Knowledge on valve selection for coal chemical plants

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Due to the special nature of coal chemical processes, stable and safe operation is of paramount importance, with control valves playing a key role in this regard. As a key component in coal chemical industries, control valves are exposed to extremely harsh operating conditions. They typically handle multiphase flows consisting of gases, liquids, and solids, and operate in environments characterized by high temperatures, high pressures, and high corrosion levels. Next, we will introduce the specific details and current application status of various key control valves in different unit areas of coal chemical industries. First, let’s take a look at the current application status of control valves in the coal chemical industry. The application areas for valves specifically designed for coal chemical industries include the following categories: slurry (powder) transportation, including water-coal slurry (indirect liquefaction) and oil-coal slurry (direct liquefaction), etc ； Soot blowing system ； Wastewater treatment system ； Slag discharge system. The characteristics of the media controlled by valves in the coal chemical industry are as follows: 1. The media temperature is high; the maximum conveying temperature can reach 350℃ ； 2. The gas-liquid-solid three-phase fluid regime is complex ； 3. High operating pressure, with a pressure difference of over 10 MPa, resulting in high flow rates of the medium ； 4. The medium contains strongly corrosive substances such as chloride ions, ammonia, potassium salts, hydrogen sulfide, or phosphoric acid. The current situation of valves used in the coal chemical industry can be described as follows: 1. The critical valves in coal chemical plants rely mainly on imports; due to their high cost and short service life, these imported valves lead to frequent shutdowns. Moreover, since the manufacturers are located far from China, after-sales service is delayed, which becomes a bottleneck for the proper operation of these plants. 2. In today’s coal chemical plants, it is necessary to shut down the plants for maintenance every 3 to 6 months. The key control valves are severely eroded and corroded, preventing them from operating stably over long periods of time; they represent the bottleneck devices that hinder the proper operation of the facility. Next, we will take a closer look at the selection of various control valves in different unit areas of coal chemical industries. I. Boiler Installation Area: In coal chemical projects, the boiler feed water pressure is 16 Mpa. The steam produced by the boilers has a pressure of approximately 9.8 Mpa and a temperature of 540°C; this steam is mainly used to drive turbine engines, while the remaining portion is used in production after undergoing multiple pressure reduction steps. Therefore, in the boiler plant area, feedwater control valves and steam pressure reducing valves are extremely important, and they are also very costly. 1. The boiler feedwater valve is used to precisely control the level of liquid in the drum. The high feedwater pressure (16 Mpa) and high temperature (220°C) subject the valve body and its internal components to severe erosion; improper selection of the valve can easily lead to vibration. A cage sleeve can be used for the valve core structure. The connection between the valve core and the valve stem must be by welding; threads, pins, or bolts with caps are not to be used, in order to prevent the valve stem and valve core from coming apart under high-pressure erosion. The material of the valve body is usually WC6 or CF8M; the trim materials are 304 or 316. The stem material can be 304 or 17-4PH (martensitic stainless steel with extremely high strength). If 304 is selected, a strength margin of 1.5 to 2 times is required. The actuator can be an electric or pneumatic diaphragm-type actuator. Considering the requirements for control safety, it is necessary for the valve to remain open in case of an emergency. 2. Minimum return valve for the boiler feed water pump: Boiler feed water pumps are usually multi-stage pumps with 10 or more stages, resulting in a very large pressure difference between the inlet and outlet. To ensure the safe operation of these pumps, the minimum return valve serves as an important safety control component. Due to a pressure difference of up to 16 Mpa and a temperature of 220°C, there is severe erosion on the valve body and its internal components, making control difficult; in addition, the cost is high. In many cases, sleeve control valves equipped with positioners are used as a substitute. However, controlling them via positioners is difficult; this often leads to vibration and erosion, resulting in a shorter service life. The true minimum reflux valve is a self-acting valve; its valve core must be treated with tungsten carbide spraying, and the valve core as well as the valve body can be cladded with Stellite alloy. 3. Steam pressure reducing valves: Here, focus is given to valves that reduce pressure from 9.8 Mpa to 1.0 Mpa; pressure reducing valves of other pressure levels can generally meet the production requirements in China. For valves designed for pressure reduction from 9.8 MPa to 1.0 MPa, the CCI labyrinth sleeve pressure reducing valve remains the top recommendation worldwide. Although most manufacturers are capable of producing such valves and have some track record of their usage, the key core technologies lie in the structure and manufacturing process of the sleeve itself. CCI uses a labyrinth-type pressure relief sleeve; currently, CCI is still the best at designing and manufacturing this component. However, the connection between the valve stem and the valve plug should also avoid the use of pins and threads; it is preferable to use an integral or welded connection. It should be noted that other imported brands often add flow straightening or noise-reducing orifice plates at the inlet or outlet of the valve; these plates also serve to reduce pressure. However, this design presents significant problems, as such valves are usually connected to the pipes by welding, and if the orifice plates get damaged, it can have fatal consequences for the entire system. Moreover, at low loads, the pressure reduction and noise suppression effects of the orifice plate decline very rapidly, rendering it virtually ineffective; this is also an issue that needs to be taken into account when selecting such a device. II. Air separation unit area: The air separation unit area mainly consists of low-temperature valves, oxygen valves, and molecular sieve regeneration valves (the valves of the units are described separately). 