Discussing the effectiveness and necessity of relaxation pass and start-up re-torque procedure according to industry research over relaxation behavior.
You can find more information on our web, so please take a look.
Relaxation passes and start-up re-torquing procedures are used in specifications to help prevent leakage and increase sealing performance in ANSI pipe flanges, heat exchangers, and other pressure boundary joints.
Relaxation Pass: This is a circular torque pattern on a bolted flange joint assembly at the same torque value as the initial bolt torque, performed after a specific time.
Note: We typically perform this on a gasketed flange at ambient temperature and steady-state.
Start-Up Re-Torque (formerly known as hot torque): This is also a circular torque pattern on a bolted flange joint assembly at the same initial bolt torque value as it was assembled to, only done between 250F-450F. Be sure not to do this at high temperatures since the K-Factor (some mistakenly call this &#;coefficient of friction&#;) changes after 450F!
NOTE: Since there was confusion in the industry between hot bolting and hot torquing, ASME PCC-1 has modified the name of this process to ensure clarity between the two.
So, when do you perform a relaxation pass and start-up re-torque (hot torquing)? Are they worth every penny?
Hex started learning about bolt stress relaxation when we first read John Bickford&#;s &#;An introduction to the Design and Behavior of Bolted Joints.&#; John Bickford, being the grandfather of bolting.
We noticed first in the top paragraph that Bickford states experiments should be done before making a specification.
Taken from Handbook of Bolts and Bolted Joints &#; John BickfordBickford states structural steel bolts showed a loss of 11% of preload immediately after tightening and another 3.6% and the next 21 days, followed by another 2% over the next 11.4 years.
NOTE: This applies to structural steel and not a gasketed flange joint!
If you go down further, Bickford states that the Southwest research Institute suggests that fasteners lose an average of 5% of preload right after tightening in quotes &#;because of elastic recovery&#; (this shouldn&#;t be confused with elastic interaction).
We wanted to examine how much bolt preload we might lose to bolt stress relaxation while using metallic gaskets.
A good friend of ours, who operates a refinery, said they would give us six heat exchangers with the gaskets. We used a torque wrench to do the bolt tightening. We used UT measurement to measure the axial load and monitored the amount of relaxation on the bolted flange connections.
We used three different gaskets for our test: (4) kammprofile gaskets, (1) double jacketed gaskets, and (1) corrugated metal gaskets. The experiment consisted of using UT measurements on each bolt once every hour for the first day, then once a day for six days, and then once a month for the next two months.
Now we get a chance to look at the data:
Suppose you look at the measurements in the first eight hours, about 0.9% preload relaxation. You&#;ve got to remember that UT is plus or minus 3%, so we&#;re well within that 3%. We also see the same readings on day one, day two, and even though two weeks of monitoring.
On average, we saw almost zero creep relaxation on metallic gaskets and no preload relaxation with standard bolt materials (ASTM B7 and B16 bolt material).
NOTE: these pressure vessels have not seen any internal pressure or high temperature!
To understand this more, Hex had to go over to our friends at TEADIT and ask them to do some research on relaxation since we know sheet gaskets have gasket creep.
Jose Vega and his team at TEADIT went through and looked at when we should do relaxation passes. They used a 4&#; 300# flange connected to software that shows the bolt stress on each of the bolts, measuring the bolts&#; stress so we can measure relaxation.
TEADIT set the torque wrench to 120 foot-pounds and monitored the gasket relaxation. They performed a re-torque and then waited another 20 hours to record the gasket relaxation.
You can see these re-torque increments are:
These are the results:
Results of Corrugated Flexible Graphite Gasket Stress LossResults of PTFE Gasket Stress Loss Results of Spiral Wound Gasket Stress LossAs you can see, the corrugated flexible graphite and PTFE gaskets do need a re-torque, while the spiral wound gasket doesn&#;t! This is because of the effect of gasket creep on the gasket thickness between the flange faces. We tend to call this &#;cold flow&#; for sheet gaskets.
You can see that after the 15 minute re-torque, you make up a decent amount of bolt stress back.
However, it is interesting that a re-torque after that gives back just a little bit (around 4%) of bolt stress and not as much reward as we thought it would.
Because of this study, Hex&#;s recommendation is to wait an hour, go to lunch, return, and re-torque the sheet gaskets. Another practice some refineries have that has been effective for them is to have the next shift, with re-verified torque tools, re-torque the flange.
