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Analysis on the selection and application of coal chemical control valve

With the construction and driving of coal chemical projects throughout the country, the status of coal chemical industry in the national economy has become increasingly prominent. Due to the particularity of the coal chemical process, the various types of control valves used in the production process are related to the smoothness and safety of production, and play a vital role in coal chemical projects. Based on years of practical experience, the author elaborates and analyzes various key control valves in various plant areas of coal chemical industry for exchange.
First, the boiler unit area
The boiler feed water pressure of coal chemical project is 16Mpa, and the steam produced by the boiler is basically 9.8Mpa and 540°C. Most of them are used to drive turbine turbines, and the rest are used for production after repeated decompression. Therefore, the boiler unit area, the feed water control valve and the steam pressure reducing valve are very important and the cost is extremely high.
1. Boiler feed valve
It is used for precise control of steam drum level. The water supply pressure is high (16Mpa) and the temperature is high (220°C). The valve body and its internal parts are greatly washed. The improper selection of the valve is easy to generate vibration. A cage sleeve can be used for the spool structure. The connection between the valve plug and the valve stem requires welding, and the thread, pin and bolt are not used in the form of a cap to prevent the stem and the spool from falling off under high pressure erosion. The valve body material is usually WC6 or CF8M, the valve inner material is 304 or 316, the valve stem material is 304, 17-4PH (martensitic stainless steel, high strength). If 304 is selected, 1.5-2 times of strength allowance is required. . The actuator can be equipped with an electric or pneumatic diaphragm actuator, which requires the valve to remain open in the event of an accident, taking into account the need for control safety.
2. Boiler feed pump minimum return valve
Boiler feed pumps are often multi-stage pumps of class 10 or above. The pressure difference between the inlet and outlet is extremely large. In order to ensure the safe operation of the feed pump, the minimum return valve is an important safety control accessory for the feed pump. Since the pressure difference is as high as 16Mpa and the temperature is 220°C, the valve body and internal parts are greatly eroded, and the control difficulty is also large, and the price is higher. In many places, sleeve control valves with positioners are often used instead. The control by the positioner is difficult, often accompanied by vibration and erosion, and the service life is short. The true minimum return valve is a self-operated valve, the valve core must be treated with tungsten carbide, and the valve core and valve body can be stacked with PTFE.
3, steam pressure reducing valve
Here we focus on the valve of 9.8Mpa minus 1.0Mpa. Other grades of pressure reducing valves can basically meet the production requirements. For the valve of 9.8Mpa minus 1.0Mpa, the world's first push is CCI labyrinth sleeve pressure reducing valve. Although most manufacturers can also manufacture and manufacture such valves, they have certain performance, but the key core technology is still in the sleeve. Structure and manufacturing industry. The labyrinth decompression sleeve used by CCI is currently the best CCI calculation and manufacturing. However, the connection of the valve stem and the plug is also avoided by the pins and threads, preferably in a unitary or welded manner. It is noted that other imported brands may often increase the rectification or noise reduction orifice plate at the inlet or outlet of the valve. In fact, it also acts as a decompression function, but this structure brings great problems. Usually, the valve is basically welded. When connected to the pipeline, once the orifice is damaged, the problem caused by the whole system is fatal, and under low load, the decompression and noise reduction effect of the orifice plate is extremely fast, which basically does not function normally. This is also the need to pay attention to the use of the selection.
Second, the air separation plant area
The air separation unit area is basically a cryogenic valve, an oxygen valve and a molecular sieve regeneration valve (the valve of the unit is separately described)
1, low temperature valve
At present, the cryogenic valve is relatively mature and can be manufactured at home and abroad, such as Flowserve, Fisher, Yamatake, Koso, etc., such valve packing and valve stem lengthening treatment. The valve body and trim materials are mainly 316.
2, oxygen vent valve
As an air separation unit, the air is over-pressed and then vented urgently. It is characterized by large pressure difference, fast action speed and large venting amount (depending on the design load of gasification). Due to the characteristics of oxygen, for general non-ferrous metals. In principle, the line speed does not exceed 25 m / s (depending on oxygen purity and pressure, take a normal value here). Therefore, for oxygen venting valves under high pressure difference, the material is generally selected from Inconel 625 or above. Since the valve body and the inner parts are all made of such special materials, the cost is very high. Valve types can be: Globe valves, sleeve valves and ball valves, usually without ball valves (except for valves used for shut-off), of course, mainly due to price factors. At the same time, the Globe valve and the sleeve valve also have a good effect. Due to the large pressure difference, when the valve is opened and closed, the actuator needs to output a large force, and the actuator is often very large, the valve stem is also thick, and the operation time is long. Here we introduce a balanced spool that overcomes valve control under large differential pressures.
