A Heintz steam trap is a 19th-century bucket-type condensate drain that automatically discharges water from a steam line while holding back live steam. An open bucket inside the trap body floats on collected condensate and rises until a lever closes the discharge valve; when steam fills the bucket it sinks and the valve opens to push condensate out. The trap exists to prevent water hammer, slugging, and heat loss in steam mains. A correctly sized Heintz trap on a heritage 100 psi line will pass 200–800 lb/h of condensate without leaking live steam.
Heintz Steam Trap Interactive Calculator
Vary orifice diameter, pressure drop, and condensate load to see trap capacity, required seat size, margin, and valve force.
Equation Used
This calculator uses the article sizing point for a Heintz trap valve seat: a 5/16 inch orifice passes about 600 lb/h of condensate at 80 psi. Capacity scales with orifice area and the square root of pressure drop, so larger seats or higher DeltaP increase discharge capacity.
- Empirical condensate sizing relation calibrated to the article value of 5/16 in for about 600 lb/h at 80 psi.
- DeltaP is the net pressure drop across the valve seat after return-line backpressure.
- Condensate is water-like and the lapped seat opens cleanly without scale blockage.
- Capacity is approximate trap discharge capacity, not a live-steam leakage estimate.
Operating Principle of the Heintz Steam Trap
The Heintz steam trap works on a buoyancy and weight reversal — the same physics as the later inverted bucket traps from Armstrong, but Heintz used an open-top bucket inside a sealed cast-iron body. Condensate enters the body from the drip leg, fills around the bucket, and the bucket floats. As it floats, a lever arm rotates and seats a small valve plug against the discharge orifice. Steam can't escape because the valve is shut. When more condensate arrives and overflows the bucket rim, water inside the bucket gets heavy enough that the bucket sinks. That pulls the lever the other way, lifts the valve off its seat, and line pressure forces condensate up through the discharge port until the bucket is light again and refloats. The cycle repeats every 30 seconds to several minutes depending on condensate load.
The critical tolerance is the valve seat — the orifice must be lapped flat to within 0.0005 inch and the plug must seat concentrically, otherwise live steam wire-draws across the seat and the trap blows steam continuously. You'll hear it as a hiss instead of a periodic chug. The bucket lever pivot also has to be free; corrosion or scale on a 1/8-inch pin will stop the bucket sinking even when full, and the trap will back up the line. If you notice water hammer in the main downstream of a Heintz trap, the bucket is stuck floating. If you notice live steam blowing from the discharge, the seat is cut.
Why use this design at all? Because it's mechanically simple, has only one moving assembly, and tolerates dirty condensate better than a thermostatic bellows trap. The downside is size — a Heintz trap rated for 400 lb/h condensate at 80 psi is roughly the size of a 5-gallon pail, where a modern inverted bucket trap doing the same job fits in your hand.
Key Components
- Cast-iron body: Pressure-containing shell, typically cast in two halves with a bolted flange and gasket. Working pressure ratings on original Heintz traps ran from 25 psi up to 150 psi, with the flange bolts torqued to seal a compressed asbestos or graphite gasket.
- Open bucket: Sheet-brass or copper open-top vessel suspended on a lever. The bucket's empty weight must be light enough to float on cold condensate but heavy enough to sink reliably when full — typically 60–70% of its displaced-water weight when empty.
- Lever arm and pivot pin: Connects bucket motion to valve. Pivot pin runs in two bronze bushings; clearance must stay under 0.010 inch or the valve won't seat squarely. Pin and bushings should be inspected at every annual teardown.
- Valve plug and seat: Hardened steel or bronze plug seating into a lapped orifice. Orifice diameter is sized to the condensate load — 3/16 inch for ~200 lb/h at 80 psi, 5/16 inch for ~600 lb/h. Seat flatness within 0.0005 inch is mandatory.
- Discharge port: Threaded outlet, typically 1/2 inch or 3/4 inch NPT, leading to the condensate return line. Must be piped with a swing check valve immediately downstream to prevent return-line backflow lifting the bucket.
- Inlet strainer: Y-strainer mounted upstream of the trap inlet, with a 20-mesh screen. Catches scale and pipe debris before it can lodge under the valve plug, which is the single most common failure mode on heritage installations.
Industries That Rely on the Heintz Steam Trap
Heintz traps showed up wherever 19th and early-20th century steam plant needed to drain condensate from long horizontal mains, jacketed vessels, and marine auxiliaries. They were favoured on dirty or scale-prone systems because the open bucket isn't fouled by debris the way a thermostatic bellows would be. You still find them in service or being recommissioned at heritage sites, museum ships, and demonstration plants — and the sizing rules haven't changed.
