High Temperature Gasket Torque 2

[jbirdphoenix]

Regards best practices for hot bolt torquing and general questions on how different companies approach torque values for a Plant site and any rules of thumb applied.

First question: When do you apply hot torquing? My experience says the rule of thumb is when processes hit the 400F mark. This is specific to un-insulated flange joints.

Second question: Dual threat, sealing at ambient temperatures and then heat-up. The yield stress of the bolted assembly changes with temperature; at about 400F the change is dramatic. You still need to maintain gasket crush to seat the gasket, however, there is a risk, upon heat-up of yielding the bolt at temperature. How do you address this? Do you use the yield stress value at temperature when hot torquing?

Third question: General rule of thumb for bolt stress is to keep it between 40-70% of yield. I've heard many reasons for this, but the answers sometimes conflict. Example: if you go too low, you don't get bolt stretching and may be prone to loosening. If you go too high, there is a risk of overtightening and yielding the bolt due to variations inherent with torquing.

Thanks in advance.

 

[r6155]

1) See ASME PCC-1
2) My comments (for many years) on this forum are to avoid torque (worst procedure), use bolt elongation.

Regards

 

[XL83NL]

1) See ASME PCC-1
2) My comments (for many years) on this forum are to avoid torque (worst procedure), use bolt elongation.

Regards

Say you have an NPS 2" Cl 300 flange. Per the analysis of HexTech (who have based their research on sales volumes of gasket suppliers), this flange size is one of the most used, if not the, flange size out there, meaning most flanges in a refinery or of this size and pound class.
In reference to PCC-1, several sections indicate tensioning is not per the way to good for all applications. Refer to e.g. chapter 9, app. F-5, app. Q-3.3 or Q-4.2/Q-4.5, etc. So how could bolt tensioning for an application like a NPS2" CL300 WNRF be considered best practice? In essence, tensioning is better, but not for all applications and purposes. Don't apply a one-size-fits-all methodology as you're advocating the wrong policy (IMO).

 

[Krausen]

First question answer: You can apply it wherever & whenever you want. Note that the design temp for your bolts should be lower than the design temp of your process. B31.3, for example, states a 80% value for flange bolt design temp compared to fluid design temp I believe (double check me on this). You could also just take an IR gun out to the bolts & measure what they're operating at to get a true operating value before torquing anything.

Second question answer: Use the bolt yield stress value at your actual operating temp to establish your bolt torque value. Keep in mind to adjust your nut factor for the operating temp as well. PCC-1 discusses the variation of nut factors at temperatures above ambient.

Third question answer: If you have any concerns about sealing the flange joint, target 70% of bolt yield stress at least. But make sure to also check your gasket type & determine your optimum seating stress for gasket (these values are published by gasket mfg-ers). I would target only enough bolt torque to optimally seat your gasket. Note some gasket types can be overtorqued. Sprial-wounds, for example, cannot handle as much seating stress/torque as Kammpro gaskets. If you're gasket can handle it, I'd recommend going as high as 80% bolt yield stress even. Beyond that, you need to be careful about how accurate your nut factor value is in your bolt torque calcs, since there will be natural variations in friction/nut factor. Going higher than 80% of bolt yield stress will run risk of overstressing/permanently stretching/shearing your bolts. PCC-1 has good info on establishing your nut factor in this calc.

 

[r6155]

I'll take the example of XL83NL, 5/8" studs x L= 3.5"
1) Calculate the required elongation
2) Using a hand wrench, turn the nut to get the final length (measure with a Vernier caliper).
It's very simple.

 

[A8yssUK]

By hot torquring do you mean torquing the bolts when they are above 400F ?
or do you mean re-torquing the bolts again while at room temperature but after they have seen 400F+ (retorque after a heat cycle)

Be very careful if its while at heat, as over crushing the gasket is easily done here and the system might just go POP. The normal installation torque cannot be used "hot" a reduced torque must be used, however as most gasket manufacturers basically just flat out state "don't hot torque" you might be hard pushed to get a "reduced torque" value from them.

Also would the system be under pressure etc while torquing.... isn't that slightly insane ? a bit like hitting a bomb with a little hammer..