TST Engineering Services

Module 39 — Pumps and Pump Types

A structured marine engineering recap covering centrifugal pump operation, NPSH, cavitation, priming, positive displacement pump types, relief valves, maintenance and shipboard systems.

Core

Centrifugal

Core

PD Pumps

Safety

Cavitation

Oral focus

Faults

Updated imagery

Pump visual comparison

TST marine pumps and pump types infographic showing centrifugal pump parts and piston positive displacement pump parts

Updated TST imagery comparing centrifugal-pump construction and piston-type positive displacement pump construction for rapid visual revision.

Centrifugal pump focus

Identify suction eye, impeller, casing/volute, discharge, shaft, bearings, wear rings and seal area. Link each part to head generation, leakage control and common maintenance checks.

Piston PDP focus

Identify cylinder, piston/plunger, suction valve, discharge valve, crank/connecting rod, packing and relief protection. Link each part to fixed-volume displacement and pressure rise.

Section 1

Marine pump families

Positive displacement

Uses expanding and contracting volumes or moving pockets to physically move liquid from suction to discharge. Excellent for viscous fluids, good suction lift and high/medium pressure.

Rotodynamic

Uses velocity energy, usually from an impeller, then converts kinetic energy into pressure in a volute, diffuser or guide-vane arrangement.

Axial flow

Uses a propeller to move large quantities at low head, commonly associated with condenser circulating or salvage-type large-flow duties.

Section 2

Centrifugal pump operation and construction

1. InletLiquid enters the eye of the impeller through a flooded/primed suction.

2. AccelerationRotating vanes accelerate liquid radially outward.

3. ConversionVolute or diffuser converts velocity into pressure/head.

4. DischargeFlow leaves at a head set by pump curve and system curve.

5. MonitoringCompare suction, discharge, flow, speed and power against normal values.

Impeller

Open, semi-open, fully shrouded, single-entry or double-entry depending on duty, solids, efficiency and suction condition.

Wear rings

Replaceable close-clearance parts that reduce recirculation leakage from discharge back to suction.

Seal/gland

Prevents leakage along the shaft; mechanical seals need correct fitting and rotation sense.

Bearings

Support shaft alignment and absorb radial/axial loading from hydraulic and mechanical forces.

Section 3

NPSH, cavitation and priming

NPSH = absolute suction head − vapour pressure head

NPSH available must exceed NPSH required with margin. Low tank level, high temperature, blocked strainers, air leaks and suction restrictions reduce NPSH available.

Cavitation symptoms

Rattling/gravel noise and vibration.
Fluctuating suction/discharge pressure.
Falling flow/head and unstable current.
Impeller pitting, erosion and seal/bearing distress.

Water-ring primer

Eccentric rotor and water ring evacuate air from suction. Atmospheric pressure then pushes liquid up and floods the centrifugal pump.

Central priming

Several pump suctions connect to a common vacuum tank through valves, allowing grouped bilge/ballast/fire pump priming.

Eductor primer

Compressed air through an eductor removes air from a vacuum tank until a level switch permits main pump start.

Section 4

Positive displacement pumps

Type	Operation	Typical marine systems	Maintenance focus
Reciprocating / piston	Piston alternately draws in and discharges through suction/discharge valves. Double acting pumps use both sides.	Bilge, stripping, sludge, low-pressure boiler feed, fuel injection as a special case.	Valves, piston rings, packing, air vessel, relief valve, crank gear and lubrication.
Gear	Fluid is carried around the outside between gear teeth and casing; close clearances limit slip.	Boiler fuel oil, lube oil, purifier supply, hydraulic oil.	Cleanliness, bearing bushes, end clearances, relief valve, drive alignment, internal leakage.
Screw	Meshing screws move liquid axially with smooth, low-pulsation output.	Fuel oil transfer, lube oil service/transfer, boiler fuel, hydraulic/lubricating liquids.	Relief valve, thrust bearing, timing gears where fitted, viscosity/temperature, screw clearances.
Sliding vane	Spring-loaded or centrifugal vanes form pockets in an eccentric stator.	Air compressors, hydraulic oil pumps.	Vane wear, cam ring, side plates, cleanliness and correct fluid viscosity.
Progressive cavity / Mono	Helical rotor in rubber stator creates progressing cavities that move liquid axially.	Sludge, sewage, oily water and difficult contaminated fluids.	Stator wear, dry running, universal joints, rotor/stator condition and pressure protection.

Section 5

Performance monitoring and fault diagnosis

Five key parameters

Suction pressure: reveals blocked strainers, low tank level, air ingress and NPSH margin.
Discharge pressure: proves head and shows throttling, blockage or wear.
Flow: confirms the system is receiving required capacity.
Pump speed: needed for curve comparison and affinity-law effects.
Power/current: indicates overload, dry running, cavitation, viscosity change or mechanical drag.

Section 6

Shipboard systems and performance measurement

Centrifugal / rotodynamic duties

Commonly used for high-flow services such as sea water cooling, fresh water cooling, ballast transfer, bilge/general service, fire main, condenser circulating, evaporator feed and domestic/service water.

Positive displacement duties

Used where controlled flow, suction lift or viscous fluids are important: fuel oil transfer and circulation, lube oil transfer, purifier feed, sludge, oily bilge stripping, hydraulic oil and dosing/chemical pumps.

Special pump applications

Axial-flow pumps suit very large volume and low head duties. Eductors and ejectors support stripping, tank cleaning, priming or vacuum duties. Screw pumps suit steady fuel/lube oil delivery with low pulsation.

MIMIC / control-room trends

Use the MIMIC screen to trend suction and discharge pressure, flow rate, tank level, valve status, pump running feedback, standby/auto status, alarms and trips against the expected system line-up.

Electrical loading

Monitor motor amps, kW, overload relay condition and starting current. Rising amps can indicate overload, high viscosity, blocked discharge or mechanical drag; falling amps can indicate air locking, loss of prime or low flow.

Mechanical condition monitoring

Use vibration, bearing temperature, seal leakage, gland condition, noise, casing temperature, coupling condition and alignment checks to identify cavitation, bearing distress, imbalance, misalignment or worn clearances.

Parameters to compare

Suction pressure / vacuum and NPSH margin.
Discharge pressure and generated head.
Flow rate against system demand.
Motor amps / kW against normal running load.
Vibration and bearing temperature trend.
Seal leakage, gland drip rate and casing noise.

Performance judgement

A healthy pump is judged by comparing live readings with historical trends, pump curves and the system curve. The key is not one reading alone, but whether pressure, flow, amps, vibration and temperature all agree with the duty being performed.