CENTRIFUGAL PUMP and BALANCING

CAUSES OF CAVITATION

• low NPSH due to reduction of flow at the suction and increase in fluid velocity
• increase in temp of the fluid
• running the pp at higher speed than reqd
• obstructions or sharp elbows in the suction pipe.
• too long distance of suctin pipe.

Why horizontal centrifugal pumps are palced only in fwd-aft direction
If placed in port-stbd dir, then the pump axial force and the ships longitudnal force will act together with a resultant force having a gyroscoping effect and turns the pump body. In fwd-aft dir, both forces coincide each other and resultant force.

Centrifugal pump consists of vanes/impellers which imparts Kinetic energy to the fluid. This K.E is converted to pressure energy by using a volute casing or diffuser.
Diffuser is a stationary guide vane surrounding the impeller.
A small clearance is provided btw the impeller and the casing to enable free rotation. To maximise the efficiency, it is important to minimise the leakage passing thru this clearance from high pr to low pr side of the pump. The leaking liquid at high press erodes the surface in the clearance. This will increase the clearance and affect the pp efficiency. To minimise the pump cost, wearing rings are provided both for impeller and casing.
BALANCING OF CENTRIFUGAL PUMPS
 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 / casing cover. This high pressure liquid exerts pressure on the outlet passages and shrouds of the

impeller resulting in generation of two forces, one in lateral and another in longitudinal

direction with respect to shaft axis. 

 

The force generated in lateral direction is due to dissimilar pressure generation in volute and called as Radial thrust. Radial thrust is dependant solely on pressure head generated by pump and width & dia of the impeller.Radial thrust can be satisfactorily reduced to harmless extents by using 

-double volute casings

-diffuser type casings.

Axial hydraulic thrust is the summation of unbalanced impeller forces acting in the axial direction. It is dependent on the many aspects viz. shroud and casing clearances, peripheral shroud speeds, head developed by the pump, impeller geometry etc. 
Theoretically  double suction impellers are in in axial hydraulic balanced with the forces one one side equal to and counter balancing the forces on other side.But is not practical
Balanced by - 

BALANCING HOLES
In a single suction impeller, A pp can be provided with 2 wear rings front and back. To equalise the thrust area, the inner dia of both the rings are made same.   Pressure equal to suc pr is maintained in a chamber located on the back wear ring by drilling balancing holes thru the impeller. Leakage past the back wearing ring, through this chamber, is returned into the suction area thru these holes.The balancing holes have a total csa greater than csa of the annular space  btw the wearing ring and the chamber. This results in decreased pump efficiency as some liquid is allowed to escape from discharge to suction.
 
Another method is to use pump out vanes on the back shroud. This results in suction pres on both sides of the impeller hub.No loss in pump efficiency. In fact pump out vanes favour the pumping process.

STATIC BALANCE

A pump shaft along its rotor or impeller is said to be statically balanced if  when supported on parallel and level knife edges, it remains static in any axis.

DYNAMIC BALANCE

A pump shaft with rotor is in dynamic balancing if all the various centrifugal forces resulting from the rotation balances each other and rotor runs at full speed without vibr.

1.       M/E LO Sump centrifugal pump/ screw pump.(Mitra)

2.       multistage pump axial thrust(Mitra)

3.       Why gear pump used for engine(Mitra)(chances of loosing suction; lack of head)

4.       In vertical centrifugal pump, who takes the thrust(Paul and RKD and Sinha)(balancing holes; in general bearings)

5.       diffuser in centrifugal pump.(Battu)

6.       why two impellers used in centrifugal pump in series.(Battu) head,without increasing pump dimensions ;thrust balance;

7.       Centrifugal pp clearances(BND)

8.       CF pump opend,wat chks +to carry out,which clearences to chk nd dhud b how much,wat to chk in mech seal etc.(BND)

9.       Multistage centrifugal pump... Diagram and water flow path(Bhoumick)

10.   screw pump ..different parts..where used(Bhowmick)

