1. Why does a double-seat valve hunt at small openings?
For a single plug, flow-to-open gives good stability and flow-to-close poor stability. A double-seat valve has two plugs — the lower in flow-to-close, the upper in flow-to-open — so at small openings the flow-to-close plug induces vibration. That is why double-seat valves are unsuited to small openings.
2. Why can't a double-seal valve serve as a shut-off valve?
The double-seat plug balances forces and allows high differential pressure, but its outstanding weakness is that two sealing surfaces never contact well simultaneously, causing high leakage. Forcing it into shut-off service — even heavily modified versions such as double-seal cage valves — is inadvisable.
3. Why do linear-stroke valves clog easily while quarter-turn valves resist clogging?
A linear valve throttles vertically while the medium flows in horizontally and out horizontally, so the flow path inside the body must turn and twist into a reversed-S shape full of dead zones where deposits settle until it clogs. A quarter-turn valve throttles horizontally — medium in and out horizontally — carrying dirt away with a simple path and little space for settling.
4. Why are linear-valve stems so slender?
Simple mechanics: sliding friction is large, rolling friction small. A linear stem slides up and down; even slightly tightened packing grips it hard, creating large hysteresis. Hence very slender stems and low-friction PTFE packing to reduce hysteresis — but slender stems bend and packing life is short. The best fix is a rotating stem, i.e. a quarter-turn valve: stems 2–3 times thicker with long-lived graphite packing — stiffer, longer-lived, and actually lower friction torque and hysteresis.
5. Why do quarter-turn valves have a higher shut-off differential?
Because the resultant force of the medium on the plug or disc produces very little moment about the shaft, they can withstand a larger differential.
6. Why do rubber-lined butterfly valves and PTFE-lined diaphragm valves fail quickly on demineralised water?
Demineralised water carries dilute acid or alkali, which corrode rubber badly — swelling, ageing, loss of strength — so rubber-lined valves perform poorly. PTFE-lined diaphragm valves resist corrosion better, but their diaphragms crack under repeated folding — mechanical failure shortening valve life. Today the best choice is a dedicated water-treatment ball valve, good for 5–8 years.
7. Why should shut-off valves use metal seals?
A shut-off valve wants the lowest possible leakage. Soft seals leak least and cut well, but wear badly and are unreliable. Judged by both criteria — low leakage and reliable sealing — soft sealing loses to metal sealing. The full-capability light-duty control valve, for example, carries wear-resistant alloy facing on the seat: reliable, with leakage down to 10⁻⁷, fully meeting shut-off requirements.
8. Why didn't cage valves replace single- and double-seat valves?
Introduced in the 1960s, widely used in the 1970s and common in 1980s petrochemical imports, cage valves were once expected to become the second generation replacing single- and double-seat valves. Not so today: all three are used equally. The cage valve only improved the throttling form, stability and maintainability over the single-seat valve; in weight, anti-clogging and leakage it matches them — so it cannot replace them, and they coexist.
9. Why is selection more important than calculation?
Compared with calculation, sizing and selection matter far more and are far more complex. Calculation is a simple formula whose precision matters less than whether the process data fed in are accurate. Selection involves much more; one careless step causes mis-selection — wasting manpower, materials and money while delivering poor reliability, life and performance.
10. Why are piston actuators increasingly used on pneumatic valves?
A piston actuator exploits supply pressure fully: more compact than a diaphragm for the same duty, greater thrust, and its O-rings are more reliable than a diaphragm — hence its growing use.
