Positive displacement rotary pumps utilize rotating mechanisms to displace fluid. These pumps trap fluid inside the pump casing. Then they move it from the intake to the discharge side due to their rotary motion. An example of this is the gear pump.
This pump has moderately low discharge pressures and can have very high discharge rates.
8.1.3 Other Piping System
Free flow systems are usually found on large crude carriers, where the cargo piping is not used for the discharge of cargo. Instead, gate valves are provided on the bulkheads of the tanks which when opened allow the oil to flow freely in the aft most tanks and into the cargo oil pump. Single line to single tank systems are normally found on chemical ships but can be found on product tankers too. This is a single line servicing an individual tank through an independent submersible / deep well pump.
They cannot be used for stripping as they lose suction once the liquid level falls too low.
Positive displacement pumps are typically used where the discharge rate is small to medium, and where the viscosity of the fluid is high.
8.1.4 Drop Lines
Drop lines connect the above-deck piping with the below-deck piping without going through the pumproom. They are only used to load cargo and their main function is to limit the amount of under- deck lines used in loading to give greater flexibility on discharge. They can also be used to circulate cargo.
8.1.7 Deep Well / Submersible Pumps
Deep well / submersible pumps are centrifugal pumps, where each tank is fitted with a dedicated pump. This improves the flexibility of the chemical / product carrier and minimizes the possibility of contamination. With the installation of these pumps, the need for a traditional pump room is eliminated, along with the hazards and complications of that space. Unlike standard Centrifugal pumps, modern versions of these pumps are self-priming, so will take suction from a full cargo tank even if they are started empty of liquid and have much better stripping capabilities.
8.1.6 Positive Displacement Pump
Positive displacement pumps are self-priming pumps and can be used as priming devices.
8.1.5 Centrifugal Pumps
A reciprocating pump is a specific category of positive displacement pump that functions by using a reciprocating motion to effectively displace fluid.
These are the most common type of tanker cargo pump and are designed primarily to move large volumes of liquid. Uses an impeller moved by means of a motor. Centrifugal pumps are not self-priming and need to be externally primed.
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