Structural forms and working principles of steam traps

Thread Content

Structure and working principle of steam traps. Steam traps serve to prevent steam from entering while allowing water to be drained in steam heating systems; selecting the appropriate steam trap can help steam heating equipment achieve its highest efficiency. To achieve the most 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 performance characteristics. When selecting a steam trap, it is first necessary to ensure that its properties meet the requirements for optimal operation of steam heating equipment; only after that should other objective factors be considered. This approach ensures that the steam trap chosen is the right one and effective. 　　For a steam trap to function as a vapor barrier and water drain, it must be able to “identify” steam from condensed water. “The detection 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 steam traps: Mechanical steam traps, also known as float-type traps, 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 achieving the purpose of preventing steam from passing while allowing water to flow out. 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 enables 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 come in various types, including 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 this type of steam trap is simple; it contains only one moving component – a finely ground stainless steel hollow ball that serves both as a float and as the valve element. There are no vulnerable parts, so its service life is very long. The “Silver Ball” brand of steam traps are equipped with a Y-series automatic air discharge device, which makes them highly sensitive and capable of automatically discharging air, thereby ensuring high performance. 　　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 passing through 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 to the maximum operating pressure, continuous drainage is possible without being affected by temperature or fluctuations 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 of over 85%, it is one of the most ideal steam traps for heating equipment in manufacturing processes. 　　2. Free semi-floating ball type steam trap: The free semi-floating ball type steam trap has only one semi-floating ball bucket as the moving component; this bucket is open at the bottom, 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 in 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 re-enters the drum, the valve closes again, enabling 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 features a float connected to a lever that drives the valve core, enabling the valve to open and close as the level of condensed water changes. The lever float type steam trap utilizes dual valve seats to increase the condensate discharge capacity, resulting in a compact design with high discharge rates; its maximum steam discharge capacity is 100 tons per hour, making it the ideal steam trap for large heating systems. 　　4. Inverted bucket type steam trap: The inverted bucket type steam trap has an inverted bucket as the level-sensitive element; 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 discharge air, is not sensitive to water hammer, and has good anti-fouling properties. It has a low supercooling degree, with a steam leakage rate of less than 3% and a maximum back pressure ratio of 75%; it features numerous connecting parts, and its sensitivity is lower than that of a free-floating ball type steam trap. 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 started, air and low-temperature condensate in the pipeline enter the drain valve; the inverted bucket drops due to its own weight, and the lever connected to the bucket drives the valve core to open, allowing the air and low-temperature condensate to be discharged quickly. When steam enters the inverted bucket, the steam in the bucket generates an upward buoyant force, causing the bucket to rise; this in turn moves the connecting lever and closes the valve core, thereby shutting off 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 cycling and intermittent drainage. 　　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 closes 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 downward as the level falls, and the secondary valve opens. Steam enters the upside-down bucket inside the upper main valve chamber through the steam inlet pipe; this bucket generates an upward buoyant force, which drives the valve core to close the main valve. When the condensate level in the secondary valve chamber rises again, the next cycle begins, with intermittent drainage. 　　II. Thermo-static 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 steam trap for steam pipes, heat tracing pipelines, small heating devices, heating systems, and small heating devices with low temperature requirements. 　　Thermostatic steam traps come in diaphragm type, bellows type, and bimetallic strip type. 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 for the opening temperature: 15°C below the saturation temperature or 30°C below it. The diaphragm-type steam trap has an extremely sensitive response, is not afraid of freezing, is compact in size, can withstand high temperatures, and can be installed in any location. 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 inside the diaphragm box begins to evaporate, causing the pressure within the box to increase. The diaphragm then drives the valve core to move in a direction that closes the valve; 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 preventing steam from passing through and allowing water to be drained. 　　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 is operated in response to changes in steam temperature; it is equipped with adjustment bolts that allow the operating temperature to be adjusted as needed. Generally, the supercooling range is between 15°C and 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 rises, the liquid inside the bellows begins to evaporate and expand, causing an increase in internal pressure. This leads to deformation and elongation of the bellows, which in turn moves the valve core in the direction of closure. The steam trap starts to close before the condensate reaches its saturation temperature; it controls the opening and closing of the valve according to changes in steam temperature, thereby preventing steam from passing while allowing water to drain. 　　3. Bimetallic sheet steam trap: The main component of a bimetallic sheet steam trap is the bimetallic element that senses temperature; as the steam temperature rises or falls, this element deforms due to heat, thereby driving the valve core to open or 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 discharging 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 different pressure differences above and below the valve disc, which in turn drives the valve disc 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 (disk type), pulse type, and orifice plate type. 1. Thermodynamic steam trap: A thermodynamic steam trap contains a movable valve element that 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 condensed water pass through, a pressure difference is created above and below the valve disc, which drives the valve 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 aside 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 moves 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 on which pressure acts is smaller than the area above it. Since the pressure in the steam chamber of the steam trap 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. Disc-type steam-insulated trap: The working principle of the disc-type steam-insulated trap is the same as that of the thermodynamic trap; it features an additional outer shell surrounding the steam chamber of the thermodynamic trap. The inner chamber of the shell is connected to the steam pipeline, and the steam within the pipeline itself is used to insulate the main steam chamber of the steam 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. Pulsating steam trap: The pulsating 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 losses. This steam trap has a high operating frequency, suffers from severe wear, and has a short lifespan. It has a small size, is resistant to water hammer, can discharge air and water at saturated temperature, enabling near-continuous drainage; with a maximum back pressure of 25%, it is rarely used by users. 　　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.