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Engineering
G-17
2-Way Flow Coefcients (Cvs)
Size
Degrees Open
10° 20° 30° 40° 50° 60° 70° 80° 90°
2" 1 7 16 27 43 61 84 114 144
2-1/2" 1.5 11 24 43 67 107 163 223 282
3" 2 15 35 61 96 154 267 364 461
4" 3 27 62 109 171 274 496 701 841
5" 5 43 98 170 268 428 775 1,146 1,376
6" 6 56 129 225 354 567 1,025 1,542 1,850
8" 12 102 241 421 680 1,081 1,862 2,842 3,316
10" 19 162 382 667 1,076 1,710 2,948 4,525 5,430
12" 27 235 555 1,005 1,594 2,563 4,393 6,731 8,077
14" 34 299 756 1,320 2,149 3,384 5,939 9,914 10,538
16" 45 397 1,001 1,749 2,847 4,483 7,867 11,761 13,966
18" 58 507 1,281 2,237 3,643 5,736 10,062 14,496 17,214
20" 72 632 1,595 2,786 4,536 7,144 12,535 18,812 22,339
Table Note:
Flow Coefcients (Cv) = The amount of water in gallons per minute, at 60°F that will pass through a given orice with a one pound
pressure drop.
2-Way and 3-Way Proportional Control Valves
Control Valves are the most important element of a uid
handling system and proper selection of these valves is
crucial for efcient operation of the process. When sizing
buttery valves for control, it is imperative to have certain
requirements of the system.
You must have:
• Maximum ow requirement: This would be equivalent
to the design ow and provided or converted to gallons
per minute.
• Maximum pressure drop allowed: The Consulting
Engineer usually provides this factor and are typically
3 to 5 psi max. However, the pressure drop should
never exceed one half of the inlet pressure.
Without these two factors, selection of a control valve
would be simply a guess.
Introduction
When selecting a buttery valve for water applications
you must rst determine the requirements of the valve
assembly. The rst question to ask is, “Will the valve be
used for “Isolation” or “Proportional Control” of the uid?”
and “Does the application require a 2-way or 3-way
assembly?”
2-Way and 3-Way Isolation Valves
When selecting a valve for isolation purposes, it is seldom
necessary to calculate ow requirements beyond the
published Cvs (ow coefcients)* of the valve. These
valves are typically line size and require the lowest
pressure drop available in the full open position. It may be
possible to supply a valve smaller than the actual line size
and still obtain a low-pressure drop. However, the cost of
reducing anges will typically offset any savings incurred
by reducing the valve size. The 2- and 3-way Flow
Coefcient charts, below and on G-16, provide Cv values
for Siemens buttery valves.
Butterfly Valves
Control Valves
Selection and Sizing