A steam trap is an automatic valve that discharges condensate, air and other non-condensable gases from steam lines and equipment while holding back live steam, and Paul Industries surveys, tests, repairs, replaces and installs steam traps for industrial plants nationwide. Traps are small and numerous, which is why they are neglected, and a failed one wastes steam without any visible sign.
What is a steam trap?
In practical terms, it is the valve at every point where steam gives up its heat and turns to water: the drip legs on steam mains, the outlets of heat exchangers, coils, jacketed vessels, presses and tracer lines. What does a steam trap do there? It opens to let condensate and air out, then closes before live steam can follow, over and over, at every load from a cold start to full production.
What is the purpose of a steam trap, beyond draining water? Three jobs. It keeps steam spaces free of condensate, so heat transfer surfaces see steam rather than a layer of water. It vents air, which mixed into steam lowers the temperature at the heating surface below what the pressure gauge suggests. And it protects the piping, because condensate left in a steam main is picked up as slugs that cause water hammer. The steam trap function is the same on a 2 inch drip as on a large process exchanger; only the load, pressure and trap type change. In the boiler house, a boiler steam trap on the drip leg at the end of the steam header removes the water carried over from the boilers before it reaches the plant mains. Where steam traps sit on the distribution system, and how drip legs and condensate returns are laid out, is covered on our steam piping page.
How does a steam trap work?
Every trap tells steam from condensate by one physical difference: density, temperature or velocity. That gives the three families that ISO 6704 uses to classify automatic steam traps, and it answers the common question of how many types of steam traps there are: three families, each with two or three common designs.
- Mechanical traps sense density. A float or bucket rises and falls with the condensate level and moves the valve, so they discharge condensate at steam temperature as it forms.
- Thermostatic traps sense temperature. An element closes the valve when steam reaches it and opens as condensate cools a few degrees below steam temperature.
- Thermodynamic traps sense velocity and pressure. Flashing condensate moving fast under a disc snaps it shut, and it reopens as the steam above it condenses.
A working principle chosen for the wrong duty is the root of most trap problems, so the family matters more than the brand.
Steam trap types compared
The table compares the common designs on the points that decide selection.
| Trap | Family | Discharge | Air venting | Superheat | Freezing outdoors | Water hammer |
|---|---|---|---|---|---|---|
| Inverted bucket | Mechanical | Intermittent, near steam temperature | Slow, through a small vent hole | Needs an inlet check valve; can lose its water seal | Can be damaged in exposed positions | Tolerant |
| Float and thermostatic | Mechanical | Continuous, at steam temperature | Excellent, through its thermostatic vent | Not normally applied | Can be damaged by severe freezing | Modern welded floats are robust |
| Balanced pressure | Thermostatic | Intermittent, slightly subcooled | Excellent, wide open when cold | Tolerates moderate superheat | Unlikely to freeze | Welded stainless capsules resist it |
| Bimetallic | Thermostatic | Subcooled, more so at high back pressure | Good | Suitable | Will not freeze | Withstands it |
| Thermodynamic disc | Thermodynamic | Intermittent, near steam temperature | Limited | Suitable | Resists freezing | Withstands it |
Mechanical steam traps
An inverted bucket steam trap is the classic mechanical steam trap. The bucket hangs upside down over the inlet, inside a body full of water. Steam entering under the bucket floats it and pulls the valve shut; as that steam condenses or bleeds out through a small vent hole in the top of the bucket, the bucket loses buoyancy, sinks and opens the valve, and condensate is pushed out until steam arrives again. The vent hole is also why it vents air slowly. The trap depends on its water seal, or prime: on superheat or a sudden pressure drop the seal can flash off and let steam blow through, so superheated service takes an inlet check valve.
