What is a steam trap
What is a steam trap?
The steam trap is a kind of valve, also known as the drain valve. The drain valve is used to discharge the condensate, air and carbon dioxide gas in the steam system as soon as possible, and at the same time, it can automatically prevent the leakage of steam to the maximum extent. There are many kinds of traps with different performances.
When choosing a steam trap, you should first choose its characteristics to meet the best operation of the steam heating equipment, and then consider other objective conditions. Only in this way can you choose the steam trap you need correctly and effectively.
Function of steam trap
Reduce steam consumption of steam trap
Steam consumption of steam trap itself generally refers to steam leakage, which is the sum of steam consumption and heat loss required by steam trap action.
Quickly discharge the air and low-temperature condensate in the equipment at the beginning of start-up
The air and low-temperature condensate in the equipment at the beginning of start-up are quickly discharged, so as to shorten the preheating operation time.
At the beginning of steam supply, the steam transmission pipeline and steam using equipment are filled with air. If they are not removed, steam can not be delivered.
In addition, in the process of steam pipeline and steam equipment heating up to steam temperature, the initial low-temperature condensate produced should be discharged quickly, so that the equipment can operate normally in a short time, which is an important condition to improve production efficiency.
Especially in the case of intermittent production, because the preheating time is shortened, the operation time of each time is shortened, and because the treatment times are increased, the output can be increased finally.
Quickly discharge the steam and use the condensate generated in the equipment
Quickly discharge the condensate generated in the steam using equipment to keep the heating efficiency of the steam using equipment in the best state, so that the condensate in the equipment does not form retention, and the steam space in the equipment is ensured to the maximum extent, so that the highest heating efficiency can be always maintained.
Once the steam trap can not fully play its role, due to the retention of condensate, not only the performance of steam using equipment is greatly affected, sometimes even the production equipment is completely paralyzed.
Classification of traps
According to the different principle of action, the traps can be divided into three types:
Mechanical drain valve
Mechanical type also known as float type, is to use the density difference between condensate and steam, through the condensate level change, float lifting and driving valve disc open or close, to achieve the purpose of steam blocking and drainage. The mechanical drain valve has a small supercooling degree, which is not affected by the change of working pressure and temperature. Water is discharged. There is no water in the heating equipment, which can make the heating equipment achieve the best heat exchange efficiency. The maximum back pressure rate is 80%, and the working quality is high, which is the most ideal drain valve of heating equipment in production process.
Mechanical drain valves have free floating ball type, free semi floating ball type, lever floating ball type, inverted bucket type, etc.
(1) Free floating ball drain valve:
The structure is simple, and there is only a stainless steel hollow floating ball with fine grinding of movable parts. It is not only float but also an opening and closing part, without wearing parts, and has a long service life. The drain valve has Y Series automatic air discharge device, which is very sensitive and can automatically exhaust air and has high working quality. When the equipment is just started, the air in the pipeline is discharged through Y-series automatic air discharge device, low-temperature condensate enters the drain valve, the liquid level of condensate rises, the floating ball rises, the valve opens, the condensate is discharged rapidly, the steam enters the equipment quickly, the equipment rises rapidly, the temperature sensing liquid of Y Series automatic air discharge device expands, and the automatic air discharge device is closed. The drain valve starts to work normally, the float ball rises and decreases with the condensate level, and prevents steam from draining. The valve seat of free floating ball drain valve is always below the liquid level, forming water seal, no steam leakage, and good energy saving effect. The minimum working pressure is 0.01Mpa, and the continuous drainage is not affected by the fluctuation of temperature and working pressure from 0.01Mpa to the maximum operating pressure. It can discharge condensate at saturated temperature, with minimum supercooling degree of 0 ℃, and no water is stored in heating equipment, so that the heating equipment can achieve the best heat exchange efficiency. The back pressure ratio is more than 85%, which is one of the most ideal drain valves in the heating equipment of production process.
(2) Free half floating ball drain valve:
The free half floating ball drain valve has only one half floating ball type ball bucket as the movable part. The opening is downward. The ball bucket is the opening and closing part and the seal. The whole spherical surface can be sealed, with a long service life, water hammer resistance, no wearing parts, no fault, durable, no steam leakage. The back pressure ratio is more than 80%, which can discharge the condensate at saturated temperature, with the minimum supercooling degree of 0 ℃, and no water is stored in the heating equipment, which can make the heating equipment achieve the best heat exchange efficiency.
