Conservation of flow in a pump

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Discussion Overview

The discussion revolves around the conservation of flow in a centrifugal pump, particularly in the context of a simulation involving transient temperatures in a surge tank connected to a recirculation loop. Participants explore the implications of flowrate measurements in relation to conservation of mass versus conservation of volume, especially considering changes in fluid density due to temperature variations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether a centrifugal pump's flowrate of 500 GPM should be interpreted as conservation of mass or conservation of volume, noting the influence of fluid density.
  • Another participant asserts that for incompressible fluids like water, conservation of mass and conservation of volume are equivalent since density remains constant.
  • A subsequent participant raises a concern about changing density due to temperature increases, asking if the pump is moving 500 GPM in terms of volume or the mass equivalent over time.
  • Another participant clarifies that a gallon is a unit of volume, not mass, which may imply a distinction in how flowrate is understood in this context.
  • A participant shares a link to a CFD simulation, suggesting it may be relevant or interesting, though it is noted as potentially off-topic.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between flowrate, mass, and volume, particularly in the context of changing fluid density. There is no consensus on whether the flowrate should be interpreted as conservation of mass or volume under the given conditions.

Contextual Notes

The discussion highlights the complexity of interpreting flowrate in systems where fluid density may change, and the implications of using volume versus mass flowrate. There are unresolved aspects regarding the impact of temperature on density and how that relates to the measurements taken from the pump.

natedogg1002
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Hello.

I am writing a relatively simple simulation code for looking at transient temperatures in a surge tank, which reflects a real-world system. The tank is hooked up to a recirculation loop with a pump and a heat exchanger. The pump has an average flowrate of 500 GPM. Despite the temperature therefore density of the water it is pumping, the pump moves "500 GPM".

So my question is: When a centrifugal pump has a set flowrate of 500 GPM, is that equivalent to the conservation of mass or conservation of volume?

No amount of Googling seems to have brought me any conclusions, though I have come to understand that positive displacement pumps are different than centrifugal pumps when it comes to this particular topic.
 
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For water (or any other "incompressible" fluid), the two are the same since density doesn't vary.

In a gas, this would refer only to volume flow rate.
 
Ok, but what if the density does change over time? For example, my tank is hooked up to a recirculating loop with a heat exchanger. As time goes by, the water temperature in the tank rises about 100°F -- corresponding to a change in density of ~2 lb/ft³.

Is the pump actually moving 500 GPM, or the mass equivalent over time?
 
A gallon is a unit of volume, not mass.
 

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