Valve stem sealing
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When the valve stem is made of stainless steel, especially materials of the Cr13 series, the surface in contact with the packing is often subject to corrosion. This corrosion occurs because the packing becomes saturated with water after the hydrostatic test. Corrosion occurs during the storage phase before use, but it will not occur if it is put into service immediately after the hydrostatic test. My question is: Generally, valves are subjected to sealing (water pressure) tests before leaving the factory, right? But during transportation and installation, and by the time of commissioning and operation, wouldn’t the valve stem have corroded by then? I’m not sure what the actual situation is; do anyone have any control measures in place? This post was last edited by flybird524 on 2009-4-2 16:51]
Generally, valves are subjected to a sealing (water pressure) test before leaving the factory, right? That’s what should happen. However, the valves purchased are tested under water pressure by the buyer before installation, so that those that fail the test can be sent back to the manufacturer. If the valves you purchase pass the water pressure test but are not used and instead stored, then have you carried out any drying treatment on those valves? :handshake
Cr is hard and brittle, with strong corrosion resistance. Moreover, after pressure testing, the water is dried using compressed air or wiped dry – so how can there be corrosion? I’m not sure what the original poster means
Theoretically, stainless steel located within moist packing corrodes because the surface of the valve stem surrounded by the packing is in a deoxygenated environment. This environment affects the activation and passivation properties of metals. Many small anodes formed at the oxygen-deficient sites on the oxide protective layer of stainless steel; together with the large amount of residual passive metal that underwent anodic reactions, these anodes created a galvanic cell effect within the metal. Graphite, which is commonly used in fillers, acts as an anode material; the cathodic field generated by the valve stem steel enhances the current strength of the galvanic cell. Thus, **it exacerbated the corrosion at the original corrosion site. What do you think?
The hydrostatic testing pressure for valves should be determined based on actual operational requirements. For cryogenic liquid throttling valves used in deep cooling applications, hydrostatic testing is not advisable. The length of the packing chamber is approximately one meter, and it is difficult to prevent water from accumulating within this chamber during hydrostatic testing. Once water accumulates there, it is hard to remove it even when the area dries out. Once such valves are put into use, not only corrosion can occur, but the low temperatures can also cause the valves to freeze, resulting in the valve stem getting stuck and rendering the valve unusable. Therefore, pressure testing requires differentiation based on operating conditions. Using a pressure 1.15 times higher than the normal operating pressure also complies with relevant regulations.
All I know is that after pressure testing in the garment factory, rust-proof oil is sprayed, and all valve stems are made of 316 material! Oil is applied when adding fillers; I’m not sure what the original poster means – I’ve never encountered that before!