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                                 Centrifugal Pump Principles:-

1. Transferring liquid from one point to another point.

2. Imparting Energy to liquid.

3. Takes , Diesel Engine, Turbine.

                                                           Pump Fundamental:-

1 - Atmospheric Pressure ( ATM.PR.)

-  One Atmospheric Pressure a sea level :- =   1.033 Kgf \cm2
                                                                      =   760 mm of Hg
                                                                      =    10.33 MWC

-   One Technical Atmospheric Pressure     :- =    1.0Kgf \ cm2
                                                                      =      735.5mm of Hg
                                                                       =    10MWC 

2- Priming ( For Negative Suction Only )

Can Centrifugal pump suck liquid on its own?  NO.
Process of creating vacuum inside pump & there by brining liquid up to Impeller Eye area is called priming of th pump.
 - Priming is achieved by        -  Foot Valve
                                             -  Vacuum Pump.
                                             -   Priming Tank.

3 - Vapor Pressure at given Temperature 

a) Every molecule of liquid , like us enjoys freedom by flying in the air.Meaning every liquid wants to become vapour & fly.

  b) But it remains liquid because of pressure acting on it's surface . In majority of cases it is ATM.PR>

  c) If pressure acting on it's reduce to a level where liquid molecules starts flying , it;s vapor pressure at the temperature.

  

                                           SPECIFIC  GRAVITY (SG):-

- Pump develop same head independent of SG but the pressure in Kg \ cm2 is proportional to the SG.

- Pump delivers the same quantity by volume independent of SG but the quantity by weight will be proportional to the SG.

- Efficent is un - affected by the SG.

- Power is directly proportional to SG.

- Permissble lifts varies with SG. As SG increase , suction lift decrease.

Concept of Head:- 

i) Pressure converted in to MLC is called static Head:
            Hm  = P(Kgf\cm^2) X 10 \ Sp.Gr.

ii) Velocity Head : Static head if completely converted into velocity.
             Hv   = V^2 \ 2g.

         ## Why is pressure always converted into mtrs. of liquid colum while deling with centrifugal pumps?.

  
     A centrifugal pump will lift the liquid to same height irrespective of it's specific gravity. Only thing it will proportionately exert more or less pressure & required more or less power.
     That is why we must always think in terms of mtr of liquid colum when dealing with pumps.
     So , In pumps Engineering , we convert all engine into it's equivalent static head, such as velocity head, suction head, discharge head, frictional head etc.

                HEALTHY SUCTION & NPSH \ NP SHR

I repeat that Pump cannot suck liquid on it's own.Liquid is required to be brought up to suction of the pump . This is not sufficient, the liquid near impeller Eye area should have sufficient positive pressure head ( energy ) to overcome entry losses at the impeller Eye, frictional losses inside the Impeller & afetr Subtracting these losses, LIQUID SHOULD REMAIN AS. 

Efficiency / BKW

1.0 BKW=[Q(m3/hr) X H(m)/367.2Xeff.]  X Sp.Gr. 

2.0 Pump Efficiency: = eff(m) X eff(v) X eff(h)

eff(m)=mechanical efficiency 
 Accounts for all losses in bearings stuffing box ,all disc friction losses including wear rings, balancing devices
Accounts for leakage through wear rings , balancing devices and stuffing boxeff(h)=Hydraulic efficiency

Accounts for liquid friction losses in all through flow passages

                                                                                                      

Pump with variable frequency Drive:-

 

Essentially in this we change pump speed by use of VFD on motors.

Why change RPM?

1] Actual duty point varies from designed duty point.

2] Systems vary continuously or intermittently.

Result à More flow at less efficiency à More power à

More radial thrust  à  higher NPSHR  à  pump life deteriorates, etc.

Indirect effect  à  more frictional loss à  undesired head à

SOLUTION – USE OF VFD

Pump Sizing & selection W.R.T. SRC :-

- Most important is to plot system resistance curve.

- Select Pump for highest flow & highest head condition.

- Study performance curve carefully.(Only pump manufacturer can do this and should do it)

- Duty point should be selected on left side of BEP  if constant flow is needed  and on right side of BEP if constant pressure is needed.

- Also we can make pump follow SRC path for additional power saving (only Inconstant pressure system).

- When we reduce RPM of Pump, ability of pump to deliver flow & develophead will reduce based on affinity laws mentioned earlier.

- Draw pump curves at highest rpm and lowest rpm by using affinity laws.Superimpose these curves on SRC.

- PUMP CURVE  should have good peak Efficiency Range.

- - - Give Example of constant flow for RPM range Vs flow range. Ratio of RPM reduction should be same  as ratio of duty point flow to end 

 

Role of Pump Manufacture :-

Role of pump manufacturer is most important because he can make pump run at various rpm in a preferred range of curve to ensure lowest radial load,lowest vibration,no cavitation, and highest power saving. 

CONCLUSION

If system has more frictional head component, or if Head variation is high, or if both vary, or if same pump has to do many functions then VFD is ideal choice
 

VFD when Pumps are in parallel operation

Ideally employ all Pumps with dedicated VFD.Most important reason is that you can bring Second Pump into operation OR you can cut off Second Pump, w.r.t. flow OR pressure such that new duty point do not go close to minimum flow OR new duty point has fair efficiency. Equal load sharing, Curves remain same over long period etc.

THANKS,

DINESH SINGH
ROLL No:- 17