Dynamic Balancing of Centrifugal Pump Impeller

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 6, June 2012)

409

Dynamic Balancing of Centrifugal Pump Impeller Amit Kalmegh

1, Santosh Bhaskar

2

1Dept. of Mechanical Engineering, S.R.E.S. College of Engineering, Kopergaon, Pune University

2Prof. Dept. of Mechanical Engineering, S.R.E.S. College of Engineering, Kopergaon, Pune University

[email protected]

[email protected]

Abstract— Vibration caused by mass imbalance in rotating

machinery is an important engineering problem. The

objective of balancing is to reduce rotor vibration to a

practical minimum. Reducing rotor vibrations generally

increases the service life of the rotating machinery. The

fundamental difference between a centrifugal sewage pump

impeller and those of its clear water cousins is its ability to

pass solid materials that would normally clod later. Due to the

unbalance in the impeller, vibration occurs and leads to

decrease in fluid velocity and local pressure which may cause

an undesirable turbulence and possible cavitation. Hence, to

remove the unbalance in rotor is necessary. In this paper the

focus is given on dynamic balancing of centrifugal pump

impeller.

Keywords—Impeller, dynamic balancing, vibration,

unbalance, balancing tolerance, residual unbalance.

I. INTRODUCTION

A centrifugal pump is one of the simplest pieces of

equipment in any process plant. Centrifugal pump comes

under the category of rotating machinery. Its purpose is to

convert energy of a prime mover first into velocity or

kinetic energy and then into pressure energy of a fluid that

is being pumped. The energy changes occur by virtue of

two main parts of the pump, the impeller and the volute or

diffuser. The impeller is the rotating part that converts

driver energy into the kinetic energy. The volute or diffuser

is the stationary part that converts the kinetic energy into

pressure energy.

Fig.1 Centrifugal pump impeller [3]

Rotating machinery is commonly used in mechanical

systems, including industrial turbo-machinery, machining

tools, and aircraft gas turbine engines. Vibration caused by

mass imbalance is a common problem in rotating

machinery. Imbalance occurs if the principal axis of inertia

of the rotor is not coincident with its geometric axis. Higher

speeds cause much greater centrifugal imbalance forces,

and the current trend of rotating equipment toward higher

power density clearly leads to higher operational speeds.

Therefore, vibration control is essential in improving

machining surface finish; achieving longer bearing,

spindle, and tool life in high-speed machining; and

reducing the number of unscheduled shutdowns. A great

cost savings for high-speed pumps, turbines, compressors,

and other turbo machinery used in industries can be

realized by removing the unbalance. [1]

Balancing is defined as ―the process of adding (or

removing) mass in a plane or planes on a rotor in order to

move the center of gravity towards the axis of rotation.‖ As

the definition of balancing implies, material is either added

to or removed from the rotating element to attain an

acceptable balance level.

To balance the rotor the amount of mass has to be

removed or added in the rotor for which it is necessary to

know the amount of unbalance along with the acceptable

tolerance. This has to be done by the experimental method.

The result shows whether the rotor is balanced or

unbalanced.

II. ROTOR BALANCING METHODS

Rotor dynamics is the study of rotating machines and

has a very important part to play throughout the modern

industrial world. A great deal of resources put into the

study of rotor dynamics to calculate safe operating ranges

and unbalance before the machines goes into service and

also methods of detecting imminent failure.



410

Rotor balancing techniques can be mainly classified as:

On-line balancing methods

Off-line balancing methods

Fig.2 Rotor balancing methods [1]

The off-line rigid rotor balancing method is mostly used

in industrial applications. The rotor is modelled as a rigid

shaft that cannot have elastic deformation during operation.

In this method, any imbalance distribution in a rigid rotor

can be balanced in two different planes. Rigid rotor

balancing is again categories as single plane and two plane

balancing. Here, we are performing two plane balancing on

the pump impeller.

III. TYPES OF UNBALANCE

A. Static Unbalance

A condition of static unbalance exists when the mass

center does not lie on the axis of rotation. Static unbalance

is also known as Force Unbalance. As defined, static

unbalance is an ideal condition, it has the additional

condition that the axis of rotation be parallel to the central

principal axis - no couple unbalance. [4]

Fig.3 Static unbalance [4]

B. Couple Unbalance

A specific condition that exists when the central

principal axis of inertia is not parallel with the axis of

rotation. As defined, couple unbalance is an ideal

condition. It carries the additional condition that the mass

center lies on the axis of rotation – no static unbalance. [4]

Fig.4 Couple unbalance [4]

C. Dynamic Unbalance

The most general case of unbalance in which the central

principal axis is not parallel to and does not intersect the

axis of rotation. Dynamic unbalance is also referred to as

two plane unbalance, indicating that correction is required

in two planes to fully eliminate dynamic unbalance.

Dynamic unbalance captures all the unbalance which exists

in a rotor. This type of unbalance can only be measured on

a rotating balancer since it includes couple unbalance.

