Impellers Of Centrifugal Pumps Engineering Essay

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Centrifugal pumps can be classified based on the manner in which fluid flows through the pump. The  manner in which fluid flows  through the pump is  determined by the design of the pump casing and the impeller.  The three types of flow through a centrifugal pump are radial flow, axial flow, and mixed flow.

Radial  Flow  Pumps In a radial flow pump, the liquid enters at the center of the impeller and is directed out along the impeller blades in a direction at right angles to the pump shaft

Axial  Flow  Pumps In an axial flow pump, the impeller pushes the liquid in a direction parallel to the pump shaft.

Axial  flow  pumps  are  sometimes  called  propeller  pumps because  they  operate essentially the same as the propeller of a boat.

Mixed  Flow  Pumps Mixed flow pumps borrow characteristics from both radial flow and axial flow pumps. As liquid flows through the impeller of a mixed flow pump, the impeller blades push the liquid out away from the pump shaft and to the pump suction at an angle greater than 90o

Multi-Stage  Centrifugal  Pumps A centrifugal pump with a single impeller that can develop a differential pressure of

more than 150 psid between the suction and the discharge is difficult and costly to design and construct. A more economical approach to developing high pressures with a single centrifugal pump is to include multiple impellers on a common shaft within the same pump casing.   Internal channels in  the  pump  casing  route  the  discharge  of  one  impeller  to  the  suction  of  another  impeller.  The water enters the pump from the top left and passes through each of the stage impellers in series, going from left to right.   The water goes from the volute surrounding the discharge of one impeller to the suction of the next impeller. A pump stage is defined as that portion of a centrifugal pump consisting of one impeller and its associated components.   Most  centrifugal pumps  are single-stage  pumps, containing  only one impeller.   A pump containing seven impellers within a single casing would be referred to as a seven-stage pump or, or generally, as a multi-stage pump

Pumps Impellers  can  be  open,  semi-open,  or  enclosed.

The  open  impeller  consists  only  of  blades attached to a hub.

The semi-open impeller is constructed with a circular plate (the web) attached to one side of the blades.

The enclosed impeller has circular plates attached to both sides of the blades.    Enclosed  impellers  are  also  referred  to  as  shrouded  impellers

Impellers  of  pumps  are  either Single-Suction and Double-Suction Impellers based on the number of points that the  liquid  can  enter  the  impeller and    also    on    the    amount    of webbing    between    the impeller blades. Impellers   can   be   either   single- suction    or   double-suction. A single-suction impeller allows liquid  to  enter  the  center  of  the blades from only one direction.  A double-suction    impeller    allows liquid  to  enter  the  center  of  the impeller  blades  from  both  sides simultaneously

The impeller sometimes contains balancing holes that connect the space around the hub to the suction  side  of  the  impeller.    The  balancing  holes  have  a  total  cross-sectional  area  that  is considerably greater than the cross-sectional area of the annular space between the wearing ring and the hub.   The result is suction pressure on both sides of the impeller hub, which maintains a hydraulic balance of

Diffuser

Some  centrifugal  pumps   contain diffusers.A  diffuser  is  a  set  of stationary vanes  that surround the impeller. The   purpose   of   the diffuser is to increase the efficiency of the centrifugal pump by    allowing    a    more    gradual expansion  and  less  turbulent  area for the liquid to reduce in velocity. The diffuser vanes are designed in a manner that the liquid exiting the impeller  will  encounter  an  ever- increasing  flow  area  as  it  passes through the diffuser.  This increase in flow area causes a reduction in flow  velocity,  converting  kinetic energy into flow pressure.

