1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Selection of Pump for Parts Cleaning Machines

  1. Dec 6, 2013 #1

    Budding entrepreneur here looking for help.

    I've had this doubt for a long time. I can read the pump performance curves with respect to Head & Q. So if I have to select a pump that gives me 200 LPM @ 10 bar pressure, how do I select a pump from the curves. Is selecting a pump that matches 100 m head @ 200 LPM the correct way of doing it?

    Help is appreciated! :)
  2. jcsd
  3. Dec 7, 2013 #2


    User Avatar
    Science Advisor

    If a 100m head of water has a hydrostatic pressure of about 10 bar, then yes.
  4. Dec 9, 2013 #3
    A couple questions:

    Do you need 200 lpm @ 10 bar at some instrument/machine/device? Or has it been determined that, accounting for line losses and elevation changes, the pump needs to output at 10 bar?
    Is it water, if not, what's the SG of the liquid?
  5. Dec 10, 2013 #4

    I need 200 LPM @ 10 bar for my Parts Cleaning Machine. This output shall include whatever losses I incur due to bends and elevation changes. Liquid shall be water.
  6. Dec 10, 2013 #5
    Sorry, I'm not clear on the first question. But no matter, you want to take the pressure requried at the machine and add to it the expected losses (as you noted). That will tell you the pressure your pump must develop at that flow rate. If your system does not fluctuate much, then yes, that's all you need to do. Select a pump that matches those conditions.

    But for a more in depth look at it, here's some other considerations:

    Pump efficiency -/Many pumps, especially pumps which have motors/gearboxes/sheaves which drive them at various speeds, will match your criteria. You will want to select a pump which meets your operating conditions at or near the pump's BEP, or Best Efficiency Point (typically it is advised that, if you are going to select an operating point not on the BEP--that is, if you can't control exactly the speed the pump will operate at--then select a point to the right of the curve, not the left.

    If the pump itself (not the motor) will be using a gearbox or sheave/pulley system to control speed (and therefore operating conditions) then I'd recommend not dropping the speed any lower than 40% of the nominal pump speed (ie. if the curve is shown for 1800 rpm, be wary of running the pump at anything lower than 800. Some pumps will do it fine, but check with the manufacturer.

    As for VSD's (Variable Speed Devices, such as a variable frequency device/control) it's not typically recommened that you drop the motor speed any lower than ~50% nominal speed, depending on the type of motor. If it's TEFC, or totally enclosed-fan cooled, (as many off the shelf units are) then what happens is the lower operating speed may not be enough to cool the motor internals and you could wind up having loads of issues with temperature alarming and faulting (which I hope you consider implementing)

    Motor size -/ A pump will require a certain horsepower (or kW) at the operating point, this is generally known as hydraulic power, the power required to move the liquid. see here. You'll need to account for the efficiency of your pump which is determined by the pump design and where exactly on the pump curve your operating point is.

    Then you'll want to include a motor service factor. In many industries, a value of 1.15 is used. So if your pump requries 100 HP at the shaft (after accounting for the pump efficiency), then you would size your motor off of a service requirement of 115 HP.

    Many times, though, motors will be sized to ensure they are not overworked (and therefore prone to failure and damage due to overheating) by sizing the motor based on the hydraulic power calculated using run-out flow condition (that is, the point on the pump curve furthest to the right) which will generally be much higher than the power required at the operating point. This is of course not making very efficient use of your motor, and may lead to a shortened operating life, but to some people it's worth it not to have to worry about your motor throughout any operational condition.

    Operation - does the pump actually work at 200 lpm at 10 bar at all times? Will you have methods to shut down the pump when the parts cleaning machine is not in use so that the pump doesn't dead-head for extended periods of time? Or will you have a reticulating line? Does the machine vary it's water usage at all? If so, what methods of flow control do you have, or have you determined that the machine will suffice in controlling it's own flow (i.e. restrict nozzles -> Higher head -> Lower Flow, open nozzles -> lower head -> more flow from pump, etc).

    Just some things to think about.
  7. Dec 17, 2013 #6
    I agree with everything you said but this. I'm not sure about positive displacement pumps (gear pumps, piston/plunger pumps, etc.), but for centrifugal pumps it's actually the opposite due to the design of the impeller inlet. Damage is usually more severe when you're beyond BEP by 10%, normally where "Shockless Flow" is set at. The preferred operating range for centrifugal pumps will vary a bit depending on the application and industry the pump is for, but most follow API 610's guidelines of being able to operate in the range of 80-110% of BEP.

    All that being said, looking at the conditions he's giving here (though I don't know motor speed), a plunger pump would probably be the best suited. And if you look at most home/commercial pressure washers, that's what they use. If a centrifugal was picked, it would probably be ok to run it out, simply because it's going to be so small that the energy involved probably won't cause all that much damage, but the application itself falls more into where a plunger pump would work better.
  8. Dec 17, 2013 #7
    Yea, you're absolutely right. For some reason I combined my advice to stick as close as possible to the BEP with the secondary advice to stay to the right of the flat section toward the dead-head end of the curve. Thanks for pointing that out!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook