pump.doc

7/27/2019 pump.doc

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C E N T R I F U G A L P U M P S

Pump:

It is a device used to increase the potential (pressure) energy of a liquid. It canbe used to drive the fluid into a higher-pressure region or to an elevated level.

Types of pumps:

There are basically two types of pumps

1) Rotodynamic pumps2) Positive displacement pumps

Rotodynamic pumps:

In this type of a pump, the kinetic energy of the liquid is transformed intopressure energy.

Positive displacement pumps:

In this type of a pump, a moving element displaces the liquid against thedischarge side liquid under pressure. The pressure developed by the pump isslightly higher than the backpressure of liquid.Centrifugal Pump:

Centrifugal pump is a radial flow rotodynamic pump. In some centrifugal pumps,mixed flow and axial flow also exist.

In a centrifugal pump, the impeller rotating at high speed imparts KE to the liquid;this KE of the liquid is converted into the pressure energy form. The gradualenergy transformation is achieved in the volute casing / diffuser casing. Different parts of a centrifugal pump and their functions:Shaft:

It holds all the rotating elements of a pump. It drives the impeller.

Shaft sleeve:It covers the shaft in the region where it is prone to corrosion, erosion or abrasion. It can be replaced when it wears out without replacing the entire shaft.Impeller :The impeller driven by the shaft imparts kinetic energy to the fluid by its dynamicaction.Casing:It converts the KE of the fluid into pressure energy. The available crosssection/volume for the liquid generally increases in the casing towards thedischarge nozzle.

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Pump out vanes / Expeller vanes :

These vanes provided at the rear side / discharge side of the impeller drives thefluid away thereby reducing the pressure at the back of the Impeller. Thesevanes help to restrict the leakage of pumped liquid into the stuffing box thus

reduces the load on the seal.

Auxiliary impeller :

It is a separate impeller positioned at the back of the impeller. It serves the samefunction of pumpout vanes.

Stuffing box:

The leakage of the pumped liquid into the atmosphere through the gap betweenshaft and casing is arrested in the stuffing box. It may house gland packing or a

mechanical seal.

Impeller wear rings:

These are fitted at the suction side and discharge side of an impeller to protect itfrom wear and to reduce recirculation of discharge fluid back to suction.

Casing wear rings:

These are fitted in the casing at the front side and rear side of the impeller toprotect the casing from wear. These wear rings have little diametrical clearances

with the Impeller wear rings restricting the leakage resulting in improvedvolumetric efficiency of the pump.

Vortex breaker / stop piece:

It is provided in the suction pipe near the impeller eye to avoid pre-rotation or whirl of the fluid entering the impeller. This avoids turbulence in the fluid andreduces entry losses.

Casing throat bush:

This bush provided in the casing rear, before the stuffing box with littlediametrical clearances with the shaft restricts the leakage of fluid into the stuffingbox. This is generally made of a soft material like aluminum.

Lantern ring:

This is provided in stuffing boxes housing gland packing. This ring providescooling medium to the packing and also prevents the entry of air into the pump.

Deflector:

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It is a ring fastened to the shaft at bearing end covers, which prevents the entryof water and dust into the bearing housing.

Bearings:They take the axial loads and radial loads on the shaft. They also provide support

for the static loads on the shaft.

Gland packing:Soft and self-lubricating packing rings are provided in the stuffing box around theshaft sleeve to reduce the leakage of pumped fluid into the atmosphere.

Mechanical seal: A rotating face fixed to the shaft and a stationary face in the stuffing box alongwith a set of ‘O’ rings seal the pumped fluid from escaping into the atmosphere.This assembly is called a mechanical seal.

Coupling:This is a mechanical member which transmits the torque from the driver (motor /turbine) to the pump shaft. Couplings are generally designed to take slightmisalignment between the driver and driven shaft centerlines.

Balancing disc / drum / piston:These devices are used to balance the net axial thrust in multistage centrifugalpumps.

Minimum bypass valve:It is provided in the discharge pipeline of a multistage pump to avoid over heating

of the pumped fluid in case of closure of discharge valve. It is a safety deviceinstalled for pumped fluids operating near their boiling points to avoidvaporization of the fluid.

Some concepts related to a centrifugal pump

Cavitation:Vapor bubbles of the fluid form at low-pressure regions and gradually the bubblepressure rises as it moves into a high-pressure region where the bubblescollapse while flowing through the impeller and in the casing. These explosionsimpart high velocities to the fluid particles around and create turbulence. This

phenomenon can occur at any stage of pumping and erodes the impeller andcasing. This can occur due to the ingress of air also. This occurs mainly whenNPSHA is less than the NPSHR for the pump. Cavitation results in pump noise,vibrations, fluctuation of amperage and discharge pressure and fall in pump flow.Cavitation occurs when flow, fluid temperature, speed increases, and specificgravity decreases.

Priming:

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Filling the suction pipe and pump casing with the liquid to be pumped anddisplacing the air from it is called priming. Improper priming leads to cavitation.