When the device is just started, the air and low-temperature condensate in the pipeline enter the drain valve through the launching pipe. The bimetal drain element in the valve pops the ball barrel open, the valve opens, and the air and low temperature condensate are discharged rapidly. When steam enters the ball barrel, the ball barrel produces upward buoyancy, while the temperature in the valve increases, the drain element of bimetal plate shrinks, the ball poke drifts to the valve port and the valve closes. When the steam in the drum becomes condensate, the ball barrel loses buoyancy and sinks down. The valve opens and the condensate is discharged rapidly. When the steam enters the ball drum, the valve is closed, interrupted and continuously operated.
(3) Rod float ball drain valve:
The basic characteristics of lever floating ball drain valve are the same as that of free floating ball type. The internal structure is that the floating ball connects the lever to drive the valve center, and switches the valve with the liquid level of condensate. The lever floating ball drain valve can increase the condensate discharge by double seats, which can achieve small volume and large displacement, and the maximum drain capacity reaches 100 tons / hour. It is the most ideal drain valve for large heating equipment.
(4) Inverted bucket drain valve:
Inside the inverted bucket drain valve is a inverted bucket as a liquid level sensitive part, the bucket opening is down, and the inverted bucket connects the lever to drive the valve center to open and close the valve. Inverted bucket drain valve can exhaust air, not afraid of water hammer, good anti pollution performance. The supercooling degree is small, the steam leakage rate is less than 3%, the maximum back pressure rate is 75%, the connection parts are more, and the sensitivity is not as good as the free floating ball drain valve. Because the inverted bucket drain valve is closed by steam upward buoyancy, the working pressure difference is less than 0.1MPa, it is not suitable for selection. When the device is just started, the air and low-temperature condensate in the pipeline enter the drain valve, and the inverted bucket falls down by its own weight. The connecting lever of the inverted bucket drives the valve center to open the valve, and the air and low temperature condensate are discharged rapidly. When steam enters the inverted bucket, the steam of the bucket will produce upward buoyancy, and the rising connecting lever of the inverted bucket will drive the valve core to close the valve. A small hole is opened on the inverted bucket. When one part of steam is discharged from the hole, the other part of steam generates condensate, and the bucket loses buoyancy and sinks by its own weight. The connecting lever of the inverted bucket drives the valve center to open the valve, circulates and drains water intermittently.
(5) Combined superheated steam drain valve:
The combined superheated steam drain valve has two isolated valve chambers, which are connected with the upper and lower valve chambers by two stainless steel pipes. It is a combination of floating ball type and inverted bucket type drain valve. The valve has advanced and reasonable structure. Under the working condition of overheating, high pressure and small load, the condensate formed when the hot steam disappears can be discharged in time, effectively preventing the superheated steam leakage and high working quality. The maximum allowable temperature is 600 ℃, the valve body is all stainless steel, the valve seat is cemented carbide steel, with long service life. It is a special drain valve for superheated steam, and has obtained two national patents, which filled the blank in China.
When condensate enters the lower valve chamber, the float ball of the auxiliary valve rises with the liquid level, and the floating ball closes the inlet pipe hole. The condensate rises to the main valve chamber through the inlet pipe, and the inverted bucket falls by its own weight, which drives the valve center to open the main valve and discharge the condensate. When the condensate level of the auxiliary valve chamber drops, the float ball decreases with the liquid level, and the auxiliary valve opens. Steam enters the inverted bucket in the upper main valve cavity from the steam inlet pipe, and the inverted bucket produces upward buoyancy. The inverted bucket drives the valve center to close the main valve. When the condensate level of auxiliary valve chamber rises again, the next cycle begins again, and the drainage is interrupted.
Thermostatic drain valve
This kind of drain valve is to use the difference between steam and condensate to cause the change of temperature sensing element or expansion to drive the valve core to open and close the valve. The supercooling degree of thermostatic drain valve is relatively large, generally, the supercooling is 15-40 degrees. It can make use of part of condensate to show heat. There is always high temperature condensate in front of the valve, without steam leakage, and the energy saving effect is remarkable. It is the ideal drain valve for small heating equipment with low temperature requirements in steam pipeline, tracing pipeline, small heating equipment, heating equipment and temperature requirements.
The thermostatic drain valve has membrane box type, corrugated pipe type and bimetal plate:
(1) Membrane box drain valve:
The main action element of the membrane box drain valve is metal membrane box, which is filled with a liquid with lower gasification temperature than water saturation temperature. There are two kinds of options: open valve temperature is lower than the saturation temperature of 15 ℃ and 30 ℃. The membrane box drain valve has a very sensitive reaction, is not afraid of freezing, small volume, and can be installed at any position. The back pressure rate is more than 80%, which can exhaust non condensing gas, the membrane box is strong, the service life is long, the maintenance is convenient and the use range is very wide.
