Criticality of Torque and Load in Bolted Joints

“Bolts are really the only way to join big pieces of equipment, which allows you to take it apart to maintain the equipment, but there are many factors that must be considered,” said Gans. “Bolted joints must stay together until a decision is made to take them apart. Regardless of the equipment or situation, the purpose of every industrial bolting job is 100% joint reliability.”

KEYS TO JOINT RELIABILITY

There are three major components necessary to make this happen. The first is to be sure that the finest quality, properly calibrated tools are always used. When used correctly, state-of-the-art equipment will consistently deliver measurable and verifiable results under all conditions.

That leads to the second necessary component: correct use. That means that well trained, qualified and tested workers who have demonstrated knowledge, skill and ability in “best practice” bolting are using those tools. They must also have the capability to self-evaluate results. Gans pointed out that, while welders have to be certified, anybody in a plant can pick up a tool and close a joint. Certification would go a long way toward ensuring that only qualified workers are doing this critical work.

Finally, Gans said, “It’s essential that standards are complied with. There must be demonstrable adherence to process and material standards for the industry, proper records have to be kept, and audit trails must be maintained.”

Summarizing these components, Gans said, “It’s not enough to get it tight. You have to do it right, and you have to have to prove it.”

THE CHALLENGE OF GASKETED FLANGES

While mechanical assemblies, like those in machines, and structural assemblies, like those in bridges and buildings, are also bolted joints, perhaps the most complex and difficult bolting challenges are those in gasketed flanges on pressure vessels and piping. “The integrity of a safe seal depends upon selection of correct components, careful preparation and assembly, and correct bolt tightening and loading,” said Gans.

The goal of any joint assembly on pressurized systems and piping is to obtain a leak-free seal that will be maintained over time despite temperature, pressure, corrosion, vibration or other challenges. That seal must also be easily separated and assembled for re-use.

CHOOSING BOLTS

It is essential when choosing bolts, nuts and even washers for a job to look for the grade or property class of a bolt. Gans advised: “Don’t put a mixture of bolt materials in the same assembly. You will have problems in the future.”

Nuts and bolts are designed so that the bolt will break first by tensile fracture, which is sudden, rather than by thread stripping, and nuts should be matched properly with the bolt.

A BOLT IS A SPRING

According to Gans, a bolt is actually a spring. When installing it, you are in effect stretching the bolt. As it springs back, it squeezes the joint together. The key is, there must be enough spring action to hold the joint together but not too much stretch so that the bolt yields and no longer springs back. This spring action is called the load, or clamping force, and the total load must be greater than the pressures trying to escape. Just tightening the bolts may not be enough to stop a leak.

Gans pointed out that most springs (including bolts and flanges) behave in predictable ways according to Hooke’s law, which states that strain (ΔL/L) is proportional to stretching force (F) and by extension, also to stress (F/A). Therefore, when engineers design a joint, they consider:

“Elasticity.” A material’s ability to revert to its original shape after an applied load has been removed. “Springy” or “stretchy” is not the same as “elastic” (rubber band vs. piano string).
“Stiffness.” A material’s resistance to deflection when subjected to external forces. Opposite is flexibility not “elasticity.” Stiffness is not the same as strength (ceramic vs. mild steel).

TORQUE

When deciding how much torque is right for a particular bolt, consideration must be given to, among other things, bolt condition, diameter and strength, the desired tension, thread pitch and the amount of lubrication.

BOLTING PATTERNS

In addition to torque, the actual pattern of bolting is important, because what is done to one bolt does have an effect on others in a connection. “Consistency of load is more important than how tight the bolts are,” said Gans. He recommended tightening according to patterns including the Plus 4 method approved in ASME PCC-1 for circular patterns, and in the case of irregular or non-circular bolt patterns, to generally move from the center outward and to alternate between opposite sides of the pattern.

There are so many considerations, including enough but not too much tension, and the three component groups of a joint--flanges, fasteners and gaskets--must operate together as a system. The bolting sequence is critical. Each part must be right to obtain a seal, and over-compensating for a problem in one part of a system cannot compensate for a problem in another part of the system. It is absolutely critical that flanges be aligned properly and even though a system can get up and running if there are radial or lateral misalignments, the chance for leaks or problems later on is huge. “Bolts,” said Gans, “are not flange alignment tools and the proper torque is absolutely essential.”

TRAINING

Gans recommended some specific training programs including ASME’s official seminar on “Bolting Principles and Practices per ASME PCC-1-2010” and OSHA’s “Safe Bolting: Principles and Practices” course. Graduates of these courses are granted a Certificate of Training.

ASME is planning the first actual third-party certification in pressure vessel joint assembly only. It is currently awaiting final approval of Appendix A, but it will be renewable every 3 years, like welders. Instruction will be by independent schools and training programs and will require “mechanical tensioning device” training.

While it is not yet a fait accompli, it seems that certification is in fact on the way, providing the all-important “correct use” factor that is critical to joint reliability, Gans concluded.

Jon Gans, Ph.D. is Technical Training Manager at Granite Services, Inc.

Kate Kunkel is Senior Editor of VALVE Magazine. You may reach her at kkunkel@vma.org.