Steam Traps: Types, Selection and Why 20% of Them Are Failed Right Now

Quick Answer

A steam trap is an automatic valve that discharges condensate, air and other non-condensable gases from a steam system while holding back live steam. Five families dominate: thermodynamic (disc) traps are compact and robust, handle high pressure and freeze-resistant installations, but are noisy and unsuitable below about 0.3 bar g; float and thermostatic (F&T) traps give continuous modulating discharge and are the best choice for heat exchangers and any process where condensate must leave the instant it forms; inverted bucket traps handle dirt and waterhammer well and fail closed, but lose their water seal on sudden pressure drop; thermostatic (balanced pressure) traps are small, handle wide pressure ranges and discharge air well, making them the standard for steam tracing and radiators; bimetallic traps suit high pressure and superheat but respond slowly. In surveyed industrial plants, typically 15-30% of steam traps are failed at any time if no monitoring programme exists, and a single failed-open 15 mm trap on a 10 bar g system can pass roughly 30-40 kg/h of steam, costing in the region of 6,000-9,000 USD a year in fuel. Sizing uses a safety factor over calculated condensate load — typically 2× for process and 3× for steam mains warm-up — because a trap sized exactly for the running load cannot clear a cold start.

An edible oil refinery in North Africa had 412 steam traps. They had never been surveyed. The maintenance manager's position, entirely reasonably, was that traps are cheap, simple, and someone would notice if one failed.

An ultrasonic and thermal survey found 97 failed traps — 23.5%. Of those, 61 were failed open, blowing live steam straight to the condensate return. The surveyor's calculation put the steam loss at approximately 2,100 kg/h. At their boiler efficiency and gas price, that was about 340,000 USD a year.

Nobody had noticed because a failed-open steam trap makes no noise you can hear over a plant, causes no production problem, and does not leak to the floor. It quietly sends fuel to the condensate tank and vents it as flash steam. The only symptoms were a condensate return system that ran hotter than it should and a deaerator that vented more than expected — both of which had been accepted as normal for years.

Steam traps are the most neglected component in industrial plants, and the most reliably profitable thing to pay attention to.

What a Steam Trap Has to Do

Three jobs, and the design tension between them explains every trap type:

1. Discharge condensate immediately. Condensate in a heat exchanger or a jacket occupies surface area that should be transferring heat, because the heat transfer coefficient of condensate is a small fraction of that of condensing steam. A flooded heat exchanger loses capacity dramatically. In steam mains, condensate accumulation causes waterhammer — slugs of water accelerated to high velocity by steam flow, which destroy fittings, valves and traps.

2. Vent air and non-condensable gases. Air entering during shutdown, and CO₂ from carbonate decomposition in the boiler, accumulate at heat transfer surfaces. Air is an insulator: a film of air a fraction of a millimetre thick can have the thermal resistance of a thick layer of steel. Worse, Dalton's law means air in the steam space reduces steam partial pressure, so the actual steam temperature is below the gauge pressure's saturation temperature. 10% air by volume in a 3 bar g steam space drops the temperature from about 144°C to roughly 139°C — and the process runs cold while the gauge reads correctly.

3. Do not pass live steam. Every kilogram of steam that escapes through a trap is fuel burned for nothing, plus water treatment chemicals, plus make-up water, plus load on the condensate system.

No single mechanism does all three perfectly, which is why there are five families.

Thermodynamic (Disc) Traps

A single hardened disc sitting over inlet and outlet ports in a small chamber.

Operation. Cold condensate lifts the disc and flows out. As the condensate approaching the trap gets hotter, it flashes to steam as it passes under the disc; the high-velocity flash steam creates a low-pressure region under the disc (Bernoulli effect) while flash steam fills the chamber above, and the combination snaps the disc shut. The disc stays closed until the steam in the control chamber condenses, then reopens. Discharge is a sharp intermittent blast.

Strengths

• Very compact, light, and inexpensive
• Operates across a wide pressure range, up to 40 bar g and beyond
• Handles superheat well
• No freezing risk if mounted with the disc vertical and discharging downward — nothing holds water
• Single moving part; can be rebuilt by replacing the disc
• Works in any orientation
• Fails open (safe in terms of waterhammer, costly in terms of steam)

Limitations

• Will not operate reliably below about 0.3 bar g, or where back pressure exceeds roughly 80% of inlet pressure
• Noisy — the sharp discharge is audible and can be objectionable
• Intermittent discharge is unsuitable for applications needing continuous drainage
• Sensitive to dirt scoring the seating face; needs an upstream strainer
• Air venting is mediocre, and cold air can hold the trap closed (air binding) on start-up unless an air vent is fitted

Use: steam mains drainage, steam tracing, high-pressure applications, outdoor locations where freezing is a risk. The most widely installed trap type in steam distribution.

