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Section view of a two-stage centrifugal main cargo pump showing flow.

The bulk of the cargo discharge will be done with the ship's main cargo pumps. On most tankers the main cargo pumps are centrifugal pumps, located at the bottom of a pumproom at the after end of the cargo tank range and powered by a drive shaft from a turbine or electric motor located in the engine room. Some smaller tankers and particularly chemical or product carriers are fitted with deepwell centrifugal pumps in some or all cargo tanks. While both of these pump types operate on centrifugal force principles, their operation is quite different.

5.26.1 Centrifugal pumps
Centrifugal pumps are not self priming and will not operate efficiently unless the cargo will flow freely to them by gravity. These pumps do not 'suck' the cargo from the tanks. They expel the cargo from their impellers, creating a void into which oil flows from the cargo tanks under the force of gravity and atmospheric pressure. The pumps must be carefully checked before starting to ensure that their chambers are full of cargo. If the pump is started without filling the chamber it can be 'burned out' in a matter of minutes, requiring an expensive overhaul before it can be used again.
When the cargo discharge is started, the full cargo tanks provide adequate positive head to move the cargo freely to the pump. As discharge proceeds it is important to establish and maintain a good trim by the stern so that the pump inlet remains below the cargo suction inlets in most or all of the cargo tanks.
Once the cargo is flowing freely through the pump, they require little attention, aside from regular inspections, until the tanks feeding them reach a low level. Pumping problems may be encountered when the cargo surface reaches the upper level of the tank bottom framing. While the cargo level is above the frames it can flow freely over them and into the section of the bottom framing containing the suction bellmouth. Once the cargo level falls below the top of the framing, the cargo must flow through the lightening holes and limber holes to reach the suction pipe inlet. If the pump speed is not reduced before the cargo surface reaches the level of the bottom framing, then the suction inlet framing section will be quickly emptied and the pump will 'lose suction'. When that happens the pump usually overspeeds and the turbine overspeed safety trips the throttle closed, to the considerable annoyance of the engineer (who is invariably doing some repair at the other end of the engine room when this happens).
To avoid the embarrassment of explaining why the pump oversped, the cargo watch officer must know the height of the ship's bottom framing and slow down the pump while there is still a meter of cargo above that level. The pump speed control and the pump discharge valve can then be manipulated to maintain pump suction and cargo flow until the tank is nearly empty. Main cargo pumps should not be used to drain the tank. The stripping pumps are provided for that purpose. If a full cargo tank of the same grade is available, its suction valve can be opened slightly to 'feed' the pump until the low tank is reduced to stripping level.
Centrifugal pump performance is determined by a combination of factors included on the following table, along with their range of values and influencing factors:

Factor Liquid column Remarks
Net positive suction head (NPSH) 4 - 5 m Single stage pump; function of pump design and discharge rate
Pipeline friction (PLF) 1 - 5m Tank distance from pump and discharge rate
True vapour pressure (TVP) 5 - 9m Gulf area crude oils
Pump + line above bottom (-h3) 2 - 2.5m Depends on ship design
Trim (+h2) 0 - 3.5m Depends on location of tank and phase of the discharge
Liquid level (+h1) 0 - tank ht. Depends on the phase of discharge
Atmospheric pressure (P) +/- 10m Depends on weather and IG pressure

NPSH factors for a centrifugal pump.

If the values of these variables are known, pump performance can be predicted by the following formulas (H = h1 + h2 - h3):

P + H > NPSH + LF + TVP - pump operating normally
P + H = NPSH + LF + TVP - pump begins cavitating
P + H < NPSH + LF + TVP - pump has no suction

Cavitation causes vapour bubbles to form in the pump as the cargoes light ends boil off. The pump will make an audible (loud), rattling sound and its speed and discharge pressure will fluctuate. If allowed to continue, the cargo watch officer will shortly receive a call or a personal visit from the chief engineer demanding to know why the cargo pumps are being abused!
In some tankers the main cargo pumps are equipped with vacuum or vapour extractors which can be used to enhance the draining capabilities of the pumps. However, the most efficient (shortest), discharge will usually be achieved by using the centrifugal pump for what it was designed to do: move the bulk cargo. The stripping pumps should be used for their designed function of draining the last of the cargo from the tanks.

