Compressed Air Required For Fire Sprinkler System Application?

I would suspect that you would have an electronically controlled valve preventing the water from entering the system until the demand becomes necessary, ie: a fire melts the little fuse link at the sprinkler head where the action is. This is called a dry system and is required where freeze conditions exist. Some compressed air/nitrogen must exist in the line to prevent tripping the electronic signal, that is what alerts the solenoids to act, opening the main valve that permits the water to enter the pipeline. It is not a great deal of pressure or you would have a real maintenance problem with the sprinkler heads.

TMF

Pls specify why you need compressed air. If it is just for the sprinkler pipe to keep it dry and pressurised, you only want a makeup - which is not major. I think there is some mention of the air makeup required in NFPA13/850.

Question: is it the air as a fail safe pump?

In this case it depends on the maximal pressure you can generate.

The time is long so that you can consider an isothermal pressure reduction and you can write that the volume at high pressure is equal to the volume at low pressure multiplied by the pressures ration. Up to you to choose which way you accumulate the air. Of course a safety coefficient of at least 1.25 should be considered since over long time periods leaks can occur. The best would be to have a small compressor and fill automatically the high pressure reservoir if pressure drops under a safe threshold.

Between high and low pressure you should have a valve limiting the low pressure to the level you want to maintain it.

Multiply the 475 Gals/Min x 60 Min to get the volume of water you need to replace with air, convert it to cubic feet and convert it to SCF. As long as your air receiver pressure does not drop below 60 PSIG plus line losses to water tank with this Std Cubic Feet of volume removed you are okay.

Note, at a previous company I worked at we used a similar system for Fire Suppression in a Hyperbaric Chamber.

One gallon(US) is .13386 cubit foot of volume. So 475 gpm of water is 63.498 cfm of water.

I would rate the compressor about 20% larger in cfm.

Check with the licensed qualified individual with your company.He/she is qualified to make the calculations required.If you or your company are not licensed, you are breaking the law and are putting lives and structures at risk.