Sequential vs flow-through
Ballast water exchange is still used on some ships that have not yet fitted a treatment system, or as a contingency. Sequential exchange means emptying a tank and refilling it with mid-ocean water, so about twice the tank volume is pumped. Flow-through exchange keeps the tank full and overflows at least three times the tank volume so that about 95% of the original water is replaced. Stability, strength and weather must be checked before any tank is emptied at sea.
How this calculator works
Volume to pump = Tank volume × 2 for sequential, or × 3 for flow-through. Time = Volume to pump ÷ Pump rate. Real time is longer if two tanks share one pump, if the ship is rolling, or if the overflow is restricted. Record start and stop times in the Ballast Water Record Book. Treatment-system ships must follow the BWMP and D-2 standard, not this exchange estimate.
Who should use it
Useful for officers planning an exchange sequence. Always use the approved Ballast Water Management Plan and the Master’s stability limits.