What is the need for circulation pumps in the fire fighting tanks?

for cleaning and preventing the foaling on the tank surface.

from NFPA

((((((((((((((((((Where a continuous solvent flow circulation is maintained by means of a circulating pump, solvent coolers shall be provided to maintain a solvent temperature not exceeding90°F (32.2°C). Visual and audible alarm devices shall be provided to warn the operator when the solvent temperature exceeds90°F (32.2°C).)))))))))))))))))) 

PS. I never install that kind of pump and i am only traing to help a friend

In the event of putting fire out, the delivery pressure from the water hydrant is very critical as the firemen progress to extinguish the fire. The circulation pumps help to maintain the water delivery pressure to ensure continuous flow of water as the firemen combat the fire.