Introduction to Steam Traps
Steam traps are a type of automatic valve that filters out condensate (i.e. condensed steam) and non-condensable gases such as air without letting steam escape.
Steam provides a means of transporting controllable amounts of energy from a central, automated boiler house, where it will be efficiently and economically generated, to the point of use. Steam generated by a boiler contains heat energy which is used to heat the product. When steam loses it energy by heating the product, condensate is formed. Also, a part of energy contained by steam is lost through radiation losses from pipes and fittings. After losing this heat, steam gets converted into condensate. If this condensate is not drained immediately as soon as it forms, it can reduce the operating efficiency of the system by slowing the heat transfer to the process. Presence of condensate in a steam system can also cause physical damage due to water hammer or corrosion.
A steam trap holds back steam & discharges condensate under varying pressures or loads. The steam traps should have good capacity to vent out air and other non-condensable gases quickly while holding back the live steam. A Steam Trap is an integral part of a steam system. Steam traps play the important role in maintaining the productivity and efficiency of steam system.
Types of Steam Traps
The essential property of a steam trap is to be able to distinguish between steam, condensate and air. Different types of steam traps employ different working principles and mechanisms to distinguish between steam, condensate and air. When classified according to these operating principles, each design has advantages and limitations which must be considered while selecting a steam trap for a specific application.
Steam traps are categorized as follows
- Mechanical Traps
- Thermostatic Traps
- Thermodynamic Traps
Mechanical Traps (operated by changes in fluid density)
Mechanical steam traps operate by sensing the difference in density between steam and condensate. These steam traps include.
- Ball float traps
- Inverted bucket traps
Ball Float Traps
On start-up air is quickly discharged through the thermostatic air vent (membrane or bimetal type). Cold condensate fills the steam trap body. As soon as a certain water level is reached, the float rises and opens the valve. The cold condensate is discharged through the open valve and the open air vent.
When the condensate reaches saturation temperature, the air vent closes and condensate is discharged only through the main valve orifice. The condensate forms a water seal inside the trap body, which prevents live steam loss at all times.
The opening degree of the valve is regulated by the water level inside the trap body. Condensate is discharged continuously. As long as air enters the trap and accumulates at the top of the trap body, the temperature cools down a little bit and the air vent, which opens slightly below saturation temperature, begins to discharge the air from the trap.
Inverted Bucket Traps
On start-up the bucket is down and the valve is open. Low temperature condensate and air, later high temperature condensate enter the trap. The condensate fills the bucket and the trap body completely. As the bucket is completely submerged in the water, it lies on the bottom of the trap, the valve is wide open and condensate will discharge.
Steam enters the trap under the bottom of the bucket. The more steam is entering the trap, the more it collects at the top of the bucket, causing the bucket to move upwards (buoyancy of the bucket inside the water). At the top position of the bucket the valve will close the seat.
Air and gases pass through a small hole in the top of the bucket and collect at the top of the trap. Steam is also passing through the hole and condensing. When more condensate is entering the trap, the bucket will loose its buoyancy and will move down. The valve will open and condensate will discharge.
Thermostatic Traps
Thermostatic steam traps operate on the temperature difference between steam and condensate. The operation of the steam trap is regulated by a thermostatic element inside the trap. The temperature of saturated steam is determined by its pressure. In the steam space, steam gives up its enthalpy of evaporation (heat), producing condensate at steam temperature. As a result of any further heat loss, the temperature of the condensate will fall. A thermostatic trap will pass condensate when this lower temperature is sensed. As steam reaches the trap, the temperature increases and the trap closes.
Upon start-up in the presence of cold condensate, the capsule element is contracted and the valve plate has moved away from the seat. The wide open valve discharges condensate and air rapidly. As the temperature inside the trap increases, the capsule element will start to expand, moving the valve plate toward the seat.
Just before the condensate reaches saturation temperature, the valve plate will close the seat completely. Steam can not enter the trap, ensuring zero steam loss. As the temperature inside the trap decreases, the capsule element moves away from the seat and the condensate will be discharged. During normal operation steps 3 and 4 will repeat continuously.
Thermodynamic Traps
Thermodynamic steam traps operate on the basis of the Bernoulli principle, depending on the relationship between the velocity and the pressure exerted by the condensate and steam inside the steam trap. They have only one moving part – the disc. The traps may operate up to a back pressure of 80% of the inlet pressure, but for smooth operation it is recommended that the back pressure does not exceed 50% of the inlet pressure. Thermodynamic steam traps discharge the condensate intermittently.
At the time of start up the pressure of the incoming cold condensate and air raise the disc and water and air are discharged quickly. When hot condensate flows into the trap, the trap is still open and the hot condensate can be discharged quickly.
After hot condensate flows into the trap, steam enters it. As the velocity of the fluid increases, the pressure under the seat exerted by the steam decreases. At the same time the pressure in the pressure chamber above the disc increases. The disc is pressed down and closes.
While hot condensate flows into the trap, the trap remains closed for a certain period, as far as the steam inside the pressure chamber does not condense. The more condensate flows into the trap, the more the temperature cools down. The steam inside the pressure chamber also cools down and condenses. As a result, the pressure of the incoming condensate raises the disc and condensate is discharged. Cycles 2, 3 and 4 repeat.
