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steamfittersteamfitter Member Posts: 85
The literature and study materials regarding centrifugal pumps state that the inlet is usually smaller than the outlet. The pump/circulator causes a pressure differential and provides flow in the piping.

How does the circulation work on pumps that have the same size inlet and outlet?

If i'm not mistaken I believe there are small circulators as well as pretty large ones with inlets and outlets of the same size. Is the inside of the pump different than the other pumps with different size outlets?


  • steamfittersteamfitter Member Posts: 85
    pumps/circulators CORRECTION

    Sorry guys- got that last post backwards. As you know the outlets are smaller than the inlets.

    Looking for an explanation of the pumps with same size inlet and outlet. How do they work?

  • Jamie HallJamie Hall Member Posts: 6,030
    Size of the pipes...

    has very little to do with the flow characteristics of the pump, at least within reason (assuming that one takes into account Bernoulli effects, that is).

    The pump innards --whatever kind of pump -- convert the power coming in from the motor to power in the form of an increase in total energy for the flowing liquid.  They do this in various ways -- and with varying efficiencies and characteristics.  The mass flow through the pump doesn't change for a given set of operating conditions at the pump, regardless of pipe size -- so many gallons per minute in, the same out (unless you have a colossal leak!).

    The flow velocity will change with pipe size; bigger pipe, lower velocity.  But that doesn't change the rate of flow (gallons per minute).

    Not sure whether that helps?  More questions?

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • RJRJ Member Posts: 459

    water enters the center (eye) of the impeller from the suction side of the pump, on larger pumps with limited horizontal piping a suction diffuser is needed to direct water to the center. the centrifugal force created by the impeller imparts velocity to the water at the outlet of pump, creating a differential across pump. ( differential machine )         Its crucial that the suction press at pump does not go into a negative which will cause pump problems.  I have additional info on reading pump curves if needed  plus all that Jamie said
  • Charlie from wmassCharlie from wmass Member Posts: 3,833
    it is easier to push then to pull

    so the inlets of pumps are larger then the outlets to minimize the resistance on the suction end as there is a large delta P for pumps. The delta P for circulators is just enough to move the water in the pipe back to itself.
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  • steamfittersteamfitter Member Posts: 85
    easier to push

    Thank you kindly. I can understand the reason for larger inlets better as you explain the fact that there is less resistance due to the larger outlet. "Easier to push than pull," is a great way to think about the function as well as remembering the concept. Thanks again!
  • steamfittersteamfitter Member Posts: 85

    Thanks for the info. very helpful!

    I'm glad you mentioned velocity, because i'm still trying to wrap my head around the whole velocity thing. feet per second, milinches, gpm.

    If gpm represents flow rate and feet per second represents velocity, what is their relationship?

    If larger pipe gives you slower velocity, how do you maintain a set gpm on a given pump/circulator? If the water is moving slower wouldn't it circulate less gallons per minute?

    Studying pumps and hw systems, but still getting hung up on a few items.

    I appreciate your help!
  • steamfittersteamfitter Member Posts: 85

  • JasonJason Member Posts: 290
    Pipe size

    Now I am confused. Pipe size has to affect gpm. If I use smaller pipe the velocity changes which in itself will not affect flow, agreed.

    If velocity increases so does resistance to flow ( head), does that not affect gpm flow?
  • Jamie HallJamie Hall Member Posts: 6,030

    now you are getting just a little bit complicated. For a centrifugal pump, in order to determine the gpm (flow) which a given pump will produce, you must examine the head against which it is operating; that is, the total pressure difference between the inlet (measured in a straight section of pipe at least four diameters from any bends) and the outlet (also measured that way).  Then you will take that head difference, and look it up on the pump curve for the specific pump.  That will give you the gpm which the pump will operate at under those conditions.

    Change the head, you change the flow..  It makes no difference to the pump how the head difference for the flow is created; a long big pipe or a short little pipe, partly closed valve, lift to an open reservoir -- makes no difference at all.

    Yes, a smaller diameter pipe will have more friction loss at a given flow than a bigger one (it's more or less proportional to the square of the inside diameter).  But it is the overall head difference which is of concern, not the sizes of the inlet or outlet (within limits -- as someone else pointed out, a small diameter inlet may lower the pressure enough that you get some real problems in the eye of the impellor -- which can destroy a pump in a matter of minutes).

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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