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How do you calculate flowrate exiting a pipe?

  1. Feb 12, 2013 #1
    It has been several years since I have dealt with fluid mechanics. I have a 4hp pump that circulates the water in a 4ft swimming pool. A 2in pipe is connected to the outlet which runs up about 1ft. It passes through a shutoff valve and is then split into two streams, both 2in pipe, by a Y-fitting. These two pipes continue to run vertically for another 2ft until they run into a 90° elbow, which turns the flow horizontal. There is a nozzle after another 1ft of pipe that reduces both streams to 1in. I need to calculate the flowrate exiting the two nozzles.

    I would like to know how to calculate this precisely, but for now I just need an estimate. Can I assume that the Y-fitting splits my stream evenly into two equal flowrates? Will the elbow significantly reduce my flowrate, or is it negligible in comparison to the reducer?
  2. jcsd
  3. Feb 12, 2013 #2
    Those connections aren't going to have too much of a measurable effect on your flowrate in this system. The main factors are:

    -Where is the pump getting it's feed water (i.e. what elevation)
    -Where is the pump discharging the water
    -What does the pump curve look like

    Probably the pump is rated for a specific flow rate due to the relatively constant geometry of pool pumping systems. Ask the vendor/pool store.

    The pump controls the flow, if it puts out 100 gallons per minute for your system conditions, then each wye section will see approximately 50 gallons per minute in both the 2 inch sections and the 1 inch sections. The reducer is there to increase the velocity of the water so that the pool gets a bit of circulation going, so the water doesn't remain stagnant. It does this to help the suspended particulates reach the filter rather than settling to the bottom.

    Your flow rate will split more or less evenly among the two sections after the wye, it wont be exact (that's life) due to the fact that your system isn't exactly a precision-built one, but it'll be pretty close.

    It is important here to note that the reducer doesn't act to reduce the flowrate (**read below), it transitions the pipe to a smaller diameter pipe where the flow velocity is increased, the effect of the water having to change direction is measurable, but in this case probably insignificant.

    **You are probably aware that pumps work on a curve. If you know your system conditions, you can calculate where on that curve your pump is sitting (a bit iterative, since flowrate is found on the curve based on system head, system head is partially determined by pipe friction losses, and pipe friction losses are determined by flow rate!). The losses from all of your elevation changes, fittings, elbows, and friction due to flow rate will allow you to calculate the total head of the system, and thus get a good idea of where on its curve your pump should be operating.

    To answer your question about the elbows/valves/reducers effect on flow rate:
    When we discuss things like this, we don't discuss how they affect flow rate, we care about how they affect losses.
    Elbows will have the least effect of the three (arguably, if the valves are ball valves, the elbows and valves will result in similar losses) with the valve coming in a close second.

    As I mentioned before, the reducer will have only a slight effect on the system head (similar to an elbow), however because the downstream pipe is now smaller diameter, the flow velocity within it will be increased (since the whole system maintains an equal flow rate). This increased velocity results in additional friction due to the interaction with the viscous water and the pipe walls (and its imperfections), resulting in added system head that the pump must overcome, driving the pump back on its curve and resulting in a flowrate that is lower than if those 1" sections were 2" sections, but discharging at a higher velocity.

    In short: You need the pump curve to determine the flowrate without actually physically measuring it.
  4. Feb 12, 2013 #3
    Thank you for the information. That about sums up everything I wanted to know.
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