Pumps: Types, Working and Maintenance
Overview
Pumps are mechanical devices that transfer fluids from low-pressure to high-pressure regions by converting mechanical energy into hydraulic energy. In ITI Fitter Semester 3, candidates study pump types (centrifugal, reciprocating, gear, submersible), their working principles, constructional details, performance parameters, and systematic maintenance procedures. Understanding cavitation, priming, head, discharge, and efficiency is essential for both theory papers and practical assessments under NCVT guidelines.
Typed Notes
Types of Pumps and Classification
Pumps are broadly classified into Dynamic (Rotodynamic) and Positive Displacement pumps. Dynamic pumps include centrifugal, axial-flow, and mixed-flow pumps; they use rotating impellers to add velocity to fluid. Positive Displacement pumps include reciprocating (piston/plunger) and rotary (gear, vane, screw) pumps; they trap a fixed volume and force it out. Centrifugal pumps are used in water supply, cooling systems, and sewage. Reciprocating pumps are used in hydraulic presses and fuel injection. Gear pumps are used in lubrication systems of machine tools and engines.
Centrifugal Pump: Construction and Working
A centrifugal pump consists of an impeller, casing (volute or diffuser type), shaft, bearings, mechanical seal or gland packing, and suction/delivery pipes with foot valve and strainer. Working principle: the impeller rotates at high speed inside the casing; centrifugal force throws liquid radially outward, creating low pressure at the eye (centre) that draws liquid in from the suction side. Kinetic energy is converted to pressure energy in the volute casing. Key parts: Impeller (open, semi-open, closed types), Volute casing (converts velocity to pressure), Foot valve (prevents back-flow during priming).
Reciprocating Pump: Construction and Working
A reciprocating pump has a cylinder, piston or plunger, suction valve, delivery valve, connecting rod, and crank. During the suction stroke, the piston moves back, creating a vacuum; the suction valve opens and liquid enters the cylinder. During the delivery stroke, the piston moves forward; the suction valve closes, delivery valve opens, and liquid is discharged under pressure. Single-acting pumps deliver liquid in one stroke per revolution; double-acting pumps deliver in both strokes. Reciprocating pumps give pulsating flow; an air vessel is fitted to smoothen flow and reduce water hammer.
Pump Maintenance, Faults and Remedies
Routine maintenance includes: checking and lubricating bearings, inspecting mechanical seals or gland packing for leakage, checking impeller for wear or cavitation damage, verifying alignment of pump and motor shafts, and cleaning strainer/foot valve. Common faults: (1) Pump not delivering water — cause: improper priming or choked strainer; remedy: re-prime, clean strainer. (2) Excessive noise/vibration — cause: cavitation or misalignment; remedy: increase suction head, realign. (3) Overheating of bearings — cause: lack of lubrication or overloading; remedy: lubricate, check load. (4) Reduced discharge — cause: worn impeller or air leakage in suction pipe; remedy: replace impeller, seal joints.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Classification of Pumps
- Dynamic (Rotodynamic) pumps — centrifugal, axial, mixed flow
- Positive Displacement pumps — reciprocating, gear, vane, screw
- Selection criteria — head, discharge, fluid viscosity, pressure requirement
A centrifugal pump of 5 HP motor is used in a municipal water supply system to lift water 20 metres; a gear pump of 1 HP is used in a lathe machine's lubrication circuit.
02Centrifugal Pump — Parts and Working
- Impeller types — open, semi-open, closed (closed most efficient)
- Volute casing — converts kinetic energy to pressure energy
- Priming — filling casing with water before start; foot valve retains prime
A 2-stage centrifugal pump used in a high-rise building (15 floors) develops a total head of 50 m; priming is done by pouring water through the priming plug on the casing top.
03Reciprocating Pump — Parts and Working
- Single-acting vs double-acting cylinder arrangement
- Suction and delivery valves — non-return (check) valves
- Air vessel — fitted on suction and delivery sides to reduce pulsation and water hammer
A double-acting reciprocating pump with 100 mm bore and 150 mm stroke running at 60 RPM delivers approximately 84.8 litres per minute (theoretical), used in a hydraulic press shop.
04Pump Performance Parameters
- Total Head (H) = Suction head + Delivery head + Friction head losses
- Discharge (Q) — volume of liquid delivered per second (m³/s or LPM)
- Efficiency — Manometric efficiency, Mechanical efficiency, Overall efficiency
A centrifugal pump with suction head 4 m, delivery head 16 m, and friction losses 2 m has Total Head = 22 m; if motor input is 5 kW and water output power is 3.5 kW, overall efficiency = 70%.
05Pump Maintenance and Fault Diagnosis
- Preventive maintenance — lubrication, alignment check, seal inspection every 500 hours
- Cavitation — identification by crackling noise, pitting on impeller; remedy: reduce suction lift below 6 m
- Common faults — no delivery, low discharge, overheating, excessive vibration and their remedies
During routine maintenance of a centrifugal pump in a factory, the fitter finds pitting marks on the impeller vanes — this confirms cavitation damage; the suction pipe is repositioned to reduce suction lift from 8 m to 4 m to solve the problem.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Centrifugal pump works on centrifugal force; impeller rotates and throws liquid outward — most common pump in industry
- 02Reciprocating pump is a positive-displacement pump; delivers fixed volume per stroke — used where high pressure is needed
- 03Priming: filling pump casing with liquid before starting to remove air — essential for centrifugal pumps, NOT needed for self-priming pumps
- 04Cavitation: formation and collapse of vapour bubbles inside pump due to low suction pressure — causes noise, vibration, and impeller damage
- 05Total Head = Suction Head + Delivery Head + Friction Losses; measured in metres of liquid column
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is priming in a centrifugal pump and why is it necessary?
What is cavitation? State its causes, effects, and remedies.
Differentiate between single-acting and double-acting reciprocating pumps.
Name the main parts of a centrifugal pump and state the function of the volute casing.
List four routine maintenance checks a fitter must perform on a centrifugal pump every 500 operating hours.
Pumps convert mechanical energy into hydraulic energy to move fluids. Centrifugal pumps (rotodynamic, need priming) and reciprocating pumps (positive displacement, high pressure) are the most exam-important types. Key parameters are Total Head, Discharge, and Efficiency. Cavitation and improper priming are the most common problems. Regular maintenance — lubrication, alignment, seal check, and strainer cleaning — ensures long pump life.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which type of pump requires priming before starting?
Q2.An air vessel is fitted in a reciprocating pump to:
Q3.Pitting marks found on the impeller of a centrifugal pump indicate:
Q4.Which pump is most suitable for delivering a small quantity of liquid at very high pressure?
Mark karo — dashboard par progress dikhega.