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Centrifugal pump restricted output

  1. Oct 30, 2016 #1
    I am also constructing a small boat as a test bed for a much larger houseboat. If we forget about practicality issues.... and accepting that my BS econ degree from the 60's isn't helping me much, I have a simple question with a difficult answer I guess.

    2 stage centrifugal pump. (Standard purchase)
    7.5hp. 100m³hr flow rate. 4 inch inlet. 4 inch outlet. ~50 psi. 0 to 3500rpm variable...

    If I restrict the output with a fixed nozzle of, say, 2 inches, will I get the needed increase in velocity without stalling the pump or causing cavitation in the pump? Of course there are a lot of othere related questions. Learning here is as important as the answer....

    And, please, no boat forums. They know less than I do but don't realize it.
     
  2. jcsd
  3. Oct 30, 2016 #2

    Bystander

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    "One hundred cubic meters/hr?" Sounds like a set of maximum ratings, no two of which hold for any given operational arrangement --- that's given that maximum flow is actually specified as "cubic meters per hour." Can you double check that bit, please?
     
  4. Oct 30, 2016 #3

    russ_watters

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    Welcome to PF!

    Bystander is almost certainly correct; those are probably maximum ratings of separate points, not an actual single performance point. You will need to locate a performance curve for the pump (or do some testing).

    Then...."needed increase in velocity" to what? For what?
     
  5. Oct 30, 2016 #4

    Averagesupernova

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    In my experience with centrifugal pumps, the pressure does nothing but increase from wide open flow to completely blocked off with no flow where the pressure peaks. Obviously there is cavitation throughout this range. The impeller turns and there is no or reduced flow. The efficiency (forward speed of the boat vs. horsepower demand) will vary quite a lot I am sure throughout this range. I would assume if you vary the nozzle size with the speed of the pump you can ride the top of the efficiency curve. Sorry, I don't have the skills to lay this out for you but I can tell you that there will always be some cavitation in the pump when you start restricting its flow.
     
  6. Oct 30, 2016 #5

    Bystander

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    I had my brain set for firefighting, bilge-pump, whatever, and --- it's a propulsion system. o:):doh:o:):doh:o:):doh:
     
  7. Oct 30, 2016 #6

    russ_watters

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    Propulsion was my guess, but unless I missed something I don't know that we can judge that yet...
     
  8. Oct 30, 2016 #7

    Bystander

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    "Dollars to doughnuts ...."
     
  9. Oct 30, 2016 #8

    Averagesupernova

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    My guess too.
     
  10. Oct 30, 2016 #9
     
  11. Oct 30, 2016 #10
    I verified it. its 100 cubic meters per hour. yes. I would think that this is maximum possible at ideal conditions. 4 inch input and 4 inch output. normal numbers on head etc.
     
  12. Oct 30, 2016 #11
    I
     
  13. Oct 30, 2016 #12
    I can be ridiculed in boat forums as well! lol. in the next 20 yrs, with global warming etc., living on the water isn't a bad idea. doing it cheaply and with a small footprint is the goal. a pump like this can be acquired for a reasonable price, a 10th of the cost of a normal proposition unit. so I came to you for help. I have spent hrs doing research to little help. all I want to know is if I can increase the velocity of the output by restricting the nozxle size without burning up the pump. im not looking for speed but instead power. I would like to know a few things BEFORE I purchase the pump. that is all. I would be happy with a 50% chance of success.
     
  14. Oct 30, 2016 #13

    Bystander

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    ... for propulsion? Of course.
    PV "dot" ~ hp, and m3 x ΔP (= 4-5 atm, atm = 105Pa, ....). Hour = 3600 s. "hp" = 33,000 ft.lbs/min, or 550/s.
    You want to play with it from here for a while? I'll be right here.
     
  15. Oct 30, 2016 #14
     
  16. Oct 30, 2016 #15
    263 g per meter³ gives you 26300 ghr, 7.3 g/sec, 65.3 lb/sec. That is coming out of a 4inch pipe. Even with 50% efficiency, thats over 30.0 lbs/sec. At this number, can you compute the velocity? With the 4 inch outlet pipe 1 ft long from the pump?
     
    Last edited: Oct 30, 2016
  17. Oct 30, 2016 #16
    I could tell ya what Stocks NOT TO BUY but not the velocity.....
     

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  18. Oct 30, 2016 #17
    In 12"x4" pipe, 75 inches³' 1728 inches³=1 ft³, .04ft³ of water in the pipe.
    If 50m³ water per hour, 10.5 ft/sec. If right = 7mph..... I THINK I need 3 times that speed or 20 mph...
     
  19. Oct 30, 2016 #18

    russ_watters

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    Ridiculed? Maybe that's the problem you had at the boating forums: you misread people trying to help you as ridicule. Because you've received none here.

    So anyway, we are trying to help you but as we said we need you to help us to help you otherwise we can't. Can you provide the make and model of the pump?
     
  20. Oct 30, 2016 #19
    Sir, I Am Another Boat Person Trying To Re-invent The Wheel. I Understand That. The Lol After My Statement Was Show Understanding, Not Distain. I appreciate all the assistance I can get which I feel tthat have recieved so far. what specific spec on the pump do you need as make and model has not been established?
     
  21. Oct 30, 2016 #20

    Bystander

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    Ah-hah --- looking before the leap.
    I'd say you've done fine so far. What factor do you need to reduce the area by to exceed the minimum acceptable speed? Present that value to the forum for our opinion(s), and go from there.
     
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