Warren Brown and Tze Lim with Integrity Engineering Solutions (a mechanical engineering consulting firm) did an excellent study, where they presented the following information at the ASME Pressure vessel and Piping Conference. Brown and Lim looked at different ASTM A193 bolt materials at 725 degrees Fahrenheit (385 Celsius) to determine how much bolt stress relaxation one should see at that temperature.
&#;Quantifying Bolt Relaxation During High Temperature Operation&#; Warren Brown, Tze-Yew LimASTM A193, B7 bolt material relaxed 60%, B16 bolt material relaxed about 25%, and the B8M bolt material gained a little load.
Most specifications in plants that Hex works with state they upgrade bolt materials between 700F and 800F process temperature. These are good for a minimum, but if you think that high temperature is a culprit for a flange connection leakage rate, my suggestion is to look at upgrading the bolt material.
Interesting: Hex Technology has seen that some plants put Heavy Hex nuts on B16 studs instead of Grade 4 or Grade 7 nuts. That&#;s a bad idea as there is relaxation in the Heavy Hex Nuts that you don&#;t see in the Grade 4 or Grade 7 nuts.
Question #1: Should you do a Relaxation Pass on a bolted flange joint?
Answer: Metallic gaskets do not need a relaxation pass. The amount of gasket creep is nominal and within our margin of error for measuring with UT!
Question #2: When should I do a startup re-torque on a bolted flange joint?
Answer #2: The answer to this question has two parts.
We typically recommend performing a bolted flange joint analysis before making these a common task in your specification. Mechanical Engineering should look at items like flange material, operating conditions, flange rotation, and possibly even do a finite element analysis to ensure you get the correct clamping force without deformation of the flanges.
Important notes
For more information about relaxation pass procedure and start-up retorque, don&#;t hesitate to contact us at [ protected]!
See also:
Flange Stud Bolt Lengths: What do I Need to Know?
Kammprofile Gaskets, Explained
Bolt Lubricant and Torque: A Comprehensive Guide
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mechanical joints vs flanged
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(Civil/Environmental)
(OP)
31 Aug 06 17:29I'm working on a water distribution system for a subdivision. I typically work on roadway alignment stuff, so this is a bit out of my range. When do I use mechanical joins vs flanged joints at T's, hydrants, etc? Any input would be greatly appreciated.
Replies continue below
(Civil/Environmental)
31 Aug 06 17:37generally, flanged is used above ground. Mechanical is usually used where thrust restraint is needed. The more common type is bell and spigot joints which are the cheapest and easiest to install. These are only suitable for long runs of straight pipe where thrust restraint is not necessary.
(Civil/Environmental)
31 Aug 06 20:01Flanged Joints are rigid, but are also restrained. They do not allow for any deflection which may come about due to minor misalignments and/or settlement which are common in underground installations. They are only used in aboveground installations.
Mechanical Joints are flexible, but do not provide any restraint by themselves (The follower gland just presses the gasket so that it seals the joint). They are able to compensate for the minor misalignments and/or settlement in underground installations. There are retainer glands available that do provide restraint for mechanical joints. These are similar to the standard follower gland, but have set-screws, or a variation there of, that dig into the barrel of the pipe to provide the restraint.
Compared to push-on joints (the other common joint used in underground installations) the mechanical joint is labor intensive and is only used at fittings, at least in my area. The only advantage over push-on joints is that in pipe sizes commonly used in distribution systems (say 16&#; and smaller) mechanical joints have a slightly larger allowable deflection.
The water supplier should have standard details that would cover almost all common situations (tees, valves, bends, hydrants etc.).
For what it's worth, the general practice in my area is for pipe to have push-on joints and fittings to have mechanical joints w/ retainer glands. Thrust blocks would be used at all fittings. Hydrant leads would be the same as above, but short hydrant leads (say under 20 ft long) would also have the auxiliary valve rodded back (ductile lugs and all-thread) to the tee and the hydrant rodded back to the auxiliary valve.
(Civil/Environmental)
1 Sep 06 01:44Zabrab&#;
Flanges ARE used underground (e.g. for joining valves to tees). However, as you point out, flanges don't allow deflection. Consequently, the length of flanged assemblies should be kept to a minimum so that differential settlement doesn't overstress the piping system.
Fred
(Civil/Environmental)
4 Sep 06 19:10fed3,
You are correct for a tapping sleeve and valve. In my area a flanged joint would not be used between a standard tee and valve. In my area, it would be MJ tee, spool piece, MJ valve.
