Butterfly Valves
Care must be taken to not exceed the torque limit. Exceeding this value can permanently deform the rubber or even damage the seat during assembly or operation. Using lubricated bolts or antiseize compounds may also influence the torque required, since frictional resistance is reduced. From equation 4, we can calculate the value of sealing rubber deflection( ∆L) due to bolts tightening. So, lateral strain can be calculated in the following equation:
� �
�L L
L – Rubber seal profile length before deformation( shown in Figure 5).
Poisson ratio created a relation between lateral deflection and longitudinal deflection:
� � � �
�
� � � � ω – Poisson ratio δ – Lateral strain ε – Longitudinal strain
�L L
Equation 8 �
E �
�
� � E ��
σ – Compression stress effected by retainer on the rubber seal( N / mm 2) E – Young’ s modulus( N / mm 2)
The retainer maintains a fixed force acting on the rubber seal to fix the rubber on the valve disc: Equation 9
Fretainer
� �
A contact
F retainer
- Normal force due to bolt tightening
( N)
A contact
- Contact area between retainer and rubber seal( mm 2) From equations 8 & 9:
F � E�� � A retainer contact
Therefore, acting force per bolt can be calculated by the following equation:
F Fa
�
NO.
retainer bolts
F a
- Preload acting per bolt to achieve tightening between retainer and rubber seal( N).
Figure 5: BFV disc seal cross section
Now, tightening torque per bolt can be calculated as following: Equation 10
T �T �T
1 2
T- Total tightening torque( N. mm)
T 1
- Friction torque between bolt and threads( N. mm)
T 2
- Friction torque between bolt head and retainer( N. mm)
Equation 11 T � F �r
1 a m 1 1
��
� tan� ����tan� r m1
- Bolt mean radius( mm) μ '- Inclined friction coefficient between threads
Figure 6: Comparison between experimental seal expansion and calculated seal expansion at the same tightening torque values for different BFV sizes.
46 Valve World May 2025 www. valve-world. net