1. Low-temperature valves: These valves are now quite mature, and can be manufactured both domestically and internationally by companies such as Fox, Fisher, Yamatake, and Koso. Such valves feature special packing as well as extended valve stems. The materials for the valve body and internal components are mainly 316. 2. Oxygen vent valve: Used for emergency venting in case of overpressure of oxygen in the air separation unit. It is characterized by a large pressure difference, fast response time, and a large venting volume (depending on the designed capacity for gasification). Due to the properties of oxygen, for ordinary non-ferrous metals. In principle, the line speed should not exceed 25 meters per second (this depends on both the oxygen purity and pressure; here a typical value is used). Therefore, for oxygen relief valves operating under high pressure differences, materials such as Inconel 625 or Menel 500 and above are generally chosen. Since both the valve body and internal components are made from this special material, the cost is very high. Valve types can include globe valves, sleeve valves, and ball valves; ball valves are generally not used (except for valves intended for shut-off purposes), and this is mainly due to cost factors. At the same time, Globe valves and sleeve valves also exhibit excellent performance. Due to the large pressure difference, a significant amount of force is required from the actuator when the valve is opened and closed; as a result, the actuator is often very large, the valve stem is thick, and the operation time is long. 3. The three-bar valve used in the molecular sieve purification system is chosen based on the characteristics of the molecular sieve regeneration process: the pipes are usually quite large in diameter (generally over 900 mm). Up to 1500 mm in size), with a high regeneration frequency, relatively clean media, and moderate pressure and temperature. The valve is required to have zero leakage and can be sealed individually. For such large-diameter pipes, butterfly valves are typically used; for stricter leakage requirements, triple-eccentric butterfly valves can be chosen. A three-bar valve is a special type of butterfly valve with a triple-eccentric design; its rotating shaft is completely offset from the center, resembling the structure of a robotic arm. During operation, the valve disc first moves in the opposite direction to the flow of the medium, causing it to separate from the valve seat; thereafter, the valve disc is flipped, overcoming the friction between it and the valve seat. III. Gasification Unit The gasification unit is the most complex component and the core device in coal chemical processing. It requires a wide variety of valves, with very strict specifications, and its design is also closely related to the type of gasifier used. 1. Slurry and coal powder system: A control system that uses slurry as a raw material; slurry is used as a medium for transportation and flow regulation, with valves serving to control the circulation of slurry and to shut it off. Given the characteristics of coal slurry, ball valves are the preferred choice – specifically two-way hard-sealed fixed ball valves. The internal components can be made of material 316 or higher grade, and can be hardened through nitriding or surfacing with Stellite alloy ； The control system for systems using coal powder as a raw material is relatively complex. Moreover, the wear caused by coal powder is quite severe. Of course, considering the characteristics of the 9 main types of valves, ball valves remain the preferred choice. They are double-sealed fixed-ball valves, and their internal components can be made of materials such as 316 or higher grades; these components can also be hardened through nitriding, surfacing with Stellite alloy, or even coated with tungsten carbide. Several points need to be emphasized here: 1. When selecting bearings for valves, it is important to ensure good sealing of these bearings, in order to prevent coal dust from entering them and causing wear that could accelerate valve damage ； 2. For the material of the butterfly spring in the valve seat, it is recommended to use high-speed steel such as SKH-23 ； 3. When installing the valve, install brackets or hangers for the actuator to prevent excessive weight from affecting the valve stem and bearings. These three points also apply to the gasification slag locking valve, and the selection criteria are basically the same as those for the slag locking valve. Two types of valves are recommended here for reference; they may yield better results. 1. Orbital ball valve, also known as elastic ball ball valve. It is important to note that for ball valves used in rail systems, it is necessary to take into account the impact of the material on the bearings, valve stem, and actuator; sealing measures need to be implemented in this regard ； 2. Disc valves have a proven track record of use and perform well, but it is necessary to choose ones with dual sealing; attention also needs to be paid to the sealing of the valve stem and actuator. IV. Synthesis and Other Associated Equipment: The selection of valves for methanol synthesis and other related equipment follows the basic requirements for valve selection. Given the characteristics of the process, the following points require attention: 1. Temperature – when selecting materials for valves used in low-temperature environments, 304L or 316L should be used ； 2. For corrosive media containing hydrogen sulfide, in addition to 316L, 317L or duplex steel can be preferentially chosen ； 3. For media with high toxicity or strong irritancy, a bellows-sealed type can be used ； 4. For the production of polypropylene using the gas method, high-frequency valves are required, as they need to be opened and closed frequently; this aspect needs to be taken into consideration when selecting such valves.