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Flange torque specifications

M. Roberts

Cast Iron
May 11, 2021
Hello all. I hope this is the appropriate place for this question. Take a look at the picture attached. Roughly 9" diameter flanges, 8, 3/4" bolts. Between the flanges is an 1/8" thick Viton rubber gasket, designated as a #150 size. What I am looking for is the recommended torque spec for the bolts on the flange. I have contacted the equipment manufacturer (it is vacuum going thru the system), as well as the gasket manufacturer, and neither has an answer...does anyone have any information?


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A well designed gasketed flange doesn't just pinch the gasket. It should bottom out on a shoulder or other metal component when the gasket is compressed the designed amount. If this is the case with your flange, just torque what the bolts are good for. If it is not, chances are the torque will be very light before the gasket is extruded out of the flange. I'd measure the gasket, and evenly tighten the bolts until the flange gap is roughly 20-30% less than the uncompressed gasket. This is just a guess though, and depends on a lot of things.
unless someone else designed and built the system you may be out of luck on a "spec". The gasket maker and the flange maker only control their product and likely won't get involved as a CYA measure. You can make your own spec however. Vacuum is only 14.5 psi in the negative so let's go a safety factor of 3 rounded up and say 50 psi.
Torque your bolts with a torque wrench set at a arbitrary but reasonable torque for the fastener, but keep it on the light side. Pressure up the system, or just that section with some blind flanges if not possible to do the whole system, to 50 psi and spray the area in question with soapy water. Tighten if she blows bubbles. I don't know how dangerous your device is so use your own judgment as to whether this is good advise or not.
Thank you. Well, apparently, these aren't well designed...no "hard stop" integral to the flange. I did attempt to use a smoke pencil to check for gross vacuum leaks, but I was unable to see any evidence of a leak. Pressurizing the system is not an option.
Cross-tightening is very important so you don't build a wrinkle in the part.
For example, tightening a big heavy-duty mill cutter going one way from start and it is not uncommon the whole cutter will be out of flat when done.
I like "Stros" suggestion of taking it to 25 to 30% less than original thickness...I wonder If I could use gage pins as stops....obviously tightening them in a cross pattern...
as has been said a 25% compression of the rubber is desired. If you can’t figure a way for a positive metal to metal stop, perhaps deleting the gasket and make provision for an o-ring and groove. Still shoot for 25% compression.
I like "Stros" suggestion of taking it to 25 to 30% less than original thickness...I wonder If I could use gage pins as stops....obviously tightening them in a cross pattern...
I have a part I make that seals to a flange mount hydraulic pump. I have round gaskets that have an undersized OD, and a washer plate that fits the flange bolt pattern. As the flange is tightened and the gasket is compressed, it gets wider, pressing on the pilot of the pump and spreading out into the empty space at the ID of the washer plate. The washer plate forces an even gap, even if the bolts aren't evenly tight, and prevent damage to the seal.

If simply giving it your best shot at tightening the bolts isn't adequate, I'd be inclined to try making a washer plate and using a smaller OD seal.
PVC irrigation flanges about that size have "Tighten to 35 Lbs torque" cast into the flanges. The irrigation gaskets are about 1/2 that thickness. Bolts are similar in size. You have steel flanges and twice the gasket thickness, it could be completely different. Might be a starting point.
You need to incorporate a steel spacer ring into the gasket,or redesign the whole idea to use an O ring and spacers..........Its quite possible the gasket will have to take up misalignment in the piping system ,and never be satisfactory
Your flange is designed for steam, water, or oil service. The flat flanges do show up on older vacuum installations. Modern vacuum ASA flange designs use o-rings.

The flat flanges are machined with a series of shallow grooves on the face to capture the face seal and prevent a seal blowout. The flange bolts do need to be torqued to a specified value for the seal to be embedded in the flange grooves.

You can use either a flat gasket or a captured o-ring inside a aluminum disk.
This webpage has bolt torque recommendations for the flat face seals in 250 psi pressure applications. The required torque is 90 ft-lbs.
A vacuum application will not require a face seal pressure this high.

McMaster offers inexpensive adhesive backed face seals for flat ASA flanges that would be adequate for vacuum use. They do not specify recommended torque values.

This is an example of the captive o-ring in a aluminum plate design:

The rubber gasket used on your pump manifold may have been improvised.
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Thanks to all of the replies! :) I do believe that the flanges are flat, without grooves...but I will double check. These systems are at least 20 years old...I wouldn't be surprised if there are better connections out there...that is worth looking into...
I have routinely tested similar joints with deep vacuum and electronic vacuum gage. You are overthinking it. Go for slight crush. Torque will be low for rubber gasket. It will help to scuff a shiny gasket with sand paper to keep from extruding out. You can always tighten a bit more if it leaks. Pressure test with nitrogen and a small amount of refrigerant. Use an electronic leak detector sensitive to a few parts per million.
One more requirement.
The shear force tending to slide a bolted connection apart should be resisted by friction in the joint rather than by shear force across the bolts. Bolt torques are specified to insure that this condition occurs.

There are at least two reasons for this.
The first is that a bolted joint with high friction is a more efficient structure. It does not need the large diameter bolts that would be required in a joint that carries shear through the bolts.

The second reason is that the accuracy of the bolt pattern and the clearance in each bolt hole will only insure that two of the bolts in the flange will actually carry the shear load.

The same idea applies to bolted pipe connections. The vibration in the flange caused by the roots blower and its backing piston pump will cause the flanges in the pipe connection to slide within the clearance of the bolt holes and eventually lead to seal failure if there is insufficient torque applied to the bolted joint.