When the device is started, the pipeline has low temperature condensate, the liquid in the membrane box is in the condensation state and the valve is in the open position. When the temperature of condensate increases gradually, the filling liquid in the membrane seal starts to evaporate, and the pressure inside the membrane seal rises. The diaphragm drives the valve core to move towards the closed direction. Before the condensate reaches the saturated temperature, the drain valve starts to close. The membrane box controls the valve switch with the change of steam temperature, which can prevent steam and drain.
(2) Bellows drain valve:
The stainless steel bellows of bellows type drain valve are filled with a liquid whose vaporization temperature is lower than the water saturation temperature. The valve switch is controlled with the change of steam temperature. The valve is equipped with adjusting bolt, which can adjust the operating temperature according to the needs. Generally, the adjustment range of supercooling is lower than the saturation temperature of 15 ℃ -40 ℃. The back pressure ratio is more than 70%, which is not afraid of freezing, small volume, and can be installed at any position, and can exhaust non condensing gas and long service life.
When the device is started, the cooling condensate appears in the pipeline, the liquid in the bellows is in the condensation state, and the valve center is in the open position under the spring force. When the temperature of condensate increases gradually, the liquid filled in the bellows starts to evaporate and expand, the internal pressure increases, and the deformation is extended, which drives the valve core to move towards the closed direction. Before the condensate reaches the saturated temperature, the drain valve starts to close, and the valve switch is controlled with the change of steam temperature to prevent steam drainage.
(3) Bimetal drain valve:
The main component of bimetal drain valve is bimetal plate temperature sensing element. It is deformed with the rise and fall of steam temperature, and pushes the valve center to switch valve. The bimetal drain valve is equipped with adjusting bolts, which can adjust the operating temperature according to the needs. Generally, the adjustment range of supercooling is lower than the saturation temperature of 15 ℃ -30 ℃, the back pressure rate is more than 70%. It can discharge non condensing gas, be free from freezing, small volume, water hammer resistant, high pressure resistant, and can be installed at any position. Bimetal sheet is fatigue and must be adjusted frequently.
When the device is just started, the pipeline has low temperature condensate, and the bimetal sheet is flat. The valve core is in the open position under the spring force. When the temperature of condensate increases gradually, the bimetal temperature sensing element begins to bend and deform, and pushes the valve center to the closed position. The drain valve begins to close before the condensate reaches saturation temperature. Bimetal sheet controls valve switch with the change of steam temperature, which prevents steam from draining.
(4) Inverted bucket trap
Inside the inverted bucket trap is an inverted bucket, which is a liquid level sensitive part. The bucket opening is downward, and the inverted bucket connecting lever drives the valve core to open and close the valve. Inverted bucket trap can exhaust air, not afraid of water hammer, good anti pollution performance. The steam leakage rate is less than 3%, the maximum back pressure rate is 75%, the connection is more, the sensitivity is not as good as the free floating ball trap. Because the inverted bucket drain valve is closed by the upward buoyancy of steam, it is not suitable to use when the working pressure difference is less than 0.1MPa.
Thermal power type drain valve
According to the phase change principle, the different thermodynamic principles of flow rate and volume change of steam and condensate flow rate are used to drive the valve plate to switch the valve. Because the working power of thermal power drain valve comes from steam, steam waste is relatively large. The structure is simple, water-resistant, the maximum back is 50%, noise, frequent valve plate work, short service life.
The thermal power type drain valve has the thermal power type (disc type), pulse type and orifice plate
(1) Thermal power drain valve:
There is a movable valve plate in the thermal power drain valve, which is not only sensitive part but also action actuator. According to the different thermodynamic principles of the flow rate and volume change of steam and condensate passing through, different pressure difference is generated between the valve plate and the valve plate, and the valve disc is driven to switch the valve. The steam leakage rate is 3%, and the supercooling degree is 8 ℃ -15 ℃.
When the device is started, the cooling condensate appears in the pipeline, and the condensate pushes the valve plate by working pressure to discharge rapidly. When the condensate is discharged, the steam is discharged later. Because the steam is larger than the volume and flow rate of condensate, the pressure difference between the valve plate and the valve plate will be closed rapidly under the suction of steam flow rate. When the valve plate is closed, the valve plate is under two pressures, and the force area under the valve plate is smaller than the upper stress area. Because the pressure in the steam chamber of the drain valve comes from the steam pressure, the upper force on the valve plate is greater than that under, and the valve plate is tightly closed. When the steam in the steam chamber of the drain valve is cooled to condensate, the pressure in the steam chamber disappears. The condensate pushes the valve plate by working pressure, and the condensate continues to discharge, circulates and drains continuously.