A float and thermostatic steam trap, usually called an F&T steam trap, works on condensate level: a sealed ball float rises with the water and opens the valve in proportion, so discharge is continuous and follows the load, while a separate thermostatic element at the top vents air. It is the first choice wherever equipment must drain freely. A ball float steam trap without the air vent element needs a separate air vent. In both mechanical designs the orifice is sized for a maximum differential pressure, and above that the float or bucket cannot pull the valve open, so the trap stays shut and floods what it drains.
Thermostatic steam traps
In a thermostatic steam trap the valve opens only when condensate has cooled below steam temperature. The balanced pressure thermostatic steam trap uses a capsule or bellows filled with a liquid that boils just below the steam temperature at any pressure, so it adjusts itself as pressure changes, sits wide open when cold and makes an excellent air vent. Older bellows elements were easily damaged by water hammer and corrosive condensate; welded stainless capsules largely solved that. The bimetallic steam trap uses strips of two metals that bend with temperature. It stands up to water hammer, corrosive condensate and high pressure, but it responds slowly and holds condensate back, so it is normally fitted at the end of an uninsulated cooling leg of roughly 1 to 3 meters, longer where back pressure is high. A liquid expansion trap, the third type, discharges at an adjustable temperature well below saturation and suits tracing and drains where using some of the sensible heat in the condensate is acceptable.
Thermodynamic steam traps
A thermodynamic steam trap, often called a disc steam trap, has one moving part: a flat disc above two concentric seat faces. Cold condensate and air lift the disc and flow out. Hot condensate flashes as it accelerates under the disc, lowering the pressure beneath it, and steam trapped in the control chamber above snaps it shut; as that steam condenses, the disc lifts again, with an audible click that helps testing. Disc traps are compact, cheap and handle high pressure, superheat, water hammer and vibration, but they need a minimum inlet pressure of about 3.6 psig (0.25 bar), a back pressure below about 80 percent of the inlet pressure, and protection from rain and wind, which cool the cap and make the trap cycle too fast; makers offer insulating caps for exposed locations.
Thermostatic vs thermodynamic steam trap
The choice turns on subcooling and pressure. A thermostatic trap backs condensate up until it cools, which is useful on tracers, where the extra heat is wanted, and on air vents, but floods a heat exchanger. A thermodynamic trap discharges close to steam temperature and copes with superheat and outdoor service, but works poorly at low inlet pressure or high back pressure. Against a float trap, which drains continuously at any load, a disc trap is a mains-drip and tracing trap.
Which steam trap is best for each application?
The answer depends on the application, not on the trap. The selections below are the usual starting point; pressure, back pressure and location settle the final choice.
| Application | Usual first choice | Alternative | Why |
|---|---|---|---|
| Steam main drip legs | Thermodynamic disc or inverted bucket | Float and thermostatic with a check on the outlet | Small steady loads, high pressure, dirt and water hammer |
| Heat exchangers and process coils with modulating control | Float and thermostatic | Pump trap where the steam pressure can fall below the return pressure | Must drain continuously at every load; air venting at start-up |
| Jacketed vessels and batch heating | Float and thermostatic | Inverted bucket with a thermostatic air vent | Large start-up loads and air to clear |
| Steam tracing on product lines | Thermostatic (balanced pressure) | Thermodynamic disc | Small loads; some subcooling is acceptable |
| Non-critical tracing and freeze protection drains | Bimetallic or liquid expansion | Thermodynamic disc | Uses sensible heat; tolerates freezing |
| Superheated steam lines | Thermodynamic disc | Inverted bucket with inlet check valve | The trap must not lose its seal on superheat |
| Air venting of steam spaces | Balanced pressure thermostatic | Float trap thermostatic element | Fully open when cold |
An industrial steam trap on a high pressure steam trap duty, such as a drain on a high-pressure turbine supply line, needs a body and seat rated for the pressure and temperature as well as the right type; a low pressure steam trap on space heating or air handling coils is almost always a float and thermostatic design, because the control valve can drop the coil pressure close to zero. Sealed stainless steel steam traps are common on drips and tracing, where they are replaced rather than repaired, and on corrosive condensate; sanitary traps for clean steam are a separate class, covered under pharmaceutical clean steam systems.
