Feedwater is drawn into the pump through this connection.
The type of bearing used depends upon the pump design. Due to the usage of multiple impellers, some axial thrust is transferred to the bearings. Thus a suitable bearing should be chosen to handle both axial and radial thrusts.
Mechanical seals are used to seal the space between the pump shaft and casing. The seals prevent feedwater leaking out of the intended flow area.
The pump shaft is installed through the centre axis of the pump. The impellers, mechanical seals and bearings, are all installed onto the shaft. Shaft keys are used to mechanically connect each impeller to the shaft.
1st Stage Impeller
The first centrifugal impeller that the feedwater comes into contact with is referred to as the 1st stage impeller. Fluid flows into the centre eye of the impeller.
The casing must withstand the high system pressures to which it is subjected, as well as high temperatures. A boiler feedwater pump will typically discharge at pressures exceeding several hundred bar (>2900 psi), the casing must be able to withstand this pressure.
After passing through several centrifugal pump impeller stages, feedwater is discharged through this connection.
As liquid is discharged from the impeller, it flows through the diffuser. The shape of the diffuser causes the liquid’s velocity to decrease and its pressure to increase (Bernoulli’s principle); a volute casing performs a similar function, but it is not always practical due to its size.
Fluid flows into the eye of the impeller and then outwards radially. As fluid moves outwards through the impeller vanes, its kinetic energy is converted to pressure energy. There are three types of centrifugal impeller, these are the closed, partially closed and open types; the type used depends upon what fluid is being pumped.
Fluid discharged from the 5th stage impeller flows through the channel (bypass line) shown above this annotation.
After flowing through the bypass line, fluid enters into the 6th stage impeller.
Fluid is discharged from the 10th stage impeller through a passage in the casing located directly below this annotation.