(2) Disc steam insulated drain valve:
The working principle of the disc steam insulated drain valve is the same as that of the thermal power type drain valve. It adds a layer of shell outside the steam chamber of the thermal power trap. The inner chamber of the shell is connected with the steam pipe, and the main steam chamber of the drain valve is insulated by the steam of the pipeline itself. Make the temperature of main steam chamber difficult to cool down, keep the steam pressure and close the drain valve tightly. When condensate water is generated in the pipeline, the shell of drain valve is cooled, and the drain valve starts to drain; if there is no condensate in the superheated steam pipeline, the drain valve will not open and the working quality is high. The valve body is alloy steel, the valve core is cemented carbide, the maximum allowable temperature of the valve is 550 ℃, durable, long service life, is a special drain valve for high pressure and high temperature superheated steam.
(3) Pulse drain valve:
The pulse drain valve has two orifice plates to adjust the valve switch according to the change of steam pressure drop. Even if the valve is completely closed, the inlet and outlet are connected through the first and second holes, and are always in incomplete closing state. The steam escapes continuously and the steam leakage is large. The drain valve has high frequency, strong wear and short service life. Small volume, water hammer resistant, can discharge air and saturated temperature water, close to continuous drainage, maximum back pressure of 25%, so users are very few.
(4) Orifice type drain valve:
Orifice type drain valve is designed to control the drainage amount by selecting orifice plates with different pore sizes according to different drainage volume. The structure is simple, and it is not suitable for intermittent production steam equipment or steam equipment with large fluctuation of condensate water.
How to choose the right trap?
Water volume of steam trap
When selecting the steam trap, it is necessary to select the drainage capacity of the steam trap according to the steam consumption of the equipment per hour multiplied by 2-3 times of the selection ratio as the maximum condensate volume. Only in this way can the condensate be discharged as soon as possible and the temperature of heating equipment be raised rapidly.
Due to the insufficient discharge energy of the steam trap, the condensate can not be discharged in time, which reduces the thermal efficiency of the heating equipment.
When the steam heating equipment just begins to send steam, the equipment is cold and full of air inside. The air needs to be quickly discharged by the steam trap, and then a large amount of low-temperature condensate is discharged, so that the equipment gradually heats up, and then the equipment enters the normal working state.
Due to the large amount of air and low-temperature condensate and low inlet pressure during the start-up, the drain valve is overloaded. At this time, the drain valve is required to discharge more water than it normally works. Therefore, the drain valve is selected according to the selection ratio of 2-3 times.
Working pressure difference of steam trap
When selecting a steam trap, it is not allowed to select a steam trap according to the nominal pressure, because the nominal pressure can only indicate the pressure level of the shell of the steam trap, and the difference between the nominal pressure and the working pressure of the steam trap is very large.
So according to the working pressure difference to choose the drainage of the trap. The working pressure difference refers to the difference between the working pressure in front of the trap and the back pressure at the outlet of the trap.
The calculation method of the back pressure behind the trap is: when the condensate after the trap is discharged into the atmosphere, the back pressure at the outlet of the trap is zero. If the condensate discharged from the steam trap is recycled, the outlet back pressure of the steam trap is the sum of the resistance of the return pipe, the lifting height of the return pipe and the pressure in the secondary evaporator (return tank).
Seat number of mechanical steam trap
According to different working pressure differential sections, mechanical steam trap is divided into “seat numbers” of various specifications of seat aperture. Each working pressure differential section and “seat number” form a coordinate curve of displacement, and the drainage amount of different “seat numbers” is very different.
The mechanical steam trap should choose the appropriate “seat number” according to the corresponding coordinate curve of the maximum working pressure difference and the maximum displacement of the process conditions.
The “valve seat number” cannot be determined by the nominal pressure. If the “valve seat number” is selected incorrectly, the steam trap may not work or the equipment may store water, which may affect the normal operation of the equipment.
Operating temperature of steam trap
When selecting the steam trap, the steam trap that can meet the requirements of process conditions should be selected according to the maximum temperature of the steam in the pipeline.
The maximum temperature of pipeline steam exceeds the saturated steam temperature corresponding to the nominal pressure, which is called superheated steam. When selecting steam trap for superheated steam pipeline, special steam trap for high temperature and high pressure superheated steam shall be selected.
Connection dimension of steam trap
After the process conditions of the steam trap are determined, the specification and size of the steam trap shall be selected according to the working pressure difference before and after the steam trap, the drainage amount and the “seat number” and the technical parameters of the steam trap manufacturer.
