Larger industrial sized bolts are usually tightened in one of two ways when hand torque wrenches cannot provide sufficient power, hydraulic torque tools and hydraulic bolt tensioners. Hydraulic bolt tensioners are ultra-high-pressure cylinders which grip the bolt and stretch it using up to 20,000 psi of pressure. The nut is then manually tightened, and the hydraulic cylinder released, leaving the required residual bolt load. Specially designed hydraulic bolt tensioners are also available for subsea applications.
This type of industrial bolting approach is generally used on more critical applications where residual load is critical. The approach is usually less cost effective as a minimum of 50% consecutive coverage and longer bolts are required. This does however assure against leaks.
What is bolt tensioning?
The best way to understand hydraulic bolt tensioning, is to think of a bolt as a powerful spring. Bolts can be stretched if enough force is applied, and as long as they aren’t stretched beyond the point where the material will yield, they will spring back to their original length.
Tensioning takes advantage of this phenomena by using extremely high-pressure cylinders (up to 20,0000 lbs per square inch) to stretch the bolts. The nut is then hand tightened and the pressure released, allowing the bolt to spring back applying a residual load to the nut and bolt and holding the joint closed tight.
How do Hydraulic Bolt Tensioners Work?
Hydraulic bolt tensioners are basically high-pressure hydraulic cylinders that are screwed onto the studs on a joint, above the nut. The standard hydraulic bolt tensioner consists of two main components, the puller (hydraulic cylinder) and the bridge which straddles the standard nut allowing the nut to be hand tightened, when the stud is under tension.
Normal practice is to fit numerous hydraulic bolt tensioners covering 50% of the fasteners, so 6 x tensioners on a 12-fastener flange. The hydraulic bolt tensioners are then daisy chained together using short hydraulic hoses and then attached to a high-pressure hydraulic pump. The pump pressure is pre-set to give the desired pulling force at the hydraulic bolt tensioners, and ultimately the desired residual load after the pressure is released once the nut has been tightened by hand.
The hydraulic bolt tensioners are then moved to the other 50% of fasteners and the process is repeated. The multiple tensioners are more time consuming however, they enable parallel joint closure, avoiding the expense of re-tightening in the event of a leak during post-joint closure pressure testing.