Steam Traps and Steam Leaks: How Failed Traps Waste Energy, Cause Water Hammer and Hide in Plain Sight

• A steam trap lets condensate and air out of steam lines and equipment while holding steam in. It fails either open, wasting steam, or closed, backing up condensate.
• In systems unmaintained for three to five years, 15 to 30% of traps may have failed. A regular program keeps leaking traps under 5%.
• Temperature alone misleads: condensate below a healthy trap is still hot. A cold trap inlet on a live line is the more useful temperature clue.
• Airborne ultrasound separates healthy cycling from a continuous blow through, which is why it is a standard trap test.

Steam is still used in older commercial buildings, on district steam networks and in specific equipment such as heat exchangers, kitchens, laundries and humidifiers. It is less common than it was, but far from gone.

• Older commercial buildings heated by steam.
• Buildings on district steam, for example in large city cores.
• Heat exchangers that convert steam to hot water for heating.
• Kitchen and laundry equipment.
• Steam humidifiers, which some data centers and labs use for humidity control.

Wherever there is steam, there are traps. A building with steam heating can have hundreds of them, spread across mechanical rooms, risers and ceiling spaces. Many sit above ceilings or behind panels, which is exactly why they go untested for years.

A steam trap is an automatic valve that lets condensate and air out of steam lines and equipment while holding steam in. It is what keeps a steam system working efficiently.

As steam gives up its heat, it condenses into water. That condensate has to be removed, or it collects in pipes and equipment, blocks heat transfer and causes water hammer.

Air has to be removed too, especially at start up. Air in a steam space insulates heat transfer surfaces and slows warm up.

The trap's job is to open for condensate and air and close for steam. When it gets that wrong in either direction, it has failed.

The main types of steam trap are mechanical, thermostatic and thermodynamic. Each senses the difference between steam and condensate in a different way, and each cycles or discharges in its own pattern.

• Mechanical traps, such as float and thermostatic or inverted bucket traps, respond to the density difference between steam and water.
• Thermostatic traps respond to temperature, opening when cooler condensate and air reach them.
• Thermodynamic traps, or disc traps, respond to velocity and pressure as steam and condensate pass a disc.

Knowing the type matters for testing. A healthy disc trap cycles sharply; a float trap may discharge more continuously. A test has to judge the pattern against the type.

A trap that fails open passes live steam; a trap that fails closed holds back condensate. Both are failures, with very different symptoms.

Failed open, or blow through

Live steam passes straight into the condensate return, wasting energy and over pressurizing returns. Nothing drips, so there is often no visible sign at the trap itself.

Failed closed, or plugged

Condensate backs up into the equipment, cutting heat transfer. It causes water hammer and risks freeze damage in cold weather, when backed up water sits in coils exposed to outside air.

Failed open traps waste money; failed closed traps break things. A survey should look for both, and record which way each failed trap went, because the fix and the urgency differ.

In steam systems not maintained for three to five years, 15 to 30% of traps may have failed, according to DOE Steam Tip Sheet #1. With a regular program, leaking traps should be under 5% (DOE Steam Tip Sheet #1).

How much steam escapes

DOE's leak table gives steam loss through a 3/16 inch (4.8 mm) orifice to atmosphere at several pressures:

• 15 pounds per square inch gauge (psig), about 1 bar: about 30.7 lb/hr (14 kg/hr).
• 100 psig (about 6.9 bar): about 119 lb/hr (54 kg/hr).
• 150 psig (about 10.3 bar): about 170 lb/hr (77 kg/hr).
• 300 psig (about 20.7 bar): about 326 lb/hr (148 kg/hr).

What it costs

Cost depends on the price of steam. One DOE example values a failed trap at about $6,640 a year, using a steam cost of $10 per 1,000 lb (Reliability Magazine).

That is an example, not a typical figure. A low pressure heating trap with a small orifice loses far less, but a building with dozens of failed traps still adds up quickly.

Steam traps fail mostly from dirt, wear and the conditions around them. A trap has small moving parts working against hot, dirty, fast moving fluid every day.

• Dirt and scale from the piping, which jams valves open or blocks the outlet.
• Wear of seats, discs and valve heads from years of cycling.
• Wrong type or size for the load, so the trap cannot keep up or cycles too hard.
• Water hammer damage to floats, bellows and internals.
• Freezing of traps on outdoor or unheated lines in cold weather.

