In order for a coupling to resist internal or external hydrostatic pressure and root intrusion, a minimum contact pressure (P1) is required between the coupling and pipe. To ensure a satisfactory design life, the minimum contact pressure should be used over a 50 year duration. The pressure (P1) is a function of the pressure applied to the elastomeric sleeve by the tension clamps (P2) and the stress relaxation characteristics of the elastomer.
The pressure applied by the clamp is a function of the tension (T) in the clamp band combined with the width of the clamp and the diameter of the pipe. The tension is created by torque (N) which is applied using the tightening mechanism. The required torque is a function of the length of the band between tightening mechanisms, the friction between the band and elastomer and also the efficiency of the tightening mechanism.
There are three types of clamps available depending on the size of the coupling. The first type is a unique design of bolted clamp. The two other types are worm drive clamps. A worm drive clamp uses perforated bands as this design is self clearing if the clamp is coated in mud or sand. In addition all elastomeric sleeves are coated in silicon to reduce the friction between the clamp and the sleeve.
These use heavy duty 'Hi Torque' worm drive clamps irrespective of the assembly torque required. For diameters over 180mm all clamps incorporate double worm drives. This is due to the nature of their application on public sewers, to improve their tolerance range and reliability of seal.
The recommended tightening torque required to give a contact pressure sufficient for satisfactory long term performance is shown in Graph 1 (overleaf). Installation tools are available and recommended from Flexseal.
These use a new unique bolting clamp system that optimise the sealing qualities of the coupling and provide ease of assembly and tightening on site. The design enables the bolt to be set at the perfect angle to the tangent of the coupling which optimises the conversion of torque applied to the tension in the clamping bands. This maximises the sealing pressure for a given torque.
The bolt is coated with a metal based lubricant which reduces the friction within the bolting mechanism and stops the tendency for stainless steel components to seize.
The recommended tightening torque required to give a contact pressure sufficient for satisfactory long term performance is shown in Graph 2 (overleaf).
All bolted clamps have dual bolt mechanisms that provide optimum performance and reliability.