How to choose valves for hydrogen production systems?

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With the increasing emphasis on clean energy, hydrogen, as an efficient and clean energy source, is playing an increasingly important role in the energy mix. As a key component of the hydrogen energy industry chain, every detail of the design and operation of hydrogen production systems is of great importance. Today, we will explore the key considerations for selecting valves in hydrogen production systems. I. Factors to consider when selecting valves The selection of valves is based on operational and safety considerations, as well as economic feasibility, and is the result of a comprehensive comparative analysis. The following factors must be considered before selecting a valve. 1. Properties of materials ① State of the material: a. The state of gaseous materials includes data related to their physical properties, such as whether it is a pure gas or a mixture, whether there are liquid droplets or solid particles present, and whether there are components that tend to condense. b. The material state of liquid materials includes relevant physical property data, whether they are pure components or mixtures, whether they contain volatile components or dissolved gases (which can separate out to form a two-phase flow when pressure decreases), whether they contain solid suspensions, as well as the viscosity, freezing point, or pour point of the liquid, etc. ② Other properties include corrosiveness, toxicity, solubility in the structural materials of valves, and whether it is flammable or explosive. These properties sometimes not only affect the material, but also give rise to special structural requirements or the need for a higher grade. 2. Process operating conditions ① Temperature and pressure under normal operating conditions, as well as special operating conditions such as those during startup/shutdown, regeneration, or coking. a. The outlet valve of the pump should take into account the pump’s maximum shut-off pressure, etc. b. When the regeneration temperature of the system is much higher than normal while the pressure decreases, for this type of system, the combined effect of temperature and pressure must be taken into account. c. The degree of continuity of operation, that is, the frequency at which the valve is opened and closed, also affects the requirements regarding wear resistance; in systems where the valve is opened and closed frequently, it should be considered whether to install a dual-valve system. ② The pressure drop allowed by the system. a. When the system allows for a low pressure drop, valve types with a low pressure drop should be selected, such as gate valves and straight-through ball valves. b. When flow rate adjustment is required, globe valves or similar valves with good regulating performance should be selected. c. Environment where the valve is located: In environments involving flammable and explosive substances, toxic chemicals, or outdoor areas in cold regions, cast iron valve bodies should not be used. 3. Valve functions ① Shutting off: Almost all valves have the function of shutting off flow. When used solely for shutoff without the need for flow regulation, gate valves, ball valves, etc. can be selected ； When a rapid shutoff is required, stopcocks, ball valves, butterfly valves, etc. are more suitable. A globe valve can both regulate flow and be used to shut off flow. Butterfly valves can also be used for regulating large flow rates. ②To change the flow direction, the use of two-way (L-shaped channel) or three-way (T-shaped channel) ball valves or stopcocks allows for rapid adjustment of the material flow. Since one valve can serve the function of multiple straight-through valves, this simplifies operations, ensures accurate switching, and reduces the space required. ③Control: Globe valves and plug valves can meet the requirements for general flow regulation, while needle valves can be used for regulating very small flow rates. For stable regulation of pressure and flow over a wider range of flow rates, throttle valves are the appropriate choice. ④Check valves can be used to prevent the backflow of material. In certain special cases, valves with additional features can be selected, such as valves with jackets, drain ports, and bypasses, as well as valves with purge ports to prevent the settling of solid particles. 4. Power for operating valves: For valves that are operated locally, hand wheels are used in the vast majority of cases. When the installation conditions prevent access with the hand, sprockets or extended rods can be employed. Large-diameter valves should use electric valves, and explosion-proof motors of the appropriate rating should be used in explosion-proof areas. Remote control valves: The power sources used include pneumatic, hydraulic, electric, etc. Among these, electric valves are further divided into solenoid valves and valves driven by electric motors. It should be chosen based on needs and the available energy source. III. Characteristics and Application Areas of Various Types of Valves 1. Gate Valve A gate valve, also known as a gate plate valve, is characterized by the use of a gate plate for opening and closing. Based on the position of the threads on the valve stem, they are divided into hidden-stem and exposed-stem types; based on the structure of the gate, they can be further classified into wedge-type and parallel-type types. The advantages of gate valves are low resistance, slow opening and closing, no water hammer phenomenon, and a wide range of applicable diameters as well as pressure and temperature conditions. Its disadvantages are a relatively complex structure, difficulty in manufacturing and maintenance, high cost, as well as a tall valve body that requires a large amount of space. When a gate valve is partially open, the valve core is prone to vibration; therefore, it is suitable only for fully open or fully closed positions, and not for applications that require flow regulation. The gate valve body has grooves and is not suitable for fluids containing solid particles. In recent years, gate valves with blow ports have become suitable for such situations. Single-gate gate valves can be installed on horizontal or vertical pipelines. Gate valves with actuators and double-gate gate valves without actuators should be installed vertically on horizontal pipelines. 