Sunday, November 25, 2012

Piping Design - Stream Traps and its Types

The function of a steam trap is to discharge condensate from steam piping or steam heating equipment without permitting live steam to escape. Some principal types of steam traps are:

  • Float
  • Thermostatic
  • Thermodynamic
  • Inverted bucket

The float type (FIGURE 1) consists of a chamber containing a float-and-arm mechanism which modulates the position of a discharge valve. As the level of condensate in the trap rises, the valve is opened to emit the condensate. This type of valve tends to discharge a steady stream of liquid since the valve position is proportional to the rate of incoming condensate. Because the discharge valve is below the waterline, float-type steam FIGURE 2 Float steam trap. (Spirax traps must employ a venting system to Sarco Inc.) discharge non-condensible gases. This is generally accomplished with a thermostatic element which opens a valve when cooler non-condensible gases are present but closes the valve in the presence of hotter steam. 

Float steam trap. (Spirax Sarco Inc.)
FIGURE 1: Float steam trap. (Spirax Sarco Inc.)
The thermostatic steam trap (FIGURE 3) contains a thermostatic element which opens and closes a valve in response to fluid temperature. Condensate collected upstream of the valve is sub-cooled  cooling the thermostat, which, in turn, exposes the discharge port. When the cooler condensate is discharged and the incoming condensate temperature approaches the saturation temperature, the thermostat closes the discharge port. Because of its principle of operations, the thermostatic trap operates intermittently under all but maximum condensate loads.

Thermostatic steam trap. (a) balanced pressure thermostatic trap. (Spirax Sarco Inc.)Thermostatic steam trap. (b) Trap strainer unit; 
FIGURE 2: Thermostatic steam trap. (a) balanced pressure thermostatic trap. (Spirax Sarco Inc.) (b) Trap strainer unit;
Inverted bucket steam trap. (Spirax Sarco Inc.)
FIGURE 3: Inverted bucket steam trap. (Spirax Sarco Inc.)
The inverted bucket steam trap (FIGURE 3) consists of a chamber containing an inverted bucket (the opening at the bottom) which actuates a discharge valve through a linkage. The valve is open when the bucket rests at the bottom of the trap. This allows air to escape during warm-up until the bottom of the bucket is sealed by rising condensate. The valve remains open as long as condensate is flowing, and trapped air bleeds out through a small vent in the top of the bucket. When steam enters the trap, it fills the bucket, causing the bucket to float so it rises and closes the valve. The steam slowly escapes through the bucket vent and condenses, thus allowing the bucket to sink and reopen the valve for condensate flow. Small amounts of air  and non-condensible gases (such as carbon dioxide) that enter the trap during normal operation are also vented through the small opening in the top of the bucket, which prevents the trap from becoming air-bound.

Thermodynamic steam trap (with integral strainer). (Spirax Sarco Inc.)
FIGURE 4: Thermodynamic steam trap (with integral strainer). (Spirax Sarco Inc.)
The thermodynamic steam trap is illustrated in FIGURE 4. In this type of trap, flashing of the hot condensate tends to force a small piston into the discharge opening when the temperature of the condensate approaches within about 30°F (15°C) of the saturation temperature. As soon as the condensate collected in the drain system cools sufficiently below the flash temperature, the trap opens and discharges the accumulated water until the temperature of the condensate once more approaches the saturation temperature and flashes, thereby closing the trap and again repeating the cycle. A small orifice permits a continuous discharge of steam, flashed vapor, or non-condensable gas when the trap is closed.

Single orifices are sometimes used to remove condensate from high-pressure, high-temperature steam lines. Where the drains are required only in  bringing the line up to temperature, the use of orifices, in conjunction with valves, is particularly desirable.

Air (drain) traps are used to discharge condensed liquid—from a gas system. The drain trap operates on the same principle as the float steam trap does, except that the drain trap does not contain a thermostatic clement.

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