The Sellers' Restarting Injector is a steam-driven feedwater appliance that forces water into a live boiler against its own pressure without any moving pumping parts. It works by accelerating steam through a converging nozzle to supersonic velocity, condensing that steam into an incoming water stream inside a combining cone, and converting the resulting kinetic energy back into pressure inside a delivery cone. The restarting feature lets it re-prime automatically after a momentary break in feed — critical on a working locomotive where vibration and heat upsets routinely interrupt the jet. William Sellers patented it in Philadelphia in 1887, and it remained the standard locomotive injector on US roads for 50 years.
Sellers' Restarting Injector Interactive Calculator
Vary steam-nozzle throat, boiler pressure, cone match, and check-valve crack pressure to see injector feed capacity and restart margin.
Equation Used
This calculator uses the article's injector sizing examples as an empirical throat-capacity curve. The steam throat diameter sets feedwater capacity, boiler pressure adjusts the available jet energy, and the check valve must see delivery pressure above boiler pressure plus crack pressure. The cone mismatch output compares the combining-cone throat to the steam-nozzle throat; small Sellers injectors require very close matching.
- Capacity is interpolated through the article points: 4.5 mm throat gives 4000 lb/hr and 6.5 mm gives 9000 lb/hr at 150 psig.
- Feed capacity scales with the square root of boiler pressure for this simplified engineering estimate.
- Steam jet velocity is normalized to 1200 m/s at 150 psig saturated steam.
- Combining cone match is judged by absolute throat mismatch from the steam nozzle.
Inside the Sellers' Restarting Injector
An injector looks like it should not work. You take steam from the boiler, point it at a tube full of water, and that mixture pushes itself back into the same boiler at higher pressure than the steam you started with. The trick is energy form — the steam nozzle converts pressure into velocity (roughly 1,200 m/s for saturated steam at 150 psig), the combining cone condenses that high-velocity steam into the cold feedwater so the mixture stays liquid but keeps most of the momentum, and the delivery cone slows that liquid jet down again so its kinetic energy reappears as pressure. The check valve on the boiler end opens once delivery pressure exceeds boiler pressure, and water flows in.
The restarting feature is what separates a Sellers from a plain Giffard. On a locomotive the jet breaks constantly — a slug of hot water from the tender, a knock from the rail joints, a steam pressure dip when the regulator closes. A plain injector that breaks its jet stays broken until the fireman shuts it off and re-cracks the steam valve. The Sellers design adds an overflow chamber with a hinged or weight-loaded flap valve between the combining cone and the delivery cone. When the jet breaks, water and steam dump out the overflow at atmospheric pressure, the combining cone re-fills, and the jet re-establishes itself in under a second without crew intervention.
If the cone tolerances are wrong, the whole thing fails. The combining cone throat must match the steam-nozzle throat to within roughly 0.05 mm on a 5 mm bore — too wide and the jet won't pull a vacuum to lift water, too narrow and the condensed mixture chokes and dumps to overflow continuously. Common failure modes are scale build-up inside the cones (you'll hear the injector start hot and quit within 30 seconds as the throat closes), a leaking overflow flap (steam roars out the overflow even when the injector is supposedly running), and a worn steam-nozzle tip from wet steam erosion which broadens the jet and drops delivery pressure below boiler pressure so the boiler check stays shut.
Key Components
- Steam Nozzle (Converging): Accelerates boiler steam from rest to roughly 1,000–1,400 m/s at the throat. The throat diameter sets the steam mass flow and therefore the injector's water-handling capacity — a 4.5 mm throat suits a 4,000 lb/hr feed, a 6.5 mm throat suits 9,000 lb/hr at 150 psig.
- Combining Cone: Mixes the high-velocity steam with cold feedwater so the steam condenses and gives up its momentum to the water. Throat must be matched to the steam nozzle within ±0.05 mm on small injectors; surface finish of Ra 0.4 µm or better is required to prevent cavitation pitting.
