Centrifugal Pumps Study Guide

Introduction to Centrifugal Pumps

Overview of Pumps

• Definition:
  • A pump is a mechanical device or arrangement that causes water to flow at increased pressure.
  • The process of moving water using a pump is referred to as pumping.
• Types of Drivers:
  • Pumps can be driven by engines or electric motors.
• Four Principles Involved in Pumping Water:
  • Atmospheric pressure, centrifugal force, positive displacement, and movement of columns of fluid due to differences in specific gravity.

Need for a Pump

1. To pump liquid from a lower to a higher pressure area.
2. To increase flow rate.
3. To move liquid from a lower elevation to a higher elevation.

Pump Characteristics

Flow Rate (Capacity)

• Definition:
  • Flow rate is the amount of liquid delivered by the pump per unit time, measured in $m^3/h$ (cubic meters per hour) or $lps$ (liters per second).

Head (h)

• Definition:
  • Head refers to the energy content of a liquid in reference to an arbitrary datum. It indicates how high the pump can lift the liquid.

Total Head

• Definition:
  • The total head is the sum of the discharge head, suction lift, and friction loss.
• Discharge Head/Delivery Head:
  • The vertical distance to which the pump is able to lift liquid.
• Friction Head:
  • The pressure required to overcome the resistance to flow within the piping system.

Pump Terminology

Basic Terminology

1. Static Discharge Head:
   • The maximum vertical distance from the pump center line to the point of free discharge.
2. Static Suction Head:
   • The distance from the center line of the pump up to the free level of the liquid source above the pump.
3. Static Suction Lift:
   • The distance from the pump center line down to the free level of the liquid source below the pump.

Total Dynamic Head

• Definition:
  • Total head while pumping, which is the sum of dynamic discharge head and dynamic suction lift (or - dynamic suction head).

Net Positive Suction Head (NPSH)

• Definition:
  • NPSH is a measure of the absolute pressure present in the liquid. It represents the Net Positive Suction Head available at the pump impeller inlet.

Water Horsepower

• Definition:
  • Water horsepower (WHP) is the minimum power required to move water.
• Formula:
  • $WHP = \frac{QH}{75}$,
    where $H$ is the change in pressure measured in meters and $Q$ is the water flow rate in liters per hour (lph).

Classification of Pumps

Types of Pumps

1. Dynamic Pressure Pumps

   • Centrifugal Pumps
   • Propeller Pumps
   • Turbine Pumps
   • Jet Pumps
   • Air Lift Pumps

2. Positive Displacement Pumps

   • Reciprocating
   • Rotary
     • Piston Pumps
     • Gear Pumps
     • Diaphragm Pumps
     • Lobe Pumps
     • Screw Pumps
     • Vane Pumps
     • Rotary Plunger Pumps

Variable Displacement Pumps/Dynamic Pressure Pumps

• Distinguishing Feature:
  • There is an inverse relationship between the discharge rate and the pressure head—when the pumping head increases, the discharge rate decreases.
• Also known as Roto Dynamic Pumps.
• Components:
  1. Impeller
  2. Volute or diffuser-style casing
  3. Shaft
  4. Shaft sleeves
  5. Bearings
  6. Sealing arrangement
• Operation:
  • Centrifugal force moves the liquid rapidly out from the center of the impeller, along the vanes, exiting at the outer diameter.

Components of a Centrifugal Pump

Impeller

• Function:
  • The impeller utilizes centrifugal force to rapidly expel liquid outward.
  • As liquid is pushed out toward the outer edge, a pressure vacuum is created in the center, facilitating the intake of more liquid, thus maintaining a continuous cycle.

Casing

• Function:
  • The casing converts energy from the rotating impeller into a controlled and pressurized flow.
• Common Type:
  • The most common casing type is the volute, which resembles a snail shell, surrounding the impeller.

Pump Shaft

• Characteristics:
  • The impeller is affixed to a shaft, typically made of steel or stainless steel, which must be appropriately sized to support the impeller without excessive vibration or wear.
• Note on Sizing:
  • An undersized shaft can cause increased vibration and reduced pump lifespan, while an oversized shaft increases cost unnecessarily.

Coupling

• Definition:
  • Component used to connect a pump with its motor or power source.

Shaft Sleeve

• Definition:
  • A component that covers the portion of the shaft under the sealing arrangement, protecting the shaft and positioning the impeller accurately. Typically made from bronze or stainless steel.

Sealing Arrangement

• Definition:
  • Area where the shaft passes through the casing, referred to as the stuffing box; requires a sealing method to prevent leakage, employing either packing or a mechanical seal.

Pump Bearings

• Function:
  • Standard ball-type antifriction bearings used to support and hold the pump shaft, allowing it to rotate and withstanding all loads from the impeller's operation.
  • These bearings are designed for essential lubrication via either grease or oil.