Strainers ahead of traps catch much of the dirt, and cleaning them is part of trap maintenance. A trap that fails repeatedly in the same spot often has a strainer, sizing or drainage problem behind it.

Steam from a failed open trap goes into the condensate return, where it raises return pressure and temperature and eventually escapes or is wasted. The building pays for steam that did no useful work.

Over pressurized returns push back on other traps, which then struggle to discharge. One bad trap can make good traps on the same return look like they are failing.

Much of the escaped steam ends up venting at a receiver or tank as a larger than normal plume. On district steam, every pound of it is metered and billed.

That is why a trap survey often starts at the return. A condensate receiver venting heavily, or return lines that are hotter than usual, point to failed open traps upstream.

Steam traps should be tested on an interval set by system pressure, with higher pressure systems tested more often. DOE's recommended intervals are:

• High pressure, 150 psig (10.3 bar) and above: weekly to monthly.
• Medium pressure, 30 to 150 psig (2.1 to 10.3 bar): monthly to quarterly.
• Low pressure, below 30 psig (2.1 bar): annually.

Most building heating steam is low pressure, so an annual survey is the baseline. Critical traps, such as those on heat exchangers or humidifiers, may deserve more frequent checks.

DOE lists four basic trap test methods: temperature, sound, visual and electronic. Most good surveys combine at least two.

Temperature alone cannot tell you if a trap is working because condensate downstream of a healthy trap is still hot. A hot outlet does not prove a leak.

Condensate leaves the trap close to steam temperature, and some of it flashes to steam as pressure drops. The outlet of a good trap and a blowing trap can both read hot.

The more useful temperature clue is at the inlet. A cold trap inlet suggests a blocked or failed closed trap, or a closed valve upstream.

Thermal imaging is still valuable for that inlet check and for spotting cold, waterlogged equipment. It just cannot judge blow through on its own.

Ultrasound tells a good trap from a bad one by the pattern of its sound. A healthy trap discharges intermittently in a pattern that matches its type; a trap that is blowing through makes a continuous rush.

Airborne and contact ultrasound pick up the high frequency turbulence of steam and condensate passing the trap. The technician listens for the cycle: open, discharge, close.

A disc trap that clicks open and shut on a steady rhythm is healthy. The same trap with a steady, unbroken roar is passing steam.

Combined with an inlet temperature check, sound gives a confident verdict for most traps without taking anything apart.

You run a steam trap survey by listing every trap, testing each one with at least two methods, and tracking failures until they are fixed. The list is the foundation, and most sites do not have one.

1. 1. Build a trap list with a tag, location, type, size and service for every trap.
2. 2. Test each trap with sound and temperature, noting the inlet temperature and the discharge pattern.
3. 3. Classify each trap as good, failed open, failed closed or out of service.
4. 4. Replace or repair failed traps, starting with failed open traps on higher pressure lines.
5. 5. Record the test date on the trap tag and in the list.
6. 6. Repeat at the interval set by pressure.

The failure rate from each survey is the key number. It shows whether the program is holding traps near DOE's under 5% target or drifting toward the 15 to 30% of an unmaintained system.

Visible steam leaks are dangerous because the steam closest to a high pressure leak is invisible. It only becomes a visible plume a short distance away, once it condenses.

That makes the most dangerous part of the jet the part you cannot see. Anyone checking a steam leak should keep a safe distance and never probe for it by hand.

Steam leaks at valves, packing and joints are common, especially after work on the system. Leaking steam valves that pass when closed also overheat spaces and waste energy, covered in Part 9.

Routine visual inspection from a safe distance finds plumes at vents, valves, packing and joints. Recording each one gives the repair list.

Water hammer in steam systems is caused by condensate backed up behind a failed closed trap being slammed by steam. The result is a loud bang that can crack fittings and blow gaskets.

Steam moving at speed picks up a slug of water and drives it into an elbow, valve or the end of a pipe. The impact is violent and repeated.

Water hammer often appears during warm up, when condensate is greatest. Banging at start up is a strong sign that traps or drainage are not keeping up.

Water hammer is covered further in the Cooling Plant Performance sub-series. Here the point is that a failed closed trap is often the cause, and the pressure gauges and sound during warm up point to where.

Failed traps cost fuel and damage in both, with different risks.

In an office building

Higher fuel bills, overheated spaces, water hammer damage, and frozen and burst coils from failed closed traps in cold weather are the main costs. Burst coils in an air handler can flood a mechanical room.

