External Pressure and Balancing Forces on Ducting/Pipes

I am installing 1400mm ducting onto a flexible and the other side welded onto a vapour inlet duct. i ahve been told useing a flexi i will be putting external forces onto the ducting thus will have to allow for a greater load on the floor steels. i understand due to flexi contracting at 1mBar vacuum there will be a significant pull on the flexi. the flexible i have at the moment is:

and the ducting it will connect onto is:

How can i actually determin what the forces will be, if you look at the flexi it has some support rods, they would usually stop the flexi collapsing to much but can i use this a justification to say theyre will be hardly any movement.

the reason why i am putting the flexi in was due to expansion of vessel and ducting, but it is turning out to be a nightmare.

does anyone have any recommendations on calculating the actaul forces, or advise on the flexi or othe rtypes of connections???

Some advice i have already recieved.

Depending on your ducting layout you can get unbalanced forces, and flexibles cause real problems.

Imagine this you have a length of 60" ducting, capped off at both ends and the whole thing is on sliding supports, not bolted down. You now pull 1 mbar vacuum on the inside of your ducting and what happens? Nothing, you have a rigid structure and all forces are balanced.

Now you put a flexible in the middle of the ducting and pull the vacuum again, what happens? The flexible is compressed because the forces on the capped off ends do not balance each other until the flexible is compressed sufficiently to transfer the required force through it.

This however is not the purpose of the flexible, it is to allow for minor movement due to thermal changes etc. and the flexible should not have major stresses like this applied to it.

So we need to bolt down the two ends of the duct so the unbalanced forces cannot be transmitted through the flexible. What force does the holding down arrangement need to with stand? Answer = cross sectional area by pressure = 60" x 60" x Phi / 4 x 14.5 psi = nearly 41,000 lbs, about 18.5 ton.

In the case of a 90 degree bend with flexes each end and the bend unsupported you would get two 18.5 ton forces acting at right angles to give about 26 ton at an angle of 45 degrees.

Bottom line is it depends on your ducting layout and design, especially if there are flexibles, and is the equipment at either end fixed down and capable of absorbing the forces.