A Park Injector is a steam-driven feedwater injector of the lifting type, patented by John Park in the late 19th century to deliver cold water into a boiler against its own working pressure. It works by passing live steam through a converging steam cone, which entrains water drawn up from the tank, then a combining cone condenses the mixture and a delivery cone converts the high velocity into pressure greater than boiler pressure. It exists to feed locomotives, traction engines and portable boilers without any moving pump parts. A typical Park Injector delivers 200–800 gallons per hour at 100–180 psig.
Park Injector Interactive Calculator
Vary boiler pressure, suction lift, feed rate, and jet speed to see the injector pressure recovery requirement and animated flow through the cones.
Equation Used
This calculator estimates the pressure the delivery cone must exceed: boiler pressure plus suction lift head. It compares that demand with the ideal kinetic pressure equivalent of the injector jet and reports the minimum recovery fraction needed. Real Park Injectors also depend on cone geometry, condensation, air leaks, water temperature, and scale.
- Water density is 1000 kg/m3.
- Suction lift is converted directly to static head.
- Jet kinetic head is an ideal equivalent used for teaching pressure recovery.
- US gallons are used for feedwater flow.
How the Park Injector Works
The Park Injector sits between the saddle tank or water cart and the boiler clack valve, and it does its job with no pistons, no cams, no rotating shaft — just three carefully bored cones in series. Steam enters the steam cone at boiler pressure, accelerates through the converging throat, and exits as a high-velocity jet at maybe 1200 m/s. That jet drags water up through the suction line by viscous entrainment — this is why the Park is called a lifting injector, it can pull water up roughly 1.5 to 2.5 metres of vertical lift before priming fails. The water and steam meet in the combining cone, the steam condenses on contact with the cold water, and the resulting liquid jet now carries nearly all the original momentum but at a much smaller volume. That liquid jet then enters the delivery cone, a diverging passage that trades velocity for pressure, and exits at a head higher than boiler pressure — which is the only way it can push past the boiler clack.
Why three cones and not two? Because the steam cone has to be sized for sonic flow, the combining cone has to give condensation enough length to complete before the delivery cone, and the delivery cone has to recover pressure without flow separation. Get the cone bores wrong by even 0.2 mm on a small injector and the thing will not pick up — it will blow water back out the overflow chamber and refuse to feed. The overflow chamber is the tell-tale: when the injector is properly singing, overflow is a thin dribble or nothing at all; when it's failed to pick up, overflow gushes and you can hear the steam roaring straight through.
Common failure modes you would see on a working engine: scale buildup in the combining cone bore narrows the throat and chokes flow, a worn steam cone tip lets steam bypass the entrainment zone, feedwater above about 50°C kills the condensation step entirely because the steam cannot dump its latent heat fast enough, and a leaking suction-side union sucks air instead of water and the injector simply will not lift. The non-lifting variant sits below tank level and avoids the lift problem, but the Park's whole selling point is that it lifts.
Key Components
- Steam Cone: A converging nozzle, typically 3–8 mm throat diameter on locomotive sizes, that accelerates boiler steam to sonic velocity. Throat tolerance is tight — a worn or scaled cone increases throat area, drops jet velocity, and the injector loses its ability to lift water.
- Combining Cone: Where steam jet meets cold feedwater and the steam condenses on the water surface. Length is critical — typically 4–6 throat-diameters long — because condensation must complete before the mixture enters the delivery cone, otherwise you get two-phase flow and the jet breaks up.
- Delivery Cone: A diverging passage that recovers static pressure from the high-velocity liquid jet. The divergence half-angle is held to roughly 4–7° to avoid flow separation; steeper and the jet detaches from the wall, pressure recovery collapses, and the injector kicks back through the overflow.
- Overflow Chamber: Open to atmosphere and fitted between combining and delivery cones. During normal operation it sees a small dribble; during failed pickup it discharges the full steam-and-water flow. It is your primary diagnostic — listen and look at the overflow before anything else.
- Suction Pipe & Strainer: Carries water from tank to combining cone. Must be airtight on the pressure-drop side; a single weeping union ruins the lift. Strainer mesh is typically 40–60 holes per inch to keep grit out of the cones.
- Feed Check Valve (Clack): Mounted on the boiler shell, opens only when delivery pressure exceeds boiler pressure. A sticking clack masquerades as a failed injector — always lift the clack manually before condemning the cones.
Where the Park Injector Is Used
The Park Injector earned its keep on portable and traction engines where a mechanical feedpump was either absent or only ran when the engine was turning. Heritage operators still rely on Parks today because they will feed a stationary boiler with the engine stopped — try doing that with a crosshead pump. You will find them on agricultural traction engines, road locomotives, steam wagons, vertical-boiler launches, and stationary mill auxiliary boilers.
- Heritage Traction Engines: A Burrell 7 NHP General Purpose engine at the Great Dorset Steam Fair uses a No. 7 Gresham & Craven Park-pattern injector as its primary feed when running the saw bench.
