**Faebian Bastiman**

Earlier this week I was asked how to tighten a CF-40 flange. The implied but unvoiced condition being: and ensure that the Cu gasket is evenly clamped thus creating a leak-free seal. I happily replied with the adage from my early MBE days: “do one miss one” in regard to the order to tighten the nuts, however realised as I said it that when taken literally one would only ever tighten 3 of the 6 nuts. The more correct but slightly longer adage would seem to be: “Do one miss one, until you land back where you started…when you do, do not do that one but instead the next one, then do one miss one in the opposite direction and try to ensure even loading by only increasing the tightness by ¼ a turn on each nut” Needless to say this is expressed more simply in a diagram, and so I direct you attention to figure 1. Once you reach nut 6, restart again from nut 1 and continue until the copper of the gasket is just visible between the flanges (~1.5 mm) or for those of you with a torque wrench set it to 20 Nm.

Figure 1: CF-40 flange nut tightening sequence

However that made me think: what is the correct sequence to use on other flanges? Well both a CF-16 and CF-40 flanges have 6 nuts so the sequence for both is shown in figure 1. Generalising the sequence you would come up with a do one miss (n/3) -1, where n is the total number of nuts on the flange. Or put a different way, tighten every (n/3)^{th} nut. Great (!) but what about when the number of nuts is not divisible by 3? Like on a CF-63, 100 or 160 flange? Hmm. Well the whole point of the sequence is to evenly bite the gasket. So for any flange the “triangular-reverse-direction” loading would seem logical. Hence for a CF-63 flange you would use the sequence in figure 2 and for a CF-100 the sequence in figure 3. This same selection rules can be applied to CF-160 flanges.

Figure 2: CF-63 flange nut tightening sequence

Figure 3: CF-100 flange nut tightening sequence

Once you reach CF-200 you have 24 holes (n/3 = 8) and the sequence is a multi-pass version of the original CF-40 flange shown in figure 4. Here I have made the first half of the sequence green and the second half black so you can see the pattern is still essentially “do one miss one” at the halfway point.

Figure 4: CF-100 flange nut tightening sequence

Armed with these sequences and a good set of spanners I wish you a leak free maintenance cycle.

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Hi Faebian,

Thanks for the article. However, I don’t quite understand the tightening sequence of CF100 and CF200. Take CF100 as an example, the 4th nut to tighten according to my understanding would be the nut labeled as 7 in your graph.

Do you have a tightening sequence for a 13.25” cf flange with 30 bolt holes?

Thanks. Looking forward to your reply.

Hi Jeff,

Thanks for the comment. The primary objective here is to avoid see-sawing. To do this one needs to maintain some kind of triangular sequence. This is easy when the number of holes is divisible by 3. The CF-100 with 16 holes does not fit in the simple model. For the CF-100 you would need to tighten every 5.333 nuts. Therefore one needs to make an approximation, either tighten the 5th, then 6th, then 5th nut, and repeat. Or as I attempted in the diagram to tighten every 6th nut, and switch to the opposite side when I land on a nut I have already tightened. Perhaps I should replace the CF-100 sequence with the former version?

As for the flange with 30 nuts: you will need to tighten every 10th nut with a similar sequence to how one tightens every 8th nut on a CF-200 flange.

Ultimately one should bear in mind that CF flanges are somewhat more forgiving than other flanges. The two important rules are do not tighten each nut as tight as you can each time (ideally use a torque wrench at 20 Nm until you get a feel for the torque needed) and try to tighten a nut 60* away from the most tight nuts on the flange.

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