector-mandated independent second feed.
- Marine Steam: Steam launches on Lake Geneva and Lake Constance fitted Schaeffer and Budenberg non-lifting injectors below the waterline tank to feed vertical boilers when engines were stopped.
- Traction Engines: German Kemna and Lanz road locomotives specified Schaeffer and Budenberg restarting injectors for use while the engine was stationary at a threshing site.
- Industrial Process Steam: Brewery and sugar-refinery package boilers in central Europe used Schaeffer and Budenberg fixed-cone injectors rated 100-2000 kg/h delivery.
- Heritage Restoration: Restored 5-inch and 7¼-inch gauge live-steam locomotives at clubs such as the Bracknell Railway Society fit miniature Schaeffer and Budenberg-pattern injectors at 8 bar working pressure.
The Formula Behind the Schaeffer and Budenberg Injector
The headline number on any injector is delivery rate — kilograms of water per hour into the boiler. The classical Rankine-Kennedy expression links delivery rate to steam-cone throat area and boiler pressure. At the low end of the typical operating range, around 4 bar, the same injector delivers roughly 60% of its nameplate flow because the steam jet velocity scales with √P. At the nominal design point, usually 8-10 bar, the cones operate at peak efficiency. Push beyond the high end, around 14 bar, and you don't gain proportional flow — feedwater temperature rise inside the combining cone starts to defeat condensation and the overflow begins weeping. The sweet spot for a Schaeffer and Budenberg fixed-cone unit sits within ±15% of its rated boiler pressure.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ṁw | Feedwater delivery rate | kg/s | lb/h |
| Cd | Discharge coefficient of steam cone (typically 0.85-0.92) | dimensionless | dimensionless |
| As | Steam cone throat cross-sectional area | m² | in² |
| ρs | Steam density at boiler pressure | kg/m³ | lb/ft³ |
| Pb | Boiler gauge pressure | Pa | psi |
| R | Water-to-steam mass entrainment ratio (typically 8-12 for saturated steam) | dimensionless | dimensionless |
Worked Example: Schaeffer and Budenberg Injector in a recommissioned 1908 hop-kiln boiler
You are sizing the delivery rate from a recommissioned 1908 Schaeffer and Budenberg Class 4 lifting injector being refitted to a vertical cross-tube boiler at a heritage hop-drying kiln in Faversham, Kent, where the boiler supplies low-pressure process steam to the kiln plenum and the trustees want to confirm the original injector can keep pace with steady evaporation across a typical Sunday demonstration day. The injector has a steam-cone throat of 6.5 mm and the boiler runs nominally at 7 bar gauge with a working envelope of 4 to 10 bar.
Given
- dthroat = 6.5 mm
- Pb,nom = 7 bar gauge
- Pb,low = 4 bar gauge
- Pb,high = 10 bar gauge
- Cd = 0.88 dimensionless
- R = 10 dimensionless
Solution
Step 1 — compute the steam-cone throat area from the 6.5 mm bore:
Step 2 — at the nominal 7 bar gauge (8 bar absolute), saturated steam density is roughly 4.16 kg/m³. Apply the delivery formula:
That's the design point. The injector will lift cold water roughly 2 m, condense the jet cleanly, and feed 81 kg of water per hour into the boiler — enough to match the steady evaporation of a small hop-kiln package boiler running mid-load.
Step 3 — at the low end of the operating range, 4 bar gauge, steam density falls to 2.67 kg/m³ and pressure drops:
49 kg/h is roughly 60% of nominal — you can feel the difference as a noticeably softer hiss at the overflow during pickup, and the delivery clack on the boiler ticks more slowly. At this end of the range the injector is close to its starting threshold; below about 3 bar a Class 4 fixed-cone unit will refuse to pick up at all.
Step 4 — at the high end, 10 bar gauge, steam density rises to 5.64 kg/m³:
In theory 112 kg/h. In practice on a fixed-cone Schaeffer and Budenberg you'll see 95-100 kg/h once feedwater warms to 35°C in the suction line, because the entrainment ratio R drops as inlet temperature rises. The combining cone runs near its condensation limit and the overflow weeps a thin steady drip — that drip is your signal you're at the top of the working envelope.
Result
At the 7 bar nominal point the injector delivers about 81 kg/h of feedwater into the boiler, comfortably matching the kiln's mid-load evaporation. Across the working envelope you'll see roughly 49 kg/h at 4 bar and a theoretical 112 kg/h at 10 bar, with the practical sweet spot landing between 6 and 9 bar where the cones run cleanly and the overflow stays bone dry. If your measured delivery comes in 20% below predicted, check three things in order: a worn or wire-drawn steam cone throat (any visible scoring on the brass means scrap and replace, not lap), a mismatched cone gap after a previous strip (the 1.5-2.5 mm annular distance between steam and combining cones is set with a feeler gauge, not by eye), and feedwater temperature creeping above 50°C from a hot supply tank, which collapses the entrainment ratio and starves delivery.
