Analysis on design and selection of shut-off valve for coal water slurry gasification unit
Time: 2019-08-13 22:50:17 Source: Yonglong Valve Reading: zero second
Analysis on design and selection of shut-off valve for coal water slurry gasification unit
Coal water slurry gasification technology is one of the more advanced entrained flow gasification technologies in the world. It will provide key technologies for the development of coal liquefaction, integrated gasification combined cycle power generation (IGCC), and coal based cogeneration technology in China's coal chemical industry. It will greatly promote the development of China's coal chemical industry technology, promote the technological progress of related industries, and in the process of production and operation control, The control valve plays a very important role. For the special working conditions such as high temperature, high pressure, flammability, violent reaction and slag scouring of gasification, appropriate and high-quality oxygen, coal slurry and slag control valves are selected for production operation and interlocking control to achieve safe and stable control of temperature, pressure and flow process operating conditions.
The combustion pressure in the gasifier is 4.0~6.3MPa, and the temperature is 1100~1300 ℃. It is a high-temperature and high-pressure combustion tank and a highly dangerous device. Once an accident occurs, the consequences will be unimaginable, so its safety and reliability are in the front position; Secondly, in order to maintain the normal combustion state, it is required that the injected coal water slurry flow and the mixed oxygen flow must match, otherwise the pulverized coal may not be completely burned or produce useless gas, and explosion may occur when the oxygen is excessive, so the automatic control system is required to have high control accuracy and good stability; Third, the gasifier is a continuous working device, which must have a fairly long service life. Due to the above characteristics of the gasifier device, many technical requirements have been put forward for its auxiliary equipment and related control instrument industry. The control valve directly controls the production medium and must have a special structure suitable for use under gasification conditions. Several typical control valves are introduced as follows:
1. Coal slurry cut-off valve and coal slurry ash water control valve
Coal water slurry is a mixed medium of liquid phase and solid powder phase, and the annular throttle formed between the valve core and valve seat has a high flow rate. It is faced with three problems. The coal slurry reaches a fairly high flow rate at the throttle of the valve. Because the coal slurry has a certain hardness, its erosion destructive power is great;
The coal slurry will also deposit and block the valve chamber, so the flow passage in the valve shall be smooth without dead angle; The coal slurry is very fine and can penetrate into the guide clearance of the valve core, which may cause the valve core to jam.
The coal slurry has a certain adhesive force, which forms scale on the sealing surface of the valve core and valve seat, affecting the leakage level when the valve is fully closed, so the control valve is required to automatically remove the scale.
Therefore, there are three special requirements for the grey water valve: erosion resistance, blockage prevention and self-cleaning. The grey water is almost non corrosive, so the valve body and bonnet can be made of carbon steel, and the basic materials such as valve core and valve seat can be made of ordinary austenitic stainless steel, with a layer of erosion resistant hard alloy on its surface; The adhesion of the scale formed by ash water is less than that of the coking layer, so it is unnecessary to use an actuator with large torque.
According to the above situation, the ash water control valve is generally an erosion resistant eccentric rotary control valve. The valve body cavity of this valve is similar to a cylinder, which is unobstructed without dead corners, and the ash cannot be stored in it. The valve core rotates. In the guide sleeve gap on both sides, it is difficult for ash to penetrate into the interior. Even if scale is formed, the valve core is easily crushed when it rotates due to weak adhesion; In addition, when the valve core is closed, when it touches the sealing surface of the valve seat, due to structural reasons, there is a relative movement between the valve core and the valve seat, which can grind off the scale on the sealing surface, so it has a self-cleaning function. There is another important reason for selecting this control valve. It is small in size, light in weight, and easy to install; The quality of valves with the same caliber is only about one-third of that of angle control valves. In addition, the valve body, valve cover and other parts can be made of carbon steel, and the price is much cheaper. Since the combustion rate of coal slurry in the gasifier is related to the reaction rate of other systems, as well as oxygen supply, cooling, etc., in order to maintain a stable combustion state, it constantly adjusts the flow of coal slurry, so it requires quite high adjustment accuracy, and usually uses the equal percentage flow characteristics.
2. Black water regulating valve
The feed gas and coal ash generated from the combustion of the gasifier are discharged from the lower part respectively. After being cooled by spray water, most of the coal ash is discharged after sinking into the lock hopper; However, the cooling water still contains quite a lot of coal ash, so the cooling water discharged from the gasifier is commonly called black water. It enters the next process after being adjusted by the control valve, and is generally heated to about 250 ℃. At this time, many chemical components in the pulverized coal begin to decompose and react with the water, forming a variety of highly corrosive substances. Therefore, the valve body The valve cover and all parts in contact with the hot coal slurry must be made of corrosion resistant materials, especially the valve body and valve cover. The ordinary austenitic stainless steel cannot be used, but the ferritic austenitic duplex stainless steel. It not only has extremely strong corrosion resistance, but also its mechanical strength is more than twice that of the austenitic stainless steel. According to this technical characteristic requirement, among many structural types, the angle control valve meets the requirements. For the angle control valve with single seat structure, the valve core uses the top guide structure. There is a certain gap between the valve core handle and the guide sleeve. Although it is above the main stream beam, black water will inevitably penetrate into the guide gap. Once it stops, A coking film layer is formed in the gap, with strong adhesion; When restarting, the valve element is often stuck. In order to prevent jamming, a scraping structure is designed at the guide section of the valve core, and the scraped coking powder can be discharged by itself through the escape slot, which means that this control valve also has self-cleaning capacity. Since the removal of coking requires a large thrust, the electro-hydraulic actuator with a large thrust is often used. Its thrust is far greater than that of ordinary pneumatic or electric actuators, and the thrust can be greater than 150kN.
