11.1 Low-Pressure Steam Systems, Traps, and Condensate Return
Key Takeaways
- A low-pressure steam boiler operates at 15 psig or less, which is the dividing line between ASME Section IV heating boilers and Section I power boilers.
- Steam gives up its latent heat when it condenses, and at atmospheric pressure that latent heat is roughly 970 Btu per pound.
- A steam trap must discharge condensate, air, and carbon dioxide while holding back live steam.
- A trap that fails open wastes steam continuously; a trap that fails closed waterlogs the equipment, destroys heat transfer, and invites water hammer.
- Steam mains are pitched in the direction of flow with drip legs at low points, and branch takeoffs are taken from the top of the main so condensate is not carried into the branch.
Why steam is worth 16 items
Domain 9, Steam Systems, contributes 16 of 115 items — tied for the largest domain — and references 32208 Low-Pressure Steam Systems, 32209 High-Pressure Steam Systems and Auxiliaries, 32210 Distillation Towers and Vessels, and 32403 Compressors and Pneumatic Systems. Chapters 11 and 12 of this guide cover all four.
What steam actually does
Heating water to its boiling point takes sensible heat — heat you can measure with a thermometer. Converting that boiling water into steam takes latent heat of vaporization, which changes the state without changing the temperature. At atmospheric pressure that latent heat is roughly 970 Btu per pound.
The entire usefulness of steam follows from this: when steam reaches a heat exchanger and condenses, it gives up that same large quantity of latent heat at constant temperature. A pound of steam delivers far more heat than a pound of hot water, and it delivers it isothermally.
Two consequences shape every design decision in a steam system:
- Condensate forms continuously wherever steam gives up heat — including in the distribution piping through wall losses. It must be drained.
- Steam temperature is set by pressure. Saturated steam at 0 psig is 212°F; raise the pressure and the saturation temperature rises with it.
Superheated steam has been heated above the saturation temperature for its pressure. It carries no moisture, so it is used for turbines and long transmission, but it gives up only sensible heat until it cools to saturation, which makes it a poor heat transfer medium for process heating.
Low pressure versus high pressure
The dividing line is a code definition, and it is very commonly tested:
| Category | Limit | Code |
|---|---|---|
| Low-pressure steam boiler | 15 psig or less | ASME Boiler and Pressure Vessel Code, Section IV (heating boilers) |
| Low-pressure hot water boiler | 160 psig and 250°F or less | ASME Section IV |
| High-pressure (power) boiler | Above 15 psig steam | ASME Section I (power boilers) |
Low-pressure systems serve building heat, humidification, sterilizers, and light process duty. High-pressure systems serve turbines, process reboilers, and large plant distribution.
Distributing steam without wrecking the piping
- Pitch the main in the direction of flow, commonly on the order of 1 inch in 40 feet, so condensate runs with the steam rather than against it.
- Install drip legs at every low point, ahead of every riser, ahead of every control valve, and at intervals along long runs — commonly every 100 to 150 feet. A drip leg is a full-size (or nearly full-size) pocket below the main that gives condensate somewhere to fall out of the high-velocity steam, with a trap at its bottom.
- Take branch connections from the top of the main, not the side or bottom. Condensate travels along the bottom of the pipe, and a bottom takeoff feeds it straight into the branch and into the equipment.
- Use eccentric reducers flat side down on a horizontal steam main so condensate is not dammed.
- Provide air vents at the ends of mains and on heat exchangers. Air is an insulator and it collects where steam condenses.
- Allow for thermal expansion with loops, offsets, or expansion joints, and anchor and guide the piping so the expansion goes where it was designed to go.
Water hammer in a steam system is not a nuisance; it destroys valves, fittings, and people. It occurs when a slug of condensate is accelerated by high-velocity steam and slams into a fitting, or when steam collapses suddenly into subcooled condensate. The preventions are correct pitch, adequate drip legs and traps, slow warm-up with drains open, and avoiding steam-into-water flooded conditions.
Steam traps
A steam trap is an automatic valve with one job: pass condensate, air, and carbon dioxide; hold back live steam.
| Family | Type | Operating principle | Best suited to |
|---|---|---|---|
| Mechanical (density) | Float and thermostatic (F&T) | A float rides the condensate level and modulates a valve; a separate thermostatic element vents air | Heat exchangers and process loads with varying condensate and large air loads |
| Mechanical | Inverted bucket | A bucket floats when filled with steam, closing the valve; it sinks when condensate replaces the steam | Steam mains and drip service; tolerant of water hammer |
| Thermostatic (temperature) | Balanced pressure bellows | A liquid-filled bellows expands as temperature nears saturation and closes | Low-capacity, tracing, radiators |
| Thermostatic | Bimetallic | A bimetallic stack deflects with temperature | Tracing and freeze-protection service; can be adjusted |
| Thermodynamic (velocity) | Disc trap | High-velocity flash steam under a disc creates a low-pressure region that snaps the disc shut | Drip legs and outdoor service; simple, robust, freeze tolerant |
Trap failure and how to detect it
| Failure | Consequence | Detection |
|---|---|---|
| Failed open | Live steam blows through continuously; energy loss and overloaded condensate return | Continuous blow at the trap discharge; high downstream temperature; ultrasonic listening shows continuous high-frequency flow |
| Failed closed | Condensate backs up: heat exchanger output falls, equipment waterlogs, water hammer and corrosion follow | Cold discharge line; equipment not heating; trap body cool |
Traps are tested with a combination of temperature (infrared or contact), sound (ultrasonic listening or a stethoscope), and sight glasses where fitted. Some discharge patterns are normal: an inverted bucket cycles, a disc trap snaps open and closed a few times a minute, and an F&T on a heavily loaded exchanger discharges nearly continuously. Knowing the trap type is required before judging its behavior.
Condensate return
Returning condensate saves the heat still in it, saves treated water, and saves the chemicals in that water.
| Component | Function |
|---|---|
| Condensate receiver | Collects returns and provides pump suction head |
| Condensate transfer pump | Returns condensate to the deaerator or boiler feed system |
| Flash tank | Separates the flash steam that forms when hot condensate drops to a lower pressure, so it can be reused at low pressure |
| Vacuum breaker | Admits air when steam condenses and creates a vacuum, preventing the coil from being held full of condensate |
| Air vent | Removes non-condensable gases |
| Check valves | Keep returns from different pressure sources from backing into each other |
A frequent field problem is a condensate-flooded coil that will not heat. The cause is often a trap that cannot discharge because the return line is pressurized above the trap's differential, or a missing vacuum breaker on a modulating coil that has pulled itself into a vacuum. Neither is fixed by replacing the trap with a larger one.
At what pressure does a steam boiler cross from the low-pressure category into the high-pressure category?
A heat exchanger is no longer reaching temperature, and the line downstream of its steam trap is cold. What has most likely happened?
Why must a branch line be taken from the top of a steam main rather than from the side or bottom?