Choosing the Schaeffer and Budenberg Injector: Pros and Cons
An injector isn't always the right answer. The choice between a Schaeffer and Budenberg injector, a crosshead-driven feed pump, and a small electric feed pump comes down to where your duty cycle sits, what your boiler inspector will accept, and how much pipework you want to maintain.
| Property | Schaeffer and Budenberg Injector | Crosshead-Driven Feed Pump | Electric Feed Pump |
|---|---|---|---|
| Delivery rate range (kg/h) | 50-2000 | 100-5000 | 20-50000 |
| Operating pressure ceiling | ~16 bar | ~25 bar | Boiler-limited |
| Maximum feedwater inlet temperature | 50-55°C | 90°C+ | 90°C+ |
| Works while engine is stopped | Yes | No | Yes (with power) |
| Moving parts in service | None (cones only) | Many (rod, valves, glands) | Motor + impeller |
| Typical maintenance interval | 500-1000 hours (descale) | 200-400 hours (gland packing) | 2000+ hours |
| Sensitivity to scaled or dirty water | High — cones foul fast | Moderate | Low |
| Cost (heritage-grade unit) | £300-£900 | £800-£3000 | £400-£2500 |
| Boiler-inspector acceptance as sole feed | Rarely accepted alone | Usually accepted | Usually accepted with backup |
Frequently Asked Questions About Schaeffer and Budenberg Injector
The water valve setting that worked at 6 bar is now flooding the combining cone. As boiler pressure rises, the steam jet wants more water to balance momentum, but the geometry only matches one ratio cleanly — the rest of the range needs the water valve trimmed. Crack the water valve back roughly a quarter turn, then re-admit steam.
If trimming doesn't fix it, suspect the steam cone throat is worn oversize. A throat that's grown from 6.5 mm to 6.7 mm passes 7% more steam, drowns the combining cone at high pressure, and pushes the working envelope down to 4-7 bar.
The formula assumes saturated steam at the cone, cold feedwater under 25°C, and a clean cone profile. In a real heritage installation the most common single cause of a 15-25% shortfall is feedwater preheating in an uninsulated copper suction line that runs near the firebox or smokebox.
Measure the water temperature at the strainer with a probe thermometer. If it reads above 35°C you've found your loss. The fix is either a longer suction route or a wrap of fibreglass lagging — both restore 10-15% of delivery without touching the cones.
Non-lifting, every time. A lifting injector will work in that geometry, but you're using up its suction capability for nothing — it's designed to lift water 2-3 m against gravity. With the tank below the boiler you have positive suction head, and a non-lifting unit will pick up more reliably, run cooler, and tolerate hotter feedwater (up to 60°C versus 50°C for lifting).
The only reason to specify lifting in your case is if you anticipate the tank ever being relocated above-deck. Otherwise the non-lifting variant is simpler, cheaper, and more forgiving.
Not necessarily, but something is wrong. Continuous overflow weep means the delivery cone isn't recovering enough pressure to seat the overflow valve. Three causes in order of likelihood: the overflow valve seat is pitted or has a piece of grit under it (strip and lap), the delivery cone is internally scaled and the rough surface is bleeding velocity (descale with 5% citric acid for 30 minutes), or the boiler check valve is leaking back and pressurising the delivery side from the wrong direction.
Check the clack first by closing the boiler isolating valve — if the weep stops immediately, the clack is your problem, not the injector.
No. The cone profile is matched as a set — steam cone throat, combining cone entry, and delivery cone throat work as a calibrated trio. Opening the steam cone alone unbalances the entrainment ratio, drowns the combining cone, and you'll lose pickup at the bottom of the pressure range without gaining meaningful flow at the top.
If you genuinely need more delivery, fit the next size up Schaeffer and Budenberg unit (Class 5 instead of Class 4 in the period catalogue) or run twin smaller injectors in parallel. Bored cones are a permanent downgrade.
Heat soak. The injector body itself has warmed up over the steaming day, and so has the suction pipe between the tank and the cones. Once feedwater entering the combining cone is above about 50°C, condensation can't keep up with the steam jet and the unit overflows continuously.
Quick test — feel the body of the injector. If it's too hot to hold, that's your answer. The classical fix is a small bypass that lets a trickle of cold water flush the suction line for 20 seconds before you crack the steam valve.
Restarting (also called automatic) variants have a sprung overflow that re-establishes the jet automatically if the supply burps or the boiler pressure dips momentarily. Fixed-cone units need manual reset — close steam, close water, restart the sequence.
Fit restarting on anything that bounces around mechanically (locomotives, traction engines, marine launches in chop). Fit fixed-cone on stationary boilers where the operator is right there at the firing platform — they're cheaper, simpler, and the pickup is slightly cleaner because there's one less moving part to balance.
References & Further Reading
- Wikipedia contributors. Injector. Wikipedia
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