Joint Integrity

Whether it is a pipe flange, valve bonnet, heat exchanger, or reactor manway, joint integrity relies not only on the mechanical design of the flange and its components but also on its condition, maintenance, and assembly� Piping system leakage is a common occurrence at process plants across many industries around the world and planned maintenance shutdowns are all too often followed by leaking joints on start-up�

Leaking joints are both costly-in terms of lost product and inefficient plant operation, downtime and repair costs, and potentially damaging or dangerous with safety and environmental consequences, not to mention the negative impact on corporate image� Where critical joints are concernedthat is, where leakage would cause plant shutdown, the process to be affected or danger to personnel or equipment-failure can be costly in many senses and integrity is particularly crucial� Further, achieving a leak-free start-up after a scheduled shutdown will avoid delays, reduce equipment and testing costs, avoid rework, and enable earlier demobilization of labor�

Plant personnel are looking for high joint integrity-leak-free joints with reduced numbers of incidents during start-up� To achieve joint integrity, a broader view of the bolted flange joint as a dynamic system must be adopted� A process is to be followed that manages the key elements of the bolted system, allows the design potential of the bolted joint to be realized, and helps in achieving continued leak-free operations�

A key component of these activities has been the development of guidelines for establishing an effective fluid-leak management program�

While there is a growing acceptance of the need to manage joint integrity as a key tenet of good maintenance practice, what is often not realized is the level of engineering and management required to ensure leak-free performance� Simply installing a gasket and tightening the bolts will not ensure a reliable leak-free joint�

A range of criteria will affect the level of management required for any one critical joint, from its physical size and operating pressure and temperature, to factors such as any fluctuations in temperature that it may be subjected to�

Causes of leaking flanged joints vary, but as a general rule, flange distortion, sealing surface damage, inappropriate gasket selection, incorrect bolt loads, and uncontrolled tightening methods are typically among the primary ones�

Work that needs to be undertaken during shutdown will typically include ensuring an appropriate surface finish, flatness and condition of the existing gasket face, including any re-machining as required� The rougher the surface finish, the higher are the bolt loads required to obtain a seal, for example, while any marks or defects greater than 30% of the flange sealing face width will be difficult to seal and should therefore be re-machined� Re-machining should also be considered if the flatness of the face is outside the maximum tolerance� Alternatively, if a new gasket is required, this will be inspected and installed� This is then followed by flange alignment (significant misalignment of the flange holes can require an additional load to overcome this) and controlled bolt tightening to the determined load using hydraulic tensioning or torquing� Bolt tensioning is generally accepted as the most accurate method of tightening-a technique that makes use of advanced hydraulic technology to induce accurate bolt stresses without creating torsional or bending stress� The bolt is gripped and stretched axially to the predetermined load using hydraulic pressure� Beneficially, because the stud is axially loaded no bending or torsional stress is induced, and as friction is an insignificant factor in the technique, repeatable and accurate residual bolt loads to specific requirements are obtained, and can be readily reproduced� The residual stud tension can be confirmed by ultrasonic or mechanical stress measuring equipment� A large number of tensioners can be used simultaneously to keep time to a minimum, and can be readily applied even in areas of difficult access, thanks to the design of modern strong yet compact and lightweight equipment which can meet the most stringent requirements and enable even the largest bolt sizes to be tightened to specific design requirements without resorting to wrenches or spanner extensions� On the other hand, where tensioning is not required or hydraulic tensioning equipment cannot be used, torque tightening (involving turning the nut to stretch the bolt) offers a simple and safe method of ensuring controlled tightening and loosening of bolts� A wide range of light, compact, safe, and user-friendly hydraulic torque tools and a complete range of wrenches are used for a torque load of 80,000 ft lb or 108 Nm�

The causes of leaks from pipe flange joints are

• Gasket problems: reused gasket; wrong size or pressure rating; wrong filler, metal strip, or compression stop ring materials; gasket damage during storage; absence of inner ring when needed to accommodate smooth flange surfaces; inadequate centering in four bolt flanges

• Flange surface condition problems: surface not 125-250 μin� rms; surface has gouges or scratches exceeding ASME B16�5 limits; flanges not flat within 0�015 in�

• Bolting problems: low strength bolts or soft nuts/washers; corroded threads; painted threads; stretched threads; failure to use hardened steel washer when flange surface is poor

• Misalignment and distortion problems: excessive lateral or angular misalignment

• Preload problems: too low a preload (gasket not compressed to a hard joint condition); too high a preload (yields bolts, yields flange, crushes gasket, excessive flange rotation); failure to lubricate threads and nut-to-flange interface; loss of preload during operation (gasket creep, thermal transients)

• Assembly problems: cocking due to lack of snugging joint before applying first pass; failure to follow cross-pattern loading; too few passes; too few leveling passes

These were human performance issues that could be corrected with proper training�

