If an existing tank, maximum depth may be limited by design of the shell.
Allowable freeboard for seismic sloshing is a consideration.
Interference of a floating roof with the fixed roof or foam fittings, etc. may limit depth.
Adequate freeboard to allow overflow through specified overflow fittings/weir if applicable.
Keeping the roof structure entirely out of the product may be a consideration.
The tank liquid levels are normally set up to have LLL, NLL, HLL, etc., which could be defined as below:
- LLL, the proper height above the tank outlet nozzle or the pump NPSHr requirement
- NLL, defined per the Process and operation requirements
- HLL, under the overflow nozzle or or below the tank roof
The additional levels, LLLL and HHLL may be included in the level control in order to provide adequate response time for Operation.