Traps play a vital role in steam heating systems. Their primary function is to stop the backflow of steam and efficiently discharge condensate, ensuring that steam heating equipment operates at peak efficiency. In order to select the trap that is best suited for your system, it is important to have a thorough understanding of the operating performance and characteristics of each type of trap.
The ability of traps to “recognize” steam and condensate is based on three principles: density difference, temperature difference, and phase change. Based on these principles, traps are categorized into three main types: mechanical, thermostatic, and thermodynamic.
I. Mechanical Traps
Mechanical traps, also known as float traps, work by utilizing the density difference between condensate and steam. They operate by driving the float up and down through changes in condensate level, which in turn drives the valve to open or close for the purpose of blocking and draining steam.
Free Float® Traps
Free Float traps are simple in structure, with only one stainless steel hollow float as the moving part. This float is both a float and an opening and closing part, and has no wearing parts, resulting in a long service life. This trap is equipped with an automatic air venting device inside, which is very sensitive and automatically removes air to ensure quality work. When the equipment is started up, the air and low-temperature condensate in the piping are quickly discharged. As the condensate level rises, the float rises and opens the valve, allowing the condensate to be quickly discharged while steam enters the equipment. When the equipment rapidly warms up, the automatic air venting device closes and the trap begins normal operation.
Free Float® Traps
Free Float® traps have a half-float type bucket as the moving part, with the opening facing downward. The bucket serves as both the opening and closing member and the sealing element, and the entire surface of the ball can be used for sealing, resulting in a long service life and resistance to water shock. When the unit is activated, air and low-temperature condensate from the piping enters the trap through the launch tube, and the bimetal evacuation element pops the bucket open, opening the valve. As steam enters the bucket, the bucket becomes buoyant and rises, while the temperature inside the valve rises, the bimetal evacuation element contracts, and the bucket floats toward the valve opening and closes the valve. When the steam in the bucket becomes condensate, the bucket loses buoyancy and sinks, and the valve opens again.
Lever Float Traps
The basic characteristics of the lever float trap are similar to those of the free float type, but the internal structure is different. It uses a float connected to a lever to drive the spool to open and close the valve. This type of trap utilizes a double valve seat to increase condensate discharge, and is small in size but large in discharge capacity, making it ideal for large heating equipment.
Inverted Bucket Traps
The inverted bucket trap has an inverted bucket inside as a level-sensitive part. When the device is activated, air and low-temperature condensate from the piping enters the trap, and the bucket drops down by its own weight and drives the spool to open the valve. As steam enters the bucket and generates buoyancy, the bucket rises and drives the spool to close the valve. When a portion of the steam is discharged from the small hole, another portion of the steam generates condensate and causes the bucket to lose buoyancy and sink, thus driving the spool to open the valve again, and so on.
Combination Type Superheated Steam Trap
The Combination Superheated Steam Trap consists of two isolated chambers, connected to the upper and lower chambers by two stainless steel tubes. It combines the advantages of float-type and inverted bucket traps to discharge condensate in a timely manner and prevent steam leakage under superheated, high-pressure, and small-load operating conditions.
II. Thermostatic Traps
Thermostatic traps utilize the temperature difference between steam and condensate to cause deformation or expansion of the temperature sensing element to drive the spool to open and close the valve. This type of trap has a large degree of subcooling, but is able to utilize a portion of the sensible heat in the condensate, so there is always high temperature condensate in front of the valve and no steam leakage, resulting in significant energy savings.
Diaphragm Traps
The main action element of a diaphragm trap is a metal diaphragm box filled with a liquid whose vaporization temperature is lower than the saturation temperature of water. When the device is activated, low-temperature condensate appears in the piping, and the liquid inside the diaphragm box condenses and drives the spool to open the valve. As the temperature of the condensate gradually rises, the liquid inside the membrane box begins to evaporate and generate pressure, pushing the diaphragm to drive the spool in the closing direction.
Bellow Traps
The spool of a bellows trap is made of stainless steel bellows, which is also filled with a liquid whose vaporization temperature is lower than the saturation temperature of water. As the steam temperature changes, the liquid inside the bellows evaporates and produces pressure changes, which drives the spool to open and close the valve. The valve is equipped with an adjusting bolt to adjust the operating temperature as required.
Bimetal Traps
The main component of a bimetal trap is the bimetal temperature sensing element. As the steam temperature rises or falls, the bimetal is deformed by heat and pushes the spool to open or close the valve. The valve is also equipped with adjusting bolts to regulate the operating temperature.
Thermodynamic Traps
Thermodynamic traps work on the principle of phase change, utilizing the different thermodynamic principles generated by changes in the flow rate and volume of steam and condensate as they pass through to drive the spool to open and close the valve. Since these traps are powered by steam, there is a large waste of steam. However, they are simple in structure, resistant to water strikes, and capable of discharging air and water at saturation temperature.
Thermodynamic Traps
Thermodynamic traps have a movable disc that acts as a sensitive and actuating element. The valve is actuated to open and close the valve based on the differential pressure generated by changes in the flow rate and volume of steam and condensate passing through. This type of trap quickly discharges condensate when the device is activated and quickly closes the valve to stop steam leakage once the condensate is discharged.
Disc Type Steam Holding Traps
Disc type steam insulation traps add an outer shell to thermodynamic traps for insulation. This prevents the temperature of the main steam chamber from dropping and maintains steam pressure to ensure that the trap closes tightly. When condensate is generated in the piping, the trap shell cools down and begins to drain; if no condensate is generated in the superheated steam piping, the trap will not open.
Impulse Traps
Impulse type traps use two orifices to adjust the valve opening and closing in response to changes in steam pressure drop. However, even when the valve is completely closed, the inlet and outlet are connected through small holes, so it is always incompletely closed and there is a certain amount of steam leakage. This type of trap has a high frequency of operation, is subject to rapid wear and tear, and has a short service life.
Orifice Traps
Orifice traps control the amount of water discharged by selecting different orifice sizes. However, if the trap is not properly selected, it may not drain enough or may cause a large amount of steam to escape, so it is not suitable for steam-using equipment in intermittent production or for steam-using equipment where the amount of condensate fluctuates greatly.
In conclusion, when selecting a trap, it is important to choose the right type based on the optimal operational needs of the steam heating equipment, and to consider other objective conditions to ensure the correctness and effectiveness of the trap selected.
Structural Forms and Operating Principles of Traps
