Trap principle and structure


Traps play the role of blocking steam and drainage in the steam heating system. Selecting the appropriate traps can make the steam heating equipment achieve higher working efficiency. In order to achieve better results, it is necessary to have a comprehensive understanding of the working performance and characteristics of various types of traps.

There are many types of traps, each with different performance. When choosing a steam trap, first of all, the characteristics should be selected to meet the best operation of the steam heating equipment, and then other objective conditions should be considered, so that the selection of the steam trap you need is correct and effective.

Traps must be able to “recognize” steam and condensed water in order to block steam and drain water. The “identification” of steam and condensate is based on three principles: density difference, temperature difference, and phase transition.

Therefore, three types of traps are manufactured according to three principles: classified as mechanical type, thermostatic type, and thermodynamic type.

1. Mechanical trap

The mechanical type, also known as the float type, uses the density difference between the condensed water and the steam, and through the change of the condensed water level, the float lifts and drives the valve disc to open or close, so as to achieve the purpose of preventing steam and drainage. The mechanical trap has a small degree of subcooling and is not affected by working pressure and temperature changes. Water is discharged immediately, and there is no water in the heating equipment, so that the heating equipment can achieve better heat exchange efficiency.

The maximum back pressure rate is 80% and the working quality is high. It is an ideal trap for heating equipment in the production process.

Mechanical traps include free float type, free semi-float type, lever float type, inverted bucket type, etc.:

1. Free float trap

The structure of the free float type steam trap is simple. There is only one movable part inside. The stainless steel hollow float ball is finely ground. It is both a float and an opening and closing part. There are no vulnerable parts and a long service life.

A series of automatic air exhaust devices are very sensitive, can automatically exhaust air, and have high working quality.

When the equipment is just started to work, the air in the pipeline passes through y

The series of automatic air exhaust devices are discharged, the low-temperature condensed water enters the trap, the liquid level of the condensed water rises, the float rises, the valve opens, the condensed water is quickly discharged, the steam quickly enters the equipment, and the equipment heats up rapidly, y

The temperature-sensitive liquid of the series automatic air exhaust device expands, and the automatic air exhaust device closes. The steam trap starts to work normally, the float ball rises and falls with the condensate level, and the steam is blocked and drained. The valve seat of the free floating ball trap is always below the liquid level, forming a water seal, no steam leakage, and good energy saving effect. Minimum working pressure 0.01mpa, from 0.01mpa

The back pressure rate is greater than 85%, which is one of the ideal steam traps for production process heating equipment.

2. Free semi-floating ball trap

When the device is just started, the air and low-temperature condensate in the pipeline enter the trap through the launch tube, the bimetallic sheet emptying element in the valve bounces the ball bucket, the valve opens, and the air and low-temperature condensate are quickly discharged. When the steam enters the ball barrel, the ball barrel generates upward buoyancy, and at the same time, the temperature in the valve rises, the bimetal emptying element shrinks, the ball floats to the valve port, and the valve closes. When the steam enters the ball barrel again, the valve is closed again, working intermittently and continuously.

3. Rod float trap

The basic features of the lever float type steam trap are the same as those of the free float type.

The lever float type steam trap uses double valve seats to increase the condensate displacement, which can achieve a small volume and a large displacement, and a large drain capacity of up to 100

4. Inverted bucket steam trap

The inside of the inverted bucket trap is an inverted bucket as a liquid level sensitive component. The opening of the bucket is downward, and the connection lever of the inverted bucket drives the valve core to open and close the valve. The inverted bucket type trap can discharge air, is not afraid of water hammer, and has good anti-fouling performance. The degree of subcooling is small, the steam leakage rate is less than 3%, the maximum back pressure rate is 75%, there are many connectors, and the sensitivity is not as good as that of the free float trap. Because the inverted bucket type trap relies on the upward buoyancy of steam to close the valve, the working pressure difference is less than 0.1mpa

is not suitable for selection.