- Overflow Chamber and Flap: Sits between the combining cone and delivery cone. Vents excess water and any unstable jet conditions to atmosphere, then re-seats automatically once the jet stabilises. The flap weight and seat angle are what give the injector its restarting behaviour — too light and it chatters, too heavy and the injector won't restart at low boiler pressure.
- Delivery Cone (Diverging): Slows the high-velocity water mixture and recovers kinetic energy as pressure. Outlet pressure must exceed boiler pressure plus check-valve crack pressure (typically boiler + 5 psi) for delivery to occur.
- Boiler Check Valve: Non-return valve on the boiler shell that admits feedwater only when injector delivery pressure exceeds boiler pressure. Crack pressure of 3–5 psi is normal; a check that hangs open back-feeds the injector and floods the overflow.
- Steam and Water Cocks: Crew controls. On a Sellers, opening the steam cock alone is enough to start the injector — it lifts its own water through the suction line up to about 6 ft of lift on a hot day with 100 psig steam.
Where the Sellers' Restarting Injector Is Used
The Sellers' restarting injector earned its place on rough-riding locomotives where a plain injector would quit five times an hour. It also found work on traction engines, stationary mill boilers, and small marine plant — anywhere a simple, pumpless feedwater device with no moving parts needed to keep working through vibration and pressure swings. Heritage operators still favour Sellers-pattern injectors for their tolerance to dirty water and their ability to re-prime without crew attention.
- Heritage Railways: Strasburg Rail Road's ex-Norfolk & Western 4-8-0 No. 475 still feeds with a pair of Sellers Class L injectors mounted under the cab apron.
- Traction Engines: Case 65 HP road locomotives at the Rough and Tumble Engineers' Historical Association in Kinzers, Pennsylvania run Sellers Type 1893 lifting injectors for tender feed during ploughing demonstrations.
- Stationary Mill Boilers: The Hanford Mills Museum in East Meredith, New York uses a Sellers Type N restarting injector as the auxiliary feed on its 1898 Atlas horizontal return-tube boiler when the mechanical pump is offline.
- Small Marine Plant: Replica steam launches at the Antique Boat Museum in Clayton, New York fit Penberthy and Sellers-pattern injectors as backup feed alongside crosshead pumps.
- Industrial Boiler Houses (Historical): Baldwin Locomotive Works in Philadelphia specified Sellers injectors on every domestic locomotive shipped from 1890 through the 1920s as standard non-lifting feed equipment.
- Steam Education and Training: The Railway Heritage Centre at Steamtown NHS in Scranton uses a cutaway Sellers injector for fireman-training demonstrations showing the overflow flap behaviour during restart.
The Formula Behind the Sellers' Restarting Injector
What you actually want to know is the water delivery rate the injector will give you at a given boiler pressure and feedwater temperature — because that's what tells you whether one injector can keep up with steaming rate or whether you need to run both. The governing relationship comes from a steady-flow energy balance across the injector: the enthalpy the steam gives up must equal the enthalpy and kinetic energy the water gains. At the low end of typical operating range — say 80 psig boiler pressure with 60°F feedwater — a given injector delivers near its rated capacity because the temperature lift is small. At the high end — 200 psig boiler with 110°F feedwater from a tender that's been sitting in the sun — capacity drops sharply, sometimes to 70% of rated, because the injector can only condense a limited amount of steam before the discharge becomes too hot to maintain the jet. The sweet spot for most heritage Sellers injectors is 100–160 psig with feedwater below 90°F.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| ṁw | Water delivery mass flow rate to the boiler | kg/s | lb/hr |
| ṁs | Steam mass flow through the steam nozzle | kg/s | lb/hr |
| hs | Specific enthalpy of supply steam at boiler pressure | kJ/kg | BTU/lb |
| hd | Specific enthalpy of mixed delivery water at delivery temperature | kJ/kg | BTU/lb |
| hw | Specific enthalpy of feedwater entering the suction | kJ/kg | BTU/lb |
Worked Example: Sellers' Restarting Injector in a recommissioned Sellers Class L injector
You are sizing the water delivery rate across three boiler pressures on a recommissioned Sellers Class L Number 9 restarting injector being returned to service on a 1925 Vulcan Iron Works 0-6-0T at the Boothbay Railway Village in Maine, where the locomotive runs visitor trains on a 24-inch gauge loop and the injector takes suction from a tender holding 60°F feedwater through a 1.25-inch suction line with about 18 inches of lift to the injector body. Steam-nozzle throat measures 5.6 mm.
