Bolt Torque and Bolt preload query

I've never done anything quite like this, but general practice is to design for worst case scenario. If you need and design for an overall SF of 2 at ambient, then your yield drops by 30% as you get up to operating temp, your SF isn't really 2 anymore.

Take the yield at ambient. 50% sounds like a good value. Do the PCC-1 math and go with it (using a good procedure confirming to PCC-1).

Never torque, use elongation

Example: Torque value is the same for stud 25mm diam x 75mm long or 400 mm. Ridiculous.
For stud elongation the length is the input.
Nobody mentions about this difference.

Please take a look in ASME PCC-1

@r6155: You always say this but so far have never give a decent explanation why elongation would be the appropriate method to tighten say an NPS 2” Cl. 150 flange.

@ XL83NL

I assume that you don´t read ASME PCC-1 2022

For your example Flange NPS 2 Class 150the stud diameter is 5/8”. Care must be taken for small diameter of studs because the required stress can be exceeded during installing.

ELONGATION
I need only a Vernier caliper as a measuring device to control the stud elongation. Lubrication is used only to facilitate the installing effort for a manual wrench. You can use any lubricant, accordingly.

The required elongation must be showed in the fabrication detail drawings and in the Final Data Report.

Original length of stud plus elongation (final length) must checked at every moment, as you need.


TORQUE
Torque wrench must be calibrated and inspected before use and be frequently calibrated.

Lubrication is required, and the coefficient of friction is a vague value. An error could occur if a different lubricant is used during installing.

Fabrication detail drawings must show torque value and type of lubricant (ie.: Molykote XXX).

The torque value cannot be verified after installing, unless the same torque wrench is used.


Regards

Use both. Your flange will have different loading scenarios: Assembly (ambient temperature and no pressure), relaxed and operating state. You need to make sure that the bolt does not yield in any of these conditions because otherwise your gasket will be unloaded and therefore the flange will leak if it is unloaded below the minimum acceptable operating stress.

It would be also wise to point out to follow both PCC-1, WRC 538 and 510 to make sure that your flanges stay within allowable stresses as well.

r6155 said:

@ XL83NL

I assume that you don´t read ASME PCC-1 2022

For your example Flange NPS 2 Class 150the stud diameter is 5/8”. Care must be taken for small diameter of studs because the required stress can be exceeded during installing.

ELONGATION
I need only a Vernier caliper as a measuring device to control the stud elongation. Lubrication is used only to facilitate the installing effort for a manual wrench. You can use any lubricant, accordingly.

The required elongation must be showed in the fabrication detail drawings and in the Final Data Report.

Original length of stud plus elongation (final length) must checked at every moment, as you need.


TORQUE
Torque wrench must be calibrated and inspected before use and be frequently calibrated.

Lubrication is required, and the coefficient of friction is a vague value. An error could occur if a different lubricant is used during installing.

Fabrication detail drawings must show torque value and type of lubricant (ie.: Molykote XXX).

The torque value cannot be verified after installing, unless the same torque wrench is used.


Regards

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I'm having a hard time understanding this. I honestly will not try to argue with you but instead try to learn from you.

Now, from all the things I've found in and learnt from PCC-1 (which isn't that much, but the HexTech training does make a good starting point): yes, elongation is a load control, but not per see required for all joints. It may be the technique that provides the best (i.e. most accurate) load on the gasket, but that may not mean it's the best technique., If it takes more time, and more knowledge of the operator, etc. than that has to be taken into account as well. You didn't mention how you tighten the bolts. Is that by torque, or by a tensioning device?
Looking at PCC-1 app F, table F-4-1, its clear elongation is not the appropriate method per se for all situations. Also, elongation does require calibration.
And how much elongation are you actually (accurately) going to measure with a Vernier caliper on a 5/8"? How are going to monitor the temperature? Do you require specially prepared bolts?

I hope you can elaborate a bit more so I can better understand and learn from you.

PS: I recently reviewed Shell's procedure for assembling flanged joints. By including the fact that the vast majority of all studs assembled worldwide are 5/8" or 3/4" (see this link), it became obvious to me Shell is OK with the fact that the vast majority of all studs in their plants are assembled with a torque wrench - not requiring protocol or anything special. Stud elongation isn't called out in there, and its probably for good reason.

Don't understand what you guys are talking about, but you have only two choices, using power torque wrench, or bolt tensioner (we use it for bolt 1.5" diameter or larger, some company may use it 2" and larger).
How much for the torque for torque wrench, or elongation for bolt tensioner and proper procedures, are well defined by the industrial suppliers based on material and size. We have a spec for that since we are EPC contractor, constructing so many petrochemical plants around the world.
Don't make it a rocket science. Just talk to any bolting supplier or google for help.

Bolt torque and elongation each have their own recommended applications. It's not worth to use elongation for all joints - only the critical ones. Bolt torque is enough for most applications. What you basically do is take into account that there will be a wide assembly spread if you use bolt torque to assemble a flanged joint.

