Introduction to Steam Traps
ANSI defines steam traps the following way :
“Self contained valve which automatically drains the condensate from a steam containing enclosure while remaining tight to live steam, or if necessary, allowing steam to flow at a controlled or adjusted rate. Most steam traps will also pass non-condensable gases while remaining tight to live steam.” – ANSI/FCI 69-1-1989.
In simple terms;
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.
- For more on Water Hammer, check out Water Hammer in Process Plant
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.
Why Not Use a Manual Valve ?
It is sometimes believed that the condensate can be regulated with a regular valve instead of a steam trap by simply adjusting the valve opening manually to match the amount of condensate generated. Theoretically, this is possible. However, the range of conditions necessary to achieve this are so limited that in practice it is not a realistic solution.
The largest problem with manual method is that fluctuations in the quantity of condensate formed cannot be compensated for. The amount of condensate generated in a given system is not fixed.
- The quantity of condensate formed at start-up differs from that during normal operation.
- In the case of steam transport piping, the quantity of condensate formed may differ depending on outside temperature.
If the device can’t respond to fluctuations in quantity of condensate formed, condensate that should be discharged will instead collect inside the equipment/pipe resulting in low heat transfer efficiency and water hammer.
Features Required in Steam Traps
The steam trap has to allow condensate to pass whilst trapping the steam in the process. If good Heat Transfer is critical to the process, then condensate must be discharged immediately and at steam temperature. Leakage of steam at this point is inefficient and uneconomical.
At ‘start-up’, i.e. the beginning of the process plant, the piping system is filled with air, which unless displaced, will reduce Heat Transfer and increase the warm-up time. On start-up, steam enters the piping system only as fast as the air is vented. It is preferable to purge air as quickly as possible before it has a chance to mix with the incoming steam. Following are the drawbacks of steam-air mixing :
- An air steam mixture has a temperature well below steam temperature lowering the heat transferred.
- Air is an insulator and clings to the surface of the pipe or vessel causing slow and uneven heat transfer.
- Dissolved in condensate, the non-condensable gases from acid which corrode the system.
When the basic requirements of removing air and condensate have been considered, attention may be turned to “energy efficiency”. Steam traps must have negligible steam consumption. The steam trap must ensure that steam space must be filled with clean dry steam. The type of steam trap will influence this.
Experience has shown that ‘good steam trapping’ is synonymous with reliability, i.e. optimum performance with the minimum of attention. Causes of unreliability are often associated with the following :
- Water hammer
- Dirt or debris accumulation.
Flash steam occurs when high pressure / high temperature condensate is exposed to a large pressure drop such as when exiting a steam trap. High temperature / high pressure condensate contains an excess of energy which prevents it from remaining in liquid form at a lower pressure. The result is that the excess energy causes a percentage of the condensate to flash.
Flash steam occurs because the saturation point of water varies according to pressure. For example, the saturation point of water is 100 °C (212 °F) at atmospheric pressure, but is 184 °C (323 °F) at 1.0 MPaG (145 psig). So what happens when condensate kept under pressure at 184 °C (363 °F) is released to atmosphere? The condensate contains too much energy (enthalpy) to remain entirely liquid, and a portion of it evaporates, causing the temperature of the remaining condensate to drop to the saturation temperature (i.e., 100 °C or 212 °F if discharging to atmosphere).
Flash steam forms steam cloud outside a steam trap. These steam clouds can often be misinterpreted as a live steam leak when in fact they are simply comprised of flashed condensate with fine water droplets in suspension, caused by the flashing of hot condensate being released to atmosphere.
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 operates 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 (operated by changes in fluid temperature)
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 (operated by changes in fluid dynamics)
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.
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