Pumps – A brief guide on types of Pumps, Differences & Applications

Pumps – A brief guide on types of Pumps, Differences & Applications

A pump is a device that is used to transfer fluids from one point to another. This is done by converting mechanical energy into the pressure energy of the fluid. Pumps are generally used to transfer fluids from a lower potential point to a higher potential point. There are 2 broad types of pumps.

In this article, I’ll guide you through the basic classification and working principles of different types of pumps. To keep it simple to read and understand I won’t get into much detail about any particular type of pump. But, in future articles, I’ll explain each type of pump with complete details.

Let’s get started…

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Types Of Pumps

The Pumps are broadly classified into two categories,

  1. Centrifugal/Rotodynamic Pumps
  2. Positive Displacement Pumps

Centrifugal Pumps:

Centrifugal Pumps are a type of Rotodynamic pump which works on the principle of centrifugal force. The impeller is rotated by means of a prime mover either an electric motor or any other means. Due to the rotation of the impeller, a vacuum is created at the eye of the impeller. This vacuum draws the fluid through suction piping.

Due to the action of centrifugal force, the fluid is pushed radially outward through the impeller vanes. Kinetic Energy is imparted onto the fluid while passing through the vanes. This kinetic energy is converted into pressure energy as it passes through the casing of the pump. Because of the shape of the pump casing i.e, Volute Casing. Centrifugal Pump Basic Diagram

Types of Centrifugal Pumps

Based on the type of flow, centrifugal pumps are classified into 3 categories.

  1. Axial Flow Pump
  2. Radial Flow Pump
  3. Mixed Flow Pump

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Positive Displacement Pumps

Positive displacement pumps displace a fixed amount of fluid on every stroke. It produces/built with cavities at the suction side in which fluid is drawn and displaced with high pressure and discharge side. They are self-priming pumps.

Types of Positive Displacement Pumps

Based on the type of flow they are classified into two major categories

  1. Reciprocating Pump:

    • The pumping of the fluid is established by the expansion and contraction action of the cavity. The fluid is drawn during expansion and is displaced during contraction. Examples are, Piston Pump, Plunger Pump, Bladder Pump, Diaphragm Pump, and Peristaltic Pimp
  2. Rotary Pump:

    • The pumping of the fluid is achieved by the space created either by gear teeth or screw threads. The fluid is trapped inside this space and moved along the space and displaced at the discharge side. Examples are Gear Pump, Screw Pump, Progressive Cavity Pump, Rotary Lobe Pump, Rotary Vane Pump.

Out of all the above, the pumps used widely onboard are Gear Pump, Screw Pump, and Reciprocating Pump.

Gear Pump:

Gear Pump Basic Diagram

Gear pumps are positive displacement pumps. It consists of two gears, one is driving gear and one is driven gear. Both rotate in opposite directions. At the suction side, the gear teeth get disengaged hence creating a vacuum to draw the fluid inside. The water drawn is transferred through the fine clearance between the gear tooth and pump casing. At the discharge side, the gear teeth get engaged hence discharging the fluid at higher pressure.

Oil is trapped and drawn round in peripheral cavities of the gear. Oil is discharged from the outlet. Teeth mesh at the center to preventing returning from discharge to suction

Screw Pump:

Screw Pump Basic diagram

Screw pumps are positive displacement pumps and self-priming. It consists of two or 3 shafts having a right and left-hand screw. Ball or roller bearing carries the shaft. The timing gear is fitted at the other side.

When the pump rotates, fluid is drawn from the suction, propel along the body from each end to center, discharge through the outlet. The fluid in screw pumps moves between the screw threads and is displaced axially as the screws rotate and mesh.

Reciprocating Pump:

A reciprocating pump essentially consists of a piston and a cylinder where the piston is enclosed in the cylinder. The cylinder is connected to suction and delivery pipes, each of which is provided with a non-return or one-way valve called suction valve and delivery valve respectively. The function of a non-return or one-way valve is to admit liquid in one direction only. The piston is connected to a crank by means of a connecting rod. The piston is equipped with ebonite rings that act as a sealing component with the cylinder.

The reciprocating pumps may be classified as:
(i) Single acting pump.
(ii) Double acting pump.

Double acting reciprocating pump basic diagram

The sketch is double acting reciprocating pump. It is a type of positive displacement pump. It consists a piston with piston rings, two suction valves, and two discharge valves. Each pair of the valve is at either side of the piston

When the piston is moved upward suction effect valve 1 close and valve 3 open and the other side valve 4 close and valve 2 open forcing out liquid to discharge

When the piston is moving down valve 1 open valve 4 close forcing out liquid to discharge and the other side suction effect valve 3 open and valve 2 close. The Air vessel is fitted to reduce pulsating effect at discharge.

Difference between Positive Displacement Pumps and Centrifugal Pumps

Positive Displacement Pumps Centrifugal Pumps
Self Priming type Priming arrangement is required if the fluid suction is below the pump level
The liquid is displaced by the change in volume of the chamber The Liquid is displaced by centrifugal force and tangential acceleration
Pump discharge is a non-steady flow Pump discharge is a steady flow
Air vessel is needed to reduce the pulsating effect Air vessel is not needed
A relief valve is required at the discharge side No relief valve is required
The discharge pressure head is high The Discharge Pressure head is Low
The discharge flow rate is low The discharge flow rate is high
It can be used with fluid that has various viscosity It can be used only with fluid that has medium or low viscosity
Lower cavitation problem Higher cavitation problem

 

Applications Onboard a Ship

Type of Pump Applications
Centrifugal Pumps SW pumps, FW pumps, Cargo Pumps, Bilge Pumps, Fire Pumps, Main L.O, and Camshaft L.O Pumps, Boiler Feed Water Pumps, etc. Wherever a High Flow rate is required.
Reciprocating Pumps Application on board these days is restricted to:
a) Engine room bilge pump gear driven with an electric prime mover
b) Steering compartment bilge pump operated by hand or pneumatic device
c) Stripping pumps in cargo pump room (steam-operated)
Gear Pumps These are used for Purifiers feed, Pressurized supply of FO to ME, Boiler & DG’s,
Gearbox lubrication systems, fuel oil transfer, etc
Screw Pumps Sludge pump, Main Engine LO pump, Cargo Oil pumps.
Diaphragm/WILDEN Pump Portable Pump for transferring light liquids. Compulsory requirement as per SOPEP
Trochoid Pump DG rocker arm lube oil pump, DO supply pump for incinerators.

 

Regular Maintainance of Pumps

  1. Gland packing (if applicable) to be tightened gently when leakages are seen.
  2. Periodical replacement of gland packing required.
  3. Bearings are to be greased regularly.
  4. Mechanical seal lube oil to be topped up.
  5. Check motor amperage while the pump is running (Megger readings = 3 monthly)
  6. Feel motor for vibration and check for abnormal noise.
  7. EO to regularly grease motor bearings.
  8. Record suction and discharge pressure when vessel on light load (ballast) condition and fully loaded condition as variations will be observed due to change in NPSH.
  9. Check the coupling bolts/foundation bolts regularly and replace them if required.
  10. Line filters to be cleaned frequently.

So, this was a brief introduction about the Types of Pumps, the difference between the pumps, various applications on board a ship, and routine maintenance of the pumps. Read this article for more details about the types of pumps

If you have any questions regarding the classification of the pumps, drop them in the comments. I would be happy to answer your queries.

And please let me know in the comments,  on which topic you want me to write in the next article.

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