- Steam Road Wagons: Foden C-type 5-ton steam wagons fitted Park-pattern lifting injectors to feed the cross watertube boiler from the saddle tank during stationary loading.
- Heritage Railways: Ffestiniog Railway 'Linda' and 'Blanche' carry Gresham lifting injectors of Park lineage to feed the boiler when the engine is standing at Tan-y-Bwlch.
- Portable Boilers: A Marshall portable engine on demonstration at the Lincolnshire Steam Rally uses a Park-style injector for boiler feed because the crank-driven pump only runs when the flywheel turns.
- Steam Launches: Heritage Thames steam launches such as those at Henley fit small Park injectors of 1/4 to 3/8 inch nominal size to top up vertical fire-tube boilers from a hull tank set below the boiler waterline.
- Showman's Engines: Fowler Showman's Road Locomotives running the Carters Steam Fair circuit use Park-pattern injectors to feed the boiler while the dynamo and lighting set run overnight without the wheels turning.
The Formula Behind the Park Injector
What you actually want to know in service is how much water per hour the injector will deliver at a given boiler pressure and feedwater temperature. The delivery scales with steam cone throat area and boiler pressure, and it falls off sharply as feedwater temperature climbs because the combining cone runs out of capacity to condense the steam. At the low end of the typical range — say 80 psig boiler pressure and 25 °C water — a No. 7 will deliver roughly 450 gph and pick up reliably from a 2 m lift. At nominal 130 psig and 15 °C feedwater you hit the design sweet spot, around 700 gph with a clean overflow. Push above 180 psig or feedwater warmer than 45 °C and pickup gets unreliable — the injector starts to splutter and kick back. The formula below gives a first-order steam-mass and water-delivery estimate using conservation of momentum across the cones.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ṁw | Feedwater mass flow rate delivered to boiler | kg/s | lb/h |
| ṁs | Steam mass flow rate through steam cone | kg/s | lb/h |
| hs | Specific enthalpy of saturated steam at boiler pressure | kJ/kg | Btu/lb |
| h2 | Specific enthalpy of mixture leaving combining cone | kJ/kg | Btu/lb |
| hw | Specific enthalpy of feedwater entering suction | kJ/kg | Btu/lb |
| At | Steam cone throat area (used to find ṁs via choked-flow relation) | mm² | in² |
Worked Example: Park Injector in a Burrell traction engine Park injector
You are sizing the feedwater delivery from a Gresham & Craven No. 6 Park-pattern lifting injector being refitted to a recommissioned 1912 Burrell 6 NHP single-crank compound traction engine returning to demonstration service at a heritage agricultural museum near Thursford in Norfolk, where the boiler runs at 160 psig working pressure, the water cart sits 1.8 m below the injector centreline, and the river-fed feedwater enters the suction at 12 °C. The steam cone throat measures 4.8 mm diameter and you need to know whether the injector will keep up with a sustained 6 lb/min evaporation rate while the engine runs the threshing drum.
Given
- pb = 160 psig (≈ 12.0 bar abs)
- dthroat = 4.8 mm
- Tfw = 12 °C
- hs = 2784 kJ/kg (sat. steam, 12 bar abs)
- hw = 50 kJ/kg (water at 12 °C)
- h2 = 335 kJ/kg (mixture, target ≈ 80 °C)
Solution
Step 1 — calculate the steam mass flow at the nominal 160 psig operating point. The steam cone throat is choked, so use the choked-flow relation for saturated steam (ṁs ≈ At × p0 × 0.0408 in SI mixed units, an empirical Napier-style approximation):
Step 2 — apply the energy-balance ratio to get nominal feedwater delivery:
That nominal 66 imp gph (about 80 US gph) is exactly where a No. 6 should sit at 160 psig. Against the 6 lb/min (≈ 165 kg/h) evaporation demand, the injector has roughly 80% headroom — comfortable for the threshing duty.
Step 3 — at the low end of the typical operating range, drop boiler pressure to 80 psig (≈ 6.5 bar abs). Steam mass flow scales roughly with absolute pressure, so ṁs falls to about 19 kg/h:
That is still ahead of evaporation at low pressure but the lift becomes marginal — at 80 psig the steam jet velocity is lower and the 1.8 m suction lift is right at the edge of reliable pickup.
Step 4 — at the high end, push to 200 psig with feedwater warmed to 45 °C (hw ≈ 188 kJ/kg). Steam mass flow rises to about 44 kg/h, but the enthalpy ratio drops:
In theory delivery more than doubles — in practice the warmer feedwater starves the combining cone of condensing capacity and the injector spits and breaks pickup intermittently above feedwater of about 45 °C. The honest deliverable peak is closer to 90 imp gph before reliability collapses.
