(Reference: Power Engineering Third Class Edition 2.5, Part A2, Chapter 5: Steam Traps, Water Hammer, Insulation)
James R. Stringer and diagrams recreated by Melissa Le
Steam traps are an absolute necessity in the removal of condensate from the steam system return line. Without them, water hammer can occur which can cause damage to the system’s pipes and fittings. Drain lines and traps must be provided where condensate can accumulate, such as:
- Upstream of the connection to risers
- Ends of steam header mains
- Ahead of expansion joints and bends
- Inlets to steam valves and regulators
Steam separators (aka steam purifiers; not a steam trap) remove moisture droplets and suspended impurities from the steam by changing the direction abruptly or creates a whirling motion; uses centrifugal force and gravity. The condensate accumulates and flows through a drain while the steam continues on.
Steam traps are designed to discharge condensate from drain lines, separators, and other equipment without allowing steam to escape. Steam traps should have long life, resistance to corrosion, vent air and carbon dioxide, operate against back pressure from the return line, and operate in the presence of scale/sediment.
Ball Float Steam Trap – Condensate enters the trap and a ball float will rise, opening a discharge valve for the condensate. A thermostatic bellows air vent at the top of the trap will cool when air and other gases collect, opening its discharge port and allowing the gases to escape. When the steam enters, it will heat the thermostatic valve and the discharge port will close, as well as the ball float will close the discharge port when the condensate level has dropped.
| Pros | Cons |
| Works with heavy or light condensate loads | Can be damaged by water hammer |
| Not effected by steam pressure changes | Air bellows not suitable for superheated steam |
| Won’t become air locked during start-ups | Trap can freeze; not suitable for outdoors |
Inverted Bucket Steam Trap – a bucket in the trap is upside down that holds the condensate discharge valve open. Steam that enters will cause the bucket to rise which closes the discharge. Any gases will accumulate at the top of the bucket and pass through a vent at the top. The steam will cool and turn into condensate, eventually causing the bucket to lower and the condensate drains.
| Pros | Cons |
| Simple in construction | Does not rapidly discharge gases |
| Ease of dismantling for inspection and cleaning | Can become air locked |
| Can withstand water hammer | Trap can freeze; not suitable for outdoors |
Thermostatic Steam Trap – uses a bellows filled with volatile liquid that vaporizes when heated by steam, expanding the bellows and closing off the discharge. The steam eventually condenses as does the volatile vapour causing the bellows to contract and allowing the condensate to drain.
| Pros | Cons |
| Small but handles large amount of condensate | Water hammer can damage corrugated bellows |
| Discharge large amount of air during start up | Corrugated bellows susceptible to corrosion |
| Self draining; will not freeze | High temps creates excess pressure in bellows |
Bi-Metallic Steam Trap – consists of dissimilar metals that have different coefficient of expansions. The metals are welded together and as they are heated from the steam, the metals will deflect and bend because of the unequal levels of expansion and close the discharge for the condensate. Condensate will cool the metals, causing them to straighten and open the condensate drain.
| Pros | Cons |
| Used in special applications | Does not close the discharge valve tight |
Impulse Steam Trap – uses heat energy from steam and condensate to manipulate a piston inside of a control cylinder. Condensate enters the trap, causing pressure to act on the piston disc that lifts the valve to the open position for the condensate. Some of the condensate does not drain but instead passes the piston disc and continues to the upper part of the control cylinder until it reaches a small hole drilled through the centre of the piston valve to the outlet. If the condensate is close to steam temperature, the condensate that travelled through the drilled hole will flash into steam (as it is at a lower outlet pressure) and the large volume from the steam will plug the small hole, building up pressure in the disc and force the valve to close the discharge.
Controlled Disc Steam Trap – both condensate and air pass into the trap through the control chamber and inlet orifice, lifting the disc and allowing them to leave through the discharge. Steam entering the trap and disc will have increased velocity and thus decreased pressure, allowing the pressure from the control chamber to close the orifice. The steam will over time bleed off around the disc, increasing the pressure on the disc and open the trap for condensate to discharge.
| Pros | Cons |
| One moving part (the control disc) | Low condensing capacity |
| Suited for super heated steam | Won’t operate at low pressures |
| Not affected by water hammer or vibrations | Won’t operate at high back pressures |
Liquid Expansion Steam Trap – uses a thermostatic tube element filled with a special oil that controls the discharge valve. The discharge valve will be open until hotter condensate or steam enters the trap, causing the liquid inside the trap to expand; it pushes a plunger and rod, thus closing the trap. As the steam and condensate cools, the pressure in the tube decreases, causing the plunger to contract and opens the discharge.
| Pros | Cons |
| Sealing bellows prevents leakage from tube | Tube can corrode from corrosive condensate |
| Trap protected against water hammer or over expansion by relief spring | Rod must be long for adequate movement |
| Desired discharge temperate from adjustment screw | Discharge is usually mounted on top but must be at bottom in colder climates. |
Bonus Picture!
