A float-type steam trap (form 2) is a mechanical condensate drain that uses a buoyant ball float linked to a discharge valve to release condensate continuously while blocking live steam. Typical units handle 200 to 12,000 kg/h of condensate at differential pressures from 0.1 to 21 bar. We use it on heat exchangers, jacketed kettles, and air handling coils where modulating loads demand steady drainage. Spirax Sarco's FT-14 series is the textbook example on dairy pasteurisers and HVAC steam coils worldwide.
Steam Trap Form 2 Interactive Calculator
Vary the differential pressure and FT trap reference point to see the float lift demand, equivalent seat area, and valve opening response.
Equation Used
The float must supply enough lift to overcome the pressure force holding the submerged discharge valve shut. This calculator scales the article FT-14 reference lift with differential pressure, then converts that force back to an equivalent valve seat area and orifice diameter.
- Lift demand scales linearly with differential pressure for the same valve seat.
- Reference point is the article FT-14 example: 45 N at 14 bar.
- Seat force only; lever ratio, friction, and float weight margin are not included.
The Steam Trap (form 2) in Action
The float trap works on a simple principle — condensate raises a hollow ball float, the float lifts a lever, and the lever opens a submerged orifice valve that lets condensate out. Steam stays trapped because the valve seat sits below the water line, so live steam never reaches the orifice. As condensate flow drops, the float falls and the valve modulates closed. This continuous modulation is the key reason you pick a float trap over a thermodynamic or bucket trap on modulating loads — the discharge tracks the load instead of cycling on-off.
The form 2 design adds a separate thermostatic air vent at the top of the body. On startup the trap is full of cold air, and air will not lift the float. Without a dedicated air vent the trap airbinds and the heat exchanger sits cold for 5 to 15 minutes while the system limps up to temperature. The thermostatic element — usually a balanced-pressure capsule filled with a water-alcohol mix — stays open until steam temperature arrives, then snaps shut within about 5 °C of saturation. Get the air vent wrong and you'll see slow warm-up, uneven coil temperatures, and complaints from operators that the kettle takes forever to come up.
Tolerances matter. The float-to-lever pivot must run on hardened stainless pins with under 0.05 mm radial clearance — any more and the lever flutters, the valve chatters, and the seat erodes inside 6 months. The orifice and ball valve are matched on lapping; if you swap a valve without its mating seat, expect live steam blow-through within weeks. Waterlogging — where the float sinks because it has cracked and filled with condensate — is the classic failure mode. Symptom: equipment floods, condensate backs up into the heat exchanger, and you lose half your heating capacity overnight.
Key Components
- Hollow ball float: Stainless steel sphere, typically 50 to 150 mm diameter, sealed under partial vacuum so it floats on condensate. Buoyancy lift must exceed valve seat force at maximum ΔP — for an FT-14 at 14 bar that's roughly 45 N of lift demand.
- Float lever and valve head: Pivoted lever links float motion to the main discharge valve. The valve modulates continuously between fully shut and fully open, with the orifice sized for rated capacity at design ΔP. Lever pivot clearance must stay under 0.05 mm to prevent valve chatter.
- Thermostatic air vent: Balanced-pressure capsule mounted at the top of the trap body. Opens at startup to discharge cold air, then closes within 5 °C of saturation temperature. Without this vent the trap airbinds and equipment warm-up extends by 10 to 15 minutes.
- Trap body and seat: Cast iron, SG iron, or carbon steel pressure shell rated for line pressure plus a safety margin. The valve seat sits below the operating water level so live steam cannot reach the discharge orifice — this is what distinguishes a float trap from an inverted bucket trap.
- Inlet strainer (optional integral): 20 to 60 mesh screen ahead of the float chamber to catch pipe scale and weld slag. Without a strainer, debris jams the valve open within the first month of service and you lose live steam continuously.
Industries That Rely on the Steam Trap (form 2)
Float traps shine on modulating loads — anywhere the condensate flow rises and falls with process demand rather than running flat-out. They handle the gentle dribble at low load and the flood at startup without missing a beat, which is why every textbook process heating job specs an FT trap as the default. You'll also see them on stall-prone applications where a thermodynamic trap would chatter itself to death.
