Stripping Pump

What sets a stripping pump apart

Main cargo pumps are centrifugal machines built for high flow rates against a reasonably full suction; once the tank level drops and the pump starts drawing air, a centrifugal impeller loses its prime and cannot recover on its own. A stripping pump is a positive-displacement unit -- reciprocating, screw or eductor-driven -- built to keep moving liquid even as the suction turns to a slug of vapour and residue. Its job is not speed, it is the last one to two percent of the cargo that decides how clean the tank certificate can be.

Main components

Pump element

Typically a positive-displacement reciprocating or twin-screw unit, or on many modern tankers a steam or water-driven eductor with no moving parts in the cargo stream at all.

Stripping line and manifold

A dedicated small-bore line separate from the main cargo line, run to a low point in each tank so the last liquid can be drawn without competing with main-pump suction.

Vapour handling

A gas-liquid separator or vent arrangement ahead of the pump, since stripping duty routinely ingests a mixed liquid-vapour stream that would cavitate a centrifugal pump instantly.

Selection criteria

• Cargo viscosity and vapour pressure -- eductors need a driving fluid supply and lose efficiency on high-viscosity products; reciprocating pumps handle a wider range but need more maintenance.
• Stripping rate specified by the charter party or terminal, often expressed as a minimum final ullage or a maximum time to complete.
• Compatibility with the cargo grade, since stripping systems on chemical tankers must be segregated per tank the same way the main system is.

Regulations and class

MARPOL Annex I and Annex II set the standard for how completely a tank must be stripped before ballasting or tank washing, and the stripping system's effectiveness directly determines the residue quantity that ends up in a slop tank rather than overboard. Class rules require the stripping system to be tested during cargo trials and included in the pump room's periodic survey.

Typical faults

• Air ingress through a worn suction valve, which reads as low stripping rate and is often misdiagnosed as a worn pump.
• Eductor driving-fluid pressure too low, common when the driving pump is shared with another duty and demand peaks simultaneously.
• Stripping line blockage from wax or polymer buildup on cargoes prone to solidifying at ambient tank temperature.
• Packing gland leakage on reciprocating types, a slow failure that shows up as cargo loss rather than a sudden alarm.

What to look for in a supplier

• Cargo compatibility data for the actual product range the vessel trades, not a generic chemical resistance chart.
• Spare parts commonality with the main cargo pumps where the fleet standardises on one manufacturer.
• Documented stripping performance curves at realistic end-of-discharge suction conditions, not just clean-water test figures.

A stripping pump that looks fine on a water test can still fail the charterer's dry-tank requirement on a viscous cargo -- always check the stripping curve at the actual cargo viscosity and temperature, not the manufacturer's default figure.