A Bundy steam trap is a thermostatic bellows-type steam trap that discharges condensate from a steam line by sensing the temperature difference between live steam and cooler condensate. A typical Bundy unit handles 200–600 lb/h of condensate at 15–125 psi, opening when condensate cools roughly 10–20°F below saturation. It exists to drain heating coils and steam mains without venting live steam, recovering heat and protecting downstream equipment. You will still find Bundy traps on preserved Lancashire boiler plants and hospital sterilising-room manifolds across the UK.
Bundy Steam Trap Interactive Calculator
Vary steam pressure, opening subcooling, and condensate temperature to see the balanced-pressure bellows opening point and valve lift.
Equation Used
The calculator estimates the steam saturation temperature from line pressure, then subtracts the selected thermostatic subcooling. If the condensate is cooler than this opening temperature, the bellows is shown contracting and the valve begins to lift.
- Pressure input is gauge pressure and is converted to absolute pressure.
- Water saturation temperature is estimated with the Antoine equation over the listed steam-trap pressure range.
- Valve lift is shown as a teaching estimate, scaled linearly over a 20 deg F cooling band below the opening temperature.
The Bundy Steam Trap in Action
A Bundy steam trap works on a sealed, liquid-filled bellows — a thin-wall phosphor-bronze or stainless capsule charged with a water-alcohol mixture chosen so its vapour pressure tracks the saturation curve of steam, but offset slightly cooler. When live steam contacts the bellows, the internal charge boils, pressure rises, and the bellows expands axially to drive a tapered valve head onto its seat. As condensate cools the bellows by 10–20°F below the surrounding steam saturation temperature, the charge condenses, the bellows contracts under spring-assisted pull-back, and the valve lifts off the seat to discharge condensate to the return main.
The geometry matters more than people expect. The valve seat is usually a hardened stainless insert with a 60° conical face and a lap finish below 0.4 µm Ra — anything coarser and the trap weeps continuously because the valve head cannot achieve a metal-to-metal seal. The bellows free length, when new, sits within ±0.2 mm of the manufacturer's drawing; if you pull a Bundy apart and find the bellows stretched 1 mm long, it has been water-hammered and the charge is partially lost. That trap will discharge late, back up condensate, and you will hear the heating coil knocking on every steam admission.
Why this design rather than a float trap or a thermodynamic disc trap? The bellows is a balanced-pressure element, meaning it tracks saturation temperature regardless of the line pressure. You do not need to re-range the trap when boiler pressure shifts from 80 psi running to 30 psi at low fire — the bellows simply opens at the new lower saturation temperature minus the same offset. Common failure modes are bellows fatigue cracks from cyclic operation (typical service life 5–8 years on a swinging load), seat erosion from wire-drawing when the trap weeps, and charge loss after a single severe water-hammer event.
Key Components
- Thermostatic bellows capsule: A thin-walled phosphor-bronze or stainless bellows charged with a water-alcohol mixture. The charge's vapour pressure tracks steam saturation offset by 10–20°F cooler, so the capsule expands in steam and contracts in subcooled condensate. Typical free length 25–40 mm with ±0.2 mm tolerance from new.
- Tapered valve head and seat: Hardened stainless 60° conical valve onto a lapped seat insert finished below 0.4 µm Ra. The head is carried on the moving end of the bellows and seats under spring-assisted bellows expansion. A scratched seat wire-draws within weeks under live-steam blow-by.
- Inlet strainer screen: A perforated or mesh strainer ahead of the seat, usually 20-mesh stainless, catching scale and pipe debris before it reaches the valve face. A blocked strainer is the single most common reason a Bundy trap appears to have failed closed.
- Body and cover casting: Bronze or cast-iron body with a bolted cover sealing on a soft copper or graphite gasket rated for the line pressure — typically 150 psi WSP for cast iron, 250 psi for bronze. The cover allows in-situ bellows replacement without breaking the line flanges.
- Compression spring (return spring): A small stainless return spring assisting bellows pull-back when the charge condenses. Spring rate is matched to the bellows free length so the valve lifts cleanly at the trip temperature without chattering on partial loads.
Industries That Rely on the Bundy Steam Trap
Bundy traps and their close descendants run wherever you want quiet, balanced-pressure condensate drainage on a moderate-pressure steam system — heritage plant, hospital sterilisers, brewery jacketed vessels, laundry presses, and tracer lines on fuel-oil heating circuits. They are not the right choice for superheated steam or for very low loads where a thermodynamic trap holds tighter, but on saturated steam at 15–125 psi with a swinging condensate load, the bellows trap is hard to beat for energy efficiency because it does not vent flash steam to drain.
