Short answer
A 1/8 inch trap blowing through at 100 psig wastes about 43 lb of steam per hour, or roughly $3,500 a year with steam at $10 per 1,000 lb. Enter your own pressures, sizes and fuel price in the steam trap loss calculator.
1. What a steam trap does, and how it fails
A steam trap lets condensate and air out of a steam line or heat exchanger while keeping steam in. Every trap eventually fails, and it fails in one of three ways:
- Failed open (blow-through): the valve no longer closes and live steam passes continuously. This is the failure that wastes the most energy.
- Leaking: the trap still cycles but passes some steam between discharges, usually from worn seats or dirt holding the valve slightly open.
- Failed closed (plugged): nothing passes. No steam is lost, but condensate backs up into the equipment, which cuts heat transfer and can cause water hammer.
Steam that blows through is not just lost heat. It also pressurizes the condensate return, which can stop other traps on the same return from draining properly.
2. The steam loss formula (Napier's equation)
Steam escaping through a sharp-edged orifice to atmosphere is estimated with Napier's equation:
P is absolute pressure, so add 14.7 to the gauge reading. D is the diameter of the trap's discharge orifice, which is on the manufacturer's data sheet. Two things follow directly from the formula:
- Orifice size matters most. Loss grows with the square of the diameter, so a 1/4 inch orifice loses four times as much as a 1/8 inch orifice at the same pressure.
- Pressure matters in proportion. The same trap loses about four times as much at 100 psig as at 15 psig.
Treat the result as the maximum. A trap discharging into a pressurized condensate return sees back pressure and loses less, and a trap that is only leaking loses a fraction of it.
3. Steam loss per failed trap
Steam lost through one trap that has failed fully open, in lb/h, with the annual cost at 100 psig over 8,000 hours and $10.01 per 1,000 lb of steam:
| Orifice | 15 psig | 100 psig | 150 psig | Cost per year at 100 psig |
|---|---|---|---|---|
| 1/16 in | 2.8 | 10.9 | 15.6 | $870 |
| 1/8 in | 11.2 | 43.4 | 62.4 | $3,479 |
| 3/16 in | 25.3 | 97.7 | 140.4 | $7,827 |
| 1/4 in | 45.0 | 173.8 | 249.5 | $13,916 |
4. What steam actually costs
To turn pounds of steam into dollars, work out how much fuel the boiler burns to make each pound:
Saturated steam at 100 psig carries about 1,189 Btu/lb. Feedwater from a deaerator at 220°F already carries about 188 Btu/lb, so the boiler adds about 1,001 Btu for each pound. At 80% efficiency and natural gas at $8 per MMBtu:
That is fuel only. Fully loaded steam cost also includes makeup water, water treatment, boiler electricity and maintenance, so many plants carry a higher number. If your energy team has one, use it: the result scales directly.
5. Worked example: a 200-trap plant
A plant has 200 traps on a 100 psig system that runs 8,000 hours a year. Most traps have 1/8 inch orifices. A survey finds 10 traps blowing through and 14 leaking, which is 12% of the population. We assume a leaking trap passes 25% of full orifice flow.
| Item | Calculation | Result |
|---|---|---|
| Loss per blow-through trap | 24.24 × 114.7 × 0.125² | 43.4 lb/h |
| Equivalent failed traps | 10 + 14 × 25% | 13.5 traps |
| Total steam loss | 13.5 × 43.4 | 586 lb/h |
| Annual steam lost | 586 × 8,000 | 4.69 million lb |
| Annual cost | 4,692 × $10.01 | about $47,000 |
| Fuel and CO₂ | at 80% efficiency, natural gas | 5,870 MMBtu, about 343 tons CO₂ |
Repairing or replacing 24 traps usually costs far less than one year of that loss, which is why trap programs are one of the most common quick-payback items in steam system energy assessments.
6. How to find failed traps
- Ultrasonic testing: the most widely used method. A good trap gives a clear cycle of discharge and silence (or a steady modulating sound on float traps). A continuous rushing sound points to blow-through.
- Temperature measurement: an infrared gun or contact probe on the inlet and outlet. An inlet well below the saturation temperature suggests the trap is backing up condensate (failed closed). Temperature alone cannot reliably tell blow-through from normal operation, because condensate flashing downstream is also hot.
- Visual checks: on traps that discharge to atmosphere or through a test valve, live steam shows as a clear blue jet at the outlet, while flash steam is a lazy white plume.
- Wireless trap monitors: continuous sensors on critical or hard-to-reach traps. They cost more per trap but catch failures in days instead of waiting for the next survey.
Safety first
Steam lines and trap bodies are hot enough to cause serious burns. Never open a test valve or remove a trap without isolating and depressurizing it under your site's lockout procedure.
7. Building a trap management program
- Tag and map every trap with its location, type, orifice size, pressure and application.
- Survey on a schedule, at least yearly, and more often on high-pressure and critical traps.
- Rank repairs by dollars lost, not by count. One 1/4 inch trap at 150 psig can waste more than ten small drip traps.
- Fix the cause, not just the trap: missing strainers, wrong trap type for the load, undersized condensate return and water hammer all shorten trap life.
- Track the failure rate each survey. DOE guidance says a maintained system should keep leaking traps under about 5% of the population.
8. Matching trap type to the job
- Thermodynamic (disc): compact and tolerant of high pressure; common on steam main drips.
- Inverted bucket: robust and tolerant of dirt and water hammer; common on mains and process equipment.
- Float and thermostatic: continuous discharge and good air venting; preferred on heat exchangers and modulating loads.
- Thermostatic (bimetallic or bellows): holds condensate back until it cools; used on tracing and low-load drains.
A trap that suits the load lasts longer and fails less often, so check the application whenever you replace one.
Frequently asked questions
How much steam does a failed steam trap lose?
A trap that has failed fully open passes about 24.24 × absolute pressure × orifice diameter² pounds of steam per hour. A 1/8 inch orifice at 100 psig loses about 43 lb/h, which is roughly 350,000 lb of steam a year if the line runs 8,000 hours.
How much does a failed steam trap cost per year?
With steam at about $10 per 1,000 lb, a 1/8 inch trap blowing through at 100 psig costs about $3,500 a year. The same failure on a 1/4 inch orifice costs about $14,000, because loss grows with the square of the orifice diameter.
What percentage of steam traps fail?
The U.S. Department of Energy notes that 15% to 30% of traps may have failed in systems that have gone 3 to 5 years without maintenance. With a regular survey program, leaking traps should be under 5% of the population.
How often should steam traps be tested?
Many plants survey at least once a year. High-pressure traps, critical process traps and traps on large orifices are often checked more often, because each failure on them costs more per day.
Is a trap that fails closed cheaper than one that fails open?
It wastes less steam, but it backs condensate up into the equipment. That can cause water hammer, slow heat-up, frozen coils and damaged heat exchangers, so failed-closed traps should be fixed just as quickly.
Sources and further reading
- U.S. Department of Energy, Advanced Manufacturing Office: Steam Tip Sheet #1, Inspect and Repair Steam Traps
- U.S. Department of Energy: Improving Steam System Performance, a Sourcebook for Industry
- Saturated steam properties from standard steam tables
- U.S. EPA: emission factors for greenhouse gas inventories (natural gas, about 117 lb CO₂ per MMBtu)
Price your own trap losses
Enter your steam pressure, orifice sizes, survey results and fuel price to see annual steam, cost and CO₂.