thanks f81aa very much.I appreciate you spending so much time for giving me a hand.
in the handbook "fire fighting pumping systems at industrial facilities" which is uploaded in this forum,page 32,about the fire water storage tanks,it says "Storage tanks should have a means of automatic refill once a present low level has been reached. A refill time of not more than 24hours is recommended. Low level alarms to protect the pumping unit should be provided.
so, there are lots of diffefences in different handbooks or standards.I am wondering.