Centrifugal pumps can also be constructed in a manner that results in two distinct volutes, each receiving  the  liquid  that  is  discharged  from  a  180o  region  of  the  impeller  at  any  given  time. Pumps of this type are called double volute pumps (they may also be referred to a split volute pumps).  In some applications the double volute minimizes radial forces imparted to the shaft and bearings  due  to imbalances  in the  pressure around  the impeller.   

earing W Rings

Centrifugal pumps contain rotating impellers within stationary pump casings.   To allow the impeller to rotate freely within the pump casing, a small clearance is designed to be maintained between the impeller and the pump casing.   To maximize the efficiency of a centrifugal pump, it is necessary to minimize the amount of liquid leaking through this clearance from the high pressure or discharge side of the pump back to the low pressure or suction side.Some wear or erosion will occur at the point where the impeller and the pump casing nearly  come  into  contact.    This  wear  is  due  to  the  erosion  caused  by  liquid  leaking through this  tight clearance and other  causes.   As  wear occurs, the  clearances  become larger   and   the   rate   of   leakage   increases. Eventually,   the   leakage   could   become unacceptably large and maintenance would be required on the pump. To minimize the cost of pump maintenance, many centrifugal pumps are designed with wearing rings.

Wearing rings are replaceable rings that are attached to the impeller and/or the pump casing to allow a small running clearance between the impeller and the pump casing  without  causing  wear  of  the  actual  impeller  or  pump  casing  material.    These wearing  rings  are  designed  to  be  replaced  periodically  during  the  life  of  a  pump  and prevent the more costly replacement of the impeller or the casing

Cavitation

The flow area at the eye of the pump impeller is usually smaller than either the flow area of the pump suction piping or the flow area through the impeller vanes.  When the liquid being pumped enters  the eye  of a  centrifugal  pump, the  decrease in  flow area  results  in  an increase  in flow velocity accompanied by a decrease in pressure.  The greater the pump flow rate, the greater the pressure drop  between the  pump suction  and the eye  of the  impeller.   If  the pressure  drop is large enough, or if the temperature is high enough, the pressure drop may be sufficient to cause the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped.   Any vapor bubbles formed by the pressure drop at the eye of the impeller are swept along the impeller vanes by the flow of the fluid.   When the bubbles enter a region where local pressure is greater than saturation pressure farther out the impeller vane, the vapor bubbles  abruptly  collapse.    This  process  of  the  formation  and  subsequent  collapse  of  vapor bubbles in a pump is called cavitation. Cavitation  in  a  centrifugal  pump  has  a  significant  effect  on  pump  performance.    Cavitation degrades the performance of a pump, resulting in a fluctuating flow rate and discharge pressure. Cavitation can also be destructive to pumps internal components.  When a pump cavitates, vapor bubbles form in the low pressure region directly behind the rotating impeller vanes.  These vapor bubbles then move toward the oncoming impeller vane, where they collapse and cause a physical shock to the leading edge of the impeller vane.   This physical shock creates small pits on the leading edge of the impeller vane.  Each individual pit is microscopic in size, but the cumulative effect of millions of these pits formed over a period of hours or days can literally destroy a pump impeller.    Cavitation  can  also  cause  excessive  pump  vibration,  which  could  damage  pump bearings, wearing rings, and seals. A small number of centrifugal pumps are designed to operate under conditions where cavitation is unavoidable.  These pumps must be specially designed and maintained to withstand the small amount of cavitation that occurs during their operation.  Most centrifugal pumps are not designed to withstand sustained cavitation. Noise  is  one  of  the  indications  that  a  centrifugal  pump is  cavitating.   A cavitating  pump  can sound like a can of marbles being shaken.  Other indications that can be observed from a remote operating station are fluctuating discharge pressure, flow rate, and pump motor current.  

Net  Positive  Suction  Head

To avoid cavitation in centrifugal pumps, the pressure of the fluid at all points within the pump must remain above saturation pressure.   The quantity used to determine if the pressure of the liquid   being   pumped is  adequate to avoid cavitation is  the net positive suction head (NPSH). The net  positive  suction  head  available (NPSHA) is the difference between the pressure at the suction of the pump and the saturation pressure for the liquid being pumped.The net positive suction  head  required  (NPSHR)  is  the  minimum  net  positive  suction  head  necessary  to  avoid cavitation

The condition that must exist to avoid cavitation is that the net positive suction head available must be greater than or equal to the net positive suction head required.  

