Plant maintenance personnel sometimes choose their steam traps based on the pipe connection size and trap availability in plant stores. For contractors or engineering constructors involved in new construction projects, the choice frequently is driven by cost or delivery factors.
Many makes and models of steam traps are available, but attention is not always given to the differences in features or performance between them. Sometimes plant personnel have the misconception that "a trap is a trap," and all traps operate the same.
Mechanical trap operation relies on the movement of closed or open floating objects to actuate the valve. Mechanical traps (or combination pump/traps) fall into one of three classifications:
Float and thermostatic traps, where the buoyancy of the floating object opens the valve.
Inverted bucket traps, where the buoyancy of the floating object closes the valve.
Combination pump and float traps (pump/trap), where the buoyancy of the floating object opens a main trap valve and secondary pressure motive valve. This synchronized combined action allows a higher-pressure vapor to discharge condensate through the main trap valve.
Float and thermostatic traps (F & Ts)use a sealed spherical or elliptical float that becomes buoyant when the water level in the trap rises, and a temperature-sensing thermostatic element opens to vent air and other incondensables that enter the trap during operation. The two types of F & T traps commonly used are the free float and the lever float.
Figure 1. Free Float Trap
The free float trap contains only one moving part ," the float. As its name implies, it has no levers or linkages. No lever mechanism is required to move the valve off the seat. Instead, the float itself is the valve, and the buoyant force of the condensate serves to "roll" instead of pull the valve off the seat as the level of condensate in the trap rises. The rotational movement of the float valve allows
precise opening to perfectly match flow requirements.
As the condensate load increases, the float rotates upward against the top side of the valve seat, widening the opening of the discharge area. Under heavy loads, the float completely rises off the seat. It responds immediately to changes in condensate flow or pressure differential.
The rotating action of the float also helps keep the seat clean and allows it to renew its seating surface each time it comes to rest against the seat, eliminating the concentrated wear exhibited by other types of traps. Some free float traps incorporate a three-point seat design to ensure a perfect seal of the float against the valve seat, even in no-load applications. This design is useful with superheated steam conditions. Free float traps also contain a thermostatic element for rapid venting of air and other incondensables.
The float and valve of a lever float trap are attached to the opposite ends of a lever mechanism that pivots on a fulcrum. When the float is in its lowest position, the main valve is closed.
A thermostatic element is incorporated near the top of the trap as an automatic air-venting device. The float becomes buoyant with condensate inflow and lifts the main valve off the seat to allow condensate discharge. Greater condensate flow to the trap means greater float buoyancy and a subsequent higher discharge rate. At near-steam temperatures, the thermostatic element closes the separate vent valve. When the load diminishes, the float is lowered, moving the main valve closer to its seat. The valve closes when the condensate flow ceases.
The principal advantage of the lever float is price: These traps handle high loads at a relatively low cost. On the other hand, continuous modulation of the lever mechanism will wear the linkage if it is not sufficiently designed to avoid such wear. This wear can be amplified by the effect of opposing forces that occur during valve opening and can create more pressure and wear at the fulcrum point.
Figure 2. Combination Pump/Trap
Inverted bucket trapsare old-style density traps that contain an inverted cup-like cylinder (inverted bucket) that becomes buoyant in a level of liquid when subjected to the vapor pressure of a trapped vapor inside the float. The bucket's movement inside the trap is guided by a lever arm mechanism that is attached to both the bucket and the top of the trap. The valve head is positioned on the top side of the lever arm, coming into contact with the valve seat when the bucket is fully buoyant. At the top of the bucket is a small vent hole.
At startup, the bucket is down and the valve is wide open, allowing air to be discharged. As condensate enters the trap and fills the bottom portion of the bucket, any air remaining inside the bucket becomes trapped, causing the bucket to gain buoyancy and close the valve.
The incondensable air must be expelled from the bucket by passing through the tiny vent hole, so air venting generally is very slow.
Once enough air has been displaced by condensate, the bucket loses buoyancy and drops, allowing the trap to discharge. The valve remains open while condensate accumulated at the trap inlet passes through the orifice. As steam reaches the trap, it collects in the bucket and gives it buoyancy, closing the valve. After a certain amount of the steam either condenses or passes through the vent hole, the bucket again drops and opens the valve to discharge the condensate. This intermittent discharge is indicative of cyclic operation.
Inverted bucket traps usually are used as a low-price choice for drip or tracer applications. Unfortunately, they do not vent air particularly well and are not self-draining. Because they require a water prime to close, they are more subject to freezing than other types of traps. They also can lose their prime if subjected to superheated conditions or sudden drops in steam pressure at the inlet.