Centrifugal pump restricted output

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Discussion Overview

The discussion revolves around the use of a 2-stage centrifugal pump for propulsion in a small boat, specifically addressing the implications of restricting the output nozzle size on flow velocity and pump performance. Participants explore various technical aspects, including pump specifications, potential cavitation, and efficiency concerns, while also considering the broader context of boat design and operational goals.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the validity of the maximum flow rate of 100 m³/hr, suggesting it may not hold under all operational conditions.
  • Another participant emphasizes the need for a performance curve for the pump to understand its behavior under different conditions.
  • Concerns are raised about cavitation occurring when restricting flow, with one participant noting that efficiency may vary significantly across different flow rates.
  • Several participants express uncertainty about the intended application of the pump, with some assuming it is for propulsion while others suggest firefighting or bilge-pumping roles.
  • One participant confirms the maximum flow rate but notes it is likely only achievable under ideal conditions.
  • Discussions include calculations related to flow velocity and the relationship between horsepower and pressure, with participants attempting to compute necessary speeds for effective propulsion.
  • Another participant highlights the challenge of achieving practical boat speeds with centrifugal pumps, suggesting that a speed of around 15 mph is necessary for practicality.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of restricting the nozzle size or the expected performance of the pump. Multiple competing views on the pump's application and efficiency remain, and the discussion is unresolved regarding the best approach to achieve the desired output.

Contextual Notes

Participants note that the discussion is limited by the lack of specific pump make and model details, which are necessary for more precise evaluations. There are also unresolved assumptions about the operational conditions and the definitions of performance metrics.

Dan Steill
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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.
 
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Dan Steill said:
7.5hp. 100m³hr flow rate. 4 inch inlet. 4 inch outlet. ~50 psi. 0 to 3500rpm variable...
"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?
 
Dan Steill said:
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...
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?
 
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.
 
russ_watters said:
"needed increase in velocity" to what? For what?
I had my brain set for firefighting, bilge-pump, whatever, and --- it's a propulsion system. o:):doh:o:):doh:o:):doh:
 
Bystander said:
I had my brain set for firefighting, bilge-pump, whatever, and --- it's a propulsion system. o:):doh:o:):doh:o:):doh:
Propulsion was my guess, but unless I missed something I don't know that we can judge that yet...
 
russ_watters said:
Propulsion was my guess,
"Dollars to doughnuts ..."
 
russ_watters said:
Propulsion was my guess, but unless I missed something I don't know that we can judge that yet...
My guess too.
 
Bystander said:
"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?
 
  • #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.
 
  • #11
Bystander said:
"Dollars to doughnuts ..."
russ_watters said:
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?
I
 
  • #12
Dan Steill said:
I
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. I am 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.
 
  • #13
... 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.
 
  • #14
Bystander said:
... 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
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, that's 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?
 
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  • #16
Bystander said:
... 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 can copy the specs if n

I could tell you what Stocks NOT TO BUY but not the velocity...
 

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  • #17
Dan Steill said:
I could tell you what Stocks NOT TO BUY but not the velocity...
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...
 
  • #18
Dan Steill said:
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. I am 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.
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?
 
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  • #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 received so far. what specific spec on the pump do you need as make and model has not been established?
 
  • #20
Dan Steill said:
make and model has not been established?
Ah-hah --- looking before the leap.
Dan Steill said:
10.5 ft/sec. If right = 7mph...
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.
 
  • #21
Right now, no matter the size or shape of the boat, if the stream of water exits at 7mph, the craft will not attain that speed reguardless of size. This is precisely the problem that has not been conquered in attempting to use this type of centrifugal pump. I thought that a re-visit might be approapriate. A boat speed of about 15 mph would make it practical. Under that and the fuel cost per mile traveled gets too high to be practical. I am willing to give time for speed with strong consideration to the fuel footprint. Practicality wise, a shallow draft, displacement hull is the goal. Thus I want nothing hanging down off the boat. Also, compared to other forums of power, a centrifugal pump on a small engine is both more reliable and cheaper to maintain.
Spending $3000 on a jet boat pre-engineered solution designed to go 35mph at 4mpg doesn not help the fuel footprint much...
 
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  • #22
Dan Steill said:
Right now, no matter the size or shape of the boat, if the stream of water exits at 7mph, the craft will not attain that speed reguardless of size. This is precisely the problem that has not been conquered in attempting to use this type of centrifugal pump.

Which type?
 
  • #23
Wait a minute --- you're looking at a pre-packaged power plant? And wondering whether marine propulsion engineers have failed to optimize it?
 
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  • #24
Bystander said:
Which type?
in expensive single stage centrifugal pumps hhave been tried. I am not sure about multi chamber.
 
  • #25
Bystander said:
Wait a minute --- you're looking at a pre-packaged power plant? And wondering whether marine propulsion engineers have failed to optimize it?
It is a water pump designed to move water, not propel a boat. I am just seeing if its use can expanded to another aplication.
 
  • #26
Dan Steill said:
It is a water pump designed to move water, not propel a boat. I am just seeing if its use can expanded to another aplication.
I posted a photo of it earlier today.
 
  • #27
What edison used to make a lightbulb work was not designed to be a lightbulb filament...
 
  • #28
This is the same problem I run into where ever I go. Why do it rather than how to make it work. Thanks for your time.
 
  • #29
  • #30

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