Steam trap sizing
Sizing starts from the condensate load and the real pressure difference across the trap, never from the pipe size. Sizing a steam trap takes five numbers: the maximum steam pressure and temperature the body must be rated for, the inlet pressure at the trap, the back pressure on the outlet, the maximum and minimum condensate loads, and whether the load is steady, modulating or a heavy start-up. Back pressure includes any lift after the trap, at roughly half a psi per foot of rise (0.433 psi per foot of water exactly), so a trap discharging up 10 feet into a return at 5 psig sees about 9 psi of back pressure before friction.
Capacity is then read from the manufacturer’s chart at the minimum differential, a chart based on tests to standards such as ASME PTC 39 and ISO 5117, and multiplied by a safety factor. Typical factors are about 2 to 1 for float and thermostatic traps and about 3 to 1 for inverted bucket, thermostatic and thermodynamic traps, rising to 4 to 1 where start-up loads are heavy and fast, though each maker publishes its own. Can you oversize a steam trap? Yes, and it is one of the commonest faults found in a survey: an oversized trap cycles hard and wears quickly, a disc trap chatters, and when it fails open the larger steam trap orifice passes far more steam. Undersizing backs condensate into the equipment. A steam trap sizing calculator from a maker is a useful check, provided the inputs are the real pressures at the trap and not the boiler pressure.
Steam trap installation and trap stations
Good installation follows a few rules that matter more than the trap itself. The trap goes below the outlet it drains, close to it, with condensate falling to it by gravity and no lift ahead of it. Each piece of equipment gets its own trap: group trapping, two coils through one trap, lets the coil at the higher outlet pressure hold back condensate from the other, which waterlogs. Mechanical traps are installed level and in the flow direction marked on the body; for any other position, follow the manufacturer’s instructions.
A trap station, or steam trap assembly, is the full set of parts around the trap: an inlet isolation valve, a strainer with a blowdown valve, the trap, a test valve or sight glass downstream, a check valve where the condensate lifts or the return is shared, and an outlet isolation valve, with unions or flanges so the trap can come out. A steam trap with strainer built in saves space; otherwise a Y-strainer goes ahead of it. Steam trap vs strainer is not a choice, because the strainer is what keeps scale and weld debris off the trap seat. Is a steam trap a check valve? No: some designs resist reverse flow, but where condensate rises after the trap or several traps feed one header, a separate check valve is fitted.
Where a heat exchanger’s steam pressure can fall below the return pressure, a trap alone cannot drain it and the exchanger stalls and floods. A pressure-powered pump combined with a trap, often called a steam trap pump or pump trap, uses steam to push the condensate into the return. Thermostatic traps and their cooling legs are left uninsulated so the condensate can cool.
How to test a steam trap
Testing starts with a rule: no single method is reliable alone. The US Department of Energy lists four basic ways to test traps, by temperature, sound, visual observation and electronic monitoring, and a competent test combines at least two.
| Method | What it shows | Limitation |
|---|---|---|
| Visual, at a test valve or sight glass | Whether the discharge is condensate with flash steam or a continuous steam plume | Needs a test point; flash steam is easily mistaken for live steam |
| Temperature (contact probe or infrared) | A cold trap or outlet, meaning a blocked trap or an idle line | Condensate at saturation is as hot as steam, so it cannot confirm blow-through |
| Ultrasound | The trap’s cycle and the sound of steam passing the seat | Must be read against the trap type; needs a trained tester |
| Permanent monitoring | Failure on critical traps within hours, from an acoustic or temperature sensor | Cost per point, so used selectively |
Steam trap testing in the field follows a fixed order: confirm the line is live, take temperatures, listen, then confirm at the test valve. Ultrasonic steam trap testing listens to the high-frequency sound of flow through the seat, well above human hearing, with the probe on the trap’s discharge side. Bucket and disc traps should be heard cycling, float traps flow steadily with the load, and a continuous rush from a cycling type means steam is blowing through. A steam trap tester combines an ultrasonic probe with a temperature sensor and logs both against the trap tag, which is what makes a survey repeatable year to year.
