Turbomachinery_Pumps

Turbomachinery

It is defined as, "a dynamic fluid machine that adds energy to or extracts energy from the fluid".

• Mechanical energy of rotation (rotation)   ←Transfer→   fluid flow energy.
• Mainly deals with two turbomachine pumps and turbine.

Pumps (which require power source to operate):

• Which adds energy to the working fluid.
• Usually a motor exerts a torque by using blades to force the fluid to move.

Turbines (which is the source of power):

• Which extracts energy from the working fluid.
• Fluid pushes the machine and produce shaft power.

Classification On The Basis Of Flow Direction

Turbomachines are classified on the basis of flow directions:

1. Radial Flow machine   ↔   fluid flows in radial direction.
2. Axial Flow machine   ↔    fluid flows along component axis.
3. Mixed Flow machine   ↔   some machine have both radial and axial components.

Working Principle

During the fluid-blade interaction, either the fluid is pushed or the blade is pushed due to which energy transfer takes place which will increase/decrease fluid stream velocity.

• Pump   ↔   Velocity is increased in the direction of the rotation of the rotor.
• Turbine   ↔   Velocity is increased in the direction opposite to the rotation of the rotor.

Rotodynamic or Centrifugal Pumps

It is defined as, "a turbomachinery that uses the rotation of an impeller or propeller to impart velocity to a liquid".

• Rotation is continuous   ↔   pumps the fluid continuously.
• Centrifugal Pump   ↔   If pump uses the radial flow impeller.
• High speed rotating impeller throws the incoming liquid radially outward due to centrifugal effect.

There are 2 main components of centrifugal pump:

1. Impeller (Rotating)   ↔   rotates and throws fluid outward.
2. Casing with a Volute Chamber (Stationary)   ↔   fluids impart energy with increasing area.

Working Principle

• As impeller rotates, the fluid at the center is impelled outwards due to centrifugal force.
• Empty spaces creates a suction (or negative) pressure for the incoming fluid, which fills the center space.
• The fluid is discharged into the purposefully designed volute chamber (Velocity reduced, Pressure increased as Area increased) to overcome pumping system resistance.
• Priming   ↔   filling all the part up with water before it is started (initiates working).

Centrifugal Pump Configurations

Centrifugal pumps are divided into the following types:

A. Type of Impeller

1. Open Impeller   ↔   no plates.
2. Semi-Open Impeller   ↔   plates is on one side.
3. Shrouded or Closed Impeller   ↔   plates on both side of blades.

B. Suction Sites

1. Single Suction   ↔   single eye.
2. Double Suction   ↔   two suction chamber using single pump with non-drive end bearing.

C. Staging 

1. Single Stage 
2. Multi-Stage   ↔   exit of 1st pump serves as input of 2nd pump.

Preventing Leakage

• To avoid leakage, a packing box (or Stuffing box) is used on main shaft just behind the impeller.
• Packing Glands   ↔   used to tighten up the seal with a nut as the fluid dripping out exceeds the normal range.
• Due to continuous rotation, packing rings are heated up  ↔  use Lantern Rings (have pores) normalize temp using lubricants. 

Analysis of Velocities

Euler Turbomachine Equation:

• For inflow   ↔   -ve sign.
• For outflow   ↔   +ve sign.

Ideal Pump Head

Pump efficiency or performance is usually define in terms of heads. The possible ideal head which is achieved is:

Actual Pump Head

The actual output head (ha) is always lesser than the ideal head (hi) due to the following factors:

• Skin Friction
• Unavoidable Leakages
• 3D Flow effects

Energy Analysis & Efficiency

Pump Performance Curves

For selection of best pump for your application, these curves are used.

• Losses are due to Viscosity, Surface friction, Pipe assembly configurations (pressure drop at sharp turns or T-joints and Elbow Joints).
• Characteristic or Performance curves are usually plotted b/w ha, η, Power VS Q.
• Selection of pump   ↔   specify the head required (curve H vs Q).
• Even the same pump with different rpm/diameter produces a new curve.
• If operation point is on/below the curve, pump is selected after considering efficiency.

Some useful points are:

• Shutoff Head    ↔   Head developed by the pump, when the discharge valve is close (if for more time, damage the pump).
• Operation Point   ↔   should be near the peak efficiency point.
• Brake horsepower   ↔    useful power produced by the motor.

Net Positive Suction Head

On the suction side of pump, low pressure (boiling occurs) are often developed causing Cavitation.

• Greatly affects the pump efficiency.

To avoid cavitation:

NPSHA (available)  >  NPSHR (min. value to avoid cavitation)

NPSH = (Total Head at suction side)  -  (Liquid Vapor Pressure Head)

System Equation

It relates the actual head 'ha' gained by the fluid to the flowrate 'Q'. 

Pump Arrangements

Some points for Series Arrangement are:

• Generate high heads, provide regular boosts along long pipelines without large pressure at any point.
• Combined power is the sum of brake power for operating point flow rate.

Some points for Parallel Arrangement are:

• It permits a large total discharge.
• Pumping can continue if one is not operating due to failure or planned maintenance.

References:

• Material from Class Lectures + Book named Fundamentals of Fluid Mechanics by Munson, Young & Okiishi's (8th Edition) + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube (Engineering Sights).