Float and Thermostatic (F&T) Traps

A ball float on a lever operating a valve, plus a separate thermostatic air vent, in a larger body.

Operation. Condensate raises the float, which opens the valve proportionally to the condensate level — continuous modulating discharge. The thermostatic element vents air whenever air is present at the top of the body and closes when steam reaches it.

Strengths

• Continuous modulating discharge — condensate leaves as fast as it forms, so the heat transfer surface never floods
• Excellent air venting through the separate thermostatic element
• Works at very low pressure, including vacuum, and with zero pressure differential
• Handles rapidly varying loads and sudden load surges without losing performance
• Not affected by sudden pressure or load changes
• Quiet operation

Limitations

• Susceptible to freezing — the body holds water. Needs lagging and a drain, or a different trap type, outdoors in freezing climates
• Larger and more expensive than other types
• Vulnerable to waterhammer, which can collapse the float
• Float is damaged by corrosion in untreated or acidic condensate
• Needs correct orientation and a vertical float lever plane
• Fails closed if the float sinks — causing waterlogging, which is operationally obvious but can damage equipment

Use: heat exchangers, air heater batteries, calorifiers, jacketed vessels, process plant where temperature control matters, drying cylinders, anywhere load varies and modulating drainage is required. If a process has temperature control and a modulating control valve upstream, F&T is almost always the correct trap — the alternative trap types cannot handle the low differential pressure that occurs when the control valve throttles.

Inverted Bucket Traps

An inverted bucket inside a body, connected by a lever to the outlet valve, with a small vent hole in the top of the bucket.

Operation. Condensate fills the body; the bucket sits down and the valve is open, so condensate discharges. When steam arrives it enters the bucket from below, displaces water, and the bucket becomes buoyant — it rises and closes the valve. Steam slowly escapes through the vent hole in the bucket top, so the bucket loses buoyancy, sinks, and reopens.

Strengths

• Very tolerant of dirt and scale — the discharge path is large and self-cleaning
• Handles waterhammer well; robust mechanical construction
• Good at venting air through the bucket vent hole (slowly)
• Fails closed, so a failed trap waterlogs the equipment rather than wasting steam. For plants without a monitoring programme, this is a significant practical advantage: failure is noticed.
• Long service life in dirty systems
• Discharges continuously under high load

Limitations

• Loses its water seal if pressure drops suddenly or if the trap is subject to superheat — the water in the body flashes away and the bucket sinks, allowing live steam to blow through continuously until the seal re-establishes. A check valve upstream mitigates this.
• Must be mounted vertically with correct orientation
• Freezing risk — the body holds water
• Needs priming with water before start-up
• The small vent hole limits air venting capacity on start-up, so warm-up is slow
• Larger and heavier than thermodynamic traps

Use: steam mains in dirty systems, older plants, applications where failure must be fail-closed, process service with steady loads.

Thermostatic (Balanced Pressure) Traps

A small metal capsule or bellows containing a liquid with a boiling point slightly below that of water at the same pressure.

Operation. When condensate is cool, the capsule fluid is liquid, the capsule is contracted and the valve is open. As condensate temperature approaches saturation, the capsule fluid vaporises, the capsule expands, and the valve closes. Because the fill liquid tracks the steam saturation curve, the trap works across its whole pressure range with no adjustment — hence "balanced pressure."

Strengths

• Very small and light
• Works across a wide pressure range with no adjustment, from vacuum to around 32 bar g
• Excellent air venting — it is wide open when cold
• Freeze-proof if installed to drain, because it fails open when cold
• Can be set to subcool condensate by a few degrees, which eliminates flash steam at the discharge
• Low cost, simple, easy to replace capsule

Limitations

• Discharges condensate a few degrees below saturation temperature, so condensate backs up slightly in the line — unacceptable where the heat transfer surface must be kept clear
• Not suitable for superheat, which can rupture the capsule
• Capsule is damaged by waterhammer and by corrosive condensate
• Limited capacity compared with float traps of similar size

Use: steam tracing, radiators and unit heaters, instrument and sample line drainage, small loads, air venting duty, freezing-risk locations. Standard for tracing systems because of its size, cost and freeze behaviour.

