To obtain a leak-free flange connection, a proper gasket installation is needed, the bolts must be assign on the correct bolt tension, and the total bolt strength must be evenly divided over the whole flange face.
With Torque Tightening (the application of preload to a fastener by the turning of the fastener's nut) the correct bolt tension can be realized.
Correct tightening of a bolt means making the best use of the bolt's elastic properties. To work well, a bolt must behave just like a spring. In operation, the tightening process exerts an axial pre-load tension on the bolt. This tension load is of course equal and opposite to the compression force applied on the assembled components. It can be referred to as the "tightening load" or "tension load"
Torque Wrench is the general name for a hand-guided screwing tool, and used to precisely set the force of a fastening such as a nut or bolt. It allows the operator to measure the rotational force (torque) applied to the bolt so it can be matched to the specifications.
The selection of the proper flange bolt tigtening technique requires experience. The successful application of any technique also requires qualification of both the tools that will be used and the crew who will do the work. The following summarizes the most commonly used flange bolt tightening techniques.
Torque loss is inherent in any bolted joint. The combined effects of bolt relaxation, (approximately 10% during the first 24 hours after installation), gasket creep, vibration in the system, thermal expansion and elastic interaction during bolt tightening contribute to torque loss. When torque loss reaches an extreme, the internal pressure exceeds the compressive force holding the gasket in place and a leak or blow-out occurs.
A key to reducing these effects is proper gasket installation. By bringing the flanges together slowly and parallel when installing a gasket and taking a minimum of four bolt tightening passes, following the correct bolt tightening sequence, there is a payoff in reduced maintenance costs and increased safety.
Proper gasket thickness is also important. The thicker the gasket, the higher the gasket creep which in turn can result in torque loss. On standard ASME raised face flanges a 1.6 mm thick gasket is normally recommended. Thinner gasket materials can take a higher gasket load and therefore higher internal pressures.
Lubrication reduces the friction during tightening, decreases bolt failure during installation and increases bolt life. Variation in friction coefficients affect the amount of preload achieved at a specific torque. Higher friction results in less conversion of torque to preload. The value for the friction coefficient provided by the lubricant manufacturer must be known to accurately establish the required torque value.
Lubricant or anti-seizure compounds should be applied to both the nut bearing surface and the male threads.
The first pass, lightly tighten the first bolt then move directly across or 180 degrees for the second bolt, then move1/4 turn around the circle or 90 degrees for the third bolt and directly across for the fourth. Continue this sequence until all bolts are tightened.
When tightening a four-bolt flange, use a criss-cross pattern.
In a flanged connection, all components must be correct to achieve a seal. The most common cause of leaky gasketed joints is improper installation procedures.
Before beginning a bolting process, the following preliminary steps will avoid future problems..
Irrespective which method of tightening is used, above determined checks and preparations always must be done.
Remark(s) of the Author...
My own experiences about...Torque Wrenches
Improper flange connections
the bolts are too short!
What can you do..
What is the proper length of a Stud Bolt..
As a rule, you can use.. The free threads of the bolt above the top of the nut is equals to 1/3 times the bolt diameter.