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Steam trap selection and performance comparison

It automatically discharges the condensed water and non-condensable gas generated in the steam pipeline and equipment, and at the same time effectively prevents the steam from being discharged and ensures the normal operation of the steam equipment. In chemical design, steam traps (also known as steam traps) are often encountered, but because the trap system is only a small accessory, it is often not valued by the designer. According to reports, there are about 3 million traps in use every year in China, but up to 70% to 75% are not used well. In addition to the reasons for the manufacture of traps, the main reason is in system design and selection. There is a problem.
Steam trap

Type Main advantages Main disadvantages Application recommendation
Float type (with thermostatic venting valve) traps continuously discharge condensate at saturated steam temperature with large displacement;
Can adapt to high and low load changes, not affected by pressure changes; good exhaust performance. The float is easily damaged by the water hammer;
In the exposed state, insulation is required; the thermostatic venting valve is sensitive to corrosion. a heat exchanger with a large load change;
Where there is a condensate pump downstream;
Always open a parking occasion.
Inverted bucket type trap The valve body is firm, simple in structure, resistant to water hammer, and has a long service life;
If a check valve is installed at the inlet, it can be used in a superheated steam system. Emission of non-condensable gas is slow (there is often an exception to increase the exhaust air valve); when the system pressure suddenly changes, it is easy to lose the water seal and leak steam, and a check valve must be installed at the inlet;
Exposure to cold conditions will cause freezing and must be kept warm; under small loads, steam will leak. When there is less condensate discharge.
Liquid or solid expansion thermostatic traps utilizing the thermal sensitivity of condensate;
At the set temperature, it is allowed to discharge non-condensable gas when driving; it is not sensitive to overheating, water hammer or vibration; antifreeze. The thermal element is susceptible to corrosion;
The condensate flows back into the process discharge line. Ideal for steam heating lines;
Heating equipment temperature control is not strict.
The pressure balance type steam trap is small in size, light in weight, small in installation space, large in displacement, and good in air discharge performance;
Corrosion resistant, water hammer resistant and overheated, not damaged by freezing;
Automatically adjusted according to changes in working pressure. There is no need to adjust the trap as pressure changes. The condensate discharge temperature is lower than the saturation temperature in order to make the steam space
Keep the flow smooth and add a cooling section in front of the trap;
Ordinary bellows are not resistant to freezing. Intermittent operation process
Prepare, drive quickly
Emissions are not condensed;
The heat tracing and main steam lines are hydrophobic; the ambient temperature is below freezing.
Bi-metal traps are rugged and reliable, easy to install and maintain;
Corrosion resistance, water hammer resistance and overheating, not damaged by freezing; good exhaust characteristics;
The effluent temperature is lower than the saturation temperature and part of the sensible heat can be utilized. The bimetal is slow to sense temperature and cannot react quickly to load changes; the condensate discharge temperature is lower than the saturation temperature, and in order to keep the steam space smooth, a cooling section must be added before the trap. The main steam pipe is hydrophobic; the installation environment temperature is lower than the freezing point.
Thermodynamic steam trap The valve body structure is firm, simple, light weight, long service life and low installation cost;
The operating pressure range is large, and it is not necessary to adjust or replace the valve size due to pressure changes;
Resistant to water hammer, vibration, corrosion, the body is not frozen damage, can be applied to superheated steam, can be operated in any orientation;
The only moving parts are stainless steel sheets, which ensure reliable operation, reduce maintenance and have a check valve function;
Air-removed discs to prevent air blockage;
The trap makes a “click” when switching, which helps to determine if the trap is working properly. Too low inlet pressure and excessive back pressure cannot work;
Models that do not contain a gas-proof lock disc are prone to air resistance at startup. Exposure to low temperatures can cause frequent valve switching and reduce life. A heat shield must be added. The main steam and the heat tracing line are hydrophobic;
Constant pressure and load; where the installation ambient temperature is below freezing.
From the selection and application design of the steam trap, problems should be considered in order to make the steam trap effective and stable for long-term operation, so that the plant can improve safety, reduce cost, save energy and improve economic efficiency.

1 selection
There are three main types of traps in chemical plants. See Table 1 for comparison of various types of traps.
1.1
Mechanical trap
Working mechanically, the principle is based on the difference in density between steam and condensate. Use a float or float to operate the switch of the trap and drain the condensate at a saturated temperature. Types include float and inverted buckets.
1.2
Thermostatic trap
The switch of the valve is controlled by operating the temperature sensing element of the trap with the principle that the saturated steam has a higher temperature than the condensed water and the non-condensable steam. Such traps must be cooled below the saturation temperature before the condensate drains. Types include pressure balanced, bimetallic and liquid or solid expansion.
1.3
Thermodynamic steam trap
Secondary steam is generated when the condensate passes through the trap, and the secondary steam at high speed produces a simple principle of the low pressure zone to control the switching of the valve.
2 displacement determination
The type of trap is selected after the size is determined, and the size is determined based on the condensate displacement.
The determination of the displacement of the trap is divided into two steps. First, the condensation load is calculated according to the heat balance relationship, and then the displacement of the trap is obtained by multiplying the condensing load by the safety factor. The first step can be determined according to the relevant formulas and charts according to different uses (such as main steam pipe hydrophobic, heat tracing and process equipment), and will not be described here. The following safety factors are mainly emphasized. For the main steam pipe hydrophobic and heat tracing pipelines, because the pressure is constant, the amount of water is small (generally less than 50kg/h), and the safety factor is 2~3; for the safety factor of process equipment, refer to Table 2.
In particular, the size of the trap cannot be determined according to the diameter of the connecting pipe, because the connection size of the trap is not directly related to the drainage capacity of the trap. The size of the drain of the trap depends mainly on the size of the valve hole. The difference between the pressure difference before and after the valve and the temperature of the condensate.

It is worth noting that the displacement of the steam trap is affected. The pressure difference is an important factor. The pressure difference is the difference between the inlet pressure and the outlet pressure of the steam trap. The outlet pressure is also called the back pressure. The inlet pressure of the steam trap is lower than the pressure of the steam system sent to the steam equipment. The resistance loss of the steam equipment should be considered, generally 0.05~0.lMPa. The back pressure is not equal to the pressure of the condensate system. Even if it is discharged to the atmosphere, since the condensed water is partially flashed into secondary steam, it will have a resistance to the trap, and therefore a back pressure will be generated. When the condensate is recovered, the outlet pressure is the sum of the pressure drop of the piping system, the difference in position and the pressure of the condensate recovery system. The higher the back pressure, the smaller the differential pressure of the trap and the corresponding reduction in the condensate discharge. Therefore, when selecting the type, the capacity of the steam trap should be determined according to the pressure difference and the displacement.

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