Some may claim this to be a trick question. I assure you it is not meant to be, but as I will point out in this article, having too much bolt extending beyond its nut can be an issue, as well as not having the bolt flush with the outer surface of the nut.
Have you ever come across a nut and bolt assembly where something just did not look right? How about a pipe flange or a manway on a pressurized process where the nuts are just biting onto the bolt, clearly because either the wrong bolts were used or an incorrect gasket assembly is being used and not allowing the two flanges to meet their tolerances? And by the way, this article can apply to any bolt and nut assembly, not just pressurized systems or chemical processes. Me being a safety professional, I use the principle on my kids bicycles!!! The $60,000,000 question is... how far should a bolt pass through a nut in order for that assembly to meet the full ASME design rating?
For my entire career in chemical process safety (nearly 20 years now) I was always told by my engineers that at least three (3) threads of the bolt should be showing on the outside of the nut. I also have a "Mechanics Pocket Guide" and this was the "general approach" offered in this publication and it is a GOOD ONE to follow, as it provides AMPLE safety factor in the design of the flange or manway. But what does ASME actually require if the bolt and nut assembly are on a ASME Coded Pressure Vessel and what does ANSI B31 require if the bolt and nut assembly are on a pressurized pipe flange?
I turn to one of my hidden gems, ASME PCC-1 Guidelines for Pressure Boundary Bolted Joint Assembly for answers. This ASME standard was recently revised in 2010 and is full of useful information that, in my opinion, should be incorporated into any Mechanical Integrity Program at a facility covered by OSHA's Process Safety Management and EPA's Risk Management Program. The best part of the revision is the "lessons learned" tidbits. Some of the better “Lessons Learned” tidbits that we can ALL UNDERSTAND are:
- NEVER use gaskets that are NOT rated for the pressure to be applied, even if the gasket is “temporary” for testing purposes only. Such gaskets have failed during pressure testing causing serious injuries to many and even fatalities in some rare cases.
- Do NOT reuse gaskets! Gaskets are typically designed to deform and take the necessary shape to make a seal and once they have been used, they become brittle and they have already taken the shape of the previous use. Reusing the gasket, even in the same flange that it came out of, and this may crack the now brittle gasket or require more bolting load to create a seal thus damaging the flange face. (See my previous posts on Torquing for more on this type of error)
- Nuts MUST run FREELY over the bolt/stud. Even a small imperfection in threads will have a BIG EFFECT on torque measurements leading to inaccuracy in tension loads in the studs, which can result in leakage.
- Something as simple as EXCESSIVE paint thickness on the outer flange surface under the washer can result in leakage when the paint eventually deteriorates in service. Power wire brush around the washer contact surfaces to remove excessive paint.
Flange faces should be examined for pits, gouges, dents, scratches and burrs. If the gasket is being replaced, remove ALL traces of the old one from the flange faces using solvents and soft wire brushes, as allowed in your facility’s maintenance procedures. Pay particular attention to damage that runs across the ridges as such damage provides a leak path. ASME PCC-1, Appendix D provides maximum flaw depth as a function of the radial dimension of the flaw compared to the width of the seating surface; the longer the radial dimension of the flaw, the less the depth that is acceptable.
- NO DEFECTS DEEPER than 0.050 inches deep are permitted for SOFT gaskets
- NO DEFECTS DEEPER than 0.030 inches are permitted for hard gaskets.
The standard allows larger flaws in soft gaskets than for hard gaskets. It also provides guidance on how to determine if a gasket is “hard” or “soft”.
ASME PCC-1 Appendix E addresses initial alignment of flanges. Previous requirements were based on ANSI B31 alignment requirements, but this 2010 version specifies a bolt torque limit of 20% of target load needed to bring the flange faces into parallel alignment. If more than that is required, the design engineer MUST be consulted. This recognizes the CRITICAL importance in the relationship between the initial alignment force and system flexibility. Although the standard through-hardened washers conforming to ASTM F-436 were specified in the previous version of PCC-1 (2000); these carbon steel washers are UNSUITABLE for elevated temperature service (creep range) as they will deform over time allowing bolt tension to drop to dangerous levels. They are also larger than the spot face on most flanges, so they bend into a cone during flange assembly – a VERY SERIOUS ISSUE on piping used to transfer highly hazard chemicals!