Monitoring adds permanent sensors on the traps that matter most, an approach DOE recommends for traps on important processes. Wireless steam trap monitoring uses battery-powered acoustic or temperature sensors clamped to the trap, reporting through a gateway, so no cabling is run. A steam trap monitor reports a failure to the maintenance system as it happens instead of at the next survey.
What is a steam trap survey?
A survey is a plant-wide test of every trap, in which each one is tagged, located on a drawing, identified by make, model, size and pressure, tested, and classed as working, failed open, leaking, failed closed or out of service. The survey report puts a steam loss on each failed trap, flags traps of the wrong type or size, and gives a prioritized repair list, so the repairs can be justified in dollars.
How to do a steam trap survey that is worth repeating comes down to the register. The first survey finds and tags every trap, including those buried in insulation; later surveys test against the same register, so trends show and repairs are confirmed. The DOE reports that in systems not maintained for three to five years, 15 to 30 percent of traps may have failed, that a regular program holds leaking traps under 5 percent, and that a survey of any system with more than 500 traps will probably reveal significant losses. DOE’s recommended testing intervals tighten as pressure rises, from annual checks on low-pressure systems to weekly or monthly checks at 150 psig and above.
What happens when a steam trap fails?
A trap fails in one of two directions, and they cost different things. A trap that fails open, or a partial steam trap leak through a worn seat, blows live steam into the condensate return, wasting energy every hour and pressurizing the return so other traps back up. Can a steam trap fail closed? Yes, from dirt, a blocked strainer, air binding, a collapsed float or a failed element, and a closed trap floods the main or the equipment, which brings water hammer, slow heating and, on outdoor lines, freezing. Inverted bucket traps tend to fail open; float traps can fail either way, closed if the float collapses and open if dirt holds the valve off its seat.
The steam lost through a failed-open trap depends on the orifice and the pressure. These DOE figures, from the Boiler Efficiency Institute, are for steam discharging to atmosphere, per trap, every hour it runs:
| Trap orifice | At 15 psig | At 100 psig | At 150 psig | At 300 psig |
|---|---|---|---|---|
| 1/32 inch | 0.85 lb/h | 3.3 lb/h | 4.8 lb/h | Not listed |
| 1/16 inch | 3.4 lb/h | 13.2 lb/h | 18.9 lb/h | 36.2 lb/h |
| 1/8 inch | 13.7 lb/h | 52.8 lb/h | 75.8 lb/h | 145 lb/h |
| 3/16 inch | 30.7 lb/h | 119 lb/h | 170 lb/h | 326 lb/h |
| 1/4 inch | 54.7 lb/h | 211 lb/h | 303 lb/h | 579 lb/h |
| 3/8 inch | 123 lb/h | 475 lb/h | 682 lb/h | 1,303 lb/h |
DOE’s worked example puts a single 1/8 inch trap stuck open on a 150 psig line, with steam valued at $10 per thousand pounds, at about $6,640 a year: 75.8 lb/h times 8,760 hours. Multiply by the number of failed traps a first survey usually finds and the case for a program makes itself.
