Bray 40_41 - Manual (Page 13)

Bray/McCannalok HPBV – Dynamic Torques
All information herein is proprietary and confidential and may not be copied or reproduced without the expressed written consent of BRAY INTERNATIONAL, Inc.
The technical data herein is for general information only. Product suitability should be based solely upon customer’s detailed knowledge and experience with their application. Dynamic : 13
1. Subchoked Liquid Flow, Line-size Valve.
Example Calculations: Data:
A 24” class 150 McCannalok is to be installed in a 24”
line carrying water. It has been determined that the flow
is not choked and the following pressure drops through
the valve have been calculated:
• 75 psi with the valve 20 degrees open
• 70 psi with the valve 30 degrees open
• 35 psi with the valve 40 degrees open
• 3.5 psi with the valve 55 degrees open
• 1.1 psi with the valve 60 degrees open
• .20 psi with the valve 70 degrees open
• .05 psi with the valve 80 degrees open
• .03 psi with the valve 90 degrees open
Estimate the maximum dynamic torque in Lb-inches for the valve
considering installation with the seat retainer upstream. Td = Ct D³  p
Substituting produces following results: • 20 degrees, Td = .020 x 24³ x 75 = 20,700 Lb-inches,
(tends to close the valve)
• 30 degrees, Td = .027 x 24³ x 70 = 26,100 Lb-inches
• 40 degrees, Td = .046 x 24³ x 35 = 22,300 Lb-inches
• 55 degrees, Td = .078 x 24³ x 3.5 = 3,800 Lb-inches
• 60 degrees, Td = .086 x 24³ x 1.1 = 1,300 Lb-inches
• 70 degrees, Td = .100 x 24³ x .20 = 300 Lb-inches
• 80 degrees, Td = .032 x 24³ x .05 = 20 Lb-inches
• 90 degrees, Td = -.369 x 24³ x .03 = -150 Lb-inches,
(tends to open the valve)
Thus the peak dynamic torque will occur between 30 and 40 degrees open.
Verify dynamic torque at 35 degrees:
Approximate pressure drop = (70+35)/2 = 52.5 psi.
At 35 degrees: Td = .037 x 24³ x 52.5 = 27,000 Lb-inches
The peak dynamic torque of approximately 27,000 Lb-inches occurs at about 35 degrees open.
When sizing the valve operator, total torque must be considered. The total torque when the disc is in the seat con-
sists of seating torque, stem packing torque, eccentricity torque and stem bearing torque. The total torque when
the disc is in the seat is published as seating/unseating torque (see pages 1-6. When the disc is out of the seat, total
torque consists of dynamic torque, stem packing torque, and stem bearing torque.
2. Choked Gas Flow, Reduced-size Valve.
Example Calculations:
20” (DN 500) class 300 McCannalok butterfly valve is installed in a 600 mm (24”) line flowing air and the disc is
set at 45 degrees. The flow is choked. A pressure gauge tapped into the pipe just above the upstream reducer shows
24.5 bar.
Estimate the dynamic torque in N-m, given that the seat
retainer is installed downstream. D = 500 mm
Fk = 1.0
Ct = .114
Y = .667 for choked flow
p1 = 24.5 + 1.0 = 25.5 bar absolute, (1 atm. = 1.013
bar, i.e. about 1.0 bar)
xt at 45º = .35
Calculations:
 p = p1 Fk xt Td = .0001 Ct D³ Y  p
 p = 25.5 x 1.0 x .35 = 8.9 bar Td = .0001 x .114 x 500³ x .667 x 8.9 = 8,500 N-m
Thus the dynamic torque at 45º open is about 8,500 N-m, and acts to close the valve.