Regarding making flange assembly rocket science, it actually is way more complex than it looks. After all, it is the operators that are stuck with flange leakage and reliability issues left by incorrect design and/or assembly procedures from EPC contractors.

@ XL83NL
Turn the nut to achieve the required stud elongation.

Anyone telling you to tension all your flange joint studs has never worked in a real plant before. Plants will have thousands of studs in hundreds of flange joints needing to be worked, often together in large quantities at the same time during outage/maintenance events. The time it takes to properly tension & measure every single stud in every flange joint is cost prohibitive from a schedule/timing standpoint. This is why stud tensioning is typically only done on critical flange joints with studs 1.5"-dia & larger (note these flange joints will take more time to control torque anyway). If you read through most of the major company technical standards, stud tensioning will only be allowed (or required) on flange joints with studs over 1.5"-dia for this reason.

Danmecheng said:

ask for the

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Hi Dan,

As per my understanding. You want to know whether you can use yield strength at ambient temp. for calculating the torque values using PCC-1 'O'.

First of all as we all know bolt torquing is done at ambient temp. in shutdown case or even in fabrication. So using operating yield is not justified.
Therefore for PCC-1 you have the usual 70% - 40% of Yield strength at ambient temp. for max and min values.

Although I would suggest you to look into the torquing procedure. As you mentioned than you have operating temp. at 500 °C. As you as going to do the torquing at operating you have to take bolt expansion into account. If you achieve 100% bolt torquing load in ambient conditions, there is a high risk that upon startup the bolts expand and loosen up because to thermal expansion. Therefore I think as standard practice in plants they torque/ tension it up to 70% load and then after plant achieves operating temp. they torque/tension it to 100%.

Bolts will relax after temperature increase therefore the clamp force will gradually be reduced.
Hence why they need to be pre-stressed at ambient.
A common practice is to pre-stress bolts to 310 MPa or ~ 50% Yield for B7 bolts at ambient.
Read non mandatory appendix S.

It is my impression that there is a little of confusion between bolt torque, bolt tensioning and elongation.

Flange tightening is performed by bolt torque (i.e. by wrench) or bolt tensioning (i.e. by hydraulic tensioner).
Both methods have some assumption and imprecision for instance the friction coefficient in torque (as explained by someone else) or the nut/thread scatter when you release tensioner pressure in hydraulic tensioning.

Elongation is a way to indirect check the stress in the bolts, not a tightening method. Elongation can be used wither with torqued or tensioned joint. In my opinion, elongation is not so accurate as said above.
The reason of my believe is the fact that you need to know actual young modulus, actual bolt area and actual effective length to have the actual stress. All calculation are based on assumption, thus the result is affected by these assumption.

Back to original question, I would not blind use 50% of bolt yield stress on HE girth flange. My suggestion is to check the maximum allowable load that the flange can withstand, for instance by use of WRC 538. Joint component approach of appendix O of PCC-1 and API 660 are really good starting point to develop correct tightening procedure.

As other described, at 500°C great part of the load is dispersed by relaxation. Moreover, think also that you apply a stress at the bolt equal to strain*E. When bolts go in temperature E decrease, thus stress decrease as well.

Krausen said:

Anyone telling you to tension all your flange joint studs has never worked in a real plant before. Plants will have thousands of studs in hundreds of flange joints needing to be worked, often together in large quantities at the same time during outage/maintenance events. The time it takes to properly tension & measure every single stud in every flange joint is cost prohibitive from a schedule/timing standpoint. This is why stud tensioning is typically only done on critical flange joints with studs 1.5"-dia & larger (note these flange joints will take more time to control torque anyway). If you read through most of the major company technical standards, stud tensioning will only be allowed (or required) on flange joints with studs over 1.5"-dia for this reason.

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That's my understanding too, but r6155 keeps on coming with the same reply over and over in each thread about bolting, torque, etc. His posts are usually too short to truly understand his reasoning. I sincerely hope that, based on the feedback in this thread, he can provide arguments for his points, but so far he has been reluctant

1) @Krausen
I work in pressure vessel manufacturing. The testing is more demanding than at the plant.
There is a procedure for checking bolt elongation on small flanges, without measuring all the bolts, only one of them is measured. Very easy.

2) All of you use the same torque value for a short and long studs: WRONG

Distribution of the stud stress is depending on the stud length. However, practical observation shows that longer studs are more resistant to fatigue even when tightened to a same preload (same torque), of the shorter stud.

r6155 said:

I need only a Vernier caliper as a measuring device to control the stud elongation.

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Who uses vernier caliper to measure elongation of the bolt? Ultrasonic measurement is typically used, but for large bolting only.

r6155 said:

I work in pressure vessel manufacturing. The testing is more demanding than at the plant.

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No, testing is not more demanding during manufacturing compared to the Site. They are both subjected to the same requirements.

r6155 said:

The required elongation must be showed in the fabrication detail drawings and in the Final Data Report.

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No, only target stress or torque values are required.

plus attached

"Never torque, use elongation"

I wonder what r6155 would do if confronted with a flat tire and needs to install the spare tire.


angle torque is pretty common on rod, main and head bolts on modern cars.