- Hospital sterilisation: Bellows traps on the steam supply manifold of a Getinge HS66 autoclave at Guy's Hospital London, draining the chamber jacket between sterilising cycles.
- Heritage steam plant: Bundy-pattern traps on the steam-heated feedwater preheater at Kew Bridge Steam Museum, draining condensate from the 1846 Cornish engine house pipework.
- Brewing: Thermostatic bellows traps on the kettle calandria condensate line at Fuller's Griffin Brewery in Chiswick, handling 400 lb/h of condensate at 35 psi during boil.
- Commercial laundry: Bellows traps on the head and foot of each Jensen flatwork ironer roll at a Sunlight Service Group plant, draining condensate from the 90 psi steam-heated chests.
- Petrochemical tracing: Bundy-style traps on jacketed-pipe tracer loops carrying heavy fuel oil at the Phillips 66 Humber Refinery, maintaining oil at 60°C with 50 psi steam.
- Heritage rail: Carriage steam-heating traps on Mk1 coaches running behind preserved locomotives on the Severn Valley Railway, draining the through-pipe at each coach end.
The Formula Behind the Bundy Steam Trap
Sizing a Bundy trap means matching its discharge capacity to the actual condensate load at the actual differential pressure across the trap — not the line pressure alone. At the low end of the typical operating range, around 5–10 psi differential, a given trap passes maybe 30% of its nameplate capacity because the discharge is gravity-assisted only. At nominal differential, 30–60 psi, you hit the sweet spot where the trap runs at roughly its rated figure. At the high end, above 100 psi, capacity rises again but bellows fatigue life shortens because every cycle stresses the capsule harder. The formula below predicts steady-state condensate discharge through the trap orifice once the bellows has lifted.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Qc | Volumetric condensate discharge rate when the trap is open | m³/s | ft³/s |
| Cd | Discharge coefficient of the seat orifice (typically 0.6–0.7 for a lapped 60° seat) | dimensionless | dimensionless |
| Ao | Seat orifice area at full bellows lift | m² | in² |
| ΔP | Differential pressure across the trap (line pressure minus return-line back pressure) | Pa | psi |
| ρc | Density of the subcooled condensate at trap inlet temperature | kg/m³ | lb/ft³ |
Worked Example: Bundy Steam Trap in a hospital steriliser steam manifold
You are sizing a replacement Bundy-pattern thermostatic bellows trap on the steam supply manifold of a Getinge HS66 autoclave at a district hospital in Birmingham. The line carries saturated steam at 45 psi, the condensate return main sits at 5 psi, the trap seat orifice is 4.0 mm diameter, the discharge coefficient is 0.65, and the condensate enters the trap at 140°C giving ρc ≈ 926 kg/m³. You need to know whether one trap is enough for the predicted 220 lb/h jacket condensate load during the chamber heat-up phase.
Given
- Pline = 45 psi
- Preturn = 5 psi
- ΔPnom = 40 (276,000) psi (Pa)
- do = 4.0 mm
- Cd = 0.65 —
- ρc = 926 kg/m³
Solution
Step 1 — compute the seat orifice area from the 4.0 mm bore:
Step 2 — at nominal 40 psi differential (276,000 Pa), compute the discharge velocity term and the volumetric flow:
Converting to mass flow at ρc = 926 kg/m³ gives 0.174 kg/s, or about 1,380 lb/h. That is comfortably above the 220 lb/h heat-up load — one trap is plenty, with headroom for a startup slug.
Step 3 — at the low end of the operating range, a typical 10 psi differential (69,000 Pa) during low-fire boiler operation:
Capacity halves, but you still clear 220 lb/h with margin. This is the regime where flash steam loss is lowest and the bellows runs coolest — the trap will see the longest life here.
Step 4 — at the high end, 110 psi differential (758,000 Pa) on a sudden return-line vent:
That sounds great until you remember the bellows now slams open and shut at 758 kPa working stress every cycle. Fatigue life on the capsule drops from a typical 6–8 years at nominal duty to under 2 years at this end of the range.
Result
At nominal 40 psi differential the trap discharges roughly 1,380 lb/h of condensate — more than 6× the predicted 220 lb/h heat-up load, so a single 4. 0 mm seat trap handles the autoclave manifold easily. Across the operating range, capacity moves from about 690 lb/h at 10 psi differential up to 2,290 lb/h at 110 psi, with the sweet spot sitting between 30 and 60 psi where bellows fatigue stays moderate and flash-steam losses are minimal. If you measure 220 lb/h backing up in service despite this headroom, suspect three things first: a partially blocked 20-mesh inlet strainer cutting effective Ao, a stretched bellows from a past water-hammer event causing the valve to seat late, or an air-bound return line raising Preturn above the assumed 5 psi and collapsing the differential. Pull the cover, gauge the bellows free length against the OEM drawing, and check return-main venting before condemning the trap.