Given
- Steam nozzle throat diameter = 5.6 mm
- Feedwater temperature = 60 °F
- hw at 60°F = 28 BTU/lb
- Nominal boiler pressure = 140 psig
- Low operating pressure = 80 psig
- High operating pressure = 180 psig
- Delivery temperature target = 150 °F
- hd at 150°F = 118 BTU/lb
Solution
Step 1 — at nominal 140 psig, the saturated steam enthalpy is hs = 1194 BTU/lb. Compute the water-to-steam ratio from the energy balance:
Step 2 — estimate steam mass flow through a 5.6 mm throat at 140 psig using a choked-flow coefficient of 0.0165 lb/hr per psia per mm² of throat area. Throat area = π × (5.6/2)2 = 24.6 mm², absolute pressure = 154.7 psia:
Step 3 — nominal water delivery at 140 psig:
That matches the Sellers catalogue rating of 4,300 lb/hr for a No. 9 at 140 psig with 60°F water — a comfortable margin over the locomotive's 2,800 lb/hr peak steaming rate. At the low end, 80 psig (94.7 psia), steam mass flow drops with absolute pressure and hs falls slightly to 1183 BTU/lb:
That's still adequate but only just — at 80 psig the injector is right at the edge and any further pressure dip during shunting will starve the boiler. At the high end, 180 psig (194.7 psia), steam flow rises but so does the temperature lift and the available enthalpy ratio actually tightens slightly:
At 180 psig you have headroom to spare — the injector will deliver well over the locomotive's needs and you can throttle the water cock to trim feedwater temperature.
Result
Nominal delivery at 140 psig with 60°F feedwater is 4,300 lb/hr, comfortably above the locomotive's 2,800 lb/hr peak steaming demand. At the low end of 80 psig the injector falls to roughly 2,850 lb/hr — barely enough, and a hard regulator opening will pull the boiler down. At the high end of 180 psig you have around 5,460 lb/hr, more than the boiler can ever consume, and the operating sweet spot sits at 120–160 psig where margin and water economy both look good. If your measured delivery comes in 15–20% below these numbers, the most common causes are: feedwater temperature above 90°F (every 10°F lift above design strips roughly 100 lb/hr off capacity), a partially blocked tender strainer dropping suction-line pressure so the combining cone can't hold vacuum, or an out-of-round combining cone throat — Sellers cones go oval after about 8,000 service hours and the injector becomes increasingly hard to pick up from cold even though it runs fine once started.
When to Use a Sellers' Restarting Injector and When Not To
A Sellers' restarting injector is one of three feedwater options you'd realistically consider on a working steam plant. Each has a different cost, reliability profile, and operating envelope. Here's how they line up on the dimensions that actually matter when you're sizing feed equipment.
| Property | Sellers' Restarting Injector | Crosshead-Driven Feed Pump | Steam-Driven Reciprocating Feed Pump (Worthington/Weir) |
|---|---|---|---|
| Moving parts | Zero | 5–8 (piston, rod, valves, eccentric) | 10+ (steam piston, water piston, valves, linkage) |
| Capacity range (typical) | 1,500–10,000 lb/hr per unit | 2,000–8,000 lb/hr, locomotive-fixed | 500–50,000 lb/hr |
| Maximum suction lift | 6–8 ft (lifting type), 0 ft (non-lifting) | 10–15 ft | 20+ ft |
| Feedwater temperature limit | ~110°F before capacity collapses | Up to boiling, with NPSH margin | Up to 200°F+ |
| Restart after jet break | Automatic in <1 second | N/A — pump runs continuously | N/A — pump runs continuously |
| Capital cost (heritage rebuild) | $1,500–4,000 | $8,000–20,000 (locomotive-mounted) | $15,000–60,000 |
| Service life between rebuilds | 3,000–8,000 hours (cone wear) | 10,000–20,000 hours | 20,000–40,000 hours |
| Sensitivity to dirty water | Low — passes small grit | High — scores piston, cuts seats | Moderate — depends on valve design |
Frequently Asked Questions About Sellers' Restarting Injector
That's the classic signature of a feedwater temperature problem combined with a marginal cone clearance. When the cones are cold, the steam condenses fully and the jet holds. As the injector body soaks up heat and the suction water warms past about 100°F, the steam can't condense fast enough inside the combining cone — vapour bubbles disrupt the jet and it dumps to overflow.