5.26.2 Deepwell pumps
Deepwell pumps are centrifugal pumps designed to be mounted in the cargo tank. They have a special application in chemical carriers, where each tank is fitted with a dedicated pump. This improves the versatility of the chemical or product carrier and minimises the possibility of contamination that can occur when one main cargo pump is used to discharge several products. Their design is best suited for refined products, cargoes with viscosities of 1000 SSU or less (although products up to 2000 SSU have been pumped).
Because the number of pumps is larger, the individual pumps themselves are smaller and individually require less power. This makes possible the use of explosion-proof electric motors on the tanker deck as prime movers for the pumps. Use of electric motors facilitates direct control of pump start and stop functions from the cargo control room. Power and load are indicated by volt and ampere meters in the CCR.
With a deepwell pump installation, the need for a pumproom is eliminated, along with the hazards and complications of that space.
The modern deepwell pump is made up of a several centrifugal pumping stages, mounted in series on a common vertical shaft. The shaft and impellers are mounted at the lower end of a pump assembly column, which is lowered into a deep cylindrical well (deepwell). In some installations the bottom of the pump well is recessed into the tanker's double bottom, providing a design which permits 100% drainage without use of stripping pumps.
Modern deepwell pumps are self priming, so that they will take suction from a full cargo tank even if they are started empty of liquid. The self priming feature also permits stripping of cargo tanks to much lower levels than is possible with standard centrifugal pumps. When a tank has been discharged with a deepwell pump only a small, air operated, reciprocating pump (mounted in the bottom of the tank), is needed to strip out the residue and then empty the bottom of the cargo pump well. The more common method of evacuating the deepwell is with an air or nitrogen purge system. The gas is piped through a small-bore line at the base of the pump and pushes the contents up the main discharge riser.
When a cargo tank is being stripped (or drained), with a deepwell pump, the pump cycles as its self-priming function is activated, dropping speed and pitch as it fills with liquid and then increasing in speed and pitch as is loses suction and drops the contents of its discharge column back into the well to re-prime itself. Pump speed should be reduced when stripping (if speed control is available). If the pump is powered hydraulically, it can be regulated to any speed desired to permit maximum cargo recovery.

5.26.3 Screw pumps
Several 40,000 DWT product tankers have been fitted with gear or screw pumps as main cargo pumps. This provides good cargo recovery with single pump stripping ability, plus flexibility in the use or steam or electric motors as the prime mover. Screw or gear pumps consist of two meshing gears or screws which move the cargo between the pump casing and the gears or screws teeth as the gears/screws are rotated. They are very efficient and particularly suited for pumping high viscosity oils, such as lubricating oils, or molasses.

5.26.4 Priming the pumps
Priming a deepwell pump is a simple matter of opening the pump well suction valve and venting the upper level of the pump well to allow the pumping stages to fill with liquid. Once a deepwell pump has established suction, it will re-prime itself until the tank is completely empty. Because of the re-priming feature, it is more difficult to damage the pump by running it 'dry'.
Priming a centrifugal pump is a little more difficult. It must be carefully filled with cargo before being started and when started must be carefully checked to ensure that cargo is moving through it.
If suction is lost while discharging, a centrifugal pump will not re-prime itself and the pump cannot normally be re-primed while running. It is necessary to stop the pump before it can be refilled with cargo and restarted. Centrifugal pumps are re-primed by stopping the pump and opening the vent valve at the top of the pump casing. When the vapour escaping from the vent line is replaced by a steady stream of cargo the pump is primed and ready to restart.
If a centrifugal pump is fitted with a vapour/air extraction device it should be placed in operation when the cargo level is low, before the main cargo pump is expected to begin losing suction, and operated continuously until the pump suction is finally switched to a full tank.
Centrifugal pumps depend on a continuous flow of cargo for cooling. They can be quickly damaged due to overheating if they are run 'dry', so they must be stopped as soon as it is suspected that suction has been lost. The impellers operate on very small clearances within the pump casing. If allowed to overheat, the impeller may contact the casing, causing physical damage or additional friction, heat and an explosion.

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