A decision was made to appoint a single point of responsibility for bolting methods and procedures� Thus came the appointment of a “bolting engineer,” a collateral duty for a staff engineer� The assignment of this sole responsibility proved to be a key factor in leakage reduction� It resulted in the development of a bolting expert who became the central point of contact for bolting and leak-related questions�

In order to provide the necessary support to the facility, the assigned individual must be highly knowledgeable in the design, maintenance, and behavior of bolted gasketed joints and bolting in general�

Initially, it was anticipated that existing solutions to bolting problems were readily available and that the development of a bolting program would be a brief research and implementation process� The reality was that existing solutions were scattered, poor, or nonexistent�

Plant Staff Technical Knowledge Related to Leak Repair

Overall, plant personnel had limited technical training related to bolted joint behavior or gasketed joint performance� Individuals, departments, or groups were doing the best they could with what they knew�

Bolted joint assembly was based on bolt or stud size and standard torque tables with little or no regard to the type of joint, bolt material, flange design, gasket considerations, or required bolt preload or service conditions� In many situations, this approach is satisfactory� In many, if not most, others, it is totally inadequate�

Historical Records of Leaks and Maintenance

Records of leak repairs, if they existed, contained little useful information on the as-found conditions, cause of the leak, or specific corrective action to prevent recurrence� In the absence of historical data, the same ineffective repairs or maintenance activities were being performed on leaking joints�

Insufficient emphasis was placed on gathering data with which to perform root cause determination of the leaks� Root cause determination was rarely performed prior to planning and implementing repair actions� It was usually performed some time later to meet an arbitrarily assigned due date�

Historical Records of Corrective Action

This system has become a historical archive of problems and the corrective actions that were implemented� If a failure occurs, the component ID is entered into the system and a listing of prior problems is made available� Ineffective corrective actions quickly become evident�

Procurement of Bolting-Related Supplies

Procurement of gaskets was based solely on price with little input from maintenance or engineering personnel� The purchasing department freely substituted manufacturers and products if they were approved as “equal�” “Equal” gaskets from various vendors and manufacturers were placed in the same stock bins under the same stock numbers� No attention was given to the fact that gaskets from different sources were not sufficiently identical and that they performed differently in the field or in specific joints�

No inspection criteria were employed for bolting or gasket materials except for checking that the paperwork was complete�

It is important to follow a controlled assembly sequence to ensure high joint integrity�

The following is a brief summary of the recommended procedure for joints assembled by torquing:

• Inspect all joints during disassembly for conditions that could lead to leakage�

• Obtain and inspect the new gasket; clean and inspect the flange on the gasket seating surface and where the nuts contact the flange; and clean and inspect the bolts, nuts, and washers�

• Lubricate the bolt on the threads, nut-to-flange interface, and hardened steel washers if used�

• Tension the joint� • Tighten the nuts finger tight� • Torque the nuts to about 5% of the final torque in a cross pattern to

ensure that all clearance is out of the joint before starting the main torque sequence�

• Torque the bolts to the final specified torque in a cross pattern using three passes (33%, 67%, and 100%) for most joints and four passes (25%, 50%, 75%, and 100%) for critical joints, problem joints, misaligned joints, and where the bolt preload stress will be greater than 52,500 psi�

• For misaligned joints, inspect to ensure that the flanges are parallel within 0�015 in� after three leveling passes at 50% of the final specified torque�

• Finally, torque bolts sequentially until there is no further nut rotation� If more than three leveling passes are required, this is evidence that the gasket is still not fully compressed and consideration should be given to going to the higher 60,000 psi stress and torquing the joint again after 24 h�

Most industrial plants have a significant number of high-risk flanges that warrant plant-wide concern� Generally, these fall into one or more of the following categories:

• Flanges that CHRONICALLY leak-Certain flanges are a continuing problem because of their tendency to leak in spite of ongoing efforts to fix them�

• Flanges that MUST NOT leak-Certain flanges pose such potentially serious problems that they should never leak� Such leaks could cause:

1� Physical injuries� 2� Unscheduled work stoppages-production loss� 3� Damage to plant or equipment�

4� Late schedules� 5� Fires� 6� Regulatory and/or legal problems� 7� Problems resulting from leaks in flanges can range from local in

severity to plant-wide catastrophe� Although the range of negative results can vary widely, these high-risk leaks have one thing in common-ALL LEAKS ARE PREVENTABLE�

Leaks are symptoms of a loss of clamping pressure on the sealing element under operating conditions� This can be caused by a single factor or by a combination of factors�

Quality control factors that can negatively affect joint integrity include

• Variation or errors in bolt loadings during assembly procedures • The flatness and/or texture of the flange face • Bolt metallurgy • The creep relaxation of the gasket material

Joint integrity is today firmly on the map, and is recognized as a crucial element of any plant maintenance program� There is no doubt that effective management of critical pressure-containing joints can reap rewards in terms of cost-savings and operational efficiency, not to mention removing risk� Moreover, with a leak-free start-up guaranteed to users of the management program, for example (an approach to eliminate leaking joints), the advantages of investing in such management programs become even greater