(Civil/Environmental)
4 Sep 06 21:34zabrab&#;
Interesting. Where are you located? I'm in Central California, and have worked in Southern California. We routinely bury flanged assemblies consisting of standard tees, valves, and reducers. I have even seen city standard details that require a flanged hydrant bury, though most are MJ.
Fred
(Petroleum)
5 Sep 06 01:38Petro pipelines use buried flanges in many locations. We put a flange protecting stainless steel band with grease fittings around it and fill the gap.
Going the Big Inch!
http://virtualpipeline.spaces.msn.com
(Civil/Environmental)
5 Sep 06 09:09The use of the flanged joint goes back more than 300 years. I believe the use of what became basically the now standardized mechanical joint design only goes back 85 or so years, to U.S. Patent No.1,365,530 of Mr. W. D. Moore of ACIPCO. Both joints (with some improved gasket means now available) are good joints and still used effectively for many applications today. However, in addition to what has already been said, their bolted assemblies are relatively labor-intensive and their performance rather labor-reliant compared to some contemporary and essentially boltless push-on joining structures that have been introduced into the marketplace in the decades since. I believe there will perhaps eventually also be more future preference for contemporary joints that do not require such bolting, based on ergonomics and trench safety etc.
As a result of its geometry and rigid bolting nature, the flanged joint in theory requires perfect alignment of support along the axis of the lines to avoid placing bending loads on the pipe, fittings, and flanged fabrications etc. in construction (and perfection in particularly underground construction can be difficult to obtain in some locales in even this 21st century!) I think this reason, perhaps also along with realities of differential settlement, vibrations, seismic movements, some injudicious choices of bolting material relative to corrosion etc. in some areas is why some AWWA manuals and standards contain the cautionary statement, &#;The use of flanged joints underground is generally not recommended because of the rigidity of the joint.&#;
As one poster has replied however flanged joints are still used internationally, and even in a few areas of the USA off the branches of underground tees.
(Civil/Environmental)
6 Sep 06 19:38Fel3,
I&#;m located in southeastern Pennsylvania.
I have used flanged fittings underground, but their use has been limited to very specific instances. When bringing a line up into or down out of a pump house, my early designs had the underground vertical-to-horizontal bend rodded up to the aboveground flanged bend, but this configuration looked &#;clumsy&#; coming through the floor. Later designs used a flanged bend underground for the vertical to horizontal transition with a relatively short FL x PE through the wall to a coupling just outside of the building. Nowadays, I would just use a restrained MJ bend.
How do you integrate the flanged assemblies with the remainder of the work on a regular basis? Do you use a short FL x PE spool piece from the last fitting?
To bring this around to acmish&#;s original post, the mores of the particular fields of engineering and the standard practice in various geographical areas differ, sometimes greatly. The most relevant information for your particular application will most likely be standard details from the entity that will be accepting the completed construction. Their details (or the details of a nearby water provider if they do not have standard details) should reflect what standard practice in your area is.
(Civil/Environmental)
7 Sep 06 11:35to integrate with the rest, I have used fl x pe as suggested, with a butt strap fully welded to the mainline. This was for CML&C steel pipe - for a 24" outlet from 48" main.
(Civil/Environmental)
15 Sep 06 22:55zabrab&#;I've been very busy, so I'm just now getting back to this thread. I have used a variety of means to connect flanged assemblies to the world around them. Sometimes it's per the client's preference, sometimes it's mine.Considering a flanged tee or cross with valves attached, I have used FLxMJ valves, FLxFL valves with flanged coupling adapters, FLxFL valves with FLxPE spools, duct tapeAnother regional difference is that ductile iron pipe is not used as often in California compared to locations that are closer to the manufacturers. We commonly use PVC for small diameter / low pressure (<<150 psi) applications and fabricated steel for larger diameters and higher pressures.Fred
(Civil/Environmental)
6 Oct 06 17:04acmish
First of all, be careful if you're moving out of your field of design expertise; maybe you should refer the work to another designer with the needed skills. If you're designing a subdivision you'll need to conform to the standards of the entity having jurisdiction over the approval of your design. Depending on the location of the town/city/district and their level of sophistication and experience, you might find very thorough guidance or maybe not so thorough. Anyway, the in-place standards is a good place to start. If the jurisdiction doesn't have good guidance, you should consider obtaining standrads from another area town/city/district that is experienced and has good details and specifications.
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