Structural forms and working principles of steam traps

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Steam traps serve to prevent steam from escaping while draining condensate in steam heating systems. Selecting the appropriate steam trap enables steam heating equipment to operate at maximum efficiency. To achieve the optimal results, it is necessary to have a comprehensive understanding of the operating performance and characteristics of various types of steam traps. There are many types of steam traps, each with different characteristics. When selecting a steam trap, one should first ensure that its characteristics meet the optimal operating requirements of steam heating equipment; only then should other objective factors be considered. This is how you can choose the right and effective steam trap for your needs. A steam trap must be able to “identify” steam and condensate in order to function as a vapor barrier and drain mechanism. “The identification of “steam and condensate” is based on three principles: density difference, temperature difference, and phase change. Thus, three types of steam traps were manufactured based on three principles, classified as mechanical, thermostatic, and thermodynamic types. I. Mechanical type steam traps: Also known as float-type traps, these utilize the density difference between condensed water and steam; changes in the level of condensed water cause the float to rise or fall, which in turn moves the valve disc open or closed, thereby preventing steam from passing through while allowing water to be drained. Mechanical steam traps have a low degree of subcooling; they are not affected by changes in operating pressure and temperature. Water is discharged as soon as it appears, preventing water from remaining in the heating equipment, which allows the heating equipment to achieve optimal heat exchange efficiency. With a maximum back pressure ratio of 80% and high performance, it is the ideal steam trap for heating equipment in manufacturing processes. Mechanical steam traps include free floating ball type, free semi-floating ball type, lever ball type, and inverted bucket type. 1. Free-floating ball steam trap: The structure of the free-floating ball steam trap is simple; it contains only one movable part—a finely polished stainless steel hollow ball, which serves both as a float and as the closing/opening element. There are no vulnerable parts, ensuring a very long service life. The “Silver Ball”-brand steam traps are equipped with a Y-series automatic air vent device inside; this device is highly sensitive and can automatically expel air, thereby ensuring high-quality operation. When the equipment starts operating, the air in the pipes is expelled through the Y-series automatic air exhaust device. Low-temperature condensate water enters the drain valve; as the level of this condensate rises, the float rises as well, causing the valve to open and the condensate to be discharged rapidly. Steam then enters the equipment quickly, resulting in a rapid increase in its temperature. The temperature-sensitive fluid in the Y-series automatic air exhaust device expands, which causes the device to shut off. The steam trap begins to function properly; the float rises and falls with the level of condensed water, thereby preventing steam from entering while allowing water to drain. In a free-floating ball type steam trap, the valve seat is always below the liquid level, creating a water seal that prevents steam leakage and results in good energy-saving effects. The minimum operating pressure is 0.01 Mpa; within the range from 0.01 Mpa up to the maximum operating pressure, continuous drainage is possible without being affected by temperature or variations in operating pressure. It can discharge saturated-temperature condensate water, with a minimum subcooling of 0°C; no water remains in the heating equipment, which enables it to achieve optimal heat exchange efficiency. With a backpressure ratio greater than 85%, it is one of the ideal steam traps for heating equipment in production processes. 2. Free semi-floating ball type steam trap: The free semi-floating ball type steam trap has only one semi-floating ball-shaped chamber as the moving component; this chamber is open downward, and it serves both as the valve element and as the sealing element. The entire sphere can be sealed, offering a long service life; it is resistant to water hammer, has no vulnerable components, operates without failures, is durable, and prevents steam leakage. With a backpressure ratio of over 80%, it is capable of discharging condensate at saturation temperature; the minimum subcooling degree is 0°C, and no water remains in the heating equipment, thereby enabling the heating equipment to achieve optimal heat exchange efficiency. When the device is first started, the air and low-temperature condensate in the pipeline enter the drain valve through the discharge tube. The bimetallic element inside the valve pushes the ball apart, opening the valve and allowing the air and low-temperature condensate to be discharged rapidly. When steam enters the ball tank, the tank experiences an upward buoyant force; at the same time, the temperature inside the valve rises, causing the bimetallic strip in the discharge element to contract. As a result, the ball tank moves toward the valve opening, and the valve closes. When the steam inside the drum turns into condensed water, the drum loses its buoyancy and sinks; the valve opens, and the condensed water is quickly discharged. When steam enters the ball tank again, the valve closes, resulting in intermittent and continuous operation. 3. Lever-float type steam trap: The basic characteristics of the lever-float type steam trap are the same as those of the free-float type. Its internal structure involves a float connected to a lever, which in turn operates the valve stem; this causes the valve to open and close according to the rise and fall of the condensate level. The lever float type steam trap utilizes dual valve seats to increase the condensate discharge volume, resulting in a compact design with high discharge capacity; its maximum steam discharge rate can reach 100 tons per hour, making it the ideal steam trap for large heating systems. 