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Why Can't You Throttle Back External Pumps?

Discussion in 'General Plant Topics' started by csmith, Aug 7, 2010.

  1. csmith

    csmith Guest

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    I purchased a Mag Drive model 3 external pump today for my in-progress CO2 reactor. As it's going on a 20 gallon tank, and I already have good flow coming from my filter, I made the comment at my LFS when I bought it that I'd have to turn the flow down a bit with a ball valve due to my tank size (pump is rated at 350 gph, smallest they had). I was cautioned against doing this as it would somehow adversely affect the pump. How is turning down the flow on the pump a bad thing for it, but my filter can be turned all the way down and not be affected the same way?
     
  2. Biollante

    Biollante Lifetime Charter Member
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    Pressure Rated Pumps Vs. Mag Drive

    Hi,

    I am not familiar with the Mag-3 that is an external pump. :eek:

    The general answer is that it is not pressure rated. Some pumps are designed to maximize flow not pressure. The “floating” impellors simply do not work well against pressure. :)

    Some engineer will no doubt correct the over simplification, but generally the non-pressure rated so-called magnetic drive pumps create flow with very low energy costs. :gw

    Biollante
     
  3. csmith

    csmith Guest

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    What a prankster..

    Are you saying the mag-3 isn't for external use? If they sold me this thing knowing that..

    So can you really not turn these things down?
     
  4. Biollante

    Biollante Lifetime Charter Member
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    I Like Pressure Rated Pumps

    Hi,

    I guess this is the Mag 3 I am familiar with http://www.marinedepot.com/Danner_Mag_Drive_Supreme_3_350_GPH_Water_Pump_Up_to_500_Gallons_Per_Hour_Submersible_Water_Pumps-Danner_Mfg.-DN1111-FIWPSBUF-DN1121-vi.html, I guess it only says not recommended for external use. Most of these pumps I am familiar with depend on being submerged to dissipate heat.

    Who is your LFS? :rolleyes:

    There really is no practical way to govern flow that I am aware. Some have little adjustment nozzles, but they really are not very effective. I have not experimented with them to know whether you will mess them up by placing backpressure on them.

    Biollante
     
  5. Oreo

    Oreo Guru Class Expert

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    It's total BS. You can throttle back the pump BUT...

    You must do so on the outlet side, not the inlet side. This is important.

    There are some kinds of pumps that definitely can not be throttled this way. Those would be positive displacement pumps or roller pumps. Centrifugal pumps- like 99.999% of all pumps used on aquariums actually last longer and use less power when throttled back some with a valve on the outlet.

    (I'm a professional water treatment plant operator. It's my job to know these things.)
     
  6. Biollante

    Biollante Lifetime Charter Member
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    Okay No Difference

    Hi

    I stand corrected, as you are a guru class expert I accede to your unfailing logic.and articulate explanation.

    Given the elevated nature of the discourse I will take my leave.

    Humbly and inoffensively yours,
    Biollante
     
    #6 Biollante, Aug 8, 2010
    Last edited by a moderator: Aug 8, 2010
  7. Oreo

    Oreo Guru Class Expert

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    Biollante, YOU ARROGANT BASTARD!!

    (just kidding. ;) )

    I can add, valving back the pump on the outlet side will cause the pump to sound louder and in some cases, sound god-awful like it's hurting something. You can trust that it isn't hurting anything. It's just the sound of the water moving under higher pressure through & inside the pump.

    You should avoid closing the valve completely for more then a minute or so but it's not the end of the world. One of the worst things you can do to these centrifugal pumps is to run them dry, even for a few seconds. It will wear the ceramic bearings out really fast that way.

    As for submersed vs. external use, that pump will probably be fine either way. Louder if used external of course. You may want to check if it is "thermally protected". One reason to restrict use to submersed is because the water will cool the motor efficiently.
     
  8. csmith

    csmith Guest

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    Hey guys,

    I get what you were saying now about the Mag-3 "external pump", Biollante. :eek: I read the Marine Depot disclaimer, but can't find anywhere else to make the same statement. I found a handful of threads questioning why Marine Depot made the disclaimer, however. Apparently quite a few people do indeed run it inline. Maybe it's a CYA tactic from Marine Depot? I don't know, but I'll give it a shot as I can't put this thing in the tank. My only other option would be a Maxi-Jet 400 and I know that wouldn't have enough power to push the water down 3 feet, through a reactor and back up. One thing I've done is added elbows at every junction that needed connections, so maybe that'll scrub off some of the flow.

    Oreo, If I understand correctly you're saying you have to put the ball valve on the output side, but when you put it on the output side it also makes it run louder?
     