- Dairy processing: Spirax Sarco FT-14 traps on the regenerator section of an APV plate pasteuriser at a Saputo cheese plant, draining 800 kg/h of condensate from the steam-to-water heat exchanger feeding the 72 °C holding tube
- HVAC: Armstrong J-series float traps on the steam preheat coils of a Trane CSAA air handler in a hospital in Winnipeg, where modulating control valves create stall conditions that bucket traps cannot handle
- Pharmaceutical: TLV J3X-21 sanitary float traps on a clean-steam-jacketed bioreactor at a Lonza facility in Visp, Switzerland, where steady drainage keeps the jacket temperature within ±0.5 °C of the 37 °C culture setpoint
- Pulp and paper: Velan UniversalFT traps on the drying cylinders of a Voith paper machine at a Domtar mill in Espanola, Ontario, draining 4,500 kg/h per cylinder section without flashing back into the dryer
- Food processing: Spirax Sarco FT-14HC traps on jacketed Lee Industries cooking kettles at a Campbell Soup plant in Maxton, North Carolina, where batch heating cycles drive condensate load from 50 kg/h at simmer to 600 kg/h at boil-up
- District heating: Gestra UNA 26h ball-float traps on the calorifier banks at a Vattenfall district heating substation in Berlin, handling 2,200 kg/h of condensate at 8 bar with continuous modulating discharge
The Formula Behind the Steam Trap (form 2)
Sizing a float trap comes down to matching trap capacity to peak condensate load with a sensible safety factor. The formula computes the trap capacity required at the actual differential pressure across the trap — and ΔP is where most sizing errors live. At the low end of the operating range — say, a modulating coil throttled down to 20% load — ΔP can collapse to 0.2 bar and the trap discharges sluggishly. At the nominal design point the trap sits at 60 to 70% of rated capacity, which is the sweet spot for modulating response. At the high end — startup, when condensate load can spike to 3× steady-state and ΔP is at its maximum — the trap must still pass the load without backing up. Size for startup, not for steady-state.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Creq | Required trap capacity at reference ΔP | kg/h | lb/h |
| Qcond | Peak condensate load from the equipment | kg/h | lb/h |
| SF | Safety factor (typically 2 to 3 for modulating loads, 1.5 for constant loads) | dimensionless | dimensionless |
| ΔP | Actual differential pressure across the trap (inlet minus backpressure) | bar | psi |
| ΔPref | Reference ΔP from the trap capacity chart | bar | psi |
Worked Example: Steam Trap (form 2) in a chocolate-tempering jacketed kettle
You are sizing an FT-type float trap for a 600 L jacketed chocolate-tempering kettle at a Barry Callebaut confectionery plant in Zundert, Netherlands. The steam supply runs at 4 bar gauge, the condensate return is open to atmosphere with about 0.3 bar backpressure, and the heating duty peaks at 220 kW during the initial melt-down phase. You need to choose between a Spirax Sarco FT-14-4.5 (rated 540 kg/h at 4.5 bar ΔP) and the smaller FT-14-10 frame.
Given
- Psteam = 4.0 bar g
- Pback = 0.3 bar g
- Qheat = 220 kW
- hfg at 4 bar g = 2108 kJ/kg
- SF = 3.0 modulating duty
Solution
Step 1 — convert peak heat duty into nominal condensate load:
Step 2 — calculate the actual ΔP across the trap at nominal operating conditions:
Step 3 — apply the safety factor and correct for the trap's reference ΔP of 4.5 bar:
That kills the FT-14-4.5 immediately — its 540 kg/h rating is less than half of what the application demands at startup. Move up to the FT-14-10 frame, which carries roughly 1,800 kg/h at 4.5 bar ΔP, and you have headroom.
Now check the low end of the operating range. When the kettle reaches setpoint and the modulating control valve throttles down to 15% load, heat duty drops to about 33 kW:
At this point the modulating control valve also drops the trap-inlet pressure — stall conditions can pull ΔP down to 0.5 bar. The float trap continues to modulate cleanly at this point, which is exactly why you specced an FT and not a thermodynamic disc trap. A TD trap would cycle once every 8 to 10 minutes at this load and waterlog the kettle jacket between discharges.
At the high end — cold startup with the kettle full of room-temperature chocolate mass — peak condensate load can hit 3× nominal because the jacket has to heat the steel shell as well as the product:
The FT-14-10 still passes this with ΔP at its maximum 3.7 bar. If you'd undersized to the FT-14-4.5, the kettle would flood within 90 seconds of opening the steam valve.
Result
Specify a Spirax Sarco FT-14-10 with the integral thermostatic air vent, sized for a required capacity of 1,244 kg/h at 3. 7 bar ΔP. At nominal melt-down the trap runs at about 21% of rated capacity — well inside the modulating sweet spot where the float lever sits mid-stroke. At the 56 kg/h low end the float barely lifts but discharge stays continuous, and at the 1,128 kg/h startup peak the trap is at 63% of capacity with comfortable headroom. If you commission the system and measure jacket temperature lagging by 8 to 10 °C below setpoint, the most likely causes are: (1) the air vent capsule failed shut and the trap is airbound — pull the cap and check for steam at the vent port during startup, (2) backpressure in the condensate return line is higher than the assumed 0.3 bar because someone tied a new condensate header in upstream without resizing the main, or (3) the float has cracked and waterlogged, which you will spot as a continuous full-bore discharge instead of the expected modulating dribble.