.A formula for NPSHA can be stated as the following equation.

NPSHA = Psuction - Psaturation

When a centrifugal pump is  taking suction from a tank or other reservoir, the  pressure at the suction of the pump is the sum of the absolute pressure at the surface of the liquid in the tank plus the pressure due to the elevation difference between the surface of liquid in the tank and the  pump suction  less  the head  losses  due to  friction  in the  suction  line from  the  tank to  the pump.

NPSHA    = Pa + Pst - hf - Psat

Where: NPSHA    =   net positive suction head available Pa =   absolute pressure on the surface of the liquid Pst =   pressure due to elevation between liquid surface and pump suction hf =   head losses in the pump suction piping Psat =   saturation pressure of the liquid being pumped

Preventing  Cavitation

If a centrifugal pump is  cavitating, several changes  in the system design or operation may be necessary to  increase  the NPSHA  above the  NPSHR  and stop  the cavitation.  One method  for increasing the NPSHA is to increase the pressure at the suction of the pump. For example, if a pump is taking suction from an enclosed tank, either raising the level of the liquid in the tank or increasing the pressure in the space above the liquid increases suction pressure. It  is  also  possible  to  increase  the  NPSHA  by  decreasing  the  temperature  of  the  liquid  being pumped.   Decreasing  the  temperature  of  the  liquid  decreases  the  saturation  pressure,  causing NPSHA  to  increase.    Recall  from  the  previous  module  on  heat  exchangers  that  large  steam condensers  usually  subcool  the  condensate  to  less  than  the  saturation  temperature,  called condensate depression, to prevent cavitation in the condensate pumps. If  the  head  losses  in  the  pump  suction  piping  can  be  reduced,  the  NPSHA  will  be  increased. Various  methods  for  reducing  head  losses  include  increasing  the  pipe  diameter,  reducing  the number of elbows, valves, and fittings in the pipe, and decreasing the length of the pipe.

It may also be possible to stop cavitation by reducing the NPSHR for the pump.   The NPSHR is not a constant for a given pump under all conditions, but depends on certain factors.  Typically, the  NPSHR  of   a  pump  increases  significantly  as  flow  rate  through  the  pump   increases. Therefore,  reducing  the  flow  rate  through  a  pump  by  throttling  a  discharge  valve  decreases NPSHR.

 NPSHR is also dependent upon pump speed.  The faster the impeller of a pump rotates, the greater the NPSHR.  Therefore, if the speed of a variable speed centrifugal pump is reduced, the NPSHR  of the pump decreases.   However, since a pump's flow rate is  most often dictated by  the  needs  of  the  system  on  which  it  is  connected,  only  limited  adjustments  can  be  made without starting additional parallel pumps, if available.

The net positive suction head required to prevent cavitation is determined through testing by the pump manufacturer and depends upon factors including type of impeller inlet, impeller design, pump   flow  rate, impeller   rotational  speed,   and   the  type   of  liquid   being   pumped. The manufacturer typically supplies curves of NPSHR as a function of pump flow rate for a particular liquid (usually water) in the vendor manual for the pump.

CentrifugalPump  Characteristic  Curves

For a given centrifugal pump operating at a constant speed, the flow rate through the pump is Figure 11   Centrifugal Pump Characteristic Curve dependent upon the differential pressure or head developed by the pump.   The lower the pump head, the higher the flow rate.   A vendor manual for a specific pump usually contains a curve of  pump  flow  rate  versus  pump  head  called  a  pump  characteristic  curve.    After  a  pump  is installed in a system, it is usually tested to ensure that the flow rate and head of the pump are within the required specifications.   A typical centrifugal pump characteristic curve is shown in Figure 11. There  are  several  terms  associated  with  the  pump  characteristic  curve  that  must  be  defined. Shutoff head is the maximum head that can be developed by a centrifugal pump operating at a set  speed.   Pump  runout  is  the  maximum  flow  that  can  be  developed  by  a  centrifugal  pump without damaging the pump.  Centrifugal pumps must be designed and operated to be protected from the conditions of pump runout or operating at shutoff head.   Additional information may be found in the handbook on Thermodynamics, Heat Transfer, and Fluid Flow. ME-03 Rev. 0 Page 14

