CRANE NUCLEAR, INC. REQUIRED TORQUE CALCULATION METHODOLOGY
CRANE has performed a complete evaluation of the Flowseal High-Performance Butterfly valve torque requirements including analysis of both total seating/unseating torque and total dynamic torque, with the minimum required operator torque being the larger of the two values.
To determine the maximum total dynamic torque, calculations were performed at various disc opening angles.
I. Total Seating/Unseating Torque:
The total required seating and unseating torque (Tt), in in-Ibs, is calculated as follows:
Tt = Ts + Tb + To + Tp
where: Ts = Seat torque
Tb = Bearing torque
To = Disc Offset Torque
Tp = Packing torque
A. Seat Torque, Ts
The seat torque accounts for both interference-related and differential pressure-related effects. Based on the Flowseal seat design, and when actual test data is not available, using a Seating Factor of 20 has been found to conservatively bound(l) the required torque to account for seat material, seat contact width, thickness, and the service application.
B. Bearing Torque, Tb
The shaft (stem) bearings support the differential pressure forces imposed on the butterfly valve disc (static force when the disc is fully closed and dynamic force when the disc is partially or fully open). As the valve is stroked, the combination of bearing force and bearing friction results in a bearing friction torque. At intermediate disc positions, the bearing torque is calculated using the total pressure drop across the valve, even under choked flow conditions. The bearing coefficient of friction (µ) is typically bounded by 0.25(1) for metal bearings, 0.1 for fiberglass backed TFE. For conservatism, the use of µ = 0.6 for raw water service may be used.I. Total Seating/Unseating Torque (con’t):
C. Disc Offset Torque, To
The disc offset torque is the product of the resultant differential pressure acting on the valve disc and its moment arm due to the shaft centerline offset from the disc centerline. For Flowseal butterfly valves, this offset is 0.062 inches.
D. Packing Torque, Tp
The packing torque accounts for the packing drag load on the shaft as it rotates to open or close the valve disc. The total packing torque is dependent on packing material, packing length, sectional area, shaft diameter, and system pressures. For graphitic packing materials, Crane utilizes a packing coefficient of friction of 0.2, and a packing stress of 2,000 psi, as recommended by EPRI(1). These values have been found to result in conservative calculated required torque requirements compared to the actual output torques determined from flow tests(2.3).
II. Total Dynamic Torque:
The maximum total dynamic torque is evaluated at various disc opening angles in both opening and closing directions. At any disc angle, the total dynamic torque, Td is given by:
Td = Tb + Tp + Thyd
where: Tb = Bearing Torque
Tp = Packing Torque
Thyd = Hydrodynamic torque
The bearing torque, Tb, and the packing torque, Tp, are frictional torque components which always oppose stem rotation. These components are calculated the same as described above for seating/unseating torque. Unlike frictional torque components, the hydrodynamic torque component acts in the same direction regardless of the direction of the disc rotation. Therefore, this torque component resists the disc motion in one direction and assists it in the opposite direction.
Note: In the Crane calculation methodology, hydrodynamic torque is defined as the fluid dynamic torque imposed on the disc by both incompressible and compressible fluids.
Due to the lower torque required to open or close the Crane-Flowseal High-Performance valve (owing to its double offset disc design), the preferred direction for the valve orientation is with the shaft downstream, or the seat upstream (SUS). There may be cases (such as where the valve is configured with a spring return pneumatic operator) when the preferred direction may be shaft upstream, in order to take advantage of the flow promoting disc closure.
The hydrodynamic torque coefficient (Ct value) used to determine the hydrodynamic torque at a given disc angle was determined by actual flow testing (1,2), and the calculation methodology was based on EPRI guidelines.
The required torque values are calculated based on given design conditions for both normal operating and upset conditions. In both cases, the seating/unseating and hydrodynamic torques were calculated, and the higher values with the required margins (as specified) would be used to determine operator sizing and the recommended valve orientation (shaft upstream or downstream).
See the following Exhibits for Required Torque Calculations for the Proposed Valves.
(1) “Flow Testing of One 6”-150# Motor Operated Butterfly Valve And One 12”-150# Motor Operated Butterfly Valve” for Crane Nuclear Operations, Wyle Laboratories Report No. 44696-0, August 1995.
(2) “Evaluation of Flowseal Double Offset Butterfly Valve Test Data”, MPR Associates, Inc. Summary Report No. MPR-1668, September 1995.REQUIRED SEATING TORQUE CALCULATION (14” -150# Double-Offset HPBFV)
at 100 Psig Differential Pressure
Total Seat Torque (Tt) = Ts +Tb + To + Tp
I. Seat Torque Coefficient ( Ts );
where: A = seating factor = 20
Ts = AD2
Ts = 20 (13.077)2
Ts = 3,420 in-Ibs
II. Bearing Torque ( Tb );
Tb = µ [ ( π / 4 ) D2 ] ( d/2 ) ΔP
where: µ = friction coefficient = 0.25*
= 0.25 [ ( π / 4 ) ( 13.077 )2 ] ( 1.50 / 2 ) ( 100 )
= 2,518 in-Ibs
D = disc diameter = 13.077”
d = stem diameter = 1.50”
ΔP = diff. pressure = 100 psig
III. Disc Offset Torque ( To ); where: D = disc diameter = 13.077”
To = ( π / 4 ) D2 ΔP E
= ( π / 4 ) (13.077)2 (100) (0.063)
= 846 in-Ibs
ΔP = diff. pressure = 100 psig
E = disc offset = 0.063”
IV. Packing torque (Tp);
Tp = f y σ π d L r
= (0.2) (0.5) (2000) π (1.50) (0.94) (0.75)
= 664 in-Ibs
where: f = friction coefficient = 0.2
y = radial-axial stress ratio = 0.5
σ = packing stress = 2000 psig
d = stem diameter = 1.50”
L = packing length = 0.94”
r = stem radius = 0.75”
TOTAL REQUIRED SEATING TORQUE = 7,448 in-Ibs. (621 ft-Ibs.)
“Note: Friction Coefficient of 0.25 based on Bronze Bearing material.