Result
Nominal delivery is approximately 300 kg/h, or 66 imperial gallons per hour, against a boiler demand of 165 kg/h — so the No. 6 Park has roughly 80% headroom over the threshing-duty evaporation rate, which is what you want for a long demonstration day. Across the operating range, expect 36 gph at 80 psig with cold water, 66 gph at the 160 psig design point, and a theoretical 160 gph at 200 psig that you will never actually see in service because warm feedwater breaks the condensation step. The sweet spot sits squarely at 130–170 psig with feedwater under 30 °C. If your measured delivery is 30% below predicted, suspect three things in this order: (1) the steam cone throat has eroded or scaled past 5.0 mm — pull it and gauge it with a pin gauge; (2) the suction-side union on the water cart is drawing air, easily checked by smearing soap suds on the joint while the injector is trying to pick up; (3) the feed check valve is partially seized so the injector is delivering against a higher-than-nominal back pressure, which you confirm by lifting the clack lever by hand and listening for the injector note to change.
Park Injector vs Alternatives
The Park Injector is one of three common ways to feed a small steam boiler. Each has a real operating envelope, and the right choice depends on whether the engine spends time stationary, how warm the feedwater is, and how much you trust your operator to read the overflow.
| Property | Park Lifting Injector | Non-Lifting Injector | Crosshead Feed Pump |
|---|---|---|---|
| Suction lift capability | Up to 2.5 m | 0 m (must sit below tank level) | Up to 4 m with foot valve |
| Maximum feedwater temperature | ~ 45 °C before pickup fails | ~ 55 °C | Up to 90 °C |
| Operates with engine stationary | Yes | Yes | No — only when crosshead moves |
| Typical delivery (No. 6 size, 150 psig) | 60–80 gph | 70–90 gph | Depends on stroke; ~ 50–120 gph at working RPM |
| Moving parts subject to wear | None | None | Piston, packing, valves |
| Sensitivity to scale and grit | High — cones bore directly affected | High | Moderate — valves can be lapped |
| Capital cost (heritage market) | £400–£900 | £300–£700 | £800–£1500 plus drive linkage |
| Restart after failed pickup | Close steam, vent overflow, retry — 5–10 s | Usually self-priming on retry | N/A — keeps pumping |
Frequently Asked Questions About Park Injector
The combining cone is heating up. As cone-block temperature rises past about 60 °C, the incoming feedwater warms in the suction passage on its way to the cone, and the steam can no longer condense fast enough to maintain the liquid jet. The overflow opens, the jet breaks, and pickup is lost.
Two real causes: feedwater drawn from a tank that already sits in the sun next to the firebox, or a steam cone leaking past its seat into the cone block when the regulator is shut, which slow-cooks the assembly. Check feedwater temperature at the suction union with a stem thermometer — if it climbs past 40 °C during the run, you have heat soak, not a cone problem.
Size to roughly 1.5× peak evaporation rate at the boiler's working pressure, not at maximum pressure. A boiler evaporating 100 kg/h sustained needs an injector good for 150 kg/h at the normal running pressure, because you want to refill during light-firing periods rather than running the injector continuously.
Rule of thumb for Gresham-pattern Parks: No. 5 covers boilers up to 50 sq ft heating surface, No. 6 from 50 to 90 sq ft, No. 7 from 90 to 140 sq ft. Going one size up rarely hurts — you just run it for shorter bursts. Going one size down means it runs continuously and any small fault stops the engine.
The textbook energy balance assumes the steam cone is delivering dry saturated steam. On a real traction engine, the steam at the injector valve is often wet — 5 to 10% moisture is normal if the dome is small or the regulator is partially cracked. Wet steam carries less enthalpy per unit mass, so the (hs − h2) numerator drops and so does delivery.
Diagnostic: open the steam valve fully rather than feathering it. A fully-opened injector valve gives drier steam because the velocity through the take-off pipe blows entrained water past it. If delivery jumps when you stop feathering, you had a wet-steam problem.
You can, but watch the cones. Sodium-based softeners leave a soft sludge that builds up in the combining cone over weeks of running and progressively narrows the throat. The injector slowly loses delivery — typically 10% per season — until pickup gets marginal.
Pull the cone block every winter shut-down, run a brass cone-reamer through the combining cone to its original diameter, and check the steam cone for pitting. If the throat has grown more than 0.15 mm over original spec, replace the cone — do not try to bush it.
Surge in the water cart. When the engine rolls, water in a partly-full tank slops fore-and-aft, and the suction pickup pipe momentarily breaks surface and draws air. The injector sees an air slug, loses its prime, and refuses to restart until the slug clears.
Two fixes: fit a swirl baffle around the pickup pipe at the tank floor, or keep the tank above two-thirds full when running. On Foden wagons the original solution was a fully-baffled saddle tank with the pickup at the centre rather than the end.
You are operating right at the edge of the injector's pressure or temperature envelope. A fine spray means the delivery cone is just barely recovering enough pressure to crack the boiler clack, and a small fraction of flow is escaping back through the overflow each cycle.
It usually appears when boiler pressure has climbed close to the injector's designed maximum, or when feedwater has warmed mid-run. Drop boiler pressure 10 psi by easing the firing or wait two minutes for the feed to cool, and the spray should close back down to a clean dribble.
References & Further Reading
- Wikipedia contributors. Injector. Wikipedia
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