Choosing the Bundy Steam Trap: Pros and Cons
The bellows trap competes mainly with float-and-thermostatic (F&T) traps, thermodynamic (TD) disc traps, and inverted-bucket traps on saturated steam service. Each has a clear operating regime where it wins, and a regime where it fails fast. Pick on differential pressure range, condensate temperature requirement, and how much you care about flash-steam loss.
| Property | Bundy thermostatic bellows trap | Float & thermostatic trap | Thermodynamic disc trap |
|---|---|---|---|
| Working pressure range | 15–125 psi | 0–250 psi | 30–600 psi |
| Discharge temperature vs saturation | 10–20°F subcooled | At saturation (continuous) | At saturation (intermittent) |
| Flash steam loss to return | Low — subcools first | Moderate | High — blow-through on each cycle |
| Tolerance to superheat | Poor — bellows overheats | Fair with thermostatic air vent | Excellent |
| Typical service life on saturated steam | 5–8 years | 8–12 years | 3–5 years |
| Sensitivity to water hammer | High — bellows charge loss | Moderate — float damage | Low |
| Installed cost (DN20, mid-2024 UK) | £90–£140 | £180–£260 | £70–£110 |
Frequently Asked Questions About Bundy Steam Trap
Cyclic discharge is normal, not a fault. The bellows needs the condensate ahead of the valve to cool 10–20°F below saturation before the charge condenses and the valve lifts. Once it lifts, hot condensate rushes through, the bellows reheats, and the valve closes again. You see this as a 30–90 second cycle on light loads.
It only becomes a problem if the cycle stretches past 3–4 minutes — at that point condensate is backing up far enough into the heating coil to cause stratification and uneven heating. Fix it by sizing down the trap or fitting a steam-trap monitor; do not just open a bypass.
On a steam main drip leg you usually want a thermodynamic disc trap, not a Bundy. Drip-leg condensate is small in volume but arrives at saturation temperature, and you want it cleared immediately to prevent slugs forming in the main. The Bundy holds condensate back until it has subcooled, which is the wrong behaviour here.
Reserve the Bundy for jacketed vessels, heating coils, and tracer lines where you actively want the heat recovery from subcooling and where slug formation is not a risk. The 10–20°F subcool that hurts a drip leg is exactly the energy you save on a process heater.
The formula assumes liquid-only flow through the orifice. In real installations, condensate arriving at the trap is often partly flashed because the inlet line drops pressure ahead of the seat — the moment some of the water flashes to vapour, the effective density falls and the actual mass discharge halves or worse.
Check the inlet pipework. If the trap sits more than 300 mm above the drain point or the inlet pipe size steps down close to the trap, you are flashing condensate before it reaches the seat. Drop the trap below the drain point and use full-bore inlet pipe to the cover flange.
No, and upsizing makes it worse. The bellows charge is calibrated to the saturation curve — superheat puts the capsule at a temperature well above its design vapour pressure, and the charge fluid expands until either the bellows yields plastically or the capsule ruptures. You typically get 200–500 hours before failure on 50°F of superheat.
For superheat duty switch to a thermodynamic disc trap or a bimetallic trap with an external steam-jacket cooling leg. If you are stuck with a Bundy on a line that occasionally sees superheat at startup, fit a 2 m cooling leg of uninsulated pipe ahead of the trap to drop the inlet temperature back to saturation.
Live steam blow-by on a previously good Bundy almost always traces to seat wire-drawing rather than bellows failure. A small piece of scale or pipe rust gets caught between the valve head and the seat for one cycle, the trap weeps slightly, and the high-velocity flashing steam erodes a microscopic groove across the seat lap. Once the groove forms, the valve cannot seal again no matter how cleanly the bellows operates.
Diagnose by isolating the trap, removing the cover, and looking at the seat under a 10× loupe — a fresh wire-draw groove looks like a hairline scratch radial to the bore. Replace the seat insert, do not try to lap it back; field lapping rarely restores the 0.4 µm Ra finish needed for steam-tight closure.
The practical lower limit is around 5% of the trap's nominal capacity at design differential. Below that the cycle frequency drops so low that condensate sits in the inlet pipework long enough to cool well below the bellows trip point, and a slug discharges all at once with violent banging.
If your load varies from 20 lb/h overnight to 400 lb/h during process runs, do not pick a single trap rated for 400 — fit a small bellows trap in parallel with a larger one, with the small trap handling the standby load. This is exactly how laundry steam mains and brewery night-time tracing loops are built.
References & Further Reading
- Wikipedia contributors. Steam trap. Wikipedia
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