Butterfly valves are usually bought by pipe size \u2014 and that is exactly how most of them end up wrong. A valve that matches the flange bolt circle can still be the wrong valve for the duty: too much pressure drop when it should isolate, or seat damage within months when it is asked to throttle. Sizing a butterfly valve properly takes five checks, and none of them requires software.<\/p>\n
Before looking at a catalogue, write down four numbers:<\/p>\n
For isolation duty, a butterfly valve is normally selected at line size: DN100 valve on a DN100 pipe. The disc sits in the flow path even when fully open, so a line-size butterfly valve already imposes some pressure drop \u2014 going smaller than the line adds more.<\/p>\n
The exception is control duty. A butterfly valve that spends its life at 25\u201330% open because it is oversized will regulate poorly and wear its seat. In that case it is legitimate practice to select one size down from the line \u2014 provided the velocity check in Step 3 still passes.<\/p>\n
The flow coefficient Kv states how many m\u00b3\/h of water pass through the valve at a pressure drop of 1 bar. For water service the required Kv is:<\/p>\n
Kv required = Q \u00f7 \u221a\u0394P<\/strong> (Q in m\u00b3\/h, \u0394P in bar)<\/p>\n Example: 90 m\u00b3\/h with an allowable drop of 0.1 bar needs Kv = 90 \u00f7 \u221a0.1 \u2248 285. Compare that against the valve’s published Kv \u2014 and here is the detail most specifications miss: for a regulating valve, compare it against the Kv at roughly 60\u201370% open, not at full open. A butterfly valve controls flow usefully between about 20\u00b0 and 70\u00b0 of disc travel; below about 20\u00b0 open, flow is unstable and the high local velocity across the barely-open seat invites erosion and cavitation<\/a>. If your required Kv only fits at near-closed positions, the valve is too big.<\/p>\n Divide the flow by the pipe cross-section and keep the result within the velocity range your system standard allows \u2014 building-services water lines are commonly designed to stay around 2\u20133 m\/s. Sustained higher velocity through a butterfly valve accelerates seat liner wear and raises the torque the disc must hold against. If a one-size-down control valve pushes velocity well past your design limit, go back to line size and accept the flatter control range.<\/p>\n The rating on the nameplate must cover maximum system pressure plus surge<\/em>. A quarter-turn valve can slam; the transient it creates is part of its own duty. (This is the same mechanism covered in our gate valve vs butterfly valve guide<\/a>, and why a dispositivo anticollasso idraulico<\/a> is often specified nearby.)<\/p>\nStep 3 \u2014 Velocity check<\/h2>\n
Step 4 \u2014 Pressure rating and flange standard<\/h2>\n