2. Globe valve: The globe valve is a type of valve widely used in chemical plants. Based on their structural type, they are divided into three categories: standard type, streamlined type, and straight-through type. The most commonly used is the standard globe valve. The advantages of globe valves are smooth flow regulation, excellent sealing without leaks, minimal maintenance requirements, resistance to high pressure and temperature, and suitability for various fluids. They are commonly installed at locations where flow regulation is needed, such as at pump outlets, bypass lines of control valves, and upstream of flow meters. Its disadvantages are complex construction, high cost, and significant local resistance as the fluid passes through the valve. Additionally, compared to gate valves of the same diameter, globe valves are larger in size, which limits their maximum diameter (DN150–200). During installation, care should be taken to ensure that the flow direction is consistent with the arrow on the valve body. Non-actuated globe valves can be installed at any position along a pipeline; actuated globe valves should be installed vertically on horizontal pipelines. 3. Plug valves, piston valves, and ball valves have similar functions; they are all valves that can be opened and closed quickly. The valve core has transverse openings, allowing fluid to flow in a straight line; this results in low pressure drop, making it suitable for suspensions or viscous fluids. The valve core can also be designed with L-shaped or T-shaped channels, resulting in three-way and four-way valves. They have a regular shape, making them suitable for use as jacketed valves in applications where heat retention is required. These types of valves can be easily converted into pneumatic or electric valves for remote control. The advantages of a plug valve are its simple design, low cost, fast opening and closing, small footprint, ease of inspection and maintenance, and low fluid resistance when fully open. Its disadvantage is that it cannot precisely regulate the flow rate, and the valve is difficult to operate when the diameter is large. It is mainly used in fluid pipelines carrying fluids with a temperature below 120°C and a pressure of 0.3~1.6 MPa (gauge). Like plug valves, ball valves open and close quickly, are easy to operate, present low fluid resistance, have lightweight components. Their sealing surfaces are easier to machine than those of plug valves, and they are less prone to wear. It is suitable for fluids at low temperatures, high pressures, and high viscosity, but it is not suitable for regulating flow rate. 4. Butterfly valve: It is characterized by a large diameter, light weight, rapid opening and closing, and low operating force. Due to its poor sealing performance, it is only suitable for regulating flow and cannot be used for complete shutdown. Due to the fact that its operating temperature is limited by the sealing material, it is commonly used in large-diameter pipelines for crude oil, water, air, flue gas, etc., where the temperature is below 80°C and the pressure is less than 1 MPa (gauge). 5. Check valve A check valve is a valve used to prevent fluid from flowing in the reverse direction. It is generally used to prevent contamination, temperature rise, or mechanical damage caused by fluid backflow. The commonly used types are swing-type, lift-type, and ball-type. The swing-type type has a larger diameter than the other two types; it can be installed on horizontal, vertical, or inclined pipes. When installed on a vertical pipe, the fluid should flow from bottom to top. Lift-type and ball-type types have smaller diameters and can generally only be installed on horizontal pipes (except for special versions). Check valves can only be used to prevent sudden backflow, but their sealing performance is poor; therefore, for materials for which backflow must be strictly prevented, other measures should also be taken. When the inlet of a centrifugal pump is in suction mode, in order to prevent the liquid inside the pump from flowing out and causing difficulties in suction, the check valve installed at the inlet pipe end also functions as a non-return valve. When the container is open, the bottom valve can be equipped with a filter screen. Check valves are generally suitable for clean media and not suitable for media containing solid particles or high viscosity. 6. Diaphragm valves and clamp valves: With these types of valves, the fluid comes into contact only with the diaphragm or hose, and does not touch any other parts of the valve body. They are particularly suitable for use with corrosive fluids, fluids for which leaks are not permissible, as well as viscous liquids and suspensions. However, their range of application is limited by the material of the diaphragm or hose. III. Connection methods between valves and pipelines There are three connection methods for various valves and chemical pipelines. 1. Threaded connection: Threaded connections have poor sealing properties and are difficult to assemble and disassemble; they are generally suitable for pipelines with small diameters and low pressures. Usually, male threads are machined on the pipe, while female threads are formed at both ends of the valve, allowing them to be fitted together. There are also those with male threads machined on the valve, known as external thread connections. There are several types of threaded connections. One type is tapered pipe threads, which rely on the contact and compression of the threaded surfaces to achieve sealing; this type of connection generally requires appropriate thread fillers, and it is mostly used for non-hazardous media such as water. Another type is cylindrical threads, where the threads only provide the force needed to compress the gasket, with the gasket itself serving as the sealing element. There is also the slip-on connection, which utilizes the deformation of the sleeve created by the threads and the metal of the pipe to achieve sealing. 2. Flange connection: Flange connections are commonly used for large-diameter pipelines, high-pressure pipelines, and those that need to be frequently disassembled and cleaned. In high-temperature pipelines, the flange screws expand due to heat, which reduces the compression force on the gaskets and leads to leaks; therefore, it is necessary to choose appropriate bolt materials and implement re-tightening measures for use in high-temperature conditions. 3. Welded connections: Welded connections are generally used for high-pressure pipelines to ensure safety. However, a welded joint is a permanent connection; if it needs to be disassembled, both ends of the pipes must be cut off and rewelded. 4. Collar connection: It uses a collar connection with the pipeline to prevent leaks, and is made of metal. When the nut is tightened, the collet is subjected to pressure, causing its cutting edges to bite into the outer wall of the pipe; simultaneously, the external conical surface of the collet fits tightly against the internal conical surface of the fitting under this pressure, thereby ensuring reliable leakage prevention.