4. Inverted bucket type steam trap: The interior of an inverted bucket type steam trap features an inverted bucket that serves as the level sensor; the opening of this bucket faces downward, and it is connected to a lever that drives the valve core to open and close the valve. The inverted bucket type steam trap can expel air, is not susceptible to water hammer, and has good anti-fouling properties. The degree of supercooling is low; the steam leakage rate is less than 3%. The maximum back-pressure ratio is 75%. There are relatively many connectors, and its sensitivity is inferior to that of free-floating ball steam traps. Since the inverted bucket type steam trap closes due to the upward buoyancy of steam, it is not suitable for use when the operating pressure difference is less than 0.1 MPa. When the device is first activated, air and low-temperature condensate in the pipeline enter the steam trap. The inverted bucket falls due to its own weight; the lever connected to the bucket then causes the valve stem to open the valve, allowing the air and low-temperature condensate to be discharged rapidly. When steam enters the inverted bucket, it generates an upward buoyant force; this causes the inverted bucket to rise, moving the connecting lever and thereby closing the valve. The inverted bucket has a small hole in it; as some of the steam escapes through this hole, the remaining steam turns into condensate water. As a result, the inverted bucket loses its buoyancy and sinks due to its own weight. The lever connected to the inverted bucket then causes the valve element to open, allowing for continuous drainage in a cyclic manner. 5. Combined superheated steam trap: The combined superheated steam trap features two separate valve chambers, which are connected to each other by two stainless steel pipes. It is a combination of a float-type trap and an inverted bucket type trap. Its structure is advanced and rational; it enables timely discharge of the condensate water formed when superheated steam disappears under conditions of superheating, high pressure, and low load, effectively preventing the leakage of superheated steam and ensuring high performance. The maximum allowable temperature is 600°C; the valve body is made of all stainless steel, while the valve seat is made of cemented carbide steel, ensuring a long service life. It is a specialized steam trap for superheated steam, and it holds two **patents, filling a gap in domestic technology. When the condensate enters the lower valve chamber, the float ball of the auxiliary valve rises as the liquid level increases, and the float ball seals off the steam inlet hole. The condensate rises to the main valve chamber through the inlet conduit; the hanging bucket drops due to its own weight, which in turn causes the valve core to open the main valve and allow the condensate to be discharged. When the condensate level in the secondary valve chamber drops, the float moves down along with the level, and the secondary valve opens. Steam enters the inverted bucket inside the upper main valve chamber through the steam inlet pipe; the inverted bucket generates an upward buoyant force, which drives the valve core to close the main valve. When the level of condensate in the auxiliary valve chamber rises again, the next cycle begins, with intermittent drainage. II. Thermostatic type steam traps: These types of steam traps utilize the temperature difference between steam and condensed water to cause deformation or expansion of a temperature-sensitive element, which in turn drives the valve core to open and close the valve. Thermostatic steam traps have a relatively large degree of subcooling, typically ranging from 15 to 40 degrees. They can make use of part of the sensible heat contained in the condensed water; high-temperature condensed water always remains before the valve, there is no steam leakage, and thus they offer significant energy-saving benefits. It is the ideal drain valve for steam pipes, heat tracing pipelines, small heating devices, heating systems, and small heating devices with low temperature requirements. Thermostatic steam traps include diaphragm type, bellows type, bimetallic strip type, etc. 1. Diaphragm-type steam trap: The main operating element of a diaphragm-type steam trap is a metal diaphragm; this diaphragm is filled with a liquid whose vaporization temperature is lower than the saturation temperature of water. There are two options available for the opening temperature – 15°C and 30°C below the saturation temperature. The diaphragm box steam trap features extremely sensitive response, freeze resistance, a compact size, and resistance to overheating; it can be installed in any position. It has a backpressure ratio of over 80%, is capable of removing non-condensable gases, features a robust diaphragm box, offers a long service life, is easy to maintain, and has a wide range of applications. When the device is first started, low-temperature condensate appears in the pipes; the liquid inside the diaphragm box is in a condensed state, and the valve is in the open position. As the temperature of the condensate water gradually rises, the liquid contained within the diaphragm box begins to evaporate, causing the pressure inside the box to increase. The diaphragm then drives the valve element in the direction of closure, and the steam trap starts to close before the condensate water reaches its saturation temperature. The diaphragm box controls the opening and closing of the valve in response to changes in steam temperature, thereby serving to prevent steam from passing through and to drain water. 2. Bellows-type steam trap: The valve element of a bellows-type steam trap is a stainless steel bellows filled with a liquid whose vaporization temperature is lower than the saturation temperature of water. The valve opens and closes in response to changes in steam temperature. It is equipped with an adjustment screw, allowing the operating temperature to be adjusted as needed; generally, the range of subcooling adjustment is 15°C–40°C below the saturation temperature. With a backpressure ratio of over 70%, it is resistant to freezing, compact in size, can be installed in any location, capable of removing non-condensable gases, and has a long service life. When the device is started, cooling condensate appears in the pipeline; the liquid inside the bellows is in a condensed state, and the valve core remains in the open position due to the force of the spring. As the temperature of the condensate gradually rises, the liquid inside the bellows begins to evaporate and expand, causing the internal pressure to increase. This leads to deformation and elongation of the bellows, which in turn moves the valve stem toward the closed position. Before the condensate reaches its saturation temperature, the steam trap starts to close. It then regulates its opening and closing based on changes in steam temperature, thereby preventing steam from passing through while allowing condensate to drain. 