Step 11. Install Vent Pipes.
For the vent pipes use 4 inch galvanized pipe (standard weight). This pipe is rated for 150-psi steam, 300-psi water and has a burst pressure of 1500-2000 psi. Since this pipe will be exposed to the effect of the blast wave, it needs to be able to withstand this shock. Have the pipes cut to a length of 4 feet and have one end of the pipe threaded. Install the pipe into a 150 lb. steel raised face threaded flange. (Vent Pipe) The pipe threads are tapered which means that as you turn the flange onto the pipe it will get incrementally tighter. (I recommend that you have the pipe supply store mount the flange onto the pipe, because in general, they will have the equipment to make this connection very tight.
I recommend that a 6-inch square of rubber with cutouts (for the boltholes and the 4-inch pipe openings) be used to provide shock insulation to the pipe to ceiling joint. (One for the outside and one for the inside.) The outside rubber sheet will also provide waterproofing for the joint. Place the pipe/flange assembly over the rubber sheet and mark the bolt hole locations, the flange outer circle. Cut the bolt hole locations. Put the rubber sheet back up to the flange; align the bolthole pattern and cut the inside 4-inch hole. Cut out a total of 4 of these rubber gaskets.
Place several continuous beads of 100% silicon caulking on the sheet of rubber to form a good seal. Turn this sheet over and align with the holes. This should form a watertight seal between the rubber and the 6-mil black plastic. Place several continuous beads of silicon caulking on the sheet of rubber and place the vent pipe/flange assembly onto the rubber, aligning the holes. (Detail 11.1) (Detail 11.2) This should provide a good seal between the rubber sheet and the flange.
On the inside of the shelter, place several beads of silicon caulking on the second sheet of rubber and marriage this side with the 125 lb. cast iron raised face threaded flange. Align the holes. Push this against the ceiling with the holes properly aligned. Drop a (5/8 – 11x4 Hex Bolt Zinc) bolt through each of the eight holes in the top flange, the top rubber sheet, the black plastic, the plywood, the bottom sheet of rubber and the bottom flange. Inside the shelter, thread a (5/8 – 11 Hex) nut on each of the bolts. Tighten these bolts snugly.