Header Plug Torque Calculations

[caper656]

Hello all! I found another topic with vagueness that I want to provide calculations for, with the purpose of standardizing routine torque checks... SA105 Shoulder Plugs for Boxed pressure vessels, more precisely, heat exchangers. A very grey area as far as where original calculations came from as vendors change over the past 40 years. The Torque Tables I've been sent dont mention code references or the equations where the values came from. In addition, the vendors do not know how to prove the table out and I find differences between the vendors as far as overall value suggesting there are some different methods to calculate.

That said, after reading through API661, ASME and EN1591, I havent seen anything designating the equations to be used for this specific usage. Obviously, there are general torque calculations such as Require Torque = K * D * P (where K is the lubrication of the threads coefficient, D nominal diameter of the plug, P clamping load) but I am unsure this simplified version can be used as the result are close but not consistent with the table between sizes. (Example: 1" SA105 with K as 0.15 is Torque= 0.15*1*( 0.4* Sy*Bolt Root Area); where sy=31000psi@400F and 40%of yield, bolt area=0.62413 = .15*1*7739.206= 1160.811(in.lbs) or 96.73(ft.lbs)

The approximate plug torque values from from the SA105 plug vendors shows the below.
Diameter-----Torque values(ft-lbs)
0.75--------------76
0.875------------122
1-----------------184
1.125-------------273
1.25--------------275

I did also find this from the vendor but no backing equations:

https://www.stanleyengineeredfastening.com/resources/spiralock-torque-calculator#

thoughts?

 

[SwinnyGG]

Tightening specifications for pipe plugs are not, and should not be, comparable to preload calculations for a bolt which clamps two things together (as in T=KDP).

Tightening pipe plugs using torque as the primary gauge of joint quality is, as with any other type of pipe fitting, not likely to result in reliable joints.

Torque information for pipe plugs is hard to find because you don't want to be using torque to install them.

 

[SnTMan]

OP said "Shoulder Plugs"... Likely a gasket is used. Start by identifying the compressive stress wanted by the gasket.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[caper656]

Correct! Shoulder plug with a a366 gasket. I will look at calculating the gasket compressive force needed

 

[SwinnyGG]

Whether the plug is tapered or uses a gasket doesn't change the answer- if you want reliable sealing, don't rely on torque.