How many threads of a bolt must be showing outside a nut to meet RAGAGEP?

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 $64,000,000 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 an 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:

1. 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.
2. 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)
3. 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.
4. 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.

UPDATE 2016 - Another source of good info is API 570, 5.12 Inspection of Flanged
Joints which states: Flange fasteners should be examined visually for corrosion and
thread engagement. Fasteners should be fully engaged. Any fastener failing to do so is
considered acceptably engaged if the lack of complete engagement is not more than
one thread.