Since dynamic unbalance is a combination of static and

couple unbalance and since static and couple unbalance

have different units, there are no unique units for dynamic

unbalance. It can be expressed as static and couple or in

terms of the balance corrections required. [4]

Fig.5 Dynamic unbalance [4]

D. Quasi-Static Unbalance

A special form of dynamic unbalance in which the static

and couple unbalance vectors lie in the same plane. The

central principal axis intersects the axis of rotation, but the

mass center does not lie on the axis of rotation. This is the

case where an otherwise balanced rotor is altered (weight

added or removed) in a plane some distance from the mass

center. The alteration creates a static unbalance as well as a

couple unbalance. Conversely, a rotor with quasi-static

unbalance can be balanced with a single correction of the

right magnitude in the appropriate plane. [4]

IV. UNBALANCE EFFECT

An unbalanced rotor generates an inertial force

(centrifugal) which increases with the square speed.

Fig.6 Unbalance effect [2]



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F = m.r.ω2 = U.ω

2

Where,

U = m.r = unbalance [kg.m]

ω = angular speed [rad/s]

ω = 2π.N / 60

Where,

N = revolutions/minute

F = Centrifugal force in Newton

The vector unbalance U (multiplied by the factor ω2,

square of the angular speed) originates the centrifugal force

F; this means that the load caused by the unbalance

increases with the square of the speed (doubling the

running speed the centrifugal force (inertia force) becomes

four times greater). [2]

V. BALANCING TOLERANCES

International standard ISO 1940 gives a rule in order to

calculate an acceptable residual unbalance, having

following features:

Gross unbalance deficiencies are avoided

Useless and expensive balancing works are

avoided

For each rotor type, depending on its maximum service

speed the acceptable total residual unbalance per unit of

mass is calculated [(gr.mm)/kg] (specified residual

unbalance).

The calculated value is the same mass eccentricity:

Where,

E = Mass eccentricity [microns]

U = Unbalance [gr.mm]

M = Rotor mass [kg]

According to ISO 1940 standard, all rotors are classified,

depending on their balancing requirement. Balancing

quality G is a number which defines the balancing accuracy

required; for instance G = 6.3 means that a normal

balancing is accepted. The maximum service speed is

reported on the horizontal x axis, while the acceptable

specific unbalance (acceptable unbalance per unit of mass

or acceptable residual mass eccentricity) is reported on the

vertical y axis.

The following formula can be used instead of previous

diagram:

Et = (9550 / M).G

Where,

Et [μ] = Total acceptable mass eccentricity

N [RPM] = Maximum service rotor speed

G [mm/s] = Balancing quality grade

Total residual accepted unbalance:

U[gr.mm] = Et.M

Where: M [kg] = Rotor mass

Total residual admitted unbalance in grams is m= U/R

Where, R [mm] is the compensation radius. [2]

VI. EXPERIMENTAL METHOD

TABLE I

IMPELLER SPECIFICATIONS

Sr.

No.

Regular Italic

1 Impeller type Single vane impeller

2 Balancing speed 1450 rpm

3 Length of the rotor 210 mm

4 Diameter of the

impeller

310 mm

5 Suction diameter 160 mm

6 Weight of the rotor 38.8 kg

7 Balancing grade G6.3 (As per Internal

Standard ISO 1940)

Set up the rotor in the balancer (balancing machine) and

secure it. Mount the rotor vertically on the shaft of the

balancer. Make sure that the rotor is place in proper vertical

position. There is no misalignment in the rotor and the shaft

of the balancer. Feed the balancing grade in the machine.

Make sure that the rotor is freely rotating. This machine is

the vertical axis semiautomatic machine and has the

capability of performing two-plane balancing with required

balancing speed and provides us the exact information of

unbalance amount and location on the rotor.

Fig.7 Rotor placement on machine



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Balancing machine screen provides various details of the

plane radius , rotor radius, acceptable unbalance for the

rotor. We can feed the balancing grade (G6.3 for pump

impeller) and machine automatically calculates the

tolerance per plane data and displays on the screen of the

balancing machine.

Start the balancer with the balancing speed of 1450 rpm

and read out the details on the screen. This specifies the

details about the rotor and the acceptable unbalance. Before

starting mark the location of zero on the rotor which should

matches the arrow mark on the balancing machine table.

TABLE III

BALANCING GRADES [5]

Balancing

Grades

Rotor Types

G 4000 Crankshaft drives of rigidly mounted slow marine

diesel engines with uneven number of cylinders.

G 1600 Crankshaft drives of rigidly mounted large two-

cycle engines.

G 630 Crankshaft drives of rigidly mounted large four-

cycle engines.

Crankshaft drives of elastically mounted marine

diesel engines.

G 250 Crankshaft drives of rigidly mounted fast four-

cylinder diesel engines.

