Solving the puzzle of bolted joints: when bolts, flanges, and gaskets don't seem to cooperate, there's a guide that can reveal the solution.
We might expect, then, that a puzzle of only three major components--flanges, bolts, and gaskets--would present no challenge, even for adults, but experience teaches us otherwise.
Sure, some joints are simple and work satisfactorily with little effort, like the puzzles for children, but many others prove to be more challenging. Some at times seem to need pieces that aren't in the box. Someone can spend countless hours trying to get those three components to work together properly.
This is understandable when you consider that the components come in multiples and that each one has its own set of variations. The components come in many sizes, materials, and qualities. They can be arranged in many configurations and be required to meet an assortment of operating conditions to differing design margins.
Now add in the experience, knowledge, and skill levels of the assemblers, and you are beginning to see the full picture. Perhaps one of the most telling things is the variations among the individual components. Even with the highest levels of quality control, fasteners, gaskets, and flanges perform their work to varying degrees.
Bolted flange joints are found just about everywhere in industry, so it is surprising at times to consider the lack of knowledge concerning them in the industrial world. It wasn't as surprising ten years ago, when information existed in scattered bits and people had to search for it. That situation changed in a spectacular manner early in 2001 when the newly formed ASME Pressure Technology Post Construction Committee published its first completed work, PCC-1-2000 Guidelines for Pressure Boundary Bolted Flange Joint Assembly, a non-mandatory guideline. This document presented us with knowledge and practices specific to the assembly of bolted flange joints. The puzzle solution that came from this box was the most definitive to date and allowed those dealing with bolted joints to assemble the variables by methods that had been used successfully for many years by many people.
About the same time that the original edition of PCC-1 was published, several new assembly methods were being developed that would prove faster and often yield better results than previous best methods. In 2010 a revised PCC-1 was published that brought new breadth and added clarity to the picture.
The two of us have worked with bolted joint assembly for a combined total of more than 90 years. We find today that PCC-1-2010 is helping people across industry not only to assemble bolted flange joints but also to establish joint integrity programs, procedures, and best practices.
The 2010 document contains, for instance, a whole section on bolt stress--the unit load that should be put on the bolts--and includes tables as well. In the past, we found that a majority of decision makers chose to calculate bolt stress based on bolt diameter or a percentage of minimum bolt yield strength. One predominant method many found to work was to apply 50 or 60 percent of yield, and they used it across the board, but it didn't always work well. Equally used was the method of applying 50 ksi to all bolts, which also met with limited success.
PCC-1-2010 shifts the emphasis to gasket stress and gasket type. Using this information, the bolt load required to achieve a certain level of gasket stress within a satisfactory range of flange stress and rotation, you can make a much better decision regarding the bolt load for the particular joint under consideration.
Once you have chosen a target bolt load, what tightening method do you use? PCC-1-2010 has guidance on this subject as it categorizes joints based on service applications in a table and discusses details in the "Tightening of Bolts" section. There is also a section called "Alternatives to Legacy Tightening Sequence/Pattern." Load validation methods and tools are presented in these sections as well as diagrams of load indicator bolts.
The most common method of controlled tightening is torque, and the most common question about this is what bolt torque to use. PCC-1-2010 provides instruction and information pertinent to bolt torque values. However, to be even close to having a bolt torque that achieves a given bolt load or stress, you must manage the friction causing variables.
There are at least four major considerations that influence obtaining the torque to achieve an approximate load--five, if you count load validation methods. To achieve an even gasket stress within a range of a target, first you have to create conditions that limit friction and friction variations. Next you want each bolt to encounter the same friction.
Third you need a procedure of tightening that will reduce bolt interaction and provide repeatability from bolt to bolt performed by competent people. This includes accurate, consistently performing, and reliable torque tools, gauges, and pumps used by well-trained technicians.
Fourth you should understand and use a calculation method or chart that best represents the conditions you have, and last, when deemed necessary, you should validate the load achieved by the torque applied.
The aspect of validating the load will become your educator for future choices. All of this information should be contained in a procedural document and each task recorded as completed. As stated earlier, all of PCC-1-2010 contains information pertaining to choosing a most appropriate target torque value. Be sure not to overlook the less obvious topics such as "Cleaning and Examination of Flange and Fastener Contact Surfaces," "Alignment of Flanged Joints," "Installation of Gaskets," "Lubrication of Working Surfaces," "Installation of Bolts," or the appendices on washers (Appendix M) and reuse of bolts (Appendix N). You'll find vital pieces of the puzzle in all of these pages.
It is hard to place an exact price tag on most of PCC1-2010 contents, but when you look at the alternative tightening sequences you will find methods that directly shorten the assembly time. Other benefits of using and following the guidance in PCC-1-2010 are associated with passing hydro-tests the first time, quicker, leak-free startups, reduced bolted flange joint emissions and joint failures, longer leak-free run cycles, and documentation for quality control and future reference.
If you look at the much bigger picture, you will see reduced greenhouse gases, reduced construction and maintenance costs, increased productivity, and a can-do attitude where bolted joints are the subject.
The guidance of PCC-1-2010 can be applied at just about every stage of the development and use of bolted flange joints. Following are a few of the people who can benefit from knowing this document.
DESIGN ENGINEERS can use it to explore design issues, field assembly practices, and available assembly methods as they relate to their designs. They can design with PCC1-2010 information in mind. Stated as part of the criteria, they should include a written assembly procedure based on best practices according to PCC-1-2010. Having a working knowledge of the best practices will be valuable if the vessel flange joints leak after assembly.
