Pumps and Working Principle

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Questions and Answers

What is a pump?

A pump is a mechanical device that is used to provide energy to a fluid to move it from one place to another.

Which of these is NOT a classification of pumps based on method of displacement?

  • Reciprocating pumps
  • Positive displacement pumps
  • Rotodynamic pumps
  • Centrifugal pumps (correct)

Positive displacement pumps apply direct pressure to the fluid.

True (A)

How do rotodynamic pumps function?

<p>Rotodynamic pumps increase the velocity of the liquid with the help of a rotating vane impeller, producing head and flow.</p> Signup and view all the answers

Which type of positive displacement pump uses oscillating pistons, plungers, or membranes?

<p>Reciprocating pumps (B)</p> Signup and view all the answers

Diaphragm pumps are commonly used to handle hazardous or toxic fluids.

<p>True (A)</p> Signup and view all the answers

What are gear pumps, lobe pumps, and screw pumps collectively known as?

<p>Rotary pumps</p> Signup and view all the answers

Rotary pumps require check valves for fluid movement in one direction.

<p>False (B)</p> Signup and view all the answers

What type of fluid are gear pumps, lobe pumps, and screw pumps best suited for?

<p>Viscous fluids</p> Signup and view all the answers

Which pump type is known for its ability to handle grit and slurry?

<p>Centrifugal pumps (C)</p> Signup and view all the answers

Reciprocating pumps typically require higher speeds and result in a high discharge rate compared to centrifugal pumps.

<p>False (B)</p> Signup and view all the answers

What is the main component in a centrifugal pump that rotates to generate fluid movement?

<p>Impeller</p> Signup and view all the answers

The casing of a centrifugal pump is a non-airtight chamber surrounding the impeller.

<p>False (B)</p> Signup and view all the answers

What is the purpose of the volute in a centrifugal pump?

<p>The volute gradually increases the area of flow from the impeller outlet towards the delivery pipe.</p> Signup and view all the answers

Which of these is NOT a type of head commonly discussed in the context of centrifugal pumps?

<p>Pressure head (A)</p> Signup and view all the answers

What is the manometric head (Hm) in a centrifugal pump?

<p>The manometric head represents the total head against which the pump has to work.</p> Signup and view all the answers

The pump overall efficiency is determined by comparing the power output of the pump to the power input to the pump.

<p>True (A)</p> Signup and view all the answers

What is the term for the pressure head created by the weight of lifted water in a centrifugal pump?

<p>Water horsepower</p> Signup and view all the answers

The power input to a centrifugal pump is determined by the power supplied by the electric motor.

<p>True (A)</p> Signup and view all the answers

What does the specific speed (N_s) of a centrifugal pump represent?

<p>The specific speed is a dimensionless parameter that represents the ratio of impeller speed to the flow rate and head produced by the pump.</p> Signup and view all the answers

The head coefficient in a centrifugal pump relates the head to the pump's diameter and impeller speed.

<p>True (A)</p> Signup and view all the answers

What is the significance of the power coefficient in a centrifugal pump?

<p>The power coefficient indicates the relationship between the power required by the pump and its operating conditions, including diameter, impeller speed, and fluid properties.</p> Signup and view all the answers

The capacity coefficient relates the flow rate to the pump's diameter and impeller speed.

<p>True (A)</p> Signup and view all the answers

In the main characteristic curves of a centrifugal pump, what does the curve for P(Q AND H CONSTANTS) represent?

<p>The relationship between power input and head at constant discharge rate (A)</p> Signup and view all the answers

The curve for H(Q = CONSTANT) represents the relationship between head and speed at a constant discharge rate.

<p>True (A)</p> Signup and view all the answers

What does the curve for Q(H = CONSTANT) show in the main characteristic curves of a centrifugal pump?

<p>This curve displays the relationship between discharge rate and speed at a constant head.</p> Signup and view all the answers

The pump performance curves for centrifugal pumps are typically plotted against the discharge rate (Q).

<p>True (A)</p> Signup and view all the answers

When a centrifugal pump is filled with gas or vapor instead of liquid, what condition occurs causing it to malfunction?

<p>Vapor lock (A)</p> Signup and view all the answers

Even though the head developed by a vapor-filled pump is the same as a liquid-filled pump, the pressure is significantly lower due to the lower density of gases.

<p>True (A)</p> Signup and view all the answers

What is the process of filling a centrifugal pump with liquid prior to operation called?

<p>Priming</p> Signup and view all the answers

Priming is essential for all types of pumps, including positive displacement pumps.

<p>False (B)</p> Signup and view all the answers

What is the main purpose of placing a centrifugal below the liquid level?

<p>To enable self-priming.</p> Signup and view all the answers

Priming involves keeping the impeller submerged in liquid without any air pockets during the initial startup.

<p>True (A)</p> Signup and view all the answers

What is cavitation in a centrifugal pump?

<p>Cavitation occurs when the pressure in a centrifugal pump drops below the vapor pressure of the liquid, leading to the formation of vapor bubbles that collapse violently, causing damage and noise.</p> Signup and view all the answers

Higher liquid temperatures increase the likelihood of cavitation in a centrifugal pump.

<p>True (A)</p> Signup and view all the answers

What happens when the vapor bubbles generated during cavitation collapse?

<p>The collapsing vapor bubbles create local shock waves that can damage the pump's internal components.</p> Signup and view all the answers

Cavitation can contribute to excessive noise and vibrations in centrifugal pumps.