NPSH available:It is the net positive suction head available at the eye of the impeller considering

all the head losses.NPSHA = Pa /d + hs + Ps /d - Pv /d – hf – hi Pa /d = atmospheric head acting on the liquid free surface

hs = static head from the free surface of the liquid to the centre of theimpeller.

d = density of the liquid at the operating temperaturePs = suction pressure.Pv = Vapour pressure of the pumped liquid at the operating temperaturehf = Head losses due to friction in the suction pipeline.hi = Entry losses at the pump entry due to turbulencehs and Ps terms are deducted it the suction source is below the centreline of the

pump and when the suction pressure is negative respectively.

NPSH Required:It is the pressure required at the impeller eye for satisfactory pumping. It is givenby the pump manufacturer. NPSHA > NPSHR for satisfactory operation of thepump.

a) NPSHR will increase with the fluid temperatureb) NPSHR will increase with the speed of the impeller c) NPSHR will increase with the capacityd) NPSHR will increase with decrease in viscosity

Also NPSHA decreases with increased capacity.

Characteristic Curves of a centrifugal Pump:

Efficiency, power consumed and the head are plotted against the capacity or flow. We observe that Head falls parabolically, Power increases almost linearlyand efficiency varies as the flow across the pump increases. Efficiency rises to apeak point and then falls in a parabolic fashion. The pump should be made to runclose to the peak of efficiency curve. The head at zero flow or shutoff condition of

discharge valve is called the shutoff head.Minimum bypass valve:

In multistage centrifugal pumps, the temperature of the fluid rises rapidly if thepump is run with the discharge valve closed. Due to the temperature rise of thefluid, the internal clearances will be affected owing to different thermal expansioncoefficients of static and rotary parts in the pump and may lead to rubbing andconsequent seizure of the pump. Pumps handling condensate are prone tocavitation also, as slight temperature rise may lead to evaporation of condensateoperating close to the saturation curve.

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The minimum by pass valve provided in the discharge line recycles the dischargefluid back to suction and dissipates the heat generated in the pump.

The quantity of fluid to be bypassed is designed according to the pump designflow, head and temperatures. The d/s of minimum bypass valve should not be

provided with isolation valve to avoid any inadvertent closing of the valve.

System head curve:It is the sum of all heads (absolute). The operating point is the intersection of thiscurve and the characteristic curve. This operating point is to be kept close to thehighest efficiency point for good performance of the pump.Radial thrust:

Radial thrust occurs on the shaft due to unequal forces acting side ward on theimpellers in the volute casing. This becomes more prominent when the flow is

less (i.e. head is high). Another cause of radial thrust on the shaft is itsSelf-weight acting vertically downward.

Radial thrust can be avoided / reduced by providing a double volute casingwhere the force acting on the impeller due to the volute is greatly reduced.

Another way of balancing radial thrust is providing a diffuser casing where thepressure is uniform through out the casing around the impeller.

Axial thrust and it’s balancing:

In a centrifugal pump impeller, the unequal force components acting on the face

and back of the impeller due to differential pressures lead to an axial thrust onthe shaft towards the suction. This becomes more predominant in high-pressuremultistage pumps as the number of impellers arranged in tandem increase.Thrust = Pd Ab – Pd A1 – Ps (A2+ A3)

Thrust = Pd Ab - Ps (A1+A2+ A3) – (Pd - Ps )A1

Thrust = Ab (Pd - P s) -- A1(Pd - Ps )

Thrust = (Pd - Ps) (Ab - A1 )

But Ab = pi/4 ( (impeller O.D)2

- (shaft O.D )2

)

and A1 = pi/4 ( (impeller O.D)2 - (wear ring O.D )2 )

Hence net Thrust = (Pd - Ps) pi/4 ( (wear ring O.D )2- (shaft O.D )2 )

This shows that net thrust is positive and directed towards suction.

This thrust can be balanced in small single and two stage pumps by:

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1) Maximizing A1 in the impeller 2) Using a double suction impeller 3) Using Opposed impeller design4) Providing balancing holes5) Provided both suction and discharge wear rings.

6) Reducing wear ring outer dia.

However, in multistage High-pressure centrifugal pumps, balancing devices suchas balancing disc or balancing piston are used.

1) Increase of balancing line pressure indicates wear of balance piston.

2) Balancing disc adjusts clearance automatically so as to maintainequilibrium. This cannot be used in volatile and inflammable services asrubbing between Balancing disc and counter balancing disc may lead tofire. A lift off device is required in this type of an arrangement. The lift off

device pulls the balancing disc away from the counter balancing disk for some time initially during startup of the pump. The balancing disk isreleased when sufficient pressure is developed in the final discharge. Therelease in automatic. Disc arrangement doesn’t require thrust bearings.

3) Balancing piston reduces thrust to a large extent. But during the start of apump, when the discharge pressure is not developed fully, thrust loadsexist and thrust bearings are to be installed to take care of such loads.

4) Both disk and piston arrangements are simultaneously used. The pistonacts when the disk fails.

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pump.doc