Impeller and Casing of is the most important parts of marine centrifugal pump, usually material of impeller is bronze, material of casing for fresh water is cast iron and material of casing for seawater is bronze.
(1)Pump’s Impeller
A typical marine centrifugal pump has one or more shaft-mounted vaned impellers that rotate within a stationary casing and transfer energy to the fluid flowing between their vanes.
The impellers can also be classified based on their shroud configurations: the vanes of a closed impeller are located between both front and rear shroud; vanes of a semi-open impeller are attached to a single rear shroud; and an open impeller’s vanes are attached to the periphery of a hub and, in some cases,a partial rear shroud. The impeller in high specifc-speed axial-flow pumps has no shrouds and frequently is referred to as propellers.
There are also double-suction impeller and single-suction impeller. For double-suction, liquid enters the impeller through two inlets, or entrance eyes. A single-suction impeller, however,has only one entrance eye.
(2)Pump Casing
The discharge path of a casing includes a collector to catch the fluid discharged from the impeller and a channel to guide this fluid either to the pump’s discharge connection or, in the case of a multistage pump, to the inlet of the next stage. Additionally, the casing ordinarily has some types of diffuser in which a portion of the velocity head of the liquid being discharged from the impeller is converted to static pressure head. Referred to as pressure recovery, this conversion is necessary due to the relatively high absolute velocity of fluid leaving the typical centrifugal pump impeller.
One of the most common types of collectors used in single-stage radial-flow pumps and mixed-flow centrifugal pumps is the volute, which is a scroll-shaped channel with a gradually increasing radius and cross-sectional area that surrounds the periphery of the impeller. To increase pressure recovery, the fluid leaving the volute is generally decelerated in the casing’s discharge nozzle, which forms the transition from the volute’s throat to the pump’s discharge connection. As an alternative to a spiral volute, some centrifugal pumps are furnished with a circular or concentric collector having a constant radius and cross-sectional area. In addition, a modified or semi-concentric casing design, in which the radius and cross-sectional area of the collector remain constant over only a portion of the circumference,is sometimes used.
Volutes are also used in some multistage centrifugal pumps. When a multistage-volute pump has an axially split casing, the flow passages that connect successive stages, which are referred to as crossovers, may be integrally cast with or welded onto the casing. Alternatively, a multistage pump may be fitted with multi-vaned diffuser, which is also used in some single-stage radial-flow pumps, contains a number of diverging vanes mounted in a ring that surrounds the periphery of the impeller.