Check the suction-line routing first. A line that runs along the boiler lagging or over the firebox throat sheet picks up enough radiant heat to push tender water from 70°F to 110°F by the time it reaches the injector. Re-route the line away from hot surfaces or wrap it in heat-reflective lagging. If the lift is fine, the next suspect is a steam-nozzle that's been reamed oversize during a previous overhaul — even 0.1 mm too large floods the cone with more steam than it can condense at warm-water conditions.
If the injector body sits below tender water level by at least 6 inches at minimum tender level, fit a non-lifting type — it's simpler, more tolerant of dirty water, restarts faster, and handles slightly hotter feedwater because it doesn't need to maintain a lift vacuum. If the injector has to sit above water level (common on side-tank locos and many traction engines), you have no choice but a lifting type.
The mistake people make is fitting a non-lifting injector then realising the tender drains below injector level when the train is on a downgrade. The injector then can't restart and you're shovelling cold coal trying to keep pressure up. Look at your worst-case track gradient and minimum tender level before deciding.
A clean Sellers should overflow strongly during pickup, then snap shut to a few drops per minute once the jet is delivering. Continuous overflow under running conditions almost always points to the overflow flap not seating properly. The flap is usually bronze on a bronze seat, and any grit between the two lifts the flap a fraction of a millimetre — enough to bleed perhaps 5% of delivery flow back to atmosphere.
Pull the overflow cap and lap the flap into its seat with fine paste. While you're in there, check the flap pivot pin — if it's worn the flap doesn't drop square onto the seat and you'll never get a tight seal regardless of lapping.
The energy-balance formula gives you a thermodynamic ceiling assuming perfect mixing and no losses. Real injectors hit about 65–75% of the ideal water-to-steam ratio because some steam escapes condensation in the combining cone and ends up in the overflow during steady running. The Sellers catalogue numbers already include this real-world derating, which is why a calculated 18,500 lb/hr scales down to a published 4,300 lb/hr for the same throat size.
Use the formula to predict how delivery changes with pressure or feedwater temperature — the ratios are accurate even if the absolute numbers need scaling. If catalogue data exists for one operating point, anchor your calculation to that point and use the formula to extrapolate to others.
The restarting feature depends on the overflow flap dropping closed once the cone re-fills with water. If the flap is sticking open — usually because of mineral scale on the pivot or a corroded pivot pin — the cone vents to atmosphere continuously and never builds the vacuum needed to draw the next slug of feedwater through.
Test it cold: with the steam off, lift the flap by hand and let it drop. It should fall freely under its own weight and seat with a soft click. If it hangs at any point in its travel, strip the pivot, clean it, and reassemble dry — no grease, because grease attracts grit and makes the problem worse within a few weeks of running.
Yes, but watch for two things. Anti-foam compounds based on polyglycols can leave a sticky residue inside the cones over time — you'll see the injector get progressively harder to pick up over a season's running. Oxygen scavengers like sodium sulphite are fine in solution but if dosing is uneven you can see solid sulphite crystallise out in the overflow chamber where the water sits stagnant between runs.
The fix is operational, not mechanical. Drain the injector and overflow line at the end of each running day by leaving the steam and water cocks cracked open and the overflow drain clear. A 30-second blow-through with clean steam flushes any residue out before it bakes onto the cone surfaces.
References & Further Reading
- Wikipedia contributors. Injector. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
