Centrifugal Pumps – Working, Types, Application & Repair

Centrifugal pumps

Centrifugal pumps are one of the most commonly used pumps on the earth with ever growing applications. They are simple and reliable machine of any machinery space. On ship they are the simplest of machinery installed in the engine room. They are unique for its high flow rate at variable dynamic head with less repairs. Allowing operating personnel to throttle flow rate without building pressure on pipeline.

Each centrifugal pump comprises of components such as shaft, bearing, structure and sealing arrangement. It converts energy of a prime mover to kinetic energy and then to pressure energy with help of volute casing. The impeller is the rotating components that converts shaft energy to kinetic energy of fluid. Diffuser or volute casing is that stationary part of the pump that converts these kinetic energy to pressure energy.

Is Centrifugal Pump a Type of Positive Displacement Pump?

No it is not; you won’t believe but many nontechnical but curious people asked me whether these pumps are of positive displacement type? Even having a long list of differences these pumps have; They are often misunderstood similar to other positive displacement pumps.

What makes a centrifugal pumps unique is that it can be run in either direction with its radial and axial flow designs. Other major features that makes it different than that of a positive displacement pumps are; performance, fluid viscosity and working.

The impeller suck in water creating a minor vacuum by adding kinetic energy to the water or fluid. It is then converted to pressure energy with the help of volute casing or diffuser. On other hand a positive displacement pump force a limited amount of liquid trapped in the system towards discharge port.

While in positive displacement pumps the flow rate remains same irrespective of pressure, flow rate do change with pressure in centrifugal pump. The problem with centrifugal pumps is that it can’t handle high viscous fluids while the positive displacement pumps have no problem pumping them out.

Where are Centrifugal Pumps used on Ship and Shore Industries?

These pumps are among the top used pumps in various industries for transferring fluids. They are undisputed champions for transferring low viscous fluids from one place to another across industries. They are mostly used for their advantage of caring large quantity fluid, high flow rate with dynamic head. Above all the freedom to change flow-rate without dangerously pressurizing the system make it the top choice.

They are used on shore for boiler feed applications, irrigation, waste management, as a booster pump, refineries, mining, air conditioning, fire fighting, food production and power generation. These pumps are used on ship’s for bildge, boiler feed, fire fighting, sea water suction and general service pump. They are one of the simplest machinery in engine room.

Centrifugal pumps

Different Components of Centrifugal Pumps?

Basically a centrifugal pump has two major components; rotating and stationary. While rotating components comprises of parts like impeller and shaft; stationary components have parts such as casing, cover and bearing with other small components such as impeller seal, motor, coupling, volute, stuffing box and wear ring.

A ) Primary Component

When an impeller rotates in a liquid at certain speed; transfer of energy took place from moving blades of impeller to the fluid. Kinetic energy is generated in the fluids as it moves forcing itself towards the volute casing. As the impeller moves; centrifugal force moves the liquid from near the hub towards the tip of impeller blades.

An Impeller is mounted on the shaft made of stainless steel. A shaft of optimum size is required for better performance as a longer shaft will increase unnecessary costs while smaller shafts can cause troubles like vibration, shaft breakage and lower pump life. Made of cast iron, casing converts kinetic energy into pressure energy due to its volute like structure.

An impeller is fitted inside the volute with its outer diameter closest to the casing at the time of discharge. It converts converts kinetic energy to pressure energy by providing increasing area around the impeller before leaving for discharge. A mechanical seal is provided in between the gaps of stuffing box and shaft. A stuffing box is the position in centrifugal pumps where shaft pass through the casing.

B ) Secondary Component

A mechanical seal is provided to protect the system with three main parts like gland, spring, rotating and stationary faces. A gland fits around the shaft onto the pump casing. The stationary and rotating sealing rings are sealed to the gland and shaft respectively. Now the space in between these rings are compressed with springs; to make it possible for fluids to pass.

Finally these pumps are equipped with wear rings to avoid possible wear of casing and impeller saving costs of maintenance. A wear ring is a kind of replaceable-wearable copper alloy rings placed in between the impeller and casing. They are designed to be replaced several times during a life time of centrifugal pumps. They provide a small clearance in between the impeller and the casing for problem free operation.

Operating Principle of Centrifugal Pumps

Working on the principle of centrifugal force, these pumps explains how centrifugal head is generated when a fluid is rotated by a rotating impeller throwing fluid away from its central axis. Finally sudden change in area across impeller and changing angular momentum build pressure.

First step in operating a centrifugal pump is to prime ensuring its suction, casing and a part of discharge is filled with water/fluid. It can be achieved by placing the pump itself at lower level or using a secondary priming pump specially for the work. Once it is ensured that there is no air in the system, motor or prime mover is started. If there is air pockets left in the system the pump won’t discharge.

The impeller rotates throwing fluids on the walls of the casing. The casing is specially designed in the form of volute which converts major part of fluid kinetic energy to pressure head. Now fluid leaves the impeller under pressure creating vacuum behind the impeller eye. The vacuum will pull water/fluid from the suction line to continue the above process.