According to Jorg Albrecht, managing director of Hevi Technology, a provider of joint assembly equipment and quality control training, "Being able to correctly calculate necessary bolt loads, flange stresses, and then provide torque targets is only one part of the [bolted flange joint design] process. Engineers also need to make sure that the results of their careful analysis don't get thrown to the wind--as they often do. Consider the frustrations of many of your contemporaries who've otherwise correctly designed a pressure interface, only to be told that they obviously made a mistake because the joint isn't reliable in the field."
There are holes in the puzzle between the designing of flanges and the assembly of the same; this is why PCC-1 was written. Albrecht goes on to advise other designers to complete their fine work by filling in the holes.
"Make sure that you've also got a copy of ASME PCC-12010," he said. "Don't finish your spec with a torque value as many unfortunately do. Doing so leaves too many open holes. Seal those holes by using this publication to ensure that your intentions of reliable and safe joints will be met."
No one intentionally designs a flange joint to fail, neither does one intentionally assemble flange joints to fail, but the assembly intentions of the designer must be communicated to the assembler. This often results in a change of practice and a learning experience for both the engineer and the assembler but look at what Albrecht says about the outcome.
Following the document's guidelines may require more of assemblers, but the results will be worth any extra effort. "The hard-hats will probably curse you for making their job a little more difficult," Albrecht said, "but the clients will love you because their plants won't be leaking on start-up or come crashing down in the middle of a production run.
FRONT LINE ENGINEERS can use the guidelines to ensure that all of the proper assembly elements are considered and applied. They may also be able to identify the need for an improved gasket, though there is no procedure for choosing the replacement gasket as this technology is broad and constantly changes.
Appendix D has flange face condition tolerances based on types of gaskets (soft or hard) and Appendix O illustrates assembly bolt stress determination methods based on gasket stress and type. Although these are only two of the factors in the decision making process they may be the critical ones. The document can also be used as a training manual for assembly personnel, and in the event of leakage, it contains a troubleshooting guide (Appendix P).
PROJECT MANAGERS can use PCC-1-2010 as a guide to develop assembly methods and practices, to train personnel, and to reduce overall construction cost. Assembled according to these practices, most systems will pass leak and hydrostatic tests the first time and start up leak-free. The big payoff is long-term performance and increased ease of disassembly during the maintenance of a plant.
Terry Lowery is a former project manager for Superior Plant Services, LLC and currently is with Industrial Specialty Services, LLC, a division of Brand Energy and Infrastructure Services.
"Over the past 11 years on various projects under my supervision we have developed and implemented bolted joint assembly procedures that have incorporated the principles contained in PCC-1-2000 and the revision of that document, PCC-1-2010," Lowery said. "For example, on one project in Louisiana there were over 7,000 bolted joints assembled, 930 of those were DEP tested at 2,175 psi with nitrogen, and there were only three bolted joint assemblies that leaked. Those particular leaks were on valve bonnets assembled by the manufacturer. As a project manager, I see the principles of PCC-1 as an important part of my career success."
Project managers may even find they can negotiate a bonus based on reduced bolted flange joint leakage for the first five years of operation.
MAINTENANCE AND CONSTRUCTION PERSONNEL from superintendents to plant mechanics and craftsmen are perhaps the individuals who benefit the most from acquiring the knowledge and skill levels associated with mastering the best practices contained in PCC-1-2010. By applying these practices, they will come to be associated with producing good work and leak-free bolted flange joints. When others are searching for answers they will already have the answers. As a proven skillful and knowledgeable tradesman, an employee's value is increased to his employer. He not only increases benefits to his family, but also lives in a safer community because of the reduced hazards associated with bolted flange joints.
INSPECTION PERSONNEL should study PCC-1-2010 as the industry assigns greater responsibility to them in matters of quality control.
BOLTING CONTRACTORS should know all that is contained in PCC-1-2010 as a platform for excellence in their trade, which is to help solve problems associated with bolted flange joints. PCC-1-2010 comprises the basics for successful bolted flange joint assemblies and troubleshooting.
TRAINING COORDINATORS must have PCC-1-2010 in their library as a go-to source. Training all those associated with bolted flange joint assembly on the best practices contained in PCC-1-2010 is paramount in qualifying an individual for the assembly task.
PURCHASING AGENTS AND BUYERS will also help those they work for if they know PCC-1-2010. It is important to know why those following the guidelines specify various items and services, and also important to know why it is risky to substitute or omit anything. Such things as gaskets, washers, lubricants, and specialty services fall in this category. Failure of a substituted part or quality of service can thwart the overall integrity of the joint.
As puzzles go, the assembly of bolted joints has many pieces including the variations of each component, service condition, and individual involved. The integrity of the bolted flange joint at each stage of its prolonged service life is the goal of ASME PCC-1-2010 Guidelines for Pressure Boundary Bolted Flange Joint Assembly.
Edward Hayman is a retired bolting service contractor who specialized in training and consulting on bolted joint assembly. Clyde Neely is a Fellow of Becht Engineering Co. Inc. in St. Albans, W. Va. Both are members of the ASME Pressure Technology Post Construction Committee, as well as the Subcommittee on Flange Joint Assembly, which Neely chairs.
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|Author:||Hayman, Edward; Neely, Clyde|
|Date:||Jun 1, 2011|
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