Working principle of steam traps

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This post was last edited by Jinma Water Damage Control on 2026-2-3 at 10:00. The function of a drain valve in roof systems can be described in four words: automatic vapor suppression and water drainage, that is, it only allows water (condensate) to flow out while preventing steam from escaping. Prevent direct release of steam and save on steam consumption: A leaking steam trap is like a leak in a pipe, causing valuable raw steam to be wasted. Only condensate water is drained, ensuring efficient heat transfer: A properly functioning steam trap prevents the accumulation of condensate water inside the hot cylinders and plates, allowing saturated steam to come into contact with the cylinder walls and thus heat the raw paper. This ensures efficient heat transfer, uniform and effective heating of the material, and as a result, an increased production speed and better quality of cardboard. Although there are many types and designs of steam traps, such as float-type, inverted bucket-type, thermodynamic-type, thermostatic-type, etc., the principles used to achieve the function of preventing steam from entering while allowing water to flow out are essentially just three: density difference, velocity difference, and temperature difference. 1. Principle of density difference: Water and steam have different densities, and this difference is utilized to prevent steam from entering; for example, in float-type steam traps, a float is installed inside the valve body, similar to the float in a toilet tank. When there is condensate water, the buoyancy of the water lifts the float, causing the valve nozzle to open and allowing water to be drained ； When the water is drained, steam enters the valve body, reducing the buoyancy acting on the float. Due to its own weight, the float drops down and blocks the valve outlet, preventing steam from escaping. The inverted barrel type steam trap operates on the same principle as a float-type steam trap; one only needs to think of it as a float replaced by an inverted barrel. The difference between the float-type and bucket-type lies in the position of the valve nozzle. In conventional drum-type steam traps, the drain valve nozzle is located at the top of the valve; water has to accumulate to a certain level inside the valve before the drum rises, and the linkage mechanism then activates to open the valve nozzle and allow water to be drained – this is intermittent drainage. In float-type steam traps, the drain valve nozzle is at the bottom of the valve; as long as the float rises a little, some water is discharged – this is continuous drainage. 2. Principle of speed difference: This method takes advantage of the difference in speed between water and steam to prevent steam from escaping while allowing water to be drained. For example, in diaphragm-type steam traps, when condensate flows into the valve, its flow speed is lower than that of steam, so the valve disc at the upper part of the valve remains open, allowing the condensate to be discharged ； When steam flows into the valve, since the speed of the steam flow is greater than that of the water flow, the fast-moving steam creates a low-pressure area beneath the valve disc, which pulls the valve disc down and prevents steam from escaping. Why does the steam flow cause the valve disc to be drawn down? Take an example that everyone is familiar with: just like the pulling force felt when a high-speed train passes by, this is caused by the air near the train being pulled away at high speed, resulting in a low-pressure area around the train. That’s why train staff keep reminding people not to walk inside the yellow line! ! ! It’s not that I’m afraid you’ll fall; it’s that there’s a kind of “magic” force that draws you in. The valve disc is also drawn down by a similar suction force, closing the steam trap. 3. Principle of temperature difference: This principle is utilized to prevent steam from passing through while allowing water to drain, and it is applied in thermostatic steam traps such as diaphragm steam traps, bellows steam traps, and bimetallic steam traps. The working mechanism of these traps relies on a key component – the temperature sensing element – which expands or contracts due to the difference in temperature between steam and condensed water. When steam arrives, the temperature is high; the temperature-sensitive element expands, and the valve nozzle closes. When condensate water arrives, the temperature is low, the element retracts, the valve nozzle opens, and the condensate water is discharged.