G 100 Crankshaft drives of fast diesel engines with six or

more cylinders. Complete engines (gas or diesel) for

cars, trucks and locomotives.

G 40 Car wheels, wheel rims, wheel sets, drive shafts.

Crankshaft drives or elastically mounted fast four-

cycle engines (gas or diesel) with six or more

cylinders. Crankshaft drives for engines of cars,

trucks or locomotives.

G 16 Drive shafts (propeller shafts, cardan shafts) with

special requirements. Parts of crushing machinery.

Parts of agricultural machinery. Individual

components of engines (gas or diesel) for cars,

trucks and locomotives. Crankshaft drives of

engines with six or more cylinders under special

requirements. Slurry or dredge pump impeller.

G 6.3 Parts or process plant machines. Marine main

turbine gears (merchant service). Centrifuge drums.

Fans. Assembled aircraft gas turbine rotors. Fly

wheels. Pump impellers. Machine tool and general

machinery parts. Normal electrical armatures.

Individual components of engines under special

requirements

G 2.5 Gas & steam turbines, including marine main

turbines (merchant service). Rigid turbo-generator

rotors. Rotors. Turbo-compressors. Machine tool

drives. Medium and large electrical armatures with

special requirements. Small electrical armatures.

Turbine driven pumps.

G 1 Tape recorder and phonograph drives. Grinding

machine drives. Small electrical armatures with

special requirements

G 0.4 Spindles, disks and armatures of precision grinders.

Gyro

TABLE IIIII BALANCING TOLERANCE [2]

The maximum service speed is reported on the

horizontal x axis, while the acceptable specific unbalance is

reported on the vertical y axis. The formula can be used

instead of previous figure.

Et (μ) = (9550/N).G

Et (μ) = 9550/1450×6.3 = 41.50



413

Where,

Et[μ] = total acceptable mass eccentricity

N [RPM] = maximum service rotor speed

G [mm/s] = balancing quality (grade)

Total residual accepted unbalance.

U[gr.mm]=Et.M

Where, M[kg] = Rotor mass

U[gr.mm] = 41.50×38.8

U[gr.mm] = 1610.2

Acceptable unbalance per plane

= 1610.2/2=805 gr.mm

Plane 1 acceptable unbalance

= 805/80=10.1 gr

Plane 2 acceptable unbalance

= 805/154=5.2 gr

Wait for some time to stabilize the data on the balancing

machine screen. This data gives us the amount of

unbalance and the angle of the same on the rotor. Once the

data is stabilized, stop the balancer and read the data on the

screen. The screen is mainly divided into two parts, on the

left hand side the data is for plane 1 and on right hand

screen the data is for plane 2. The data is represented in two

colours – red and green. Green colour shows that the

unbalance amount is under acceptable limit whereas the red

colour data shows the unbalance has to be removed as this

exceeds the tolerance limit.

TABLE IVV

BALANCING DATA - I

Sr.

No.

Parameter Plane 1 Plane 2

1 Amount of unbalance

[gr]

6.25 30.62

2 Angle [deg] 210 45

3 Radius [mm] 70 153

If the data shows in green, then the rotor if said to be

balanced. Otherwise we need to remove the unbalance

amount from the rotor.

Unbalance Correction Methods:

Addition of mass

Removal of mass

Repeat the same balancing process until the data comes

under the acceptable limit of the rotor as per ISO 1940/1

grade G 6.3.

TABLE V BALANCING DATA - II

Sr.

No.



[gr]

5.21 21.01

2 Angle [deg] 212.3 44.5


TABLE VI

BALANCING DATA - III

Sr.

No.



[gr]

5.15 2.11

2 Angle [deg] 213 45.5


TABLE VI shows that the unbalance amount is within

the balancing grade limit. Hence, we can say that the

impeller is balanced as per G6.3 grade.

VII. RESULTS

Rotor dynamics is the study of rotating machines and

has a very important part to play throughout the modern

industrial world. The experimental results of rotor dynamic

balancing shows the data within acceptable limit as per ISO

1940/1, grade G6.3 for pump impeller.

References

[1] Shiyu Zhou and Jianjun Shi, ―Active Balancing and Vibration

Control of Rotating Machinery: A Survey‖, The Shock and

Vibration Digest, July 2001, Vol. 33, No. 4, 361-371.

[2] Ing. G. Manni, ―Balancing Theory and Applications‖, CEMB S.p.A. – Via Risogimento, August 1999, Rev. 2.1.

[3] Joe Evans, ―Sewage Pump Impeller Selection‖, Pacific Liquid & Air

Systems.

[4] Gary K. Grim, John W. Haidler, Bruce J. Mitchell, Jr., ―The Basics of Balancing‖, Balance Technology Inc.

[5] Earl M. Halfen, ―Shop Balancing Tolerances A Practical Guide‖,

IRD Balancing.

Documents

Dynamic Balancing of Centrifugal Pump Impeller