11.   Gear p/p drawing How PR is generated in discharge(Chakravarty)

12.   difference between fire pump and boiler feed water pump.what is double volute, structure.is it necessary dat double volute have double impeller(Chakravarti)

13.   screw pump thrust load accomodation,(Mitra)
all answers were rejected by him whole day....., thrust bearing, fluid forces...etc

14.   thrust balancing of screw pump?(Mitra)  Thrust Balancing of screw pump : Hydraulic balancing : Radial force is balanced by hydrodynamic film formed by the idle rotor while rotating in the housing....Axial force is hydrostatically balanced by the differential pressure in the power rotor and idle rotor.....

LUBE OIL PUMP

Positive displ pp is suitable for oil duty due to its self priming quality but has speed restriction due to high power requirement and also excessive pr head. So, it requires a bypass valve to control the flow, if a constant AC motor is given.
A centrifugal pump with an extended shaft open end impeller serves pupose of self priming.
When l.o temp increases, its viscosity decreases... hence the pump will be able to discharge more but with a pressure drop. This can be seen in the graph and it shows that centrifugal pr drop is less compared to that of postive displ pp. It also shows that there is huge increase in quantity using a centrifugal pump while positive displ pp shows a marginal increase.

POSITIVE DISPLACEMENT PUMP
Works on the principle of constant quantity. They can create their own suction lift and able to pump air or vapour, without loss of suction head. Efficiency highly depends on the close clearances btw parts.

CAPACITY
For zero head, it gives max volumetric efficiency. But as head increases, the quantity(capacity) decreases which is due to the slip btw clearances pf gear or screws and the casing slip.
Capacity is directly proportional to speed and slip almost remains constant

POWER
Power requirement depends on frictional resistance to overcome by the parts and the pumping liquid
Power input increases with increase in speed and head.
HEAD
Theoretically head is infinite but practically, limited by the torque availability and strength of the pump parts.
Why centrifugal pump for lube oil pump?

From the curve, we can see that with increase in temp, the positive displacement pump pressure drop is high and the quantity increase is just marginal. The main engine lube oil prime purpose is to cool the engine, for which if there is an increase in temperature, the centrifugal pump pressure drop will be very less and the quantity increases very much. For this purpose, centrifugal pump is used so that we can increase in temperature the quantity also has to be increased.

AXIAL PUMP

Axial pump requires a relief valve because While starting, if the discharge valve is throttled, the head increases 3times and power 2times which trips the motor. Actually water hammering and vibr occurs and relief valve lifts.
Beyond its normal duty, power req decreases with capacity increases marginally and head falls sharply.
While in a centrifugal pump at beyond its normal duty, there is a sharp increase in power with fall in H&Q.

Application
Axial pump is ideally suited where high throughput and low head is reqd. It draws less power when run at high speed, compared to the centrifugal pump.
DISADVANTAGES
It creates a vortex at suction and hence requires a vortex destroyer in the suction.
Material
Casing & Guide vanes - Gun metal
Shaft stainless steel
Impeller Aluminium Bronze - Cu76%,Tin22%Al-2%
MIX FLOW PUMP

Application - LNG,LPG and chemical carrier. efficiency of the pump is very high as it takes less power compared to centrifugal pump or displacement pump.
WHY INDUCER?
For a given discharge, the NPSH reqd is reduced by fitting a inducer at the mouth of the shaft. It enhances the suction characteristics by collapsing any vapor/cavitation, formed in the suction, in the region btw inducer and the eye of the impeller. It reduces the NPSH reqd as inducer can handle air/vapor to a certain extent.
WHAT IS RIGID SHAFT AND FLEXIBLE SHAFT
RIGID shaft has its operating speed less than critical speed.
FLEXIBLE shaft has its operating speed more than the critical speed so that it can absorb the vibr arising out of crossing critical speed.