Bimetallic Traps

A stack of bimetallic strips or discs that deflect with temperature, operating a valve.

Operation. Cold, the bimetallic stack is relaxed and the valve open. As temperature rises, the stack deflects and closes the valve. Many designs have the valve on the downstream side so inlet pressure assists opening.

Strengths

• Very robust mechanically; handles waterhammer, superheat and corrosive condensate
• High pressure capability — suitable for 100 bar g and above in some designs
• Compact for the pressure rating
• Good air venting when cold
• Freeze resistant — fails open

Limitations

• Responds slowly, because the bimetallic stack must heat and cool
• Does not track the steam saturation curve precisely; requires adjustment, and the setting drifts with age and with back pressure changes
• Can discharge substantially subcooled condensate, backing it up in the line
• Not suitable where precise condensate removal timing matters

Use: high-pressure steam mains, superheated steam lines, power plant applications, tracing on high-temperature lines.

Selection Table

Sizing

Trap capacity depends on the differential pressure across it, and vendor capacity charts must be read at the actual differential, not at inlet pressure.

Differential pressure = inlet pressure − back pressure

Back pressure comes from condensate line resistance, static lift, and pressure in the condensate return main. A trap on a 7 bar g supply discharging into a 2 bar g return with a 10 m lift has a differential of about 7 − 2 − 1 = 4 bar, not 7 bar. Sizing at 7 bar undersizes the trap by a wide margin.

Safety factors over calculated condensate load:

The reason is start-up. A cold steam main at the moment steam is admitted condenses many times its running load — all the steel has to be brought from ambient to saturation temperature. A trap sized exactly for the running load cannot clear the start-up load, condensate accumulates, and the first slug of waterhammer arrives.

Modulating control is the other reason. When a temperature control valve throttles to meet a low heat demand, steam pressure in the heat exchanger drops — sometimes to near-atmospheric or into vacuum. Trap differential pressure collapses at exactly the moment condensate must still leave. This is why F&T traps, which work at zero differential, dominate in modulating applications, and why thermodynamic traps fail in them.

Do not oversize excessively either. A grossly oversized trap cycles with very short open periods, wearing the seat quickly, and in intermittent types it wastes steam on every cycle.

Installation

Most "trap failures" are installation faults.

Strainer upstream. Pipe scale, weld slag and rust will score a trap seat. A Y-type strainer immediately upstream, with a blowdown valve, is standard on all but the largest traps. Mount a horizontal-line strainer with the screen pocket horizontal, not pointing down, or it fills with condensate and the screen does nothing.

Drip leg sizing on steam mains. A drip leg that is too small does not collect condensate — the steam flow blows past the branch and carries the condensate with it. The drip pocket should be the same diameter as the main up to DN 100, and at least half the main diameter above that, with a minimum pocket depth of about 1.5 pipe diameters. The trap connects to the side of the pocket, above the bottom, so dirt settles below the take-off.

Drip leg spacing. Every 30-50 m on horizontal steam mains, at every low point, before every rise, before every control valve and isolation valve, and at the end of every main. Mains should fall in the direction of flow at about 1:100.

Discharge piping. Size the condensate line for the flash steam volume, not the water volume. Condensate at 7 bar g flashing to atmospheric produces roughly 13% flash steam by mass, which occupies a volume hundreds of times greater than the water. An undersized condensate line creates back pressure, which reduces trap capacity, which floods the equipment.

Lift after the trap. Allow about 1 bar of differential for every 10 m of static lift. Lifting condensate immediately after a trap on a modulating application frequently stalls it when the control valve throttles — install a pumping trap or a condensate pump instead of relying on steam pressure.

Group trapping is almost always wrong. Two or more steam spaces draining to a single trap will not work if their pressures ever differ: the higher-pressure space pushes steam through the trap and blocks the lower-pressure one, which waterlogs. One trap per steam space.

Air vents at the end of mains and at high points. A trap is not an air vent. Thermostatic air vents at the end of mains and at the top of heat exchangers dramatically improve warm-up time.

Test valves. Fitting an isolation valve and a small test valve downstream of each trap costs little and makes survey work quick and reliable.

