2r Figure 4: The direction of the lateral & longitudinal deformation due to compression force acting by the bolts. π www. valve-world. net Valve World May 2025 45
Butterfly Valves
Forces acting on BFV seal p�t
�b��L� allowed
� �
� E r
• Equation 7 illustrates the amount of sealing rubber engaged with seat.
• The term b * ∆L can be found by using CAD software by drawing the engaged rubber seal with valve seat and measuring the values of b & ∆L in the drawing.
• So, to ensure effective sealing performance [ b * ∆L ]
CAD ≥ [ b * ∆L ] allowed.
• After determination of the values of b
& ∆L, the tightening torque must be calculated to achieve these values and achieve the optimal sealing.
Figure 3: BFV rubber seal engaged with valve seat in closing position.
F P
= Pressure force( N)
F n
= Normal force( N / mm)
F t
= Friction force between rubber seal and seat( N) To ensure no leakage, the friction force must be greater than pressure force acting on the disc seal. Equation 1
Ft
� FP F ����
�D �F t seal n
µ- Coefficient of friction between rubber and valve seat
D Seal
= Rubber seal outer diameter
The normal force due to compression between rubber seal and seat can be explained according to the following equation: Equation 2
F �2�P �r�sin� n max
By substituting equation 4 & 3 in 2: Equation 5
�L Ft
���� �Dseal
�E�b� r
Equation 6 F � p�A p projected
P- Water pressure
A projected
- Seal projected Area to acting water pressure [ A projected
= π * t * D Seal
] By substituting equation 5 & 6 in 1: Equation 7
�L ���
�Dseal �E�b� � p�
A r projected
Rubber seal bolts tightening torque
Once the target expansion of the rubber is known, we need to determine how much axial force is required to produce this deformation. This force is what actually compresses the rubber between the valve body and the disc. The tightening torque applied through bolts must be enough to generate that compression force.
The required torque depends on:
• The size of the bolts
• The friction between threads and under bolt heads
• The friction between bolt head and retainer
• Number of retainer bolts
• Seal thickness
• Seal stiffness
• Contact area between rubber seal and retainer
P max
- Acting pressure between seat surface and rubber R- rubber radius θ- Shown in Figure 3
Equation 3 P max
�L � E� r
∆L- rubber seal radial expansion E- young’ s modulus of rubber
Equation 4 sin� �
b- Shown in Figure 3 b
2r Figure 4: The direction of the lateral & longitudinal deformation due to compression force acting by the bolts. π www. valve-world. net Valve World May 2025 45