Trap surveys belong on the preventive maintenance schedule alongside the rest of the heating, ventilation and air conditioning (HVAC) work covered in HVAC maintenance in office buildings. In many buildings they are the single largest energy saving left on the table.

In a data center

Steam humidifiers and steam heat exchangers on a campus are the exposure. A failed trap can disturb humidity control or a heat exchanger serving support spaces.

A humidifier trap that fails closed can send condensate into the humidifier's steam line, spitting water into the air stream instead of dry steam. A trap that fails open wastes steam the humidifier was meant to deliver. Both show up as humidity that will not hold its setpoint.

Several normal behaviors look like a failed trap. Knowing them prevents replacing good traps.

• Flash steam: a normal plume from hot condensate venting at lower pressure, as opposed to live steam blowing through a failed trap. Flash steam is lazy and billowing; live steam is a sharp, fast jet.
• A hot pipe that is hot because it should be: condensate lines downstream of good traps run hot.
• Compressed air hiss, covered in Part 7.

This is why no single sensor is enough. Sound pattern, inlet temperature and the appearance of the plume each rule out a different explanation.

A walk past can prove a visible leak, water hammer and a frozen trap. It cannot measure how much steam a trap is losing.

• Directly observable: a visible plume, a dripping or spitting trap, water hammer noise, and a frosted or frozen trap in winter.
• Strong indicator: a continuous ultrasonic rush at the trap outlet, and a cold trap inlet on a live line.
• Suspected: steam loss rate. That needs trap specific testing and the orifice and pressure data.

That is the limit of any route based check. It finds the failed traps; trap specific testing measures the loss.

A walkthrough record of trap tests and plumes turns these checks into a trend, the core of predictive maintenance.

• Listen at each trap for its normal cycling pattern versus a continuous rush.
• Trap inlet hot on lines that should be live.
• Plumes at vents, valves, packing and joints, keeping a safe distance.
• Water hammer bangs during warm up.
• Trap tags and the last test date.
• In cold weather, frost on traps and coils.

Coming Soon

1. 1. How many steam traps are on your site, and is there a list with locations?
2. 2. When was each trap last tested, and by what method?
3. 3. What did your last survey find as the failure rate?
4. 4. Do you use steam humidifiers, and who maintains their traps?
5. 5. Where does water hammer happen, and when?

What does a steam trap do?

A steam trap is an automatic valve that lets condensate and air out of steam lines and equipment while holding steam in. It keeps heat transfer surfaces clear and prevents water hammer.

How do you know if a steam trap has failed?

A failed open trap makes a continuous rush in place of its normal cycling, heard with ultrasound. A failed closed trap often has a cold inlet, backs up condensate and causes water hammer.

How many steam traps fail?

DOE says 15 to 30% of traps may have failed in systems not maintained for three to five years. With a regular program, leaking traps should be under 5%.

How much does a failed steam trap cost?

It depends on steam price, pressure and orifice size. One DOE example values a failed trap at about $6,640 a year at a steam cost of $10 per 1,000 lb. That is an example rather than a typical figure.

How often should steam traps be tested?

DOE recommends weekly to monthly for high pressure systems at 150 psig and above, monthly to quarterly for 30 to 150 psig, and annually below 30 psig. Most building heating steam is low pressure.

Can you test a steam trap with a thermal camera?

Partly. A cold inlet on a live line points to a blocked or failed closed trap. A hot outlet does not prove blow through, because condensate below a good trap is still hot. Pair thermal checks with ultrasound.

What causes water hammer in steam systems?

Condensate backed up behind a failed closed trap is picked up by fast moving steam and slammed into fittings. The bangs can crack fittings and blow gaskets, and are often worst during warm up.

Is a visible steam plume always a leak?

No. Flash steam is a normal, billowing plume from hot condensate venting at lower pressure. Live steam from a leak or failed trap is a sharp, fast jet.

Why are steam leaks dangerous?

Steam right at a high pressure leak point is invisible until it condenses a short distance away. The most dangerous part of the jet cannot be seen, which makes leaks a serious burn hazard.

• Ultrasonic Sensing in Data Centers: Hearing Faults Before They Fail
• Thermal Imaging: How It Finds a Failing Connection Before It Fails
• The Valve That Leaks Through: How Passing and Stuck Valves Quietly Waste Energy and Undermine Redundancy

• DOE Steam Tip Sheet #1: inspect and repair steam traps
• Reliability Magazine: steam trap failure statistics and DOE cost example