Detecting Failed Traps

Three symptoms, three detection methods:

Failed open (steam blowing through). Energy loss with no process symptom. Detected by:

• Ultrasonic testing — a blowing trap produces continuous high-frequency noise; an intermittent trap produces a cyclic pattern. The primary method.
• Thermal imaging — the downstream line runs at near inlet temperature rather than showing the expected temperature drop
• Visual check at a test valve, if the condensate discharges as continuous steam

Failed closed (waterlogged). Process symptom — the heat exchanger loses capacity, the jacket runs cold, the batch takes longer. Detected by temperature measurement upstream and downstream and by the process complaint.

Leaking slightly. Hard to detect; usually found by combining ultrasonic and thermal methods, and by comparing against known-good traps of the same type on the same duty.

A survey programme is the single highest-return maintenance activity in a steam plant. Typical findings in unmanaged populations are 15-30% failed. Annual surveys on large populations and continuous monitoring on high-value traps bring failure rates below 5%.

What a failed trap costs

Steam loss through a failed-open trap, approximate, for a 15 mm orifice:

Figures assume a steam cost in the region of 25 USD per tonne, which varies widely with fuel price and boiler efficiency — calculate with your own numbers, but the order of magnitude holds. One failed trap costs several times the price of a new trap every year.

Common Specification Mistakes

1. Thermodynamic trap on a temperature-controlled heat exchanger. When the control valve throttles, differential pressure collapses and the trap cannot operate. The exchanger floods, the process runs cold, and someone opens a bypass.

   Prevention: Use F&T traps on all modulating-control applications. Thermodynamic traps belong on steam mains and tracing.

2. Sizing at inlet pressure instead of actual differential pressure. A trap discharging into a pressurised return with a static lift has far less differential than its inlet gauge suggests.

   Prevention: Calculate differential as inlet minus return pressure minus static lift, and read capacity charts at that figure.

3. No safety factor for warm-up load. A trap sized for running load cannot clear a cold start, so the main waterhammers on every start-up.

   Prevention: Apply 2× for running loads and 3× for warm-up and modulating applications.

4. Group trapping several steam spaces to one trap. Pressure differences between spaces mean one waterlogs.

   Prevention: One trap per steam space, always.

5. No strainer upstream. Scale and weld slag score the seat, and the new trap fails within months.

   Prevention: Fit a Y-strainer with blowdown upstream of every trap, oriented with the screen pocket horizontal on horizontal lines.

6. Undersized drip leg on steam mains. Steam flow carries condensate past a small branch connection.

   Prevention: Drip pocket equal to main diameter up to DN 100, at least half above that, minimum 1.5 diameters deep, with the trap take-off above the bottom.

7. Condensate line sized for water flow, not flash steam volume. Flash steam dominates the volume and the undersized line creates back pressure that floods the equipment.

   Prevention: Size condensate lines on two-phase flow with flash steam calculated at the discharge pressure.

8. Float trap installed outdoors in a freezing climate. The water-filled body freezes and splits.

   Prevention: Use thermodynamic or thermostatic traps outdoors, or lag and heat-trace the trap station and provide drainage.

9. Inverted bucket trap on a superheated or sharply varying pressure line. Loss of water seal lets steam blow through continuously.

   Prevention: Use bimetallic or thermodynamic traps on superheat; fit an upstream check valve on inverted bucket traps where pressure can fall suddenly.

10. No survey programme. The traps are installed and never looked at, and 20-25% are failed within a few years.

    Prevention: Annual ultrasonic and thermal survey with a tagged register, test valves on every trap station, and continuous monitoring on the largest and most critical traps.

11. Lifting condensate after a modulating trap. When the control valve throttles, there is not enough pressure to lift, and the exchanger stalls.

    Prevention: Use a pumping trap or a mechanical condensate pump where lift is unavoidable on modulating service.

Supply from Kasko Makine

Kasko Demir Çelik Makine supplies steam traps and complete steam and condensate system equipment:

Steam traps

• Thermodynamic disc traps, DN 15 to DN 50, to 40 bar g
• Float and thermostatic traps, DN 15 to DN 100, with integral or separate air vents
• Inverted bucket traps, DN 15 to DN 50, with and without integral check valves
• Thermostatic balanced-pressure traps and capsules
• Bimetallic traps for high pressure and superheat
• Universal connector traps with swivel connectors for quick replacement
• Pumping traps and mechanical condensate pumps for lift and stall applications
• Materials: carbon steel, stainless steel, cast iron, bronze
• Screwed, socket weld, butt weld and flanged connections

