Steam and Condensate systems - Types of Steam Traps

Mechanical Steam Traps

Mechanical steam traps rely on the difference in density between steam and condensate in order to operate. They can continuously pass large volumes of condensate and are suitable for a wide range of process applications. Types include ball float and inverted bucket steam traps.

Ball float steam trap

The ball float type trap operates by sensing the difference in density between steam and condensate. In the case of the trap shown in the image on the right (Flot trap with air cock), condensate reaching the trap will cause the ball float to rise, lifting the Valve off its seat and releasing condensate. As can be seen, the Valve is always flooded and neither steam nor air will pass through it, so early traps of this kind were vented using a manually operated cock at the top of the body. Modern traps use a thermostatic air vent, as shown in the image on the right (Flot trap with thermostatic air vent). This allows the initial air to pass whilst the trap is also handling condensate.

Float trap with air cock and thermostatic air vent

The automatic air vent uses the same balanced pressure capsule element as a thermostatic steam trap, and is located in the steam space above the condensate level. After releasing the initial air, it remains closed until air or other non-condensable gases accumulate during normal running and cause it to open by reducing the temperature of the air/steam mixture. The thermostatic air vent offers the added benefit of significantly increasing condensate capacity on cold start-up.

In the past, the thermostatic air vent was a point of weakness if waterhammer was present in the system. Even the ball could be damaged if the waterhammer was severe. However, in modern float traps the air vent is a compact, very robust, all stainless steel capsule, and the modern welding techniques used on the ball makes the complete float-thermostatic steam trap very robust and reliable in waterhammer situations. In many ways the float-thermostatic trap is the closest to an ideal steam trap. It will discharge condensate as soon as it is formed, regardless of changes in steam pressure.

Advantages of the float-thermostatic steam trap

Disadvantages of the float-thermostatic steam trap

Inverted bucket steam trap

The inverted bucket steam trap is shown in Figure 11.3.3. As its name implies, the mechanism consists of an inverted bucket which is attached by a lever to a Valve. An essential part of the trap is the small air vent hole in the top of the bucket. Figure 11.3.3 shows the method of operation. In (i) the bucket hangs down, pulling the Valve off its seat. Condensate flows under the bottom of the bucket filling the body and flowing away through the outlet. In (ii) the arrival of steam causes the bucket to become buoyant, it then rises and shuts the outlet. In (iii) the trap remains shut until the steam in the bucket has condensed or bubbled through the vent hole to the top of the trap body. It will then sink, pulling the main Valve off its seat. Accumulated condensate is released and the cycle is repeated.

In (ii), air reaching the trap at start-up will also give the bucket buoyancy and close the Valve. The bucket vent hole is essential to allow air to escape into the top of the trap for eventual discharge through the main Valve seat. The hole, and the pressure differential, are small so the trap is relatively slow at venting air. At the same time it must pass (and therefore waste) a certain amount of steam for the trap to operate once the air has cleared. A parallel air vent fitted outside the trap will reduce start-up times.

Operation of an inverted bucket steam trap
Operation of an inverted bucket steam trap

Advantages of the inverted bucket steam trap

Disadvantages of the inverted bucket steam trap

Reference(s):
Spirax Sarco

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