Centrifugal  Pump  Protection

A centrifugal pump is dead-headed when it is operated with no flow through it, for example, with a closed discharge valve or against a seated check valve.   If the discharge valve is closed and there is no other flow path available to the pump, the impeller will churn the same volume of water as it rotates in the pump casing. This will increase the temperature of the liquid (due to friction) in the pump casing to the point that it will flash to vapor.   The vapor can interrupt the cooling flow to the pump's packing and bearings, causing excessive wear and heat.  If the pump is run in this condition for a significant amount of time, it will become damaged. When a centrifugal pump is installed in a system such that it may be subjected to periodic shutoff head conditions, it is  necessary to provide some  means  of pump protection. One method  for protecting the pump from running dead-headed is to provide a recirculation line from the pump discharge  line  upstream  of  the  discharge  valve,  back  to  the  pump's  supply  source. The recirculation line should be sized to allow enough flow through the pump to prevent overheating and damage to the pump.   Protection may  also be accomplished by use of an automatic  flow control device. Centrifugal pumps must also be protected from runout.   Runout can lead to cavitation and can also cause overheating of the pump's motor due to excessive currents.  One method for ensuring that there is  always adequate flow resistance at the pump discharge to prevent excessive flow through the pump is to place an orifice or a throttle valve immediately downstream of the pump discharge.  Properly designed piping systems are very important to protect from runout.

Gas  Binding

Gas binding of a centrifugal pump is a condition where the pump casing is filled with gases or vapors  to  the  point  where  the  impeller  is  no  longer  able  to  contact  enough  fluid  to  function correctly.  The impeller spins in the gas bubble, but is unable to force liquid through the pump. This can lead to cooling problems for the pump's packing and bearings. Centrifugal  pumps  are  designed  so  that  their  pump  casings  are  completely  filled  with  liquid during pump operation.   Most centrifugal pumps can still operate when a small amount of gas accumulates in the pump casing, but pumps in systems containing dissolved gases that are not designed to be self-venting should be periodically vented manually to ensure that gases do not build up in the pump casing.

Priming  Centrifugal  Pumps

Most centrifugal pumps are not self-priming.  In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function.   If the pump casing becomes  filled with  vapors  or gases,  the pump impeller  becomes  gas-bound and  incapable of pumping.   To ensure that a centrifugal pump remains primed and does not become gas-bound, most centrifugal pumps are located below the level of the source from which the pump is to take its  suction.    The  same  effect  can  be  gained  by  supplying  liquid  to  the  pump  suction  under pressure supplied by another pump placed in the suction line Summary

There are three indications that a centrifugal pump is cavitating. Noise Fluctuating discharge pressure and flow Fluctuating pump motor current Steps that can be taken to stop pump cavitation include: Increase the pressure at the suction of the pump. Reduce the temperature of the liquid being pumped. Reduce head losses in the pump suction piping. Reduce the flow rate through the pump. Reduce the speed of the pump impeller. Three effects of pump cavitation are: Degraded pump performance Excessive pump vibration Damage to pump impeller, bearings, wearing rings, and seals To avoid pump cavitation, the net positive suction head available must be greater than the net positive suction head required. Net  positive  suction  head  available  is  the  difference  between  the  pump  suction pressure and the saturation pressure for the liquid being pumped. Cavitation is the process of the formation and subsequent collapse of vapor bubbles in a pump. Gas  binding of a centrifugal pump is  a condition where the pump casing is  filled with  gases  or  vapors  to the  point  where the  impeller  is  no  longer  able to  contact enough fluid to function correctly. Shutoff  head  is  the  maximum  head  that  can  be  developed  by  a  centrifugal  pump operating at a set speed.

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