Efficiency of centrifugal pumps can be given by (n) = WQ( Hs + Hd )Shaft Power

Here;

Q = Discharge

W = Specific Weight

Hd = Head at the discharge

Hs = Head at the suction

This can be represented on graph as:

centrifugal pumps efficiency graph

The above Graph stats that discharge head is inversely proportional to the flow rate. At operating point we get the maximum efficiency for pump at minimum power consumption.

Q. What are Different Centrifugal Pump Types Used?

While there are different types of pumps used on ship, centrifugal pumps being one of them. But only few will know the different types of centrifugal pumps used on and outside of ship i.e on board and shore industries. The major classification are:-

  1. Axial Flow – Single Stage
  2. Axial Flow – Multistage
  3. Mixed Flow
  4. Radial Flow – Single Suction
  5. Peripheral – Multistage
  6. Radial Flow – Double Suction
  7. Peripheral – Single Stage
  8. Peripheral – Single or Multi Stage Self Priming Pump

Q. Why Centrifugal Pump Requires Priming?

Centrifugal pumps are the sub class of dynamic pumps which convert Roto-kinetic energy to the pressure energy of fluid to transport fluid from one place to another. This roto-kinetic energy is generated from impeller mounted on shaft running by an engine or motor. It is not a positive displacement pump; so cannot pump gas as it restricts with its ability to create necessary pressure difference.

Primming is a process of submerging pump casing, suction and a part of discharge to insure there is no air trapped in the system. This is one of the major requirement during starting; but only required for pumps having elevation of suction lower than elevation of pump. More importantly the head developed by these pumps depends upon the speed and diameter of impeller.

Now as the head generated is proportional specific weight of the liquid. This states that the pressure difference created with air would be 1/800 times that with water. This low pressure difference would be impossible to maintain steady suction; so centrifugal pumps do require priming.

Q. Why Discharge Valve are Closed During Starting?

A large part of centrifugal pumps are generally started with discharge valves shut. When examining the properties of centrifugal pump by operating characteristic curve it is clearly found that; when the amount of water delivered is zero, the power available is at minimum. So discharge valves are kept shut during the start to maintain low power demand by the pump on its motor having low power output at the start.

This helps stabilizing pumps at the starting. When the pump is stabilized the discharge valves are opened and high power availability meets the high power demand; which would be impossible for the motor to meet for the starting 4-5 seconds.

Common Problems Associated with Centrifugal Pumps

A centrifugal pump is one of the simplest installations of any plant / Engine room with the most commonly used equipment in the world. Their application is continuously expanding, and with this expanding new problem associated with centrifugal pumps are manifesting. However, it is not possible to predict any problem that may occur in the future but having the in-depth knowledge about the different factors which may affect the centrifugal pump’s performance is a smart thing to do. Read our 11 major problems with possible cause for centrifugal pumps!

Maintenance and Repairs of Centrifugal Pumps

Environment and operating condition have an great impact on pumps efficiency and life specially when it is subjected to run through the year. In such conditions, routine or preventive maintenance are done to prolong the life of the centrifugal pump. In general we routinely inspect bearing and lubricant condition, pump vibration, discharge pressure and shaft sealing.

Bearing temperature, vibration and lubricant temperature are monitored to avoid imminent failures. Mechanical seals are inspected for any signs of crack/Leakage. It is considered acceptable to have minor leakage but anything above 30 to 40 drops is seriously NO-NO. Many a times overall pump vibration is also monitored for signs of failure. Any sudden increase in vibration can be sign of resonance due to improper alignment.

Pressure difference in centrifugal pumps are considered to be the important factor stating pump condition. Difference between suction and discharge pressure is carefully monitored and logged down for years of operation. Any sudden decrease is an indication of widening impeller clearance. To maintain operation and integrity of these pumps; preventive maintenance are done on board ship.

Similar maintenance are also done on shore industries which includes steps like:-
  1. Inspecting Impeller clearance, impeller blades, shaft and volute casing for signs of pitting, erosion and wear.
  2. Erosion are fixed by using ally epoxy putty.
  3. Check all bearings for wear and change if necessary.
  4. Using feeler gauge check for wear ring clearance.
  5. Clean all the physical parts of centrifugal pump.
  6. Check all the windings of motor with help of a megger.
  7. Check for uneven or excess longitudinal or axial movement of the shaft.
  8. Assemble all the system and put the date of maintenance.

Clearance Taken in Centrifugal Pump

In Between
 Approx Clearance
> Ball Bearing and shaft
0.002 to 0.005 mm
>Ball Bearing and casing
0.003mm
>Shaft and impeller
0.12 to 0.15mm
>Sleeve and shaft
0.04 to 0.05mm
>Shaft and packing
0.07 to 0.09mm
>Packing and stuffing Box
0.015mm
>Shaft and Coupling
0.015
>Shaft and bush
0.5 to 0.6mm
>Seal Spring
3 to 4 mm
>Impeller and Hub
0.05 to 0.06mm

Conclusion

Unlike all other positive displacement pumps; a centrifugal pump converts the rotational energy of impeller into kinetic energy of fluid and then to pressure energy. Water or fluid enters through the eye of the impeller thrown towards the edge of the blades radially outwards. Then major part of kinetic energy is converted to pressure head thanks to the volute design in the pump casing.

Also Read:
Or

Why not Request your own Topic !

1 Comment

  1. ye htet

    Interesting.

    Reply

Leave a Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.