SCREW PUMP

SCREW PUMP
In a screw pump, the fluid enters the outer suction manifold and passes through the meshing worm wheels to the central discharge manifold. The two worm wheels are driven by timing gears, to ensure correct  clearance is maintained btw the screws. This clearance is responsible for the pumping action of the fluid.The most common arrangement is opposed helices (double suction) with the flow pattern being from the ends of the screw sets to the center of the pump. 

For a single screw pump, there is no radial thrust but it gives an axial thrust because it discharges from one side to the other.
In a double/triple screw, the radial thrust is taken care by providing two screws being left handed and right handed. There is no axial thrust because it takes suction from both sides and discharges from the middle.
Double/triple screw pumps are provided with hardened and ground timing gear drive inorder to maintain a fixed axis of rotation thus avoiding metal to metal contact btw the  screws and the casings.

Two screw pump hydraulic forces:
Hydraulic radial forces on a two screw pump rotor due to differential pressure are illustrated in Figure. The forces are uniform along the length of the pumping threads. These hydraulic forces cause a deflection ìd", for which running clearance must be provided in the surrounding pump body. Greater deflection requires larger clearances resulting in more slip flow or volumetric inefficiency, so ìd" must be kept to a minimum. Excessive deflection will cause damage to the surrounding body and/or contribute to rotating bend fatigue, which will ultimately result in shaft breakage.

PURIFIER

1. Basic principles of purifier? How dam ring, rpm and bowl size determined? Banerjee 

The separation principle of high speed centrifuge depends on the difference in the specific gravity of two different liquids.  

Fs ₌ ∏/6x D³ (ρw-ρ_o) g
Where Fs is the separating force, ρ_w is density of water, ρ_o is density of oil and “g” is gravitational force
The g is replaced by the centrifugal force in purifiers
F_{s =} ∏/6x D³ (ρw-ρ_o) ω² r

Therfore, Effective seperation can be achieved by increasing  ω which depends on rotation of the purifier and by increasing the density difference by increasing the temperature.

 

NOMOGRAM to select Gravity disc/Dam ring

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.....