ROTODYNAMIC PUMPS

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Rotodynamic Pumps.

Basically defined by the flow pattern;

• Axial where the outlet flow is parallel to the drive
• Radial where the outlet flow is at right angles to the drive                                                            Mixed – a bit of both

Axial

The pumping pressure is formed by the propeller lift of the vanes.

They develop a very low head (max ~ 20m), but have a very large mass flow rate

Radial

The most common form of centrifugal pump.

Water enters the eye of the impeller and is thrown out by centrifugal force. As water leaves the eye of the impeller a low pressure area is created causing more liquid to flow toward the inlet because of atmospheric pressure and centrifugal force. Velocity is developed as the liquid flows through the impeller while it is turning at high speeds on the shaft. The liquid velocity is collected

by the diffuser or volute and converted to pressure by specially designed passageways that direct the flow to discharge into the piping system; or, on to another impeller stage for further increasing of pressure.

To make the conversion from velocity to pressure more effective, stationary diffuser vanes can be installed around the rim of the impeller. This construction gives rise to the term diffuser centrifugal pump.

The head or pressure that a pump will develop is in direct relation to the impeller diameter, the number of impellers, the eye or inlet opening size, and how much velocity is developed from the speed of the shaft rotation.

Capacity is determined by the exit width of the impeller.

All of these factors affect the size of the motor to be used; the more water to be pumped or pressure to be developed, the more energy is needed.

A double-entry impeller assures extremely low NPSH values.

The impeller has an inlet on either side, lowering the friction resistance of the incoming fluid.

These pumps operate with about a 25% reduction in NPSHR compared to a similar size end suction centrifugal pump

These pumps are used for high capacity applications, or any time you need a low net positive suction head required (NPSHR).

The double suction also prevents some axial thrusting of the impeller

 For higher pressures the pumps can be arranged in multiple stages, the discharge from one stage becomes the inlet to the next

Flow control can be arranged by using multiple speed motors (variable speed is not common).

1. Flow rate is directly proportional to the rotation speed s
2. Differential head is directly proportional to the s²
3. The power required is directly proportional to (Flow x Head) i.e s3
   
   Mixed Flow
   A pump in which the head is developed partly by centrifugal force and partly by the lift of the vanes on the liquid.
   This type of pump has a single inlet impeller with the flow entering axially and discharging in an axial/radial direction