Centrifugal pump restricted output

[Dan Steill]

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.

 

[Bystander]

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?

 

[russ_watters]

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?

 

[Averagesupernova]

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.

 

[Bystander]

"needed increase in velocity" to what? For what?

I had my brain set for firefighting, bilge-pump, whatever, and --- it's a propulsion system.

 

[russ_watters]

I had my brain set for firefighting, bilge-pump, whatever, and --- it's a propulsion system.

Propulsion was my guess, but unless I missed something I don't know that we can judge that yet...

 

[Bystander]

Propulsion was my guess,

"Dollars to doughnuts ..."

 

[Averagesupernova]

Propulsion was my guess, but unless I missed something I don't know that we can judge that yet...

My guess too.

 

[Dan Steill]

"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]

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.

 

[Dan Steill]

"Dollars to doughnuts ..."

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

 

[Dan Steill]

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.

 

[Bystander]

... 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.

 

[Dan Steill]

... 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.

 

[Dan Steill]

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?

 

[Dan Steill]

... 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...

 

[Dan Steill]

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...

 

[russ_watters]

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?

 

[Dan Steill]

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?

 

[Bystander]

make and model has not been established?

Ah-hah --- looking before the leap.

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.

 

[Dan Steill]

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...

 

[Bystander]

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?

 

[Bystander]

Wait a minute --- you're looking at a pre-packaged power plant? And wondering whether marine propulsion engineers have failed to optimize it?

 

[Dan Steill]

Which type?

in expensive single stage centrifugal pumps hhave been tried. I am not sure about multi chamber.

 

[Dan Steill]

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.

 

[Dan Steill]

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.

 

[Dan Steill]

What edison used to make a lightbulb work was not designed to be a lightbulb filament...

 

[Dan Steill]

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.

 

[Nidum]

@Dan Steill : Pick a pump from here that is something like the one in the picture and get the data sheet :

http://www.dualpumps.co.uk/groups/HONDAWP__Honda_Water_Pumps

These pumps give 30 to 50 M max head / 3 to 5 bar max pressure and max flows of 120 to 700 L/min .

With definite data available we can do some proper sums .

 

[Dan Steill]

@Dan Steill : Pick a pump from here that is something like the one in the picture and get the data sheet :

http://www.dualpumps.co.uk/groups/HONDAWP__Honda_Water_Pumps

These pumps give 30 to 50 M max head / 3 to 5 bar max pressure and max flows of 120 to 700 L/min .

With definite data available we can do some proper sums .

 

[Dan Steill]

I uploaded the data sheet... Let me know if you need anything I can provide.

Preparing figures on what it is going on...

 

[Dan Steill]

Weight 600lbs
8ft by 4 ft
Flat bottom with 2ft taper bow.
2" draft.

This a test platform boat.

 

[Tom.G]

Weight 600lbs
8ft by 4 ft
Flat bottom with 2ft taper bow.
2" draft.

Hull Area = 32 sqft
Displacement = 32/6 = 5.3 cu.ft.
Weight of Water = 62 lbs/cu.ft
Weight of Displaced Water = 62 x 5.3 = 329 lbs
Looks like your Draft will be around 3.6 inches.

 

[Nidum]

I was hoping for a proper technical data sheet . One with pump performance characteristics .

Without this information we don't know the relationship between pump delivery pressure and flow rate . We could make some guesses based on known performance of similar pumps but I'd rather have solid data to work with .

Go to the actual manufacturers site and see if there is anything useful there .

 

[gmax137]

7.5hp. 100m³hr flow rate. 4 inch inlet. 4 inch outlet. ~50 psi

Pardon my units, but ...
100m³hr = 440 gpm
50 psi * 144/62 = 116 ft
water horsepower WHP = QH/3960 = 440 * 116 / 3960 = 13 BHP (assuming 100% eff.)
so you see, the listed 7.5 hp doesn't correlate to the flow & head. You really do need the pump curve from the manufacturer to begin. Plus as mentioned above, it will show you the head vs flow.