If a tank is to be filled above its normal fill level (normal capacity) up to its safe fill level (tank rated capacity), a
trained and qualified person should be assigned by the facility operator to monitor the operation and, if necessary, take action to stop or divert product flow to prevent an overflow.
The normal fill level is established at the lower of the following two levels so that appropriate action can be taken to prevent an overfill:
a. A level that allows sufficient time for the flow to be completely stopped or diverted before or when the level of the
product in the tank reaches the pre-established safe fill level. (Normal fill level never exceeds safe fill level.)
b. A level, determined by the facility operator depending on the tank's physical condition (leaks, structural strength, and
so forth) or the operating practices (e.g.~ partial or split product receipts, relief allowances, and so forth), that limits the
tank capacity to less than that normally available at the usual safe fill level.
It is general practice to operate to 90% full as the normal fill level. Safe fill level would be 100% full and the tank would start to overflow after that 100% full.
The 10% margin is a prudent margin so the liquid in the hoses can stop flowing, time to read the gage and tell the operator to stop the pump, etc all can occur. It takes a little more volume just to drain out the hoses safely without spilling.