Steam Trap (form 2) vs Alternatives
Float traps are not the only option for draining condensate. The two main alternatives — inverted bucket traps and thermodynamic disc traps — each have their place, and picking the wrong one for a modulating load is one of the most common steam-system mistakes we see in the field.
| Property | Float-Type Steam Trap (FT) | Inverted Bucket Trap (IB) | Thermodynamic Disc Trap (TD) |
|---|---|---|---|
| Discharge mode | Continuous modulating | Intermittent on-off | Intermittent blast (cycles every 20-60 s) |
| Performance on modulating loads | Excellent — tracks load down to 5% | Poor — bucket sinks at low ΔP and loses prime | Poor — chatters and erodes disc within months |
| Air-handling at startup | Excellent with integral thermostatic vent | Slow — vents only through tiny bucket vent hole | Poor — disc closes against cold air |
| Maximum operating pressure | Up to 32 bar (FT-14HC class) | Up to 45 bar | Up to 42 bar |
| Capacity range | 200 to 12,000 kg/h | 50 to 6,000 kg/h | 30 to 1,400 kg/h |
| Sensitivity to waterhammer | Moderate — can crack float | Low — robust bucket design | Low — solid disc, very rugged |
| Typical service life | 8 to 12 years on clean steam | 10 to 15 years | 2 to 5 years (disc wear) |
| Installed cost (DN25) | $$$ (highest) | $$ | $ (lowest) |
| Best fit application | Heat exchangers, jacketed kettles, AHU coils | Steam mains drip legs, tracer lines | Steam mains, superheated steam drip points |
Frequently Asked Questions About Steam Trap (form 2)
Almost always a damaged seat or a piece of pipe scale wedged between the valve head and the orifice. The float lever is trying to close the valve but cannot seat fully, so steam blows through whenever the float drops below the closing line.
Diagnose by isolating the trap and pulling the cover. If you see a wire-drawn groove across the seat face, the trap has been passing scale for some time and you need a full internals kit, not just a valve head. Fit a 60-mesh strainer upstream if there isn't one — that's the root cause 80% of the time on first-year failures.
Size for stall. When a modulating temperature control valve throttles down, trap inlet pressure can fall below condensate return pressure and the trap stops discharging — that's stall. The condensate then waterlogs the heat exchanger until the control valve opens up again.
The fix is either a pumping trap (combined float trap and condensate pump) or routing the trap discharge to atmosphere via a vented receiver. Sizing the float trap itself for stall ΔP (sometimes as low as 0.1 bar) means picking a frame one or two sizes larger than the steady-state calculation suggests, because trap capacity scales with √ΔP.
The thermostatic capsule has likely failed open. These balanced-pressure capsules are filled with a water-alcohol charge, and waterhammer or superheat above the capsule's design temperature ruptures the bellows. Once ruptured, the capsule cannot generate the differential pressure that closes the vent.
Confirm by removing the capsule and checking it against a kettle of boiling water — a healthy capsule snaps shut at around 95 °C. If yours stays open, replace it. If you keep killing capsules, check upstream for a stuck reducing valve letting superheated steam reach the trap.
Use an inverted bucket or thermodynamic trap on steam main drip legs, not a float trap. Drip legs see waterhammer slugs travelling at 20+ m/s, and a hollow ball float will crack within months under that abuse. The float fills with condensate, sinks, and the trap waterlogs — exactly the opposite of what you want on a steam main.
Float traps belong on process equipment where the steam path is reasonably quiet: heat exchangers, jacketed vessels, air coils. The continuous modulating discharge is wasted on a drip leg where the load is essentially constant and small.
Two likely culprits. First, flash steam — when condensate at 4 bar drops to atmospheric pressure across the trap, roughly 12-13% of the mass flashes back to vapour and disappears up the vent if you're measuring weight at a vented receiver. Account for this in your mass balance.
Second, you may have steam losses elsewhere on the equipment — leaking gasket on the heat exchanger, a cracked tube, or a bypass valve that isn't fully shut. Pull the trap, weigh the condensate over a known time interval at the trap outlet directly, and compare to the heat-duty prediction. If the trap discharge matches the prediction within 10% but the receiver shows less, you're losing flash. If the trap itself shows a deficit, look upstream.
It has to be installed in the orientation stamped on the body — usually with the float axis horizontal and the body upright. Lay it on its side and the float lever geometry breaks: the float either jams against the body or cannot lift enough to open the valve at rated capacity.
If you genuinely have no vertical space, look for a side-inlet/side-outlet variant like the Spirax FT-14 horizontal pattern, which has the internals geometry redesigned for that orientation. Don't improvise with a standard trap mounted sideways — it will either fail to drain or waterlog within hours.
Use a contact pyrometer on the trap inlet and outlet. If the inlet reads near steam saturation temperature (say, 152 °C at 4 bar) and the outlet reads cooler with a clear temperature drop, the trap is working but maybe undersized. If the inlet reads cool — under 100 °C — the upstream control valve is throttled or shut and there's no steam reaching the trap at all.
An ultrasonic stethoscope on the trap body confirms it: a working float trap has a continuous low-frequency rush as condensate flows through the orifice. A waterlogged trap is silent. A blocked-shut upstream valve is also silent but the pipe will be cold.
References & Further Reading
- Wikipedia contributors. Steam trap. Wikipedia
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