plumbing concerns connecting to 5k poly tanks

[mctabish]

I am coonecting 4 5k tanks for supply water for rural fire hydrant.
The tanks have a 11.75' diameter, and I am required to connect using 4" bolted double female threaded flanges. Inside flange is sch80 pvc, outside is galv steel.
my concerns are as follows:
the tank is round, yet the flanges are flat. I beleive that the tanks have enough give, yet I am wondering if it would be best to use 2 neopreme gaskets on each side instead of one.
The secnd, and bigger concern is that the bolts that hold the flanges together, dont seem to have anything for preventing water from flowing around the bolt shafts, except for the flat metal washers. Should I also use some sort of rubber washer under the bolt heads?, if so, would these "squish out" when I torque to 25lbs?


Thanks in advance!
Bruce

 

[frodo]

usually, a flange is welded onto the tank, but when you do not have a flange welded to the tank, you need a bulk head fitting

http://www.pvcfittingsonline.com/fittings/schedule-80-pvc/bulkhead-tank-adapters.html?size_in_inches=27

http://www.flexicraft.com/Braided_Flexible_Connectors/FF/

you will also need flex connectors to keep the bulk head fittings from being moved by pump vibration or ground movement
theses are just images of what you need, i did not match bolt patterns

 

[mctabish]

These are for fire reserve, and what the specs say is "Double female bolted tank fitting", that is why I am going this route...
I do like the idea of the flex, but our architect did not spec them, and now I don't have room to add.the attached Diagram is from the Fire department as rural water requirements.
My main concern is how to seal around the bolts, as they are BELOW the water in the tanks, and I don't see how they would be sealed. A local plumber looked at the diagram and said he would also suggest the bulkhead, but since we are following the diagram, he said to coat all of the bolt heads with silaflex.

Thanks in advanced!

 

[frodo]

http://www.barrplastics.com/pvc-bolted-double-flange-fittings.html#.V6aoaTVT3RY

a bulk head fitting has a gasket that encompasses the whole thing, bolts included