When the device is just started, the air and low-temperature condensate in the pipeline enter the trap, and the inverted bucket falls down by its own weight. The lever connected to the inverted bucket drives the valve core to open the valve, and the air and low-temperature condensate are quickly discharged. When the steam enters the inverted bucket, the steam in the inverted bucket generates upward buoyancy, and the upside-down bucket connects to the lever to drive the valve core to close the valve. There is a small hole on the inverted bucket. When a part of the steam is discharged from the small hole, the other part of the steam produces condensed water, and the inverted bucket loses its buoyancy and sinks down by its own weight. The inverted bucket is connected to the lever to drive the valve core to open the valve and circulate. Work, intermittent drainage.

5. Combined superheated steam trap

The combined type superheated steam trap has two isolated valve cavities, and the upper and lower valve cavities are connected by two stainless steel pipes. It is a combination of a floating ball type and an inverted bucket type. Under the working condition of small load, the condensed water formed when the superheated steam disappears can be discharged in time, effectively preventing the leakage of the superheated steam, and the working quality is high.

The maximum allowable temperature is 600℃, the valve body is made of stainless steel, the valve seat is made of hard alloy steel, and the service life is long.

When the condensed water enters the lower valve cavity, the float ball of the auxiliary valve rises with the liquid level, and the float ball closes the steam inlet pipe hole.

The condensed water rises to the main valve cavity through the water inlet conduit, and the inverted bucket falls down by its own weight, which drives the valve core to open the main valve and discharges the condensed water.

When the condensate level in the auxiliary valve cavity drops, the float ball drops with the liquid level and the auxiliary valve opens. The steam enters the upside-down bucket in the upper main valve cavity from the steam inlet pipe, the upside-down bucket generates upward buoyancy, and the upside-down bucket drives the valve core to close the main valve.

When the condensed water level in the auxiliary valve cavity rises again, the next cycle starts again, and the drainage is interrupted.

2. Thermostatic steam trap

This type of trap uses the temperature difference between steam and condensed water to cause the deformation or expansion of the temperature sensing element to drive the valve core to open and close the valve. The subcooling degree of the thermostatic trap is relatively large, and the general subcooling degree is 15

Thermostatic steam traps include bellows type, bellows type and bimetal type.

1. Diaphragm type steam trap

The main action element of the diaphragm trap is a metal diaphragm, which is filled with a liquid whose vaporization temperature is lower than the saturation temperature of water, and the valve opening temperature is 15°C lower than the saturation temperature.

Capsule-type traps are particularly sensitive in response, not afraid of freezing, small in size, resistant to overheating, and can be installed in any position. The back pressure rate is greater than 80%, it can discharge non-condensable gas, the diaphragm box is firm, the service life is long, the maintenance is convenient, and the use range is very wide.

When the device is just started, low-temperature condensed water appears in the pipeline, the liquid in the membrane box is in a condensed state, and the valve is in the open position. When the temperature of the condensate gradually increases, the filling liquid in the membrane closure begins to evaporate, the pressure in the membrane closure rises, and the diaphragm drives the valve core to move in the closing direction. Before the condensate water reaches the saturation temperature, the trap starts to close. The diaphragm box controls the valve switch with the change of steam temperature, which plays the role of blocking steam and draining water.

2. Bellows type steam trap

The stainless steel bellows of the valve core of the bellows trap is 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 adjustment bolts, which can adjust the operating temperature according to the needs. Generally, the adjustment range of the subcooling degree is lower than the saturation temperature by 15℃~40℃. The back pressure rate is greater than 70%, it is not afraid of freezing, the volume is small, it can be installed in any position, it can discharge non-condensing gas, and the service life is long.

When the device is started, cooling condensate appears in the pipeline, the liquid in the bellows is in a condensed state, and the valve core is in the open position under the elastic force of the spring. When the temperature of the condensate gradually rises, the filling liquid in the bellows begins to evaporate and expand, the internal pressure increases, and the deformation and elongation drive the valve core to move in the closing direction. Valve switch, steam blocking and drainage.

