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Why might a valve fail to open after being closed for an extended period?
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The key reason a valve fails to open after being closed for an extended period is that, due to the media and environmental influences, the valve's internal components undergo physical or chemical changes during prolonged static operation. This can cause moving parts (such as the valve stem, gate, and ball) to become stuck or locked, preventing them from operating normally.
The specific causes can be categorized into the following four categories, covering core issues such as component corrosion, media influence, and mechanical jamming.
1. "Stuck" of the valve stem and the stuffing box: the most common cause of jamming
The valve stem is the core component that connects the operating mechanism (handle, handwheel) and the valve core (gate, ball, etc.). Its exterior is in close contact with the sealing packing (such as graphite, asbestos, PTFE packing) in the "stuffing box". When closed for a long time, it is easy to jam due to the following reasons:
(1) Rust adhesion: If the valve is not treated with anti-corrosion, or the medium contains moisture or corrosive components (such as tap water, steam condensate), the contact part between the valve stem (mostly metal material) and the packing will oxidize and rust. The formed rust will fill the gap between the valve stem and the packing, "sticking" the two together, causing the valve stem to be unable to rotate or lift.
(2) Aging and hardening of the packing: The sealing packing is under pressure (the packing is compressed when the valve is closed), high temperature or medium corrosion environment for a long time, and will gradually age, lose elasticity and harden, tightly "clamping" the valve stem, forming rigid friction and hindering the movement of the valve stem. (3) Crystallization/deposition of the medium: If the medium contains easily crystallized components (such as salt water, certain chemical solutions) or impurities (such as mud, welding slag), they will be deposited in the gap between the valve stem and the packing over time, and after crystallization, they will form a hard "crust" that will jam the valve stem.
2. "Sealing surface jamming" between the valve core and the valve seat: a problem with the core sealing part of the valve
The sealing surface between the valve core (gate, ball, valve disc, etc.) and the valve seat is the key to controlling the flow of fluid. When closed for a long time, the two fit tightly and are easily "stuck" due to the action of the medium:
(1) Rust/scaling of the sealing surface: The moisture and oxygen in the medium will cause the metal sealing surface (such as copper alloy, stainless steel) to rust, and the rust layer formed will "stick" the sealing surface of the valve core to the valve seat.
If the medium contains calcium or magnesium ions (such as hard water), scale will be deposited on the sealing surface, further strengthening this "sticking", making the valve core unable to separate from the valve seat.
(2) Impurities stuck in the sealing surface: Before the valve is closed, if there are impurities (such as welding slag, mud, and metal debris) remaining in the pipeline, they will be "pressed" into the sealing gap between the valve core and the valve seat by the fluid. After a long period of static, the impurities may react with the medium (such as oxidation), or further "tighten" due to vibration, and jam the valve core like a "wedge", making it unable to move.
(3) Valve core deformation (high temperature conditions): If the valve is used in a high-temperature pipeline (such as a steam pipeline), when it is closed for a long time, the valve core and the valve seat may generate thermal stress due to uneven temperature changes (such as local excessive cooling), resulting in slight deformation of the valve core (such as warping of the gate and change in the ellipticity of the ball). The deformed valve core will form an "interference fit" with the valve seat, forcibly jamming it and making it unable to move.
3. "Lubrication failure" of valve internal components
The moving parts of the valve (such as valve stem threads, gear transmission mechanisms, bearings, etc.) need lubrication (mostly medium self-lubrication or special lubricants). Long-term closure will lead to lubrication failure:
(1) Lubricant loss/drying up: If the valve relies on medium lubrication (such as oil medium), after long-term closure, the medium cannot flow, and the oil film on the surface of the moving parts will gradually lose and evaporate.
If a special lubricant (such as butter) is used, long-term static storage will cause the lubricant to dry up and agglomerate, lose its lubricating effect, and cause the friction resistance of the moving parts to increase sharply, making it impossible to rotate.
(2) Thread pair rust and jamming: The thread pair of the valve stem and the valve cover (such as the lifting thread of the gate valve and the valve stem thread of the stop valve) will rust due to moisture in the air or leakage of the medium if it is not properly protected. The thread gap will be filled with rust, forming a "dead lock", which will cause the valve stem to be unable to be raised or lowered by rotation.
4. "Additional influences" of external environment and installation
In addition to internal valve problems, external factors may also cause the valve to fail to open:
(1) The valve is "fixed" by external forces: During long-term use, debris may accumulate around the valve (such as the fall of pipe insulation, accumulation of construction waste), or the valve and other components (such as brackets, pipes) may become "stuck" due to equipment vibration, forming external resistance and hindering valve operation.
(2) Operating mechanism failure: The valve handle, handwheel, gearbox and other operating mechanisms are exposed to the outside for a long time and may fail due to rust (such as outdoor valves being exposed to rain) or loose components (such as excessive gear meshing clearance or key pin falling off). Even if the internal components are not stuck, the operating mechanism will fail to drive the valve core to move, resulting in "failure" of the valve.