PRESENTATION ONRADIAL AND AXIAL THRUST IN CENTRIFUGAL PUMP AND

METHODS USE TO BALANCE THEM

GROUP MEMBERS :- 1) SHANKAR SHEGADE (ROLL NO: 129) 2) AKSHAY SHIKARE (ROLL NO: 130)

SHIVAJIRAO S. JONDHLE COLLEGE OF ENGINEERING & TECHNOLOGY (ASANGAON)

INTRODUCTION The Centrifugal pump is the type of Rotodynamic pump also called as velocity pump. Centrifugal pump is a device use to transport fluids by converting rotational kinetic energy into pressure energy. Both

kinetic energy and pressure energy are increased. It uses centrifugal motion of fluid to rise the water from low level to high level.

The motion of water is restricted by casing of pump, it result in increase in pressure. Main components of centrifugal pump are Impeller, Casing, Suction pipe with strainer and foot valve, Delivery pipe. Common uses include water, sewage, petroleum and petrochemical pumping; a centrifugal fan is commonly used to

implement a vacuum cleaner. The reverse function of the centrifugal pump is a water turbine converting potential energy of water pressure into mechanical rotational energy.

CENTRIFUGAL PUMP

THRUSTS IN CENTRIFUGAL PUMP

During operation and working of centrifugal pumps, the kinetic energy of flowing liquid is converted into pressure energy. This high pressure liquid is continuously flowing all over the circumference of the impeller and also gets entrapped inside the clearances between impeller and casing.

The high pressured liquid trapped between impeller and casing which exerts pressure on the outlet wide passages and shrouds of the impeller resulting in generation of two thrusts. These thrusts are – 1. Radial Thrust – The force generated in lateral direction is due to dissimilar

pressure generation in volute is called as Radial thrust 2. Axial thrust – The force generated in longitudinal direction is on account of

different areas of impeller exposed to trapped pressurized liquid called as axial thrust.

Radial Thrust in Centrifugal Pumps The force generated in lateral direction is due to dissimilar pressure generation in volute is called

as Radial thrust. The hydraulic radial thrust is due to the unequal velocity of the fluid flowing through the casing.

The unequal fluid velocity results in a non-uniform distribution of pressure acting on the circumference of the impeller.

The radial load is most influenced by the design of pump casing. The pump casing is designed to direct the fluid flow from the impeller into the discharge piping.

In a theoretical situation at Best Efficiency point (BEP), the volute casing has a uniform distribution of velocity and pressure around the impeller periphery.

Radial force depends on total head, width and diameter of the impeller.

Radial Thrust Variation of radial force with discharge in single volute casing pump

Methods of Balancing Radial Thrust

Diffuser pump

Variation of radial force with discharge

Methods of balancing radial thrust are as follows:-1. Double volute casing:- In this method the volute is divided

into two parts with the centrally provided partition which joins into a common discharge. Radial forces exerted on a shaft and bearing are very small in this type of design. Therefore such pump do not require heavy shaft and design of pump is economical.

2. Diffuser pump:- In this method the diffuser is provided in between the impeller and casing. After excursion of the liquid from impeller outlet it passes through the diffuser so due to that the liquid is not directly collapsed on the casing so there is not back jerk taking place and thus the radial forces are balanced.

Following graph shows the variation of radial force acting on shaft with percent capacity of flow at best efficiency point(B.E.P.).

The graph shows that for design capacity radial force is minimum. It decreases till that point and increases further.

Double volute casing minimum and constant radial force for large range of capacity.

Axial Thrust in Centrifugal Pumps Axial hydraulic thrust is the summation of unbalanced

impeller forces acting in the axial direction. This thrust is excessively affecting on the bearings.

This thrust is generally high at the starting and shut-off of the pump. During starting of pump there is sudden jerk of whole impeller and shaft towards driving end, as the liquid suddenly enters at impeller eye. Thus the force is exerted on the bearings which lead to the damage of bearings resulting in reducing bearing life.

As shown in Figure above the resultant unbalanced axial thrust is vector summation of the following forces –

1. Force acting on front shroud due to liquid of delivery pressure entrapped between pump casing and front shroud (F1).

2. Force acting on back shroud due to liquid of delivery pressure entrapped between casing cover and back shroud(F2).

3. Force acting in the direction of the liquid flow due to its momentum change (Fm).

Axial Thrust

Methods of Balancing Axial Thrust Methods of balancing axial thrust in Small Pumps:-

1. Ball Thrust Bearings:- It is provided in direction of axial thrust as shown in figure. This is the most efficient way to reduce the thrust on the bearing.

2. Wear Rings:- A cast iron ring in the casing is inserted which should fitted with similar ring cast integral with the impeller as shown in figure.

Balancing axial thrust with ball bearings and wear rings

Methods of balancing axial thrust in Large Pumps:-1. Balancing Hole:-

• By providing balancing holes in the impeller which allows the suction pressure to act equally from both sides as shown in figure.

• The use of drilled holes through the impeller shroud to the balancing chamber is inferior to the arrangement using a special channel to connect the balancing chamber with the suction nozzle because leakage through the holes is directed against the flow in the impeller eye, causing disturbances.

• The balance by this method is never complete. From 10% to 25% of the axial thrust always remains depending on the size of the holes. For a complete balance the diameter of the wearing rings of the balancing chamber should be greater than that at the impeller eye.

Balancing Holes

2. Back Vanes:- • In this method, radial ribs are used on the back shroud to reduce the pressure in the

space between the impeller, and the pump casing. Back vanes acts as auxiliary impeller which restricts the entry of liquid into the clearances between impeller back shroud and casing cover.

• It is evident that the balancing with wearing and holes method introduces the leakage loss of the pump which, in turn, increases as wearing rings are worn. Balancing with ribs method requires some additional power which, however does not change with time. In addition it is cheaper and more effective than the first method. In this method the power consumed is much less than the power loss due to leakage through balancing holes under normal conditions.

Back Vanes

3. Double suction impeller :-• By using double suction impeller there would be equal and opposite axial thrust acting

on each suction sides as shown in figure and hence they would balance out each other• In practice, this balance may not be achieved for the following reasons:

i. External conditions such as an elbow being too close to the pump suction nozzle may cause unequal flows to the suction eyes.

ii. The two sides of the discharge casing may not be symmetrical, or the impeller may be located off-centre. These conditions will alter the flow characteristics between the impeller shrouds and casing, causing unequal pressures on the shrouds

iii. Unequal leakage through the two leakage joints will tend to upset the balance.

Double suction Impeller

4. Even number of impellers of multistage pump:-• The number of impeller of multistage centrifugal pump are made even in number.

The suction of half of the impeller is kept on one side and the suction of remainder half of impeller is kept from other side so that total axial thrust exerted on each side will neutralize each other.

• It is similar to concept of double suction impeller. The system is shown in figure.

Balancing of Even number of impellers of multistage pump

REFERENCES Mechanical Utility System – B.L.Singhal. Analysis of Hydraulic Thrusts in Centrifugal Pump to Increase te Bearing Life;

Author:- “Prashant Sidhappa Bolade” , “Santosh J Madki”; (8 Aug 2015); https://www.ijert.org/view-pdf/13969/analysis-of-hydraulic-thrusts-in-centrifugal-pump-to-increase-te-bearing-life.

Axial Thrust in Centrifugal Pump; ”Vasant Godbole1*”, ”Rajashri Patil1” , ”S.S. Gavade2”;(27 July 2012); http://paginas.fe.up.pt/clme/icem15/ICEM15_CD/data/papers/2977.pdf.

https://en.wikipedia.org/wiki/Centrifugal_pump .

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