3. Bimetallic sheet steam trap: The main component of a bimetallic sheet steam trap is the bimetallic sheet temperature sensor, which deforms when heated as the steam temperature changes, thereby driving the valve core to open and close the valve. Bimetallic sheet-type steam traps are equipped with adjustment bolts that allow the operating temperature to be adjusted as needed. Generally, the degree of subcooling can be set between 15°C and 30°C below the saturation temperature. With a back pressure ratio of over 70%, they are capable of removing non-condensable gases; they are not susceptible to freezing, have a small size, can withstand water hammer, tolerate high pressures, and can be installed in any location. Bimetallic strips are prone to fatigue and require frequent adjustment. When the device is first started, low-temperature condensation water appears in the pipes; the bimetallic strip is flat, and under the force of the spring, the valve core keeps the valve in the open position. As the temperature of the condensate water gradually rises, the bimetallic element sensing temperature begins to bend and deform, pushing the valve core toward the closed position. The steam trap begins to close before the condensate reaches its saturation temperature. The bimetallic strip controls the opening and closing of the valve as the steam temperature changes, thereby preventing steam from passing through while allowing water to drain. III. Thermodynamic type steam traps: These types of steam traps operate on the principle of phase change; by utilizing the differences in flow velocity and volume as steam and condensed water pass through, they create a pressure difference between the upper and lower parts of the valve element, which in turn drives the valve element to open and close. Since the operating power of thermodynamic steam traps comes from steam, there is significant steam waste. It has a simple structure, is resistant to water hammer, with a maximum back pressure of 50%; it generates noise, the valve disc operates frequently, and its service life is short. Thermodynamic steam traps include the thermodynamic type (disc type), pulse type, or orifice plate type. 1. Thermodynamic steam trap: A thermodynamic steam trap contains a movable valve disc, which serves both as a sensing element and as the actuating element. Based on the thermodynamic principles related to the differences in flow velocity and volume as steam and condensate pass through, a pressure difference is created above and below the valve disc, which drives the disc to open or close the valve. The vapor leakage rate is 3%, and the subcooling degree is 8°C–15°C. When the device is started, cooling condensate appears in the pipeline; this condensate pushes the valve disc open due to the operating pressure and is quickly discharged. Once the condensate has been discharged, steam is then released. Since steam occupies more volume and flows at a higher velocity than condensate, this creates a pressure difference above and below the valve disc, causing the disc to close rapidly due to the suction force exerted by the steam flow. When the valve disc is closed, it is subjected to pressure from both sides; the area under the valve disc where pressure acts is smaller than the area above it. Since the pressure in the steam chamber of the check valve comes from the steam pressure, the pressure acting on the upper side of the valve disc is greater than that on the lower side, causing the valve disc to close tightly. When the steam in the steam chamber of the check valve cools down to become condensate, the pressure in the chamber is eliminated. The condensate pushes the valve disc open using the working pressure; it then continues to be discharged, thus creating a cyclic process of intermittent drainage. 2. Disk-type steam-insulated trap: The working principle of the disk-type steam-insulated trap is the same as that of the thermodynamic trap; it features an additional outer shell surrounding the vapor chamber of the thermodynamic trap. The inner chamber of the casing is connected to the steam pipe; the steam within the pipe itself is used to insulate the main steam chamber of the trap. This prevents the temperature in the main steam chamber from dropping, maintains the steam pressure, and keeps the drain valve tightly closed. When condensate is generated in the pipeline, the shell of the steam trap cools down, and the steam trap begins to discharge water ； On a superheated steam pipeline, if no condensate is generated, the steam trap will not open, ensuring high operational efficiency. The valve body is made of alloy steel, while the valve core is made of cemented carbide. This valve can withstand a maximum temperature of 550°C; it is durable and has a long service life, making it an ideal steam trap for high-pressure, high-temperature superheated steam. 3. Pulse-type steam trap: The pulse-type steam trap features two orifice plates that adjust the opening and closing of the valve in response to changes in steam pressure; even when the valve is completely closed, the inlet and outlet remain connected through the first and second small orifices, resulting in an incomplete closure state. As a result, steam continues to leak out, leading to high steam loss. This steam trap operates at a very high frequency; it wears out easily and has a short service life. It has a small size, is resistant to water hammer, can discharge air and water at saturated temperature, enabling nearly continuous drainage; with a maximum back pressure of 25%, it is used by few people. 4. Orifice-type steam trap: The orifice-type steam trap uses orifices of different diameters, selected based on the desired drainage volume, in order to control that volume. It has a simple structure; an inappropriate selection can lead to inadequate drainage or excessive steam leakage. It is not suitable for steam-using equipment in batch production or those with large fluctuations in condensate volume.