Steam system equipment

• Y-type and basket strainers with blowdown connections
• Thermostatic and balanced-pressure air vents
• Pressure reducing valves and steam pressure control stations
• Safety valves and relief valves
• Steam separators and moisture separators
• Flash vessels and condensate receivers
• Condensate pumps, electric and steam-powered
• Sight glasses, check valves and isolation valves
• Vacuum breakers

Associated supply

• Steam and condensate piping in carbon steel and stainless steel
• Flanges, fittings, gaskets and stud bolts
• Pipe supports, hangers and expansion provision
• Thermal insulation and cladding for steam lines

Engineering support

Send a line list or P&ID with steam pressures, condensate loads, back pressures and lift details and we will return a trap schedule with type, size, differential pressure, applied safety factor and capacity stated for each location. For an existing plant, we can also advise on a trap survey programme and provide a tagged register format.

Certification

Material certificates to EN 10204 3.1, pressure-temperature ratings and capacity test data, PED conformity documentation for European destinations, and dimensional and hydrostatic test reports where specified.

Logistics

Standard traps and strainers in common sizes ship from stock in 1-2 weeks. Large bore, special materials and high-pressure traps typically 4-8 weeks. Shipping from Istanbul by road to Europe and the Caucasus, by sea to Gulf, African and Asian destinations, and by air for urgent spares.

Send your steam system details and we will return a trap schedule with sizing basis within three working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Steam and Piping Series

• Boiler and Cooling Water Treatment
• Industrial Valves: Complete Guide
• Industrial Insulation Materials
• Carbon Steel Pipe: Grades and Standards

Frequently Asked Questions

Q: Which steam trap should I use on a heat exchanger?
A: Use a float and thermostatic trap. Heat exchangers with temperature control have a modulating steam valve, and when that valve throttles, steam pressure in the exchanger falls — sometimes to atmospheric or into vacuum. F&T traps work down to zero differential pressure and discharge continuously, so the heat transfer surface never floods. Thermodynamic traps need around 0.3 bar minimum differential and fail in this duty.

Q: How do you know if a steam trap has failed?
A: A trap failed open blows live steam continuously with no process symptom, detected by ultrasonic testing showing continuous high-frequency noise, or by thermal imaging showing the downstream line at near inlet temperature. A trap failed closed waterlogs the equipment and shows as lost heating capacity. Industrial surveys typically find 15 to 30% of traps failed where no monitoring programme exists.

Q: What safety factor should be used when sizing a steam trap?
A: Apply 2 times the calculated running condensate load for steam main drip legs and tracing, and 3 times for steam main warm-up, modulating heat exchangers, jacketed vessels and drying cylinders. The reason is start-up: a cold steam main condenses many times its running load while the steel reaches saturation temperature, and a trap sized for running load alone cannot clear it, causing waterhammer.

Q: What is the difference between a thermodynamic and an inverted bucket steam trap?
A: A thermodynamic trap uses a single disc that closes on flash steam velocity, making it compact, high-pressure capable, freeze-resistant and fail-open. An inverted bucket trap uses a buoyant bucket, tolerates dirt and waterhammer extremely well and fails closed, which makes failures visible. The bucket trap loses its water seal on sudden pressure drop or superheat, letting steam blow through until the seal re-establishes.

Q: How much does a failed steam trap cost?
A: A failed-open trap with a 15 mm orifice passes roughly 18 kg/h of steam at 3 bar g, 28 kg/h at 7 bar g and 36 kg/h at 10 bar g. At 8,000 operating hours and around 25 USD per tonne of steam, that is approximately 3,500, 5,500 and 7,000 USD per year respectively. A single failed trap therefore costs several times the price of a replacement every year it remains undetected.

Q: Can several steam-heated items share one steam trap?
A: No. Group trapping fails whenever the pressures in the connected steam spaces differ, because the higher-pressure space pushes steam through the trap and blocks drainage from the lower-pressure one, which waterlogs and loses heating capacity. Each steam space needs its own trap, sized for its own load and differential pressure.

Q: Why does air in a steam system matter?
A: Air is an effective insulator at the heat transfer surface, and under Dalton's law it also reduces steam partial pressure, so actual steam temperature falls below the saturation temperature implied by the pressure gauge. Around 10% air by volume at 3 bar g lowers temperature from about 144°C to roughly 139°C, so the process runs cold while the gauge reads normally. Thermostatic air vents at main ends and heat exchanger high points fix it.