Product Overview | Butterfly Valves
If valve flow control characteristics are important, valve sizing should take into consideration:
Butterfly valves have no appreciable pressure drop when wide open in full size pipe, handling flows and velocities of normal combustion systems. Therefore, capacity ratings usually are not a factor when selecting manual valves for individual burners or systems.
— total cfh required at high and low fire; — available air pressure drop;
— piping configuration between control valve and burners; — effect of the ratio of pressure drops in the complete system vs. those through the control valve. Where a motorized butterfly control valve serves a group of burners, system pressure drops determined by pipe size, lengths, fittings, etc. affect valve size selection in critical applications. One "rule of thumb" for combustion systems, has been to size valve (70° open) for pressure drop equal to 1/5 - 1/6 of total drop in system including, of course, drop across burners. Change in flow and pressure drop across the valve is negligible when changing from 70° open to wide open unless inlet pressure is increased substantially to maintain the 1"w.c. pressure drop.
VALVE CAPACITIES
Air capacities listed below (wide open and 70° open) are under full flow conditions: The valve installed in a pipe run of the same size as the valve with a pipe velocity of 70 ft/sec.
Butterfly valve capacities (see box above) scfh air at 1"w.c. pressure drop (70°F air; 6 osig upstream pressure)*
(70° Valve C V
Valve
Pipe size
wide open
70°
designation inches
open 255 680 1 130 1 450 2 900 4 570 7 560
open)
-0
¾
500 940
5.9
-1¾
1
15-6
TORQUE. Torques required to operate Butterfly Valves with 3 psi inlet pressure are:
-2 -3
1¼ 1½
1 900 2 800
26 33
Valve size Torque designation lb-in. -0 thru -5 2.8 -6 5.8 -7 10.0 -8 22.8
-4¾
2
5 320
66 105 173 348
-5
2½ 8 350
-6¾ -7¾
3
14 500
4
29 100 15 200
-8¾
6
78 100 45 800
1050
* See page 3 for air temperature correction factors.
Bulletin 1126A Page 2
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