3. Three-bar valve for molecular sieve purification system
According to the process characteristics of molecular sieve regeneration: the pipeline is usually thicker (generally than 900mm in diameter and large to 1500mm), the frequency of regeneration is high, the medium is clean, the pressure and temperature are moderate. Zero leakage is required for the valve and can be sealed separately. For such large diameter pipes, butterfly valves are usually used. For the leakage requirements, three eccentric butterfly valves can be used. The three-bar valve is a special three-eccentric butterfly valve. The rotating shaft is completely off center, similar to a robotic arm structure. During operation, the valve plate first moves in the opposite direction of the flow of the medium, separating the valve plate from the valve seat, and then flipping the valve plate to overcome the friction between the valve plate and the valve seat. The typical valve is the three-bar valve of Aubinos. The valve has a fireproof function. After the soft seal of the valve seat is destroyed at high temperature, the valve seat and the valve plate form a second seal to ensure that the gas does not leak.
Third, gasification unit
The gasification unit is the most complex and is the core unit of coal chemical industry. The valve used in this unit is very diverse and demanding, and it has a great relationship with the form of the gasifier. At present, the gasification furnace has Texaco, GSP and four-nozzle coal water slurry furnace.
1. Coal slurry and pulverized coal system
Coal slurry is used as a raw material control system. Coal slurry is used as a conveying and flow regulating device. The valve controls the circulation and cutting of the coal slurry. According to the characteristics of coal slurry, it is preferred to be a ball valve, a two-way hard seal fixed ball, and the inner part can be made of 316 or above. It can be used for nitriding and surfacing welding of Taili alloy for hardening treatment;
The control system of pulverized coal as raw material is more complicated, and the wear effect caused by pulverized coal is more serious. Of course, according to the characteristics of 9 types of valves, the ball valve is still the first choice. The two-way hard seal fixed ball, the inner part can be selected from 316 and above. It can be used for nitriding, surfacing stellite alloy for hardening treatment or even spraying tungsten carbide. Here to emphasize a few points: 1, the valve selection, try to ensure the bearing sealing effect, to avoid the pulverized coal into the bearing to cause wear and accelerate the valve damage 2, valve seat butterfly spring material selection, it is recommended to choose high-speed steel such as: SKH-23; 3. When installing the valve, the actuator mounting bracket or hanger should be installed to avoid the impact on the valve stem and bearing. These three points of attention are also applicable to the gasification lock slag valve, and the selection is basically the same as the lock slag valve. Two valve types are recommended for reference here and may work better. 1. Orbital ball valve, also known as elastic ball valve. It should be noted that the track ball valve needs to consider the influence of materials on the bearing and the valve stem and the actuator. Here, the seal needs to be processed; 2. The disk valve is heard to have a certain performance, and the effect is good, but the choice needs to be selected. Two-way sealing also requires consideration of the sealing of the valve stem and actuator.
2, lock slag valve
It is consistent with the ball valve selected by the above pulverized coal system and will not be repeated.
Fourth, synthesis and subsequent other devices
Methanol synthesis and subsequent equipment selection of valves basically follow the basic requirements of valve selection. According to the characteristics of the process, attention and attention should be paid to 1. Temperature, low temperature valve material selection, use 304L or 316L; 2. Corrosive medium such as vulcanization Hydrogen, in addition to 316L, can be high-selection 317L or dual-phase steel material; 3, for corrugated pipe seal type containing medium with strong toxicity or strong stimulation; 4, gas method for producing polypropylene, high frequency valve, switching times At present, only a few products have performed well, and attention needs to be paid in the selection.
In summary, the current valves are basically divided into nine categories: 1, single seat valve 2, double seat valve 3, angle valve 4, diaphragm valve 5, ball valve 6, butterfly valve 7, sleeve valve 8, eccentric rotary valve 9 three-way valve. According to the characteristics of the valve itself, according to the different requirements of the process and the characteristics of the medium, the selection and optimization. Of course, there are many valves that are derived from nine categories. For example, the flapper valve and the disc valve belong to the butterfly valve type, the plug valve belongs to the ball valve type, and the reversing valve belongs to the three-way valve type. In the valve selection, in addition to paying attention to the type and material of the valve, the valve accessories are also very important, the actuator, the positioner and the air decompression original, the valve position feedback original, etc., due to the length of the problem are not described one by one.

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