It is not allowed to select the same size of drain valve according to the connection size of the equipment. The connection diameter of the drain valve can not represent the size of the drain volume. The drain capacity of the same diameter of the drain valve may vary greatly.
Therefore, the selection of the trap must be based on the process conditions of the equipment, with reference to the parameters provided by the trap manufacturer to select the trap is the correct choice.
Common faults and maintenance methods of steam trap
Normal operation of steam trap
The condensate is discharged intermittently, and there is no steam leakage after shutdown.
Sometimes the condensate is discharged continuously, there is continuous noise, then the relevant shut-down sound, after waiting for a period of time, the cycle starts again.
Common faults and maintenance methods of steam trap
Cold valve does not drain
If the steam trap fails to drain condensate:
- The original design pressure is wrong; the pressure rises without installing a smaller valve nozzle; the pressure reducing valve fails; the reading of the boiler pressure gauge is low; after normal wear, the valve seat hole enlarges; the high vacuum in the return water pipeline increases the differential pressure, which exceeds the working differential pressure of the steam trap.
- Condensate or steam does not enter the trap: the filter in front of the trap is blocked; other valves on the pipeline with trap are damaged; the pipeline or elbow is blocked.
- Mechanical wear or defects that require repair or replacement.
- The drain valve body is blocked by dirt, so a filter should be installed to drain.
- For inverted bucket type steam trap, when the exhaust hole of floating bucket is blocked by dirt, a filter should be installed; the exhaust hole should be slightly enlarged; and metal wire should be placed in the exhaust hole.
- For the floating ball type steam trap, if the air vent is not smooth, it will produce the phenomenon of air resistance.
- For the thermostatic steam trap, the bellows element may be damaged due to water hammer, so that the steam trap is closed.
- For disc type trap, the trap may be reversed.
The heat valve does not drain
There is no condensate flow to the drain valve: the drain valve is installed on the bypass valve with leakage; the siphon discharge pipe in the drying drum is broken or damaged; the vacuum occurs in the coil of the water heater, which hinders the discharge, so a vacuum breaker is installed between the heat exchanger and the drain valve.
If fresh steam is ejected from the steam trap, it may be caused by the following reasons:
A. The valve can not be closed: the valve seat hole is blocked by scale; the parts are worn.
B. Inverted bucket trap does not work:
- If the steam trap sprays fresh steam, first close the inlet valve for a few minutes, and then open it slowly. If the steam trap can work at this time, it means that the steam trap is good.
- Inverted bucket trap does not work, generally due to sudden or frequent changes in steam pressure. If possible, the trap should be installed below the water collection point.
C. For floating ball type steam trap, the thermal static element may be damaged and cannot be closed.
If the inverted bucket type or disc type steam trap is continuously discharged, or the floating ball type or thermostatic type steam trap is fully opened for discharge, the following factors should be checked:
- A. The trap is too small: replace the large trap, or install another trap in parallel; a high-pressure trap may be installed in the low-pressure situation, so the trap with proper internal mechanism should be selected.
- B. Abnormal water supply conditions: bubbling in the boiler, causing a large amount of water to run into the steam pipe. A water separator should be installed to change the water supply conditions.
Heating is slow
The steam trap works normally, but the heating of process heating unit is not smooth.
- A. One or more units are short circuited. The solution is to install a steam trap on each pipeline.
- B. In fact, it seems that the steam trap is too small to deal with condensate effectively. Try a larger trap.
- C. The air handling capacity of the steam trap may be insufficient, or the air cannot reach the steam trap. Use the auxiliary exhaust valve.
If it is discharged under atmospheric pressure, the steam trap works normally, but there is a problem when it is connected with the return water pipeline.
Check the following points:
- The back pressure reduces the displacement of the drain valve;
- The return pipe is too thin – the steam trap is hot;
- Other traps may spray steam – trap heat;
- The drain hole on the condensate tank is blocked – the drain valve is hot or cold;
- The return water pipeline is blocked – the steam trap is hot;
- The return water pipeline is ultra vacuum and the drain valve is cold.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
If you want to have more information about the article or you want to share your opinion with us, contact us at email@example.com
Please notice that you might be interested in the other technical articles we’ve published:
- Sealing Tests for Valves
- The Origin and Development of the Valve
- Nine Anti-Corrosion Measures of Valves
- The Selection of Control Valve
- Operation and Maintenance Manuals for Gate Valves
- When the control valves are used in liquid service
- Choosing Valves Based on Industry Characteristics
- How to choose valves
- Corrosion Fatigue of Valves
- How to choose a valve