O-Ring flange screw torque

[Only-VA]

Good evening all,

I am racking my brain over something that I believe should be fairly straight forward. I have a inlet control valve with an O-ring that mates to the top of compressor via four socket head cap screws and I am trying to determine the proper bolt torque for those four screws. Please see below if I am over complicating this or if i am on the right track.
Design Parameters:
Design pressure: 2 PSI
O-RING SIZE:
.139" C.S
3.387" OD
70 DURO

Mounting bolt size and quantity as determined by the compressor.
M8 X 1.25 X 20 Qty: 4

So based off the above, my calculations are as follows.
ASME BPVC VIII Appendix 2
Hydrostatic force due to pressure is:
W=(PI/4)(G^2)(P) + (Gasket factor is zero to self energizing seal)
W=(PI/4)(3.387^2)(2)
W= 18 lbs

Parker O-Ring handbook:
Compressive force required to properly squeeze the O-Ring:
Squeeze % from interpolation: 24%

Compression load force based off Cross Section / DURO / Squeeze
50 Pounds per inch of O-ring circumference

Which gives me a load of 162 lbs to compress the O-ring to 24%

So if i am thinking about this correctly I need to develop a total clamping load of 180 lbs with the four M8 screws.
F=(180/4) =45 lbs per screw

T=cDF
c=.2
D=0.314961"
F=45

T=3 Ft-lbs.....

And this is where I am at a loss. Do I torque the bolts to 3 Ft-lbs based off the above loads or do I follow ASME PCC-1 and torque them based off of yield strength?
Thank you all in advance.

 

[BEMPE16524]

ASME PCC-1 for general industrial or pipeline application, which I think your application falls under this category.

 

[LittleInch]

I'm no flange designer here, but I would have thought that what you want to do is have the O ring groove machined in both sections to the correct depth so that when the two metal parts are then clamped together with (much) more force and torque than you are proposing, the O ring is compressed enough to resist the fluid forces and not crushed into a flat piece of rubber.

The issue I see with your calc is that any screw with that amount of torque will simply unscrew under the smallest level of vibration or risk getting over torqued by someone who doesn't know.

so like this design as you scroll down a bit. https://www.globaloring.com/o-ring-groove-design/

 

[r6155]

Nuts are tightened only enough to make flanges stay in contact. The pressure does the rest.
Lower tensing bolting is used. No special care is required.
NO calculations here.

Regards

 

[LittleInch]

A drawing or sketch might help...

 

[Only-VA]

All, I believe i have most definitely over thought this. Here is my thinking with a clear head after stepping away from the problem.

The O-ring groove in the inlet control valve will set the depth at which you can compress the O-ring before metal to metal contact occurs. So with that being said, a O-ring groove set to the depth of .103, based off parkers handbook, will give me a O-ring squeeze of roughly 25% when the flanges contact each other. Once the flanges are contacting the the four screws can be torqued to the proper torque values to develop their required stress to avoid loosening under external factors such as vibration.

 

[GD2]

Good evening all,

I am racking my brain over something that I believe should be fairly straight forward. I have a inlet control valve with an O-ring that mates to the top of compressor via four socket head cap screws and I am trying to determine the proper bolt torque for those four screws. Please see below if I am over complicating this or if i am on the right track.
Design Parameters:
Design pressure: 2 PSI
O-RING SIZE:
.139" C.S
3.387" OD
70 DURO

Mounting bolt size and quantity as determined by the compressor.
M8 X 1.25 X 20 Qty: 4

So based off the above, my calculations are as follows.
ASME BPVC VIII Appendix 2
Hydrostatic force due to pressure is:
W=(PI/4)(G^2)(P) + (Gasket factor is zero to self energizing seal)
W=(PI/4)(3.387^2)(2)
W= 18 lbs

Parker O-Ring handbook:
Compressive force required to properly squeeze the O-Ring:
Squeeze % from interpolation: 24%

Compression load force based off Cross Section / DURO / Squeeze
50 Pounds per inch of O-ring circumference

Which gives me a load of 162 lbs to compress the O-ring to 24%

So if i am thinking about this correctly I need to develop a total clamping load of 180 lbs with the four M8 screws.
F=(180/4) =45 lbs per screw

T=cDF
c=.2
D=0.314961"
F=45

T=3 Ft-lbs.....

And this is where I am at a loss. Do I torque the bolts to 3 Ft-lbs based off the above loads or do I follow ASME PCC-1 and torque them based off of yield strength?
Thank you all in advance.

There are two parts to the gasket loading/stress - seating and operating. The gasket will relax approx 15% in operation and will require to account for the bolt stress/load/torque.
There is also the question of the mating flange stresses. if you are connecting a ductile control valve flange to a brittle compressor cast iron flange, you will also need to check the flange assembly stress.
ASME PCC-1 Appendix O provides a simple rule for the assembly bolt stress.
in all cases, you will need to know the target gasket stress (usually from vendor). This value will be in the upper end of the acceptable gasket stress range.

 

[GD2]

All, I believe i have most definitely over thought this. Here is my thinking with a clear head after stepping away from the problem.

The O-ring groove in the inlet control valve will set the depth at which you can compress the O-ring before metal to metal contact occurs. So with that being said, a O-ring groove set to the depth of .103, based off parkers handbook, will give me a O-ring squeeze of roughly 25% when the flanges contact each other. Once the flanges are contacting the the four screws can be torqued to the proper torque values to develop their required stress to avoid loosening under external factors such as vibration.

Once the flanges contact each other, where do you think the bolt load will go? You will be essentially increase the tensile load on the bolt and a compressive load on the flanges without transferring any load to the gasket.