Steam Separators, Strainers and Air Vents
A steam main carries more than steam: water droplets, scale and dirt, and air. Separators take out the water, strainers catch the dirt, and air vents let the air escape. They are small, cheap fittings, and the equipment downstream depends on them.
You'll learn
- Explain what separators, strainers and air vents remove
- Place each in a steam system
- Calculate how air lowers steam temperature
- Maintain them
Before this: Wet, dry and superheated steam, Main drainage and warm-up load
Water, dirt and air
- Separator: forces the steam to change direction or spin, so heavier water droplets are thrown out and drain to a trap. Delivers dry steam to the user.
- Strainer: a fine screen, usually in a Y-shaped body, that catches scale, weld slag and dirt before they reach a trap, valve or meter.
- Air vent: an automatic valve that lets air out of the steam space and shuts when steam reaches it.
Air gets into steam systems whenever they are shut down and cool: the steam condenses and air is drawn in. At the next start-up, the steam pushes that air to the far ends of mains and into heat exchangers, where it collects unless vented.
Where they go
- Separators after the boiler where steam quality matters, before PRVs and control valves, and before critical process equipment.
- Strainers before every steam trap, control valve, PRV and flow meter.
- Air vents at the high points and far ends of steam mains, and on the steam space of heat exchangers and jacketed vessels, at the point farthest from the steam inlet.
Separators and strainers need maintaining like any other equipment. A strainer screen that is never cleaned blocks, starving the user; one that is blown down through its blowdown valve on a schedule stays effective.
What air does to a heater
When steam and air share a space, each exerts part of the total pressure, in proportion to its share of the volume. The steam condenses at the saturation temperature of its own partial pressure, not the gauge reading, so a gauge showing the right pressure can sit on steam that is cooler than expected.
- Gauge 3 kg/cm²(g), so 4.03 kg/cm² absolute in total; 10% of the volume is air.
- Steam partial pressure: 0.9 × 4.03 = 3.63 kg/cm² absolute.
- Saturation temperature at that pressure: 139.5 °C, instead of 143.2 °C for pure steam.
Key takeaways
- Separators remove water, strainers remove dirt, air vents remove air.
- Strainers before every trap, control valve, PRV and meter; separators before critical users.
- 10% air in 3 kg/cm²(g) steam lowers its condensing temperature by 3.8 °C, and the air film insulates.
- Vent air at high points, far ends and heat exchanger steam spaces.