A comprehensive guide to single-seat control valves!

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The body of a single-seat control valve contains a valve spool and a valve seat, and it features low leakage. This valve has a large unbalanced force, and its allowable pressure difference is smaller than that of double-seat valves; therefore, a valve positioner is recommended when high pressure differences and large diameters are involved. Valves with a nominal diameter of ≥25 mm have a dual-guidance structure; by changing the connection position between the valve stem and the valve core, it is possible to achieve operation in air-open or air-close mode. Working principle of electric single-seat control valve: The electric single-seat control valve features an electronic integrated design with a feedback function. It receives analog signals of current or voltage (4-20mA or 1-5V·DC) from the control system, converts these current signals into corresponding linear displacements, and automatically controls the degree of opening of the valve. This enables continuous regulation of process parameters such as pressure, flow rate, liquid level, and temperature, allowing the controlled parameters to be maintained at set values and facilitating remote automatic control. It features a wide adjustment range, with an inherent adjustable ratio of 50, and flow characteristics that can be linear or equal percentage. Electric single-seat control valves have low leakage and high unbalanced force, but their allowable pressure difference is smaller than that of double-seat valves; for high pressure differences and large diameters, sleeve control valves are the better choice. Maintenance of electric single-seat control valves: 1. Cleaning and inspection of the valve – The electric single-seat control valve removed from the process pipeline must have all its components that have been in contact with the process medium cleaned thoroughly. To prevent certain corrosive or otherwise harmful fluids from causing damage to people and equipment. At the same time, rust on the exposed surfaces of the components should be removed. 2. Valve disassembly is carried out to inspect all components in order to determine the scope of repair and replacement. First, the actuator and the electric single-seat control valve must be completely disassembled. During disassembly, it is necessary to protect precisely machined parts such as valve cores, valve seats, valve stems, push rods, and bushings, as well as the precisely machined surfaces of all components, to prevent damage and thus reduce maintenance costs. Special tools should be used to remove the valve seats. 3. Inspection and maintenance of key components: If there are minor rust spots or wear on the throttling surface of the spool in electric single-seat control valves, as well as on the sealing surfaces between the spool and the seat. It can still be repaired using ordinary machining and grinding methods. If the damage is severe, new parts must be replaced. During repair, it is necessary to ensure good coaxiality between the valve core and the valve seat. Damage to the sealing surface of the valve stem can only be repaired by replacing it with a new part. Damage to the guide and sealing surfaces of the push rod. New parts must be replaced for the reaction actuator. As for the direct-acting actuator, appropriate repairs can still be made. During maintenance of compression springs, any defects that affect strength, such as cracks, should be identified. Then it must be replaced with a new one. 4. Replacement of wear parts: The main wear parts of electric single-seat control valves include packing, O-rings, gaskets, diaphragms, and other components. During each maintenance session, the gaskets, O-rings, and washers that are removed must be replaced with new ones. After removing the diaphragm, it is necessary to check for any cracks that could indicate a potential rupture, as well as signs of aging and wear; then a decision is made regarding whether to replace it or not, depending on the specific circumstances. Generally, it must be replaced every 2 to 3 years at most. 5. Final assembly and debugging: During final assembly, appropriate lubricant should be applied to the positioning areas, guiding parts, and threaded connections of the components, to facilitate disassembly during subsequent maintenance. Special attention should also be paid to the coaxiality of the push rods, valve spool components, and valve seats within the entire unit. After completion of assembly and commissioning, the electric single-seat control valve must pass the product exit test items specified in the standards before it can be further installed and used. Maintenance of electric single-seat control valves: 1. Regularly check the pipes for rust, weld slag, dirt, and dust. 2. Regularly inspect the supports, especially those in applications with heavy self-weight and vibration. 3. The power supply must not have any faults. 4. The sealing gasket at the connection between the valve body and the upper valve cover should be replaced in a timely manner after prolonged use; to replace it, simply remove the upper valve cover and install a new sealing gasket. 5. If there is a leak in the stuffing box, the sealing packing should be replaced promptly. To do this, disconnect the valve stem, remove the pressure plate and packing gland, and then install new packing. 6. Regular maintenance. 7. When parked for an extended period, a protective cover with connecting flanges should be installed; therefore, all interfaces must be plugged with plastic plugs. When parked, ensure that the valve is in a vertical or horizontal position as a whole. 8. Do not support the valve at sensitive locations such as valve positioners, connectors, wires, etc.