An Engineer's Choice

  • Thursday, 28 April 2016

    [How To] Select Pump and Motor, Line sizings.

    The most interesting topic for an Engineer is Pumps and their Properties, And the most irritating one is theory of Fluid mechanics, May be no one will accept this but they will laugh inside when they hear this,

    I said it is the most irritating because many of the Engineers during their Engineering studies they feel it difficult to get through that subject and after once crossing that subject, it will be the most appreciable subject that a student ever choose, Now i'll go deep into matter but before that you need to gain some Knowledge regarding this topic or else it will be difficult to digest the main topic.

    The Main confusion arises and being frank many of the fresher engineers cant spot out the exact difference between the pump and the motor also, because due to a fear that has been created during their bachelors life in Engineering, But 


    a Pump is something like an equipment which is supposed to transfer the Liquid matter from a low sea level point to high sea level point, simply from bottom to top through lines applicable, in other words mechanical energy to kinetic energy.
    Here the Kinetic Energy will be delivered through the Impeller that has been Shown inside the pump cross-section. The Parts can been clearly in below pic.

    And What is a Motor??

    a Motor is the source connected to a Pump which supplies the required high frequency motion to the Pump for transferring the liquid, simply which converts Electrical energy to Mechanical Energy. 
    The Above Motor pic shows you that How the produced Mechanical Energy will be delivered through its Drive-End shaft. And if you wanna have an idea about How this Electric Energy is converted to Mechanical Energy, I'll explain you later.

    How Pumps are Classified??

    Usually pumps will be classified into 3 types, Reciprocating, Rotary, Centrifugal, but in industry point of view 99% Centrifugal pumps will be considered, and these Centrifugal pumps can be classified into two Types, 1. Self priming pumps, 2. Non- Self priming pumps. For Explanation regarding these two types see the below pic.

    If you clearly see the Self priming pumps will try to fill the impeller cavity itself before pumping to discharge end, so the advantage here is even if the suction end is empty also it can avoid priming, that means these pumps don't need marines connected to the suction end to hold on some level in line. These type of pumps will be regularly used in ETP flow lines as they will have somewhat high level suspended solids, and usually these self priming pumps will have a closed impeller.

    And while coming to Non-Self Priming pumps, these needs marines to be connected at the suction end to avoid priming, if there is no marine connected, then they may undergo priming and the impeller would run empty and there will be chance of pump failure. Usually these sort of pumps will be operated with open impellers.

    Also Read:

    Now, you will have one more question in mind, what is Priming??
    Priming is nothing but just formation of air gaps/pockets, which won't allow proper Pumping.

    So, Now you are Upto the Mark to go into the Actual Topic.
    HOW TO Select Pump and Motor, Line sizings.

    Selection of Pumps:

    Pumps will be Selected based on the requirement basis only there won't be any theoretical calculations, but you need to know some specification on which we go for pump selection, Usually a pump specification will consist of following,

    1) Pump Capacity: Pump Capacity characterizes Upto which extent it can generate a flowrate, the flowrate it will usually define is for water. Basically the rate of Pumping depends upon the Impeller diameter and the Motor RPM.

    This how an Impeller Looks like:

    2) Head: This seems very important, because without this our pump will be nothing, the pump greatness lies in the Head that it can pump. The key point here to remember is if the head of the pump is mentioned to be 20 meters then it can be used only upto that 20 meters, whatever the linesize it may be.

    3) Motor Required: Motor requirement is nothing but the capacity motor which is compatible for this pump tow work with maximum possible efficiency.

    These are the main points in a specification to observe.

    Pump Efficiency can be calculated from the Ration of Water Horse Power to Brake Horse Power.

    Efficiency = Water Horse Power / Brake Horse Power(Shaft power)

    Motor Selection:

    Usually a Motor will work on Electrical Energy and generates Mechanical Energy, but not 100%, because it will also will have some factor of efficiency, this Efficiency depends on the Brake Horse Power of Motor, Rate of Pumping depends upon the Motor RPM.

    Efficiency of Motor = Shaft Power out[Watts] / Shaft power in [Watts].

    So now everyone is well aware of the above mentioned basics,

    Now i'll Directly jump to point of calculating the Hydraulic power required to do pumping for a differential head,

    Power ( P ) = ( Q x Rho x g x h ) / (3600000 x N )

    Where, P     - power in KWatts,
                Q     - Liquid Flowrate in Cu.m/hr,
                Rho - Density in Kg/Cu.m,
                h      - Differential head in meters,
                N     - Efficiency ,

    Also Read:

    Now i'll Show you how to calculate for example i need to pump 30 Cu .m /hr at a differential head of 20 m, my solvent is water, and i'll consider the efficiency as 90%,

    So, P =  30 x 1000 x 9.80665 x 20 / ( 3600000 x 0.9) = 1.816 KW = 2.43 HP.

    My required Motor capacity is 2.43 HP ( 1.82 KW ).

    And Now another Item i need to calculate is the required Line size,

    For Pumping 30 Cu.m / hr, i need how much size??

    Usually line size can be calculated as per thumb rule, by considering the velocity in between 1.5 m/s to 2.5 m/s,

    But to provide you with some clarity in this, i'll give you a small data for considering the velocities based on viscosity,

    Velocity       Type
     0.6-0.9      Viscous
     1.5-2.5     Non- Viscous
    If you need to calculate the required line size from the velocity, use this formula,

    D, mts  = ( 1.2734 x Q / V ) ^ 0.5.

    That's It, Cheers........, Any Queries happy to hear, Will be there to Solve,

    An Excel Sheet For Our Readers From Our Side, Download It Here

    Comments are Most appreciated

    Also Read:

             How to Calculate Volume Occupied by the Torispherical Dish End? 

    About The Author

    Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
    Follow Me on Twitter AjaySpectator & Computer Innovations


    1. Hai...thank you very much...I have doubt... For high flow rate ..high line is that time can we chose the velocity as 2.3?

      1. we may consider velocity as 2.3m/sec, but that would depend on the skin friction(Friction due to Wall), and the viscosity of the Solvent, and also on the bends in the line, if we need to develop pressure we can maintain the small line size. Anything Else....???

    2. Plz give full scale up calculation like tip speed & how to scale up small scale to on plant batch which factors to be calculated details

    3. Dear ajay ji.
      Please explain the NPSH with example.many interviewer ask about this


      1. Dear Raj,

        In my next post, i'll demonstrate the NPSH, It will be typical one for sure.

        AJAY K

      2. Dear Raj,

        Follow the below link,

        AJAY K

      3. Hearthful thanks ajay ji.


      4. Dear Raj,

        Welcome & Keep on visiting for more stuff.

        AJAY K

    4. For header sizing of calcium brine (-16 degree ) at the pump outlet, say we need 50 m3/hr required in plant ,so what should be the header sizing & based on which velocity. Calculate

      1. Dear Anshu,

        Usually the velocity will vary between 1.0 to 3.0 m/sec, so consider the middle case 1.2 to 1.5 m/sec, based on the velocity go on calculating the dia of line, that's it.

        AJAY K



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