Steam trap troubleshooting
Most failures trace back to a short list of causes: dirt and scale, water hammer, wear from oversizing, corrosive condensate, freezing, loss of prime, and the wrong trap for the duty.
| Symptom | Likely cause | Check and fix |
|---|---|---|
| Continuous steam plume at the test valve | Failed open: worn seat, lost prime, debris under the valve | Clean or rebuild, clean the strainer, check sizing and superheat |
| Trap and outlet cold, equipment slow to heat | Failed closed: blocked strainer, air binding, differential above the orifice rating | Clean the strainer, check air venting, confirm the orifice suits the pressure |
| Exchanger floods and bangs at part load | Steam pressure below return pressure (stall) | Fit a pump trap, or lower the return pressure |
| Disc trap cycling rapidly | Oversized, high back pressure, or cap cooled by rain and wind | Resize, check back pressure, fit an insulating cap |
| Inverted bucket blowing steam after a pressure dip | Lost its water seal | Re-prime; add an inlet check valve |
| Thermostatic trap holding back too much condensate | Cooling leg too short, or back pressure too high | Lengthen the cooling leg or change trap type |
| Several traps on one return all running hot | One failed-open trap pressurizing the shared return | Test each trap and isolate the leaker |
Steam trap repair and replacement
Repair is worth doing when the body is sound and the trap is the right type and size for the duty. Can steam traps be repaired? Repairable designs take a steam trap repair kit of seat, valve and mechanism, fitted in place with the trap isolated, and the faces of a disc trap can be lapped if the wear is light. Universal-connector traps go further: the connector stays welded or threaded into the line and the trap module bolts onto it, so changing a trap takes minutes and no pipe work.
Steam trap replacement is the answer for sealed, non-repairable traps, for bodies that are eroded or cracked, and for traps a survey shows to be the wrong type or size, where rebuilding the same mistake makes no sense. Every replacement is a chance to correct the selection, add a missing strainer or test valve, and standardize the population to cut spares. How long do steam traps last? There is no fixed life; it depends on type, sizing, pressure, dirt and water hammer, which is why traps are managed by testing rather than replaced by age.
Steam trap maintenance
Maintenance is a program, not a task; a single steam trap inspection fixes nothing on its own. The program is a register of every trap, a test schedule set by pressure and criticality, repair or replacement of every failed trap found, and a check that the repair worked. Between surveys, strainers are blown down, test valves kept working and idle lines drained before winter. Our preventive maintenance and service contracts build trap surveys into the same schedule as the rest of the plant’s utility equipment, and failures that cannot wait go to our emergency repair service.
What Paul Industries does
Paul Industries carries out plant-wide surveys, ultrasonic and temperature testing, repair and replacement of failed traps, resizing, new trap stations, pump traps on stalled exchangers, and wireless monitoring on critical traps. The same crews correct drip legs, condensate returns and flash recovery on our steam piping and condensate work, service steam heat exchangers, install boiler feed and deaerator equipment under boiler water treatment, and plan large replacements into shutdowns and turnarounds. We work on traps from all the major manufacturers and do not manufacture traps ourselves. All of it is part of our plant utilities service. Request a quote for a trap survey, a repair program or a trap station upgrade.
Standards referenced: ISO 6704 · ISO 5117 · ASME PTC 39
Frequently asked questions
How often should steam traps be inspected?
DOE guidance is weekly to monthly at 150 psig and above, monthly to quarterly from 30 to 150 psig, and at least annually below 30 psig. Within those bands, test more often where a failure is expensive, such as large exchangers and high-pressure drips, and always after a shutdown, a water hammer event or a boiler carryover, when debris and shock damage are most likely.
Should steam traps be replaced on a schedule?
No. Traps are replaced when testing shows they have failed and cannot be repaired, or when they are the wrong type or size for the duty. Plants that standardize on universal-connector traps change modules in minutes, which makes it practical to replace a failed trap on the day it is found rather than waiting for a shutdown.
Do steam traps prevent water hammer?
They are half of the answer. A working trap on a properly sized drip leg removes condensate before steam can pick it up as a slug, but a trap cannot help if the main has no slope, sags between supports or has no drip pocket to collect the water. A failed-closed or undersized trap is itself one of the most common causes of water hammer.
Do steam traps reduce pressure?