- Heritage marine steam: Draining the saturated steam main feeding the auxiliary winches on the SS Master, the 1922 steam tug preserved at the Vancouver Maritime Museum
- Museum brewery steam plant: Drip-leg drainage on the steam supply to the copper brewing kettle at Anchor Brewing's heritage demonstration plant in San Francisco
- Heritage railway shop: Condensate removal from the workshop steam-heating main at the Strasburg Rail Road locomotive shop in Pennsylvania
- Demonstration sawmill: Drip-leg trap on the engine supply main at Hull-Oakes Lumber Company's steam-driven sawmill in Oregon, one of the last working steam sawmills in North America
- Heritage textile mill: Process steam main drainage feeding the size-box on a working power loom display at Quarry Bank Mill in Cheshire
- Museum laundry plant: Drainage on the steam supply to the calender rollers in the demonstration laundry at the Black Country Living Museum in Dudley
The Formula Behind the Heintz Steam Trap
Sizing a Heintz trap means picking the orifice diameter that will pass the actual condensate load at the differential pressure across the trap, with a safety factor for cold-start condensate surge. The condensate flow through the orifice follows the same incompressible-flow relation used for any subcooled-liquid orifice. At the low end of the typical operating range — say 50 lb/h on a small jacketed kettle — even a 1/8 inch orifice has plenty of margin and the trap cycles slowly. At the nominal middle of the range — 300 to 500 lb/h on a steam main drip — the orifice diameter must be matched carefully to avoid either backing up the line or wire-drawing live steam. At the high end — 1,000+ lb/h on a process header at warmup — the original Heintz body itself becomes the limit; you'll need two traps in parallel rather than oversizing one orifice.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| W | Condensate discharge rate | kg/h | lb/h |
| Cd | Discharge coefficient (≈ 0.65 for a sharp-edged lapped seat) | dimensionless | dimensionless |
| A | Orifice cross-sectional area | mm2 | in2 |
| ρ | Condensate density at saturation temperature | kg/m3 | lb/ft3 |
| ΔP | Differential pressure across the trap (inlet minus return-line back-pressure) | kPa | psi |
Worked Example: Heintz Steam Trap in a recommissioned Heintz trap on a museum dyehouse main
You are sizing the orifice on a recommissioned Heintz bucket trap draining the saturated-steam main feeding a recommissioned 1908 cast-iron indigo dye vat at a heritage textile-printing museum in Krefeld, Germany, where the main carries saturated steam at 90 psig with a return-line back-pressure of 10 psig and a measured peak condensate load of 400 lb/h during morning warmup falling to roughly 120 lb/h at steady running.
Given
- Pin = 90 psig
- Pback = 10 psig
- ΔP = 80 psi
- ρ = 57.3 lb/ft3 (saturated condensate at 90 psig)
- Cd = 0.65 dimensionless
- Wpeak = 400 lb/h
- Wsteady = 120 lb/h
Solution
Step 1 — apply a 2× safety factor on the peak warmup condensate load to size for cold-start surge, which is standard practice for heritage drip-leg traps:
Step 2 — rearrange the orifice equation for area at the nominal design point. Using ρ = 57.3 lb/ft3 and ΔP = 80 psi:
Step 3 — convert to orifice diameter and pick the next standard Heintz seat size up:
At the low end of the operating range — 120 lb/h steady running — that 5/16 in orifice is loafing. The bucket cycles maybe once every 4 to 5 minutes, the discharge is a satisfying short chug, and you'll see no flash steam at the return tank vent. At nominal warmup load of 400 lb/h the trap cycles roughly every 40 seconds, which is exactly where a healthy Heintz wants to run. Push past the design point to a 1,000 lb/h cold-start surge — for example if the dyehouse fires up after a weekend cold soak — and the 5/16 in orifice can't keep up; the bucket stays sunk, the valve sits wide open, and condensate backs into the main until warmup tapers. That's why heritage operators warm up the line slowly through a manual bypass valve for the first 10 minutes rather than slamming the main valve open.
Result
The correct orifice for this installation is 5/16 inch, paired with a Heintz body rated for at least 100 psi working pressure. At steady 120 lb/h running the trap cycles every 4–5 minutes with a clean chug; at 400 lb/h warmup it cycles every 30–40 seconds and sits in its design sweet spot; pushed to a 1,000 lb/h cold-start surge it loses control and floods the main, which is why slow warmup through the bypass matters. If your measured discharge sounds like a continuous hiss instead of a periodic chug, the seat has wire-drawn — pull the trap and inspect the orifice for a visible groove. If the trap cycles correctly but you still get water hammer in the main downstream, the inlet strainer is plugged with mill scale and condensate is bypassing through a leaking flange. If the bucket lever feels notchy when you rotate it by hand during teardown, the pivot bushings have corroded oversize and the valve isn't seating concentrically.
