Several common questions about control valves

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1. In terms of cut-off performance alone, soft-sealed valves outperform hard-sealed valves. However, soft-sealed valves have much lower wear resistance and reliability compared to hard-sealed valves; therefore, based on both leakage rates and reliability, hard-sealed isolation valves are superior to soft-sealed isolation valves. Many cut-off valves these days use surfacing with wear-resistant alloys, **which improves their sealing performance and is sufficient to meet the requirements of cut-off valves. 2. Can a two-seat control valve not be used as a shut-off valve? The spool of a two-seat control valve benefits from a force-balancing structure, which allows for a wide range of pressure differences before and after the valve. However, since the two sealing surfaces cannot make good contact at the same time, leakage is high, and such valves cannot serve as shut-off devices in pipelines. Even though sleeves have been added to two-seat valves to improve their performance, this does not change the limitations inherent in their structural design. 3. Comparison of clogging resistance between linear stroke control valves and rotary stroke valves? In a straight-stroke control valve, the valve spool moves vertically while the fluid moves horizontally. The flow path within the valve chamber is not straight but usually S-shaped, which creates numerous dead zones. Over time, the medium tends to settle in these dead zones, leading to blockages. In angle-seat valves, the valve plug moves in a horizontal rotational motion, and the fluid also flows horizontally; as a result, there are few dead zones, and sediment can be easily carried away by the flow. Therefore, straight-stroke control valves have much better anti-clogging performance than angle-stroke valves. 4. Why is the stem of a straight-stroke control valve relatively thin? Due to the principle of high sliding friction and low rolling friction. The valve stem of a straight-stroke valve moves up and down, which generates significant friction. Therefore, designing the valve stem to be thin and using PTFE packing with a low friction coefficient are both aimed at minimizing friction as much as possible. However, this leads to problems such as thin valve stems that are prone to bending, and a short lifespan for the packing. 5. Do angle-stroke valves have a large shut-off pressure difference? Since the torque exerted on the valve shaft by the resultant force generated by the medium acting on the valve core is very small, such valves can withstand large pressure differences; therefore, angle-type valves have a high cut-off pressure difference. 6. Why do double-seat control valves tend to oscillate when operating at a low opening degree? The reason why two-seat control valves cannot be used at low opening degrees is determined by their valve seat structure. A dual-control valve has two spools: the upper spool is in the flow-open position while the lower spool is in the flow-closed position. When the spools of a control valve are in the flow-open position, its stability is better; therefore, when a dual-seat control valve operates within a small opening range where the spools are near the flow-closed state, oscillations occur in the spools. Therefore, when selecting control valves, it is essential to avoid using double-seat control valves in the low-open-degree range.