ASME PCC-1 Appendix M provides a specification for carbon, low alloy, stainless and precipitation hardening stainless steel washers, and it provides dimensions for washers that match the facing requirements in MSS SP-9 for spot facing of flanges. Gaskets MUST be positioned so that they do NOT extend into the pipe flow stream. A light dusting of an adhesive COMPATIBLE with the process fluid(s) can be applied to one gasket surface to hold it in place. Tape strips and/or grease MUST NEVER be used across the gasket face. Bolts MUST be of sufficient length that the threads are FULLY ENGAGED. Generally this means the end of the bolt MUST be FLUSH with the nut. Although ASME B31.3 allows the threads to be one thread short, it is NOT a recommended practice and I urge you to NOT adopted this allowance, as ASME B31.5 does NOT permit this, as shown below.
ASME B31.3, 335.2.3 Bolt Length. Bolts should extend completely through their nuts. Any which fail to do so are considered acceptably engaged if the lack of complete engagement is not more than one thread.
ASME B31.5, 535.2.4 Bolt threads shall extend completely through the mating nut.
One end of the bolt should be lubricated and the nut installed so that the bolt is flush with the nut’s outer surface. All the excess threads should be at the other end of the bolt. Since bolt threads will rust in most environments, this practice makes the flush side nut easy to remove, even after years of service. Generally bolt lengths should be such that the excess threads do not protrude more than ½ inch (13 mm) to minimize the extent of corrosion that you have to deal with if the flange is disassembled. Used bolts should be examined for straightness, obvious damage and abuse; if the inspection shows ANY signs of damage or wear, a NEW bolt MUST be used. Also, nuts should always be replaced, as should damaged or deformed washers.
PLEASE NOTE that the new bolt, nut and washer MUST MEET the materials of construction, length, diameter, and strength rating spelled out in the Process Safety Information or the Recognized and Generally Accepted Good Engineering Practice adopted by the facility to manage their process design. Any change in the bolt or nut MUST UNDERGO a Management of Change review to ensure the new bolt/nut/washer is acceptable for the intended use.
After alignment and assembly, nuts should be hand-tightened and each stud numbered in preparation for torquing, then the nuts should be snugged up to 10- to 20 ft-lbs. Where the nut does not hand tighten, the bolt MUST be examined to determine the cause and repaired or replaced as needed. In this revised 2010 edition, legacy bolt tightening pattern requirements were also examined to provide alternative patterns that reduce the time and effort required from the legacy pattern in which five passes at progressively higher torques were applied. Alternative patterns such as the 4-bolt pattern reduce the passes to as little as three; research has shown that flanges tightened following the alternative patterns work just as well as those tightened using the legacy pattern and sequence.
ASME PCC-1 addresses the question of how much torque to use by providing a new a table of standard torques for coated bolts and non-coated bolts. But the standard also has an appendix that allows one to calculate the appropriate torque based on minimum required gasket stress to seat the gasket both during assembly and operation. Coated bolts have a polyimide/amide coating applied by the manufacturer and the coated torque values apply only for initial tightening. The non-coated bolt torques assume that the nuts are free-running and that a suitable thread lubricant that is compatible with the gasket has been applied. NOTE: lubricants MUST be applied to the threads AFTER insertion to avoid picking up particulate that would increase torque and lubricants MUST be suitable for the service temperature and bolting materials. Lubricants should never be applied to gaskets, and contamination of gaskets by lubricant MUST be AVOIDED.
While welders assembling pipe have to be qualified, there are no similar requirements for those who assemble flanges using blots and nuts assemblies. PCC-1 Appendix A provides requirements for a certification entity to create and administer an ASME Certified training and assessment program that provides an industry standard certification program for bolted joint assemblers. The appendix provides requirements for the minimum course content that includes a theoretical portion, practical demonstrations and practical assembly. The appendix proposes three levels of assembler qualification: Certified Bolting Specialist, Certified Senior Bolting Specialist, and Certified Bolting Specialist Instructor. While following PCC-1 fully may not be necessary for many applications, the above provides some simple things a facility and it’s contractors should do to maximize the probability of a leak-free joints. If joints still leak, Appendix P provides troubleshooting guidelines, including a useful checklist.