3. Oxygen shut-off valve and oxygen flow control valve
The gasifier burns at high pressure and speed, and a large amount of oxygen enters. In order to ensure the normal combustion state and the safety of the gasifier, the flow of oxygen must be strictly adjusted and controlled, and the oxygen must be cut off quickly and without leakage. Since the requirements for both the adjustment and the cut off functions are very high, three control valves are usually used, namely, the oxygen flow control valve, the oxygen cut off valve Oxygen vent valve, collectively referred to as oxygen valve. Compared with general industrial process control valves, there are several special requirements for oxygen valves for gasifiers:
High pressure difference: the closing pressure difference of the oxygen shut-off valve is high, up to 5.0~7.0MPa. Under such a high pressure difference condition, the leakage level is required to reach Grade VI at the same time. Generally, it is difficult to achieve this in the control valve chamber, and only the O-ring shut-off valve is used. Due to the effect of high pressure difference, the lateral thrust borne by the ball greatly reduces the friction torque between the ball surface and the valve seat. The oxygen flow control valve is mainly used to regulate the flow accurately, which requires it to have good flow characteristics; The second is the cutting off performance when closing. The leakage level is VI, so the sleeve type control valve is generally selected.
Material: High pressure oxygen flows in the valve at high speed, which may cause friction with parts and generate heat, such as carbon steel; Nickel base alloy (Inconel alloy) or ultra-low carbon austenitic stainless steel (such as 316L, 304L) shall be used. In combination with the difference of site conditions, the valve body shall be made of MONEL material, and the valve core and valve seat shall be made of MONEL400/ALLY6 material. In order to prevent the valve stem from scaling due to the constant opening for a long time, the valve stem shall be made of K500 material.
Process requirements: all parts exposed to oxygen must be de watered and de oiled to ensure cleanness. Since the flow rate of high-pressure oxygen in the valve is 20~30m/s, if high-speed friction occurs in case of burr, groove, shoulder, etc., the geometric shape of the inner wall of the flow channel and the valve internals should be smooth transition, and the surface should be smooth, so strict process procedures should be formulated in the design, processing, assembly, test, transportation and other links to ensure that the predetermined technical requirements are met. Therefore, the special requirements of oxygen valve are material and processing.
Due to the explosive characteristics of high-pressure oxygen, the oxygen valve itself is an explosive and dangerous device; To ensure safety, when selecting the nominal pressure of the oxygen valve, two pressure levels should be increased. For example, if the pressure of the gasifier is 6.3 MPa, the nominal pressure of the valve should be PN160 (ANSIClass900).
4. Syngas butterfly valve
The syngas flow at the outlet of the gasifier is large, the action is fast, and the leakage level requirements are high. Therefore, butterfly valves are used for flow control. The actuator is an electro-hydraulic actuator with a fault location. The torque is large, the vibration is small, and the normal service life can reach 1 million times; Concentric structure design of valve body, valve plate and valve shaft, 100% two-way air Class VI seal; Without bracket, it has compact structure, light weight and small volume. Pinless connection, easy to maintain; Backless design ensures no torque increase after valve installation. The butterfly valve is an imported high-performance spray plate, which is more wear-resistant than the ordinary plate, with longer service life and excellent flow characteristics.
The triple eccentric butterfly valve is selected based on the use experience. The structural feature of the triple eccentric butterfly valve is that the conical axis of the sealing surface of the butterfly plate is inclined to the cylindrical axis of the body while the double eccentric valve stem axis is eccentric. After the third eccentric, the sealing section of the butterfly plate is no longer round, but elliptical, and the shape of its sealing surface is also asymmetric, with one side inclined to the centerline of the body, The other side is parallel to the center line of the body. The major feature of the third eccentricity is that it fundamentally changes the seal structure and is no longer a position seal. Instead of relying on the elastic deformation of the valve seat, the torque seal is achieved by completely using the contact surface pressure of the valve seat. Therefore, the problem of metal valve seat leakage is solved at one stroke, and because the contact surface pressure is proportional to the medium pressure, it is also resistant to high pressure and high temperature.
5. Slag locking valve
The process medium of slag lock valve is slag water, with slag content of 20%, corrosive and volatile; The medium of flushing water valve is gray water; The operating frequency is 4 times per hour. Ball valve shall be selected, and the valve body shall be made of carbon steel plus channel surfacing Stellite alloy; The valve rod shall adopt the structural design of blowout prevention rod, made of 1Cr13 forging, plated with Ni, P; the valve ball shall adopt the fixed ball, the sealing surface and the flow channel mouth shall be welded with Stilite alloy, the thickness of the valve ball lip mouth shall be welded with Stellite alloy not less than 12mm, and the ball shall be sprayed or welded with hard alloy as a whole; The valve seat is 316 surfacing Stellite alloy, and the surfacing layer is not less than 1mm after processing. The spring material behind the valve seat is Inconel 750. There must be a sand control structure behind the valve seat to prevent ash from entering the spring and causing spring failure. The valve chamber has an automatic pressure relief function. When the pressure in the valve chamber is higher than the closing pressure after the valve is closed, the pressure can be automatically relieved to the side with low pressure. The packing is made of flexible graphite (with lubrication). The hardness requirements are valve seat HRC52-54 and valve ball HRC58-62. The selection of actuator shall have sufficient margin, and the switching times shall be 1 million without fault; The double cylinder double action electro-hydraulic actuator is adopted, with large torque, high safety and torque protection function, and the valve rod will not be twisted or deformed. The slag lock valve adopts two-way hard seal ball valve, valve ball and valve seat hardening treatment, three-stage fixed ball structure, floating valve seat, which ensures the gasification timing slag discharge and safe interlocking operation.
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