Pump Components Summary

• Impeller Vane
• Drive Shaft
• Eye
• Volute Casing
• Outlet/Discharge
• Inlet/Suction
• Volute Chamber

Working of a Centrifugal Pump

Operation Overview

• Definition:
  • A centrifugal pump is characterized by an impeller rotating within a closely-fitting case, drawing in liquid at the center and ejecting it through an opening at the casing side due to centrifugal force.
• Process:
  • When the pump is filled with water and the impeller is rotated, the blades impart high velocity to the fluid, facilitating movement.

Pressure Dynamics

• As the liquid is thrown from the impeller, the pressure at the inlet decreases, allowing more water to be drawn in via atmospheric pressure through the suction pipe.
• The liquid then transitions into the casing, where its velocity reduces and is converted into pressure before being discharged out through the discharge pipe.

Cavitation in Centrifugal Pumps

Definition and Mechanics

• Cavitation occurs at the inlet of a pump impeller when small vapor bubbles form and subsequently collapse, creating shock waves that can harm the impeller's surface.
• Lowest pressure point within a pump is located at the eye of the impeller where liquid intake occurs, and when the pressure dips too low, vapor bubbles may emerge.

Consequence of Cavitation

• As fluid is accelerated by the impeller rotation, pressure quickly rises, causing vapor bubbles to collapse.
• The collapse generates powerful shock waves, leading to potential damage over time. Extended periods of cavitation can result in significant wear, degrading performance, and shortening pump life.

Acoustic Indicators

• Cavitation can produce distinctive sounds reminiscent of small rocks rattling inside the pump casing.

Pump Priming

Definition and Importance

• Priming refers to the process of filling the centrifugal pump with water up to its casing to initiate operation. Centrifugal pumps, unlike positive displacement pumps, have limited capacity for compressing fluids, including air.

Methods of Priming:

1. Using a foot valve to retain water within the pump.
2. Employing an auxiliary piston pump to fill the pump.
3. Connecting to an external water source under pressure for filling the pump.
4. Designing a self-priming construction.

Pump Characteristics: Specific Speed

Definition

• Specific Speed (Ns) describes the relationship between flow production and head generation by an impeller.
• Definition:
  • Specific speed is defined as the speed of a geometrically similar pump delivering $1 m^3/s$ of water against a total head of one meter.

Utility

• The specific speed value is crucial for comparing various pump performances and design characteristics.

Impeller Design Considerations

• Impellers with lower specific speeds are characterized by tight clearances, ideal for high head but low flow designs.

Pump Characteristic Curves

Overview of Performance Curves

• Key Components of Centrifugal Pump Performance Curves:
  1. Head vs Flow Curve (H-Q Curve)
  2. Efficiency vs Flow Curve (Efficiency Curve)
  3. Brake Horse Power vs Flow Curve (Energy Curve)
  4. NPSHR vs Flow Curve

Classification of Centrifugal Pumps

Based on Energy Conversion

1. Volute Pumps:
   • The impeller is offset creating a curved funnel, increasing the cross-sectional area towards the pump outlet, leading to fluid pressure increase.
2. Diffuser/Turbine Pumps:
   • In diffusers, fluid pressure rises as fluid is expelled between stationary vanes surrounding the impeller.

Based on Number of Stages

1. Single Stage CF Pumps:
   • Featuring a single impeller.
2. Multi-Stage CF Pumps:
   • Incorporates multiple impellers on a common shaft, routing discharge of one impeller to another's suction, enhancing efficiency.

Based on Impeller Design Types

1. Open:
   • Vanes free on both sides, suitable for small-diameter, inexpensive pumps.
2. Semi-Open:
   • Vanes free on one side and enclosed on the other, providing higher efficiency than open impellers.
3. Closed:
   • Features vanes enclosed between two discs, best suited for large pumps with high efficiency and low NPSH required.

Impeller Design Influence

• Impeller design significantly impacts pump performance; a well-designed impeller optimizes flow, minimizes turbulence, and enhances efficiency.

Based on Action Type

1. Single Action:
   • Liquid enters the impeller blades from one side only.
2. Double Action:
   • Allows liquid entry from both sides, reducing shaft force exertion.

Based on Axis of Rotation

1. Horizontal Pumps:
   • Shafts oriented horizontally.
2. Vertical Pumps:
   • Shafts oriented vertically.

Based on Drive Method

1. Direct Connected:
2. Geared:
3. Belt or Chain Driven:

Based on Type of Flow

1. Radial Flow Pumps:
   • Liquid enters at the center and is directed outward.
2. Axial Flow Pumps:
   • Liquid is pushed parallel to the pump shaft, similar to a boat propeller.
3. Mixed Flow Pumps:
   • A combination of radial and axial flow behavior, directing liquid at an angle greater than 90 degrees.