When to Use a Heintz Steam Trap and When Not To
The Heintz trap competes with three other condensate-drainage strategies you'll find on heritage and modern steam plant: the modern inverted bucket trap, the thermostatic bellows trap, and the float-and-thermostatic (F&T) trap. Each makes a different bargain on size, dirt tolerance, and discharge behaviour.
| Property | Heintz bucket trap | Modern inverted bucket trap | Thermostatic bellows trap | Float & thermostatic (F&T) trap |
|---|---|---|---|---|
| Maximum working pressure | 25–150 psi (period castings) | Up to 600 psi | Up to 250 psi | Up to 465 psi |
| Condensate capacity (typical body) | 100–800 lb/h | 500–4,000 lb/h | 50–500 lb/h | 1,000–20,000 lb/h |
| Dirt and scale tolerance | Excellent — open bucket self-clears | Good | Poor — bellows fouls quickly | Moderate |
| Physical size for 400 lb/h | ~5 gallon pail | Hand-sized | Hand-sized | Briefcase-sized |
| Air-venting on startup | Poor — needs separate vent | Poor | Excellent | Excellent |
| Discharge pattern | Intermittent chug | Intermittent chug | Modulating dribble | Continuous |
| Service life on heritage steam | 50+ years with seat lapping | 15–25 years | 5–10 years (bellows fatigue) | 20–30 years |
| Replacement parts availability | Custom-made or salvaged | Off-the-shelf (Armstrong, Spirax) | Off-the-shelf | Off-the-shelf |
Frequently Asked Questions About Heintz Steam Trap
Probably not seized. The most common cause is that the steady-running condensate load has dropped below the trap's minimum cycling threshold. A Heintz sized for 400 lb/h warmup might only see 50 lb/h at steady running, and at that flow it can take 8–10 minutes to overflow the bucket — you assume it's stopped when it's just being patient.
Confirm by cracking the test tee downstream after 15 minutes of steady running. If condensate flows when you open it, the trap is fine and you've simply oversized for the steady load. If nothing flows, the air-vent on the body is plugged and the bucket is air-bound — vent it manually and watch for normal cycling to resume.
Functionally yes, visually no. Modern Armstrong or Spirax inverted bucket traps work on the same buoyancy principle and will outperform the original Heintz on capacity-per-pound and seat life. The problem is the body — modern traps are compact forged or cast assemblies that look obviously wrong next to riveted lagging and cast-iron flanges.
The accepted compromise on heritage sites is to keep the original Heintz body in place as a visible artefact and pipe a modern trap in parallel as the working drain, with the Heintz isolated by valves. Visitors see the period hardware, and you don't fight a 100-year-old casting every steaming season.
Pull the seat and look at it under a 10× loupe. A dirty seat shows discrete pits or scale flakes that you can wipe off with a brass scraper, and the trap will go back to normal cycling after a clean. A wire-drawn seat shows a continuous radial groove cut from the orifice edge outward, usually on one side where the plug sat fractionally off-centre — that's permanent damage and the seat must be re-lapped or replaced.
The audible test in service is also reliable: a dirty seat makes the trap chug irregularly with occasional steam blowby; a wire-drawn seat blows continuously and never fully shuts even when the bucket is clearly full.
Check the back-pressure first. The orifice equation uses ΔP, not inlet pressure — and on heritage installations the condensate return main is often partially blocked or runs uphill, raising back-pressure to 30–40 psi when the design assumed 10 psi. Cut your ΔP in half and you cut capacity by roughly √2, which is exactly the deficit you're describing.
If back-pressure checks out, the next suspect is the bucket vent hole. Original Heintz buckets had a 1/16 inch vent in the top rim to let air escape on startup. If that hole has been plugged by scale or paint during restoration, the bucket can't fully sink because trapped air keeps it buoyant, and the valve never opens fully.
Use the heat-loss method on the steam main itself. For a bare or poorly-lagged horizontal main, expect 0.5–1.0 lb/h of condensate per square foot of pipe surface at 100 psi steam. Lagged with 1 inch of asbestos or modern calcium silicate, drop that to 0.05–0.15 lb/h per square foot. Add the warmup condensate — roughly the mass of the pipe times the temperature rise times 0.12 BTU/lb·°F divided by latent heat of evaporation, applied over the warmup period.
Then double it for safety on a cold-start drip leg. Heritage operators routinely undersize traps by working from steady-state numbers and forgetting that warmup surge is 3–5× steady load on a lagged main and 8–10× on a bare one.
No. Bucket traps depend on gravity to deliver condensate into the trap body — the inlet must sit below the lowest point of the equipment being drained, with at least 12 inches of vertical drop in the drip leg. Mount it level with or above the equipment and the trap will starve of water, the bucket will sink under its own weight, and the valve will sit open and blow live steam continuously.
This is the single most common installation error on amateur restorations. If geometry forces the trap above the drain point, you need a different trap technology entirely — typically a pumping trap or an F&T arrangement with a lift fitting, not a bucket trap.
References & Further Reading
- Wikipedia contributors. Steam trap. Wikipedia
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