While other chapters of the Piping Handbook deal with the pressure integrity of the piping system, this chapter deals with managing the leak integrity of bolted flanged systems. It covers the main elements of a bolted joint system to provide an understanding of the bolted joint connection and the science of joint sealing.
This chapter focuses exclusively on bolted joints subjected to internal pressures. While integrity of mechanical (structural) joints are also critical.
Oil, gas, and power plants and other process industries are under constant pressure to work their plants at maximum design limitations and for longer periods. The bolted joint is often regarded as the weak link in the plant’s pressure envelope. Whether a pipe flange, heat exchanger, reactor manway, or valve bonnet, the joint integrity relies not only on the mechanical design of the flange and its components, but also on its condition, maintenance, and assembly. Plant personnel are looking for equipment to achieve leak-free joints with reduced shutdown periods while increasing the time between shutdowns. Similarly, flanged joints in other piping and distribution systems found throughout industrial, commercial, and residential facilities are required to maintain their structural integrity and leak tightness.
Several standards have been written to enable designers to design bolted joints. Compliance to the requirements of these standards ensures mechanical integrity of bolted joints. However, these standards do not provide adequate and effective requirements or guidelines to assure leak integrity of flanged joints.
To achieve leak integrity, a broader view of the bolted flange joint as a system must be adopted. Ideally, a process is to be followed that manages the key elements of the bolted system, which allows the design potential of the bolted joint to be realized and helps in achieving continued leak-free operation.
This chapter reviews the process required to achieve flange-joint integrity.
Some believe leaking flanges are normal and leaks cannot be prevented. Some also hold similar views about health and safety. Safety professionals now know that accidents can be prevented and that the goal of zero accidents is achievable. The goal of zero leaks is also achievable. Leaks are still very commonplace. A thorough survey throughout North American industry, performed by Pressure Vessel Research Council (PVRC), concluded that the average plant experiences 180 leaks per year. A breakdown of the severity of these leaks is shown in Fig. A7.1.
In a manner similar to accident ratio statistics, there is a relationship between minor, serious, and other dangerous events. All events represent failure in control. Failures in control that result in leaks cost industry millions of dollars yearly due to:
Control is the issue. Leaks are controllable. Control is achieved by implementation of a Flange Joint Integrity program. Joint Integrity is a control program that becomes an integral part of a plant’s safety and reliability.
To assist in managing a process, ask yourself the following questions: why, what, who, and how?
Why do we need a Flange Joint Integrity program? This was addressed in the previous section, ‘‘Cost of a Leak.’’ The stakes are enormous. A Flange Joint Integrity program will help improve plant safety and reliability while reducing its environmental impact.
What do we need to control? The operating environment, the components, and assembly all need to be controlled.
Who do we need to control? The designers, field operatives, and supervisors.
How do we control? Train personnel to required competency. Design components using latest engineering standards. Develop best practices for assembly and maintenance. Implement a quality assurance program that provides traceability and ensures compliance to specifications.
There are over 120 variables that affect flange joint integrity. These can be controlled through the following categories:
The internal environment outlines the design and operating conditions of temperature, pressure, and fluid. With the external environment, consideration is given to location of the flange, whether it is operating in air or sub-sea, and externally applied piping loads. An understanding of the environment is crucial to the design and selection of the appropriate components with the correct assembly methods.
The components include the most appropriately designed and selected flange, gasket, and bolting, commensurate with the risk dictated by the environment.
Assembly includes checking the condition of the components and proceeding according to established procedures. Proper assembly requires that
Flange faces meet the standards
Gasket-seating stress is achieved
Bolts, nuts, and gaskets are free of defects
Appropriate lubrication is used
Execution requires trained, competent people using the correct tools and following procedures.
The steps in the joint integrity process are shown in Fig. A7.2.