 | Given
the ever more challenging environmental, legislative and economic demands facing plant operators, while always striving to ensure maximum operational efficiency, joint integrity must be seen as a crucial element of any maintenance program. Effective management of critical pressure-containing joints can reap rewards in terms of cost-savings and operational efficiency, not to mention removing risk.
In Arita, Wakayama Japan a leakage and fire of naphtha and hydrogen was attributed to
uneven tightening torque of five to twenty-five bolts at the inlet flange of the reactor, where a "creep" phenomenon caused them to loosen.
Numerous incidents of leakage are recorded by the International Occupational Safety and Health Information Centre (CISDOC) and published as a Hazardous Installations Directorate to assist inspectors in assessing and inspecting accidents.
The purpose of this story is to bring awareness to practical and innovative methods that can
be used to limit the high cost of failures and make working conditions safer when using threaded fasteners as a means of joining. It is also intended to provide a basic background in the technology of threaded fastening and auditing to ensure understanding of their function.
Threaded fasteners - purpose and advantages
The threaded fastener is a prime joining system, recognized for its cost effectiveness and wide practical application. The combination of cost and the
ability of joining and disassembly make it one of the most useful methods for many applications, especially those that require ongoing maintenance. Threaded fasteners are capable of joining dissimilar material in uniform or unusual joint configurations. Fasteners, though simple in appearance, are highly sophisticated mechanical products subject to rigid standardization. They are highly interchangeable due to the range of sizes, lengths and materials.
The ability to
assemble a product that uses male or female joining elements allow multiple components that expands product design options. The differing material options (stainless, brass, plastic and exotics) further expand the options for making unique products.
The primary objective of a fastener is as a structural or load-carrying element - a device that holds, clamps and ensures that two or more parts stay together.
What Is A Joint?
A joint is all of the components
that make up a joined assembly, including the fastener, washers, gaskets and all mating material and parts that are to be held in tension.
Joint design takes into consideration where and how fasteners are to be used. The term joint comes from the concept of joining parts together, which is the purpose of a threaded fastener. This document is not intended to discuss proper joint design, but suffice it to say everything starts with the proper selection of a threaded fastener
that is intended and capable of handling the load that a product will endure over its life. If maintenance is a part of extending product life, proper fastener selection is even more important.
Exact standards do not exist for uniform joint design. However, it is known that proper joint design includes study and analysis of working forces that act on the joint. These forces or loads would include tension, shear, bending and fatigue.
Joints are sometimes categorized as
hard or soft (and of course somewhere in between) and it is the angle of their movement after initial mating of joint surfaces that determine this categorization. For understanding, a hard joint is largely a metal-to-metal connection whereas a soft joint incorporates material that causes a longer fastening time and more energy to fasten properly (a gasket for sealing purposes is an example).
Torque and Tension
For the purpose of this story we will assume that the actual
definition of torque is not all that important. What is important is that threaded fasteners need to be torqued or pulled in order to put them into tension. Practically speaking, torque (turning) of a bolt by hand or with semi or automated power tools is a useful and inexpensive method of creating tension in a joint.
Clearly there is a difference between torque and tension. Tension is the result of torque. And even if it is done perfectly it is possible to be in
error. Investigation of joints over time have fairly well established that 90% of applied torque is used to overcome friction in threaded fasteners. Friction is found in the bolt head, under the nut and in the threads of the fastener. Change in material, plating, lubricants and sealants all change the torque coefficient in the accepted formula for creating tension preload:
T=KDP: where,
T= installation torque
K= torque Coefficient
D = nominal bolt diameter
P = clamp load objective
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