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Thread: COR pump return size

  1. #1
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    COR pump return size

    I keep seeing that return piping size is mostly a bigger is better arrangement, even in the COR pump introduction video. In that video, it shows an included 1.25" return option. Is there any reason you wouldn't go up to a 2" pipe, or even 4" for that matter? I understand the concept of diminishing returns, but is there any disadvantage to me using 4" return lines?

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    Frequent Visitor bigjim's Avatar
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    I'm sure our resident engineer will step in shortly and correct me, but what I think would happen is as you increase the return pipe diameter the height and distance you are able to pump the water will diminish to the point where the volume of water that is making it to the tank will also drop.

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    Frequent Contributor zombie's Avatar
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    The detailed technical answer is that there are three major factors that contribute to head loss in a pipe besides height. The first is friction losses, which are reduced with a larger diameter pipe. The second is minor losses, which are created by fittings, unions, reducers, etc. The third is the effect of turbulence.

    The issue with using an overly large pipe is that at some point the increase in minor losses from the expanders and reducers will exceed the reduction in friction losses. Before you reach this point cost often becomes a prohibiting factor. The second issue is that a large change in pipe diameter can sometimes introduce additional turbulence not accounted for by the Reynolds number calculations near the reducer that can double or triple the minor loss on that fitting. This is incredibly complex to calculate, but suffice it to say that it makes the first problem above much worse.

    You might be an engineer if...You have no life and can prove it mathematically.

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    Frequent Visitor bigjim's Avatar
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    Quote Originally Posted by zombie View Post
    The detailed technical answer is that there are three major factors that contribute to head loss in a pipe besides height. The first is friction losses, which are reduced with a larger diameter pipe. The second is minor losses, which are created by fittings, unions, reducers, etc. The third is the effect of turbulence.

    The issue with using an overly large pipe is that at some point the increase in minor losses from the expanders and reducers will exceed the reduction in friction losses. Before you reach this point cost often becomes a prohibiting factor. The second issue is that a large change in pipe diameter can sometimes introduce additional turbulence not accounted for by the Reynolds number calculations near the reducer that can double or triple the minor loss on that fitting. This is incredibly complex to calculate, but suffice it to say that it makes the first problem above much worse.

    You might be an engineer if...You have no life and can prove it mathematically.
    What he said.

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    Thanks! I figured 4" was overkill, but 2" was where I was thinking of going in reality. I appreciate the info!

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    Quote Originally Posted by zombie View Post
    The detailed technical answer is that there are three major factors that contribute to head loss in a pipe besides height. The first is friction losses, which are reduced with a larger diameter pipe. The second is minor losses, which are created by fittings, unions, reducers, etc. The third is the effect of turbulence.
    Hello zombie,
    Thanks very much for your explanation. I wonder if you would be kind enough to elaborate a bit on the issue of frictional losses within the pipe. On an intuitive level it seems to me that a larger diameter pipe, having more surface area, should exert a high amount of friction. Is the normal force exerted against the pipe wall lower in a larger diameter pipe? I assume not since the static pressure ought to be the same. Is the boundary layer thickness relatively taller in the smaller pipe, or is there higher turbulence in the smaller pipe. This is not really clear to me.

    The reason I ask is that my tank return hole is 3/4 inch diameter but I intend to install the 1 inch flow meter. I cannot understand why a 3/4 inch sensor isn't available, but in any case I'm forced to use a combination of 3/4 inch and 1 inch diameter piping in order to complete the configuration.

    The question is whether to use only a short section of 1 inch pipe to accommodate the sensor and to use 3/4 inch in the rest of the circuit or whether to use 1 inch piping in the majority of the circuit and only use 3/4 inch adapters at the pump spud and tank entry bulkhead.

    There doesn't seem to be a lot of guidance in terms of sensor installation, so not only do I worry about flow rate loss due to improper sizing of the tubing, but also loss of accuracy of the sensor due to poor placement of fittings and so forth (I think the documents do mention to keep a certain minimum distance away from elbows).

    This also brings up the issue of, without any other frame of reference, how can one be certain that the sensor has an accurate reading based on placement within the geometry of the circuit?

    In any case, could you offer any more regarding the selection of pipe sizes?
    Greatly appreciate any assistance.

    Cheers,

  7. #7
    Frequent Contributor zombie's Avatar
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    Quote Originally Posted by orebro View Post
    Hello zombie,
    Thanks very much for your explanation. I wonder if you would be kind enough to elaborate a bit on the issue of frictional losses within the pipe. On an intuitive level it seems to me that a larger diameter pipe, having more surface area, should exert a high amount of friction. Is the normal force exerted against the pipe wall lower in a larger diameter pipe? I assume not since the static pressure ought to be the same. Is the boundary layer thickness relatively taller in the smaller pipe, or is there higher turbulence in the smaller pipe. This is not really clear to me.

    The reason I ask is that my tank return hole is 3/4 inch diameter but I intend to install the 1 inch flow meter. I cannot understand why a 3/4 inch sensor isn't available, but in any case I'm forced to use a combination of 3/4 inch and 1 inch diameter piping in order to complete the configuration.

    The question is whether to use only a short section of 1 inch pipe to accommodate the sensor and to use 3/4 inch in the rest of the circuit or whether to use 1 inch piping in the majority of the circuit and only use 3/4 inch adapters at the pump spud and tank entry bulkhead.

    There doesn't seem to be a lot of guidance in terms of sensor installation, so not only do I worry about flow rate loss due to improper sizing of the tubing, but also loss of accuracy of the sensor due to poor placement of fittings and so forth (I think the documents do mention to keep a certain minimum distance away from elbows).

    This also brings up the issue of, without any other frame of reference, how can one be certain that the sensor has an accurate reading based on placement within the geometry of the circuit?

    In any case, could you offer any more regarding the selection of pipe sizes?
    Greatly appreciate any assistance.

    Cheers,
    Friction in pipes goes down in larger pipes because the physics state that for laminar flow (which is the case for most reef pump setups) friction loss is proportional to velocity. A larger pipe has a lower velocity for the same flowrate so the losses are reduced. There is an upper and lower limit to this when cavitation in an overly large pipe or over pressurizing a very small diameter changes the flow from laminar to turbulent. For common reef flow piping this isn't something you have to worry about.

    You will get the most flow by using 1" for as much of the run as possible and reduce down to 3/4" only once. However, the difference will be relatively small and extra flow through the sump is only helpful if it bumps you into the sweet spot range of 3-5 times total water volume after head loss. It is more important that it is cleanly fit and easy to maintenance than to reduce fittings and increase pipe diameter to maximize flow.

    As long as the sensor is far enough away from fittings (unions are kind of an exception to this) that the water has a chance to "level out" and become fully laminar, you can be assured of its accuracy within reason. Since there isn't a calibration procedure for the flow sensors without doing some crazy complicated fluid mechanics calculations, take the reading with somewhat of a grain of salt and assume it has an error in the 5-10% range.

    You might be an engineer if...You have no life and can prove it mathematically.

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    Very well, thanks, yes I agree that ease of maintenance has a higher priority than squeezing out the last bit of flow rate. I'll take your advice.

    Cheers,

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