What the Fire Dept. wants is the tanks outside the building. That is what the code term "a wall between the forge and the tank" means. If you forge inside your shop, just get a bulk tank delivered from the local gas supplier, pipe the gas through the shop wall with black iron piping or copper tubing, and you will be in business ( you may have to get it inspected by the fire marshal). It is usually cheaper to have bulk tanks filled by delivery, too. If you are using the forge just outside your garage/shop, and that is why they say there needs to be a wall, just build a 3X5 foot "L" shape cinderblock wall that is 4 feet high ... and put the bulk tank on the other side. Put a quick connect propane fitting on the tank, and snap on your 20" propane hose when you need to forge. Don't leave the hose attached to the tank for many reasons ( squirrels love to gnaw on them for one).
Jewelers have had to deal with this law. We all used to put a 20# tank in the corner, and run hoses to the torches. The law made that illegal, and we had to either put the tanks outside the building and pipe in the gas, or use 16 ounce tanks inside. Most all jewelers chose the small disposable tanks.
Now, this may all be a problem of a fire marshal reading the code wrong. As I said, the code is written the way it is to make it illegal to bring a propane tank larger than 16 ounces inside a building. This is a good law ( which many knifemakers ignore). There have been tragic fires and even explosions when these rules have not been followed. The law was primarily made to prevent BBQ grills being used within 20 feet of a residential building unless the tank was separated by a wall. This prevented people in high-rise apartments from putting a gas grill on the balcony. Good rule!
If you forge outside, the fire marshal may be applying the code wrong, because your forge isn't inside a building, and thus the "wall" rule does not apply.
Make a sketch of your forging area, take photos of your forge and tank, and go see the head fire inspector, or the Fire Chief. Tell him what you do with the forge and how you store it and the tanks when not in use. Many fire inspectors just aren't used to dealing with blacksmith/bladesmith folks in most cities. He may look at the info you provide and say it is OK. If not, ask him what changes you need to make to meet code. They want to work with you, and usually will be helpful. BUT, get in their face and you have created a worst enemy that can shut you down easily.....don't go there!
I am going to get on my soapbox as the safety curmudgeon here:
IT IS FOOLISH AT BEST, AND EXTREMELY DANGEROUS AT WORST, (and illegal in most all places) - TO PUT PROPANE TANKS INSIDE A GARAGE, FULLY ENCLOSED SHED, OR IN ANY OCCUPID BUILDING.
Make a covered rack outside, preferably at least 20 feet away from the building, and store them there. If you are worried about them being stolen, run a chain and lock through them, or make an expanded metal mesh door and lock it. If you are worried about rain and snow, drop a grill cover over the rack. If you have a lot of equipment to store, make a ventilated storage shed ( top and bottom ventilation - propane is heavier than air). Running a forge inside a garage is risky without good ventilation, and putting a 20 pound propane tank in the garage with it is just stupid. Final rant - buy at least one 10# AB fire extinguisher BEFORE you buy a forge or propane tank!