No. A trap is an on-off or modulating drain, not a pressure regulator; it holds steam pressure on its inlet and discharges into whatever pressure the return is at. The pressure difference across it is what pushes condensate out, which is why a trap that sees too little differential stops draining.
Are thermostatic steam traps normally open or closed?
Open when cold and closed when hot. A balanced pressure trap is wide open at start-up, which is why it clears air so fast, and closes as steam reaches the element. It reopens when the condensate around it cools a few degrees below steam temperature.
How many steam traps do I need?
One for every piece of steam-using equipment, plus drip traps at every low point of the distribution system, at the end of each main, ahead of risers, control valves and pressure-reducing stations, and along straight runs at intervals of roughly 100 to 165 feet. A plant's trap count is usually much higher than anyone expects, which is why the first survey starts by finding them.
Can steam traps be installed vertically?
Only if the trap is built for it. Float and bucket traps work only in the orientation they were designed for, with the arrow on the body in the flow direction. Many thermostatic and some disc designs are made for horizontal or vertical lines, and the installation instructions state which positions are allowed.
How do you prime an inverted bucket steam trap?
Give it water before it sees steam. With the trap isolated, fill the body through its priming plug where one is fitted, or open the inlet slowly so condensate fills the trap before steam reaches it. Where traps repeatedly lose their prime on superheat or pressure swings, an inlet check valve keeps the water seal in place.
Can you check a steam trap with a temperature gun?
Only for gross faults. An infrared thermometer finds traps that are cold, which means blocked, isolated or flooded, but condensate at saturation is as hot as steam, so a hot reading does not prove the trap is working. Painted and bare surfaces also read differently. Use temperature together with ultrasound and a test valve.
What is steam trap monitoring?
A permanent sensor on a trap that reports its condition continuously. It pays where failures are expensive or hard to reach: high-pressure drips, large process exchangers, traps on hazardous or remote lines. The rest of the population is still covered by periodic surveys.
What does a steam trap look like?
A small valve body, usually cast or forged steel, iron or stainless, in the line just after a drip leg or equipment outlet, often with a strainer and isolation valves around it. Disc traps are small and round with a hexagonal cap, float traps are larger boxy bodies, inverted bucket traps are taller cylinders, and thermostatic traps are compact bodies with a bolted cap.
When are steam traps required?
Wherever steam condenses and the water must be removed: at every low point and the end of every steam main, ahead of risers, control valves and pressure-reducing stations, at the outlet of every heat exchanger, coil, jacket and tracer, and at separators. Without a trap at those points, condensate collects and either floods equipment or causes water hammer.
What is the difference between a steam trap and an air vent?
An air vent is a thermostatic element that releases air and other gases at high points and the ends of steam mains, where air collects at start-up. A steam trap removes condensate and, depending on type, some or all of the air with it. Float and thermostatic traps combine both jobs; inverted bucket and disc traps often need a separate air vent on equipment with a lot of air to clear.
Should a steam trap discharge to drain or to the condensate return?
To the return wherever the condensate is clean. Returned condensate is hot, already-treated boiler water; DOE notes it typically comes back at 130 to 225 F against 50 to 60 F makeup water. Condensate goes to drain only where it may be contaminated, for example from a leaking process exchanger, and then usually through a cooler.
What is the difference between an inverted bucket and an F&T trap?
An inverted bucket trap discharges intermittently, tolerates dirt and water hammer well and suits drips and steady loads, but vents air slowly and can lose its prime. A float and thermostatic trap discharges continuously in step with the load and vents air freely, which makes it the better trap on heat exchangers and modulating coils, though it is more vulnerable to freezing.
How can you tell flash steam from live steam at a trap outlet?
Flash steam leaves a test valve as a lazy, low-velocity white cloud mixed with water. Live steam blowing through a failed trap comes out as a fast jet that is clear for a short distance from the outlet before it turns white. Telling the two apart takes practice, which is why visual checks are confirmed with ultrasound.
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