The job of the inlet is to convey the liquid from the inlet pipe to the impeller entrance in a fashion that imposes minimal loss, and creates the most uniform velocity profile at the impeller entrance. Therefore, the ideal inlet geometry is a straight pipe entrance with a slight taper from the pipe flange to the impeller eye. The taper slightly increases the velocity and tends to stabilize the fluid streamlines prior to the impeller.
All curved inlets cause at least a minor penalty and in some cases a major efficiency penalty. This includes straight inlets with an elbow attached close to the suction flange. It is preferred to have at least 5 diameters of straight pipe the same size as the inlet flange, leading up to the pump inlet to prevent non-uniform velocity profile at the impeller eye. With submersible pumps the inlet configuration is planned for you by the manufacturer based on the mounting of the pump. Additional features may needed to be included in a tank mount to optimize the operation of the pump.
This diagram is a computational fluid dynamic model of the flow passing through a standard elbow. Red is High velocity fluid and blue is Low velocity fluid. The fluid is moving down the pipe to the left and exiting towards the right hand side of the page. As you can see, just after the bend the high speed flow is carried by momentum to the bottom of the pipe and the top of the pipe is filled with low speed fluid. If I pump my pump inlet just after the bend, I have high speed liquid moving into the bottom portion of the impeller and low speed liquid moving into the top. This causes one side of the impeller to act like it is pumping at a high flowrate and the other side to act like it is at a low flowrate. This causes problems with cavitation and the bearings in the pump. Moving the pump out to 5 diameters pipe away from the bend gives us a uniform flow profile and good pump performance.
Pumps mounted in tanks require that they are offset from the bottom of the tank. There is also a need for a minimal amount of submergence to prevent the pump from vortexing when operating. The physical sump dimensions are also critical for proper pump performance. An excellent source for intake and sump design can be found in the Hydraulic Institute’s ANSI/HI 9.8 Intake Design Committee Document.