<p>True (A)</p> Signup and view all the answers

What is the maximum suction lift (height) that can be tolerated to avoid cavitation in a centrifugal pump?

<p>The maximum suction lift is determined as hs = Ha - H_v - (vs^2 / 2g) - h_fs, where hs is the maximum allowable suction height, Ha is the atmospheric pressure head, H_v is the vapor pressure head, vs is velocity at the free surface, and h_fs is the head loss in the suction pipe. It ensures that the pressure at the pump inlet is sufficient to avoid cavitation.</p> Signup and view all the answers

The NPSHR is the minimum absolute head at the pump intake that is required to prevent cavitation.

<p>True (A)</p> Signup and view all the answers

What is the NPSHA, and how does it relate to cavitation?

<p>NPSHA is the available net positive suction head, representing the actual head available at the pump inlet. A higher NPSHA reduces the likelihood of cavitation, while a lower NPSHA increases the risk.</p> Signup and view all the answers

The NPSHR typically increases with flow rate (Q), while NPSHA decreases with Q.

<p>True (A)</p> Signup and view all the answers

The Thoma's cavitation factor (σ) is a dimensionless parameter that is typically smaller than the critical cavitation factor (σ_c) for proper pump operation.

<p>True (A)</p> Signup and view all the answers

Flashcards

Pump Definition

A mechanical device that adds energy to a fluid to move it.

Pump Classification

Categorization of pumps based on how they move fluids.

Positive Displacement Pump

Traps and displaces a set volume of fluid.

Positive Displacement Pump Types

Reciprocating and rotary pumps fall under this category.

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Reciprocating Pump

Uses pistons, plungers, or diaphragms to move fluid.

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Rotary Pump

Uses rotating parts for fluid displacement.

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Centrifugal Pump

Uses impeller rotation to increase fluid velocity.

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Single-Acting Pump

Delivers fluid on only one stroke of the piston.

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Double-Acting Pump

Delivers fluid on both strokes of the piston.

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Diaphragm Pump

Uses a flexible diaphragm to move fluid.

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Gear Pump

Uses gears to move fluid.

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Lobe Pump

Uses lobes to move fluid.

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Screw Pump

Uses screws to move fluid.

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Peristaltic Pump

Uses rollers to squeeze fluid through a tube.

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Pump Comparison (Centrifugal vs Reciprocating)

Centrifugal pumps are high-speed, low-maintenance, and handle grit/slurry, while reciprocating pumps are slower, more prone to wear, and need more space for multiple components.

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Study Notes

Pumps and Working Principle

  • A pump is a mechanical device that provides energy to a fluid, moving it from one location to another.
  • Pumps raise, transfer, deliver, or compress fluids, or attenuate gases, often using suction or pressure, or both.
  • Pumps increase pressure, kinetic, and/or potential energy of the fluid by converting mechanical energy.

Classification of Pumps

  • Pumps are classified based on their displacement method.

1. Positive Displacement Pumps

  • These pumps move a fixed amount of fluid by trapping it and then moving it to the discharge pipe.
  • The volume flow rate is fixed per operating cycle.
  • They apply direct pressure to the fluid.

2. Rotodynamic Pumps

  • These pumps increase the velocity of the liquid using a rotating impeller.
  • Creates head and flow by increasing the liquid's velocity.
  • Uses torque to generate rotation.
  • Impart kinetic energy through angular momentum of the impeller.

Types of Positive Displacement Pumps

  • Reciprocating pumps: Use pistons or plungers to move fluids.

    • Can be single-acting or double-acting, with discharge on one or both strokes.

  • Diaphragm pumps: Use a flexible diaphragm to move fluids.

    • Suitable for handling hazardous or toxic fluids in small amounts at moderate pressure.

  • Rotary pumps: Move fluids using rotating parts like gears, lobes, or screws.

    • Gears, lobes, and screw pumps are suitable for viscous fluids and do not usually require check valves due to unidirectional fluid movement.

Centrifugal Pumps

  • These pumps increase fluid energy by imparting angular momentum to the liquid, converting it into pressure head in the volute.
  • High velocity areas are present close to the impeller. Lowest pressure areas occur where velocity is greatest.

Components of a Centrifugal Pump

  • Impeller: The rotor with curved vanes or blades, driven by a motor.
  • Casing: An enclosed chamber surrounding the impeller, increasing the flow area (volute) from the impeller outlet to the delivery pipe.
  • Suction pipe: Connects the impeller eye to the liquid source (sump).
  • Delivery pipe: Connects the pump outlet to the required height.

Maximum Suction Lift (hs)

  • Calculations involve Bernoulli's equation to ensure that the pressure at the pump inlet is above the vapor pressure to avoid cavitation.
  • Cavitation is the formation and collapse of vapor bubbles, damaging components.

NPSH (Net Positive Suction Head)

  • It is the minimum pressure above the liquid's vapor pressure needed to prevent cavitation.
  • NPSH is affected by vapor pressure, suction head losses, and fluid velocity.
  • Higher NPSH values are required for larger pumps, faster impeller speeds, and higher discharge pressures.

Centrifugal Pump Characteristic Curves

  • Plotting curves of Head (H), Power (P), Discharge (Q), and Pump Efficiency (η) against flow rate (Q) provide a graphical representation to determine optimal operating conditions.

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