Deep-Well Cargo Pumps

Axially split

Casing or housing joint that is parallel to the shaft centerline.

Barrel pump

A horizontal pump of the double-casing type.

Barrier fluid

Fluid introduced between pressurized dual (double) mechanical seals to completely isolate the pump process liquid from the environment. Pressure of the barrier fluid is always higher than the process pressure being sealed.

BEP

Abbreviation for best efficiency point; the point of capacity at which a pump achieves its highest efficiency.

Buffer fluid

Fluid used as a lubricant or buffer between unpressurized dual (tandem) mechanical seals. The fluid is always at a pressure lower than the process pressure being sealed.

Cargo Pump Stripping

To meet requirements for residue in the cargo tank, the pump has a built-in self-closing foot valve that will seat when suction is lost. Compressed air or nitrogen is blown into the pumpstack via the flange located at the pumps discharge flange. The stripping output line is connected at deck level to the discharge line or manifold. When the stripping has been completed, the remaining residues are below requirements.

Critical speed

Rotative speed corresponding to a lateral natural frequency of a rotor.

Critical speed (dry)

A rotor natural frequency calculated assuming that the rotor is supported only at its bearings and that the bearings are of infinite stiffness.

Magnetic Drive Pump

Magnetic drive pumps are a type of glandless or sealless pump; these pumps are characterized by the fact that their design and construction physically prevents shaft leakage to the surrounding environment. Magnetic drive pumps do not employ a moving process shaft seal of any description. Leakage of process liquid is prevented by a stationary physical barrier. Mechanical seal type of shaft seal cannot in any way be considered as leakage free, even if it is barrier liquid or gas or very dilute process liquid/vapour in minute quantities that escapes. The descriptions 'glandless pumps' or 'sealless pumps' are associated with the alternative descriptions 'magnetic drive pumps' or 'canned motor pumps'. Canned motor pumps employ special electric motors that are integrated into the pump, and are not considered here.

Sealless pumps, of appropriate construction, are used for the most dangerous liquids when considering toxicity, environmental hazards and radioactivity, and, in some cases, for high or very low temperatures or very costly liquids. Sealless pumps also prevent contamination of the process liquid; there are no auxiliary systems used (barrier liquid or gas) that come into direct contact with the process liquid.

Mechanical Seal

A typical "pusher" mechanical seal consists of a rotating face, a stationary face and secondary sealing elements with adaptive metal parts such as a flange or sleeve. The stationary face is seated in a flange which is bolted onto the pump cover. For most seals the rotating face can move in axial direction and is kept in place by a spring holder and one or more springs. The rotating parts are installed on a shaft sleeve or directly on the shaft. Key to low leakage is maintenance*. The "bellows" type is very similar but uses a welded metal bellows to achieve flexibility in the design.

* Mechanical seals are seen as a maintenance intensive item, difficult to install and set up if not cartridge style, with costly spares and attendant inventory costs. Removing the mechanical seal and operating pumps of a modified design can save overall costs. Paul Barnard, Consultant, but of Exxon Chemicals writing in World Pumps January 1992, cites a case where a magnetic drive pump costs 30% of a conventional pump over a ten-year operating life.