3. Bimetallic disc trap

The main component of the bimetallic steam trap is the bimetallic temperature sensing element, which is heated and deformed with the rise and fall of the steam temperature, and pushes the valve core to open and close the valve.

The bimetallic sheet steam trap is equipped with adjustment bolts, and the operating temperature can be adjusted according to the needs. Generally, the adjustment range of the subcooling degree is lower than the saturation temperature of 15℃~30℃, and the back pressure rate is greater than 70%. It can discharge non-condensing gas and is not afraid of freezing. Small size, water hammer resistance, high pressure resistance, and can be installed in any position.

The bimetal is fatigued and needs to be adjusted frequently.

3. Thermodynamic steam trap

According to the principle of phase change, this type of trap relies on the different thermodynamic principles of flow rate and volume change when steam and condensate pass through, so that different pressure differences are generated up and down the valve plate to drive the valve plate to open and close the valve. Because the working power of the thermodynamic trap comes from steam, the steam waste is relatively large. Simple structure, water hammer resistance, 50% larger back, noise, frequent valve work, short service life.

Thermodynamic steam traps include thermodynamic (disc type), pulse type and orifice type.

1. Thermodynamic steam trap

There is a movable valve plate in the thermodynamic steam trap, which is both a sensitive part and an action execution part. According to the different thermodynamic principles of the flow rate and volume change of steam and condensate passing through, different pressure differences are generated up and down the valve plate to drive the valve plate to open and close the valve. The steam leakage rate is 3%, and the subcooling degree is 8℃~15℃.

When the device is started, cooling condensate appears in the pipeline, and the condensate pushes open the valve plate by the working pressure and discharges quickly.

When the condensed water is discharged, the steam is subsequently discharged. Because the volume and flow rate of the steam is larger than that of the condensed water, a pressure difference is generated up and down the valve plate, and the valve plate is quickly closed under the suction of the steam flow rate. When the valve plate is closed, the valve plate is subjected to pressure on both sides, and the force-bearing area under the valve plate is smaller than the force-bearing area above. Because the pressure in the steam chamber of the trap comes from the steam pressure, the force on the top of the valve plate is greater than that on the bottom. Close tightly.

When the steam in the steam chamber of the trap cools down to condensate, the pressure in the steam chamber disappears.

2. Disc type steam insulation trap

The working principle of the disc steam insulation trap is the same as that of the thermodynamic steam trap. It adds a layer of shell outside the steam chamber of the thermodynamic steam trap. The inner chamber of the shell is communicated with the steam pipe, and the main steam chamber of the trap is kept warm by the steam of the pipe itself. The temperature of the main steam chamber is not easy to cool down, the steam pressure is maintained, and the steam trap is tightly closed. When the pipeline produces condensed water, the trap casing cools down, and the trap starts to drain; if there is no condensed water on the superheated steam pipeline, the trap will not open, and the working quality is high. The valve body is made of alloy steel, and the valve core is made of hard alloy. The maximum allowable temperature of the valve is 550°C. It is durable and has a long service life. It is a special trap for high-pressure and high-temperature superheated steam.

3. Pulse trap

The pulse trap 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 small holes, and they are always in an incompletely closed state, and the steam continuously escapes. Large amount of steam leakage. The trap has a high frequency of action, severe wear and short life. Small size, water hammer resistance, can discharge air and saturated temperature water, close to continuous drainage, 25% larger back pressure, so there are few users.

4. Orifice trap

The purpose of the orifice trap is to select orifices with different apertures to control the drainage according to different drainage. The structure is simple, and if the selection is not appropriate, there will be insufficient drainage or a large amount of steam running. It is not suitable for steam-using equipment for intermittent production or steam-using equipment with large fluctuations in condensed water.

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