Closed Float Steam Traps are mechanical steam traps actuated by a float responding to changes in condensate level.

Mechanical steam traps are density detectors and therefore also have difficulties venting air and non-condensable gases. Mechanical steam traps employ either an open or a closed float to actuate a valve. Closed float mechanical steam traps usually employ a secondary thermostatic air vent which allows the trap to discharge air rapidly. The air vent, of course, is an extra component which can fail open, causing the loss of steam, or fail closed and prevent the trap from discharging condensate. Closed float steam traps are usually large in physical size. This, combined with a float that is fragile to external pressure, and the continuous presence of condensate within the trap, make this device unsuitable for high pressure applications or installations where water hammer or freeze-ups can be expected.

On the positive side, mechanical steam traps respond to changes in condensate level only, independent of temperature or pressure. They respond rapidly to changing loads. Condensate discharge temperatures follow closely the saturation curve and they have a modulating (rather than an on-off) type of discharge. They are extremely energy efficient.

Open float mechanical steam traps share many characteristics with closed float traps. One major difference, of course, is the open float as found in an inverted bucket steam trap. The open float is no longer a weak point, because it cannot be collapsed by excessive pressure. Venting is usually accomplished by means of a small vent hole in the top of the bucket. This is a compromise, as the efficiency of the trap is affected by the sizes of the vent. The larger the vent the better the air handling, but at the expense of higher steam losses. A smaller vent has the opposite effect. The end result is a trap that is relatively efficient, but which does not remove air rapidly during start-up conditions. It discharges near steam temperature with an on-off action and the discharge temperature follows the saturation curve.

All mechanical steam traps are position-sensitive and can be installed only in their intended orientation.

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Closed float steam trap is one of the oldest on the market, the closed float steam trap is still in widespread use. The opening and closing of the valve is caused by changes of the condensate level within the trap shell.

When the closed float steam trap is empty, the weight of the float closes the valve. As condensate enters the trap, the float rises and opens the valve, allowing condensate to be discharged. The float is designed to provide sufficient force to overcome the differential pressure across the valve. The internal float and valve configuration is such that the condensate level is always above the valve, thus creating a continuous water seal at its seat. Actual construction varies widely depending upon the manufacturer. While most designs employ a linkage-pivot system, one particular design uses no linkage at all and relies on a free floating ball to achieve the desired action.

An inherent disadvantage of a simple float steam trap is that it cannot discharge air or non-condensable gases. It is therefore necessary to install an auxiliary ther­mostatically activated air vent. For this reason, these steam traps are known as float and thermostatic or F&T steam traps.

Prior to selecting a steam trap for your application, review steam traps selection with its advantages versus disadvantages and additional steam trap types: Disc Steam Traps, Piston Steam Traps, Lever Steam Traps, Inverted Bucket Steam Traps, Open Bucket Steam Traps, Bimetallic Steam Traps, Bellows Steam Traps, Liquid or Solid Expansion Steam Traps (Wax Capsule Steam Trap), and Orifice Steam Traps.