Bolt Target torque - ASME PCC-1

[mech8790]

Hi everyone!

I am in dilemma, how to properly calculate a target torque for a flanged joint in accordance with PCC-1. I have read through Appendix S Asme Section VIII, Division 1 and Appendix K,J and O ASME PCC-1. I have got a joint made of two 316 S/S welding neck flanges 1500# RTJ c/w ring gasket and A-193 B8M M20 bolts. Please correct me if the way how I understand this problem below is wrong:

1) I calculate the required clamping force for pressure testing at 158 MPa (1.5 DP) using formulas in ASME Section VIII, Division 1 Appendix 2
2) Doing calcs of target torque values in accordance to appendix J and K PCC-1 I get two different values, I reckon that the value from Appendix J is more accurate than Appendix K nut factor calculations

Say, I have got 235Nm torque required to properly seal the joint during pressure testing. The thing that is confusing me is the table 1M Reference values for calculating target torque values for Low-Alloy steel bolting based on target prestress of 345MPa(Root Area). In my case the bolt I have used has got 550MPa min yield strength (Section II, part D), thus according to 12.1 target prestress clause I should obtain the target torque value for coated bolt from the table 1M with a modifing factor of 550/345. I know that the bolt stress limit during tightening should stay between 40 - 70 percent of material yield strenght so I think this table 1M base on it. For M20 bolt I should multiply 250 * 1.59 = 398 Nm

Now I have two values:

1) calculated from Appendix J - 235 Nm
2) from table 1M - 398 Nm

Which one is governing here, assuming both have got the same factor of friction applied?

 

[TGS4]

You're confusing the target bolt stress with yield stress.

Your bolt material has a yield of 550MPa. Your target assembly bolt stress should not exceed 70% of that value, or 385MPa. Table 1M is based on a target assembly bolt stress of 345MPa. I would not adjust the resultant torque values from Table 1M at all, unless you calculated that you needed more than 345MPa assembly bolt stress.

Thus, your results for coated bolts are 250N*m or 235N*m. That's pretty damn close enough for me.

Make sense?

I should also note that B8M bolts have chemistry that matches 316 SS. They would not qualify as low alloy bolts by and stretch of the imagination.

For stainless steel bolts that are at risk of galling, I would experimentally determine my nut factor, based on the lubricant and/or coating that you will actually use. You are aiming to obtain a certain bolt stress. Applying torque to a nut is an indirect method of obtaining that bolt stress. If it is a critical joint, I would recommend bolt tensioning instead.

 

[mech8790]

Hi!

Thank you for your quick reply, I really appreciate that. Yes, I was confusing the target bolt stress with yield stess. Can you tell me, is there any general rule of thumb to use 70% of yield stress in calculating maximum target assembly bolt stress? Or, any figur between 40%-70% will be good? The is not critical joint.

Am I right saying, (in theory) 385/345 factor should be use to adjust the torque values in table 1M?

B8M bolt is made of high alloy steel so the table 1M might not be applicable here?

Thanks in advance!