Fire water system design

NFPA codes are recognised as the minimum requirement but companies can increase this.

You're in luck NFPA now make any standards free! see
My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

Your best bet is to contact your fire insurance carrier. I do have guidelines, however, they are not intended for chemical plants and they are 25 years old. My guidelines are based on the number of 2-1/2" hose streams each based on 250 gpm for particular durations depending on the exposures. If you do find the requirements for the number of hose streams and the water supply duration, you must also include building sprinkler systems water demand. Normally fire insurance carrier that underwrite commercial line have at their disposal, specialists in the fire science and those are the people that you should contact. You may also check out factory mutual for further reference.