By Vincent A. Carucci
The selection of the proper bolt tightening technique requires experience and good engineering judgment. 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 table summarizes the most commonly used techniques.
A bolt up procedure should be developed for each technique to be used, even the “old reliable” manual wrench and hammer. A separate procedure should be developed for each manufacturer and model of hydraulic or pneumatic tools that may be used.
In concept, a hydraulic torque wrench is simply a conventional wrench which has been modified such that a hydraulic cylinder pushes on the end of the wrench handle.
Hydraulic bolt tensioners employ a high-pressure hydraulic cylinder that attaches to the stud that is to be tensioned and then stretches the stud directly. The nut is then tightened by hand using a short bar or bevel gear arrangement. When the hydraulic pressure is released, the load is then transferred to the nut, which then maintains load on the stud and compression of the gasket.
When a torque wrench is used for flange boltup, it is necessary to have an approximate torque value that must be applied to achieve the required preload stress for standard size bolts. Many unknowns, including the amount and type of lubrication, affect the actual torque needed to obtain the desired bolt stress. The only reliable way to determine bolt stress is to measure bolt elongation during procedure qualification, and then to adjust the torque values as necessary to achieve the desired bolt preload stress.
The majority of flange joints in process plants are assembled based on achieving a 50 ksi (345 MPa) average preload stress using ASTM A193/A193M Grade B7 or B16 bolts. Lower bolt stresses are appropriate for some combinations of flange type, rating, and bolting material. This is necessary to avoid damage to the flange, gaskets, or bolts (e.g., ring joint flanges, flange rating Classes 900 and higher, austenitic stainless steel bolts, etc.).
Selecting the appropriate boltup method and having trained crews using documented boltup procedures will help achieve leak-free flanged joints.