  9. Biollante

    Biollante Lifetime Charter Member
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    Pot Calling Whom, What?

    Oreo,

    I hope not, but I may indeed be arrogant, if I am, I hope it is honestly arrogant. Nor do I understand how my arrogance or lack thereof has any bearing on the discussion. As to my parentage, I know my parents and their legal status at the time of my birth. I am not sure how impugning the honor of my parents forwards the conversation.

    As arrogant as you who believe yourself qualified to judge, may think me, I have never been too arrogant to discuss why I think what I think and change my opinion should the facts warrant.

    Given your title and the editorial decision made by the folks that run the Barr Report, I shall withdraw from further comment.

    Biollante
     
  10. Gerryd

    Gerryd Plant Guru Team
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    Hi Bio,

    I don't think Oreo was serious....I am pretty sure he was just kidding....

    Your response #6 was extremely humble in nature and I think that was his target.....

    I am pretty sure noone here thinks you are arrogant.. You are extremely helpful to others and have a great deal of experience....
     
  11. Oreo

    Oreo Guru Class Expert

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    Biollante, I hope you'll accept my apology. I was only kidding. PM sent.
     
  12. Oreo

    Oreo Guru Class Expert

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    The reason the valve goes on the output side of the pump is to prevent "cavitation". Cavitation is when the vacuum pressure inside the pump becomes so strong that damage can occur. Centrifugal pumps are designed to contain positive pressure, not vacuum. Under negative, or vacuum pressure water can transition into gas phase so violently that steel impellers are pitted & chewed up. Practically speaking, the pump you're looking at isn't large enough or powerful enough to experience cavitation problems so the location of the valve probably isn't as critical as it is just a method of good practice.

    Throttling the pump back with a valve will make the pump run a little louder but the volume isn't related to the position of the valve on the intake or the output.**

    (**On larger pumps cavitation has a distinct sound of it's own. I'm not talking about that here.)
     
    #12 Oreo, Aug 9, 2010
    Last edited by a moderator: Aug 9, 2010
  13. nipat

    nipat Guru Class Expert

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    Thanks for the explanation of cavitation and gas phase. I was in doubt when you said putting the valve at input side was bad idea.
     
  14. nipat

    nipat Guru Class Expert

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    Well, I think you can make a kind of escape at the output side for the excess water to flow back to input.
    This way the pump runs at it optimal pressure.
     
  15. Oreo

    Oreo Guru Class Expert

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    Actually Nipat, what you describe here is less efficient then just using a ball valve on the pump output. A centrifugal pump uses energy by moving water. Less water moved, less energy used.
     
  16. milesm

    milesm Prolific Poster

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    have you considered the maxijet 1200? 295gph, 12watts (iirfc). run it externally in a closed loop. i have a mj900 (230gph) driving an inline co2 reactor. the pump is strong enough to split into 2 outputs (1 eheim spraybar and 1 mini spraybar from rapids canister) on my 20h. no need to throttle back the flow.

    mj's are inexpensive, silent, cool running, energy efficient, and extremely reliable.
     
    #16 milesm, Aug 9, 2010
    Last edited by a moderator: Aug 9, 2010
  17. csmith

    csmith Guest

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    I have two maxi-jets in my tank now, and I agree about what you've said about them. I was, however, told they couldn't be run inline/externally/whatever. How long has yours been running? Anyone else have experience running these inline?

    Edit: I'm talking about the powerheads. Do you mean the powerheads or the utility pumps (you gave stats for the powerheads).
     
    #17 csmith, Aug 9, 2010
    Last edited: Aug 9, 2010
  18. milesm

    milesm Prolific Poster

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    yes, i was talking about mj powerheads and yes, they can be run externally (3/4" input, 1/2" output). i've had mine running for about 6 months on a closed loop. not much loss in flow, but i only have 2 90 degree turns (1 in the reactor; using rex grigg's design with the reducing t) and one at a t to split the output.
     
  19. csmith

    csmith Guest

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    Shouldn't be much of an issue as far as the elbows go. I can easily remove a few in the overall setup and replace them with straight barbs. Is your maxi jet under the stand, or closer to the top of the tank? Just curious as to what kind of work I'm looking at putting this thing through. This does sound like a relatively cheap alternative.
     
    #19 csmith, Aug 10, 2010
    Last edited: Aug 10, 2010
  20. milesm

    milesm Prolific Poster

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    here's a couple of pictures of the mj and how it's connected to the tank. [​IMG]
    [​IMG]

    as can be seen in the first picture, my mj is placed just at the base of the tank. the second shows how i used various 1/2" pvc parts to construct the input from the tank to the mj.
     
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