Water flow question

[capta]

Presently I am feeding the high pressure pump on my watermaker with a water puppy (1/2" ports), with 3/4" inlet and 1/2" outlet. and it just barely supplies the 3.5 gpm needed. This pump is rated at 6.3 gpm.
Jabsco makes a Maxi puppy, rated at 9.5 gpm, (1" ports). Given all else is the same (head, intake and outlet hose size, etc.) will the reduction in hose size cut the flow so that the maxi puppy will not pump significantly more than the water puppy? I need more water flow (not pressure) to the hp pump.
Thanks.

 

[tommays]

Its about 3 times the pump the issue i see is how HOT it would get with the impeller slippage as it is rated at 10GPM minimum

 

[capta]

Its about 3 times the pump the issue i see is how HOT it would get with the impeller slippage as it is rated at 10GPM minimum

I'm sorry, I don't understand your response. The hp pump does not draw water, it only increases the pressure on the water supplied to it.

 

[tommays]

The maxi puppy is 10GPM minimum with a 21 AMP draw and not really ment to deadhead against a second device with a fixed 3.5 GPM throughput

It is going to get hot and always be at 100% load in this service so i would at the least consult with the factory on the this type of service and duty cycle


I wonder how some of the new variable speeds DC pumps would handle saltwater

BUT getting industrial we have 35 PSI of local water pressure which flows X



In this case the machine needs 55 PSI at all times with zero to 150 liters per minute water flow



The sensor controls the pump drive so that the machines water flow needs are meant and there is no damage from pressure spikes

 

[capta]

The maxi puppy is 10GPM minimum with a 21 AMP draw and not really ment to deadhead against a second device with a fixed 3.5 GPM throughput
It is going to get hot and always be at 100% load in this service so i would at the least consult with the factory on the this type of service and duty cycle
I wonder how some of the new variable speeds DC pumps would handle saltwater

BUT getting industrial we have 35 PSI of local water pressure which flows X

In this case the machine needs 55 PSI at all times with zero to 150 liters per minute water flow
The sensor controls the pump drive so that the machines water flow needs are meant and there is no damage from pressure spikes

According to the watermaker company I deal with, the hp pump does not care how much water is sent to it. According to the pump vender, there will be a decrease in flow with the reduced line size, but they couldn't say how much, nor did they mention possible damage from replacing the water puppy w/ the maxi puppy. In fact when I mentioned my needs, they were the ones who suggested the maxi puppy. I need more like 6gpm to keep the hp pump happy and was hoping that the reduced line size would give me at least that, but I do not know how to figure that out. It seems the water puppy is losing about 3 gpm as the system is presently configured (head, discharge size reduction and filters), so perhaps the maxi puppy would do the same, which would work out very well, I think.
I am certainly NOT arguing with you, and I am listening to what you say. Unfortunately, a centrifugal pump will not work in this system, so I am trying to come up with a viable solution where none has been attempted before.

 

[Allan12210]

Once upon a time we had to insure a pump that fed a process would not dead head, or run at restricted capacity, causing it to burn out. We created a bypass loop connecting the output back to the input side with a throttling valve. The pump could run at maximum capacity all the time it was needed and the subsequent process would draw what it needed when it needed it. I don't know if this would help, but its a fairly easy experiment with a couple of Tees and a valve.

If your Puppy is pushing directly against a process that has less demand that it can deliver, then in essence it might be deadheading and it is then cavitating inside resulting in poor output. Bigger might not be better as it will cavitate even more.

 

[tommays]

I am certainly NOT arguing with you

I did not take it as arguing at all as it is very difficult to discus things like this with a keyboard

I guess the biggest thing is if your can put up with the 20 amp power draw while making water

 

[Ralph Johnstone]

We created a bypass loop connecting the output back to the input side with a throttling valve. The pump could run at maximum capacity all the time it was needed and the subsequent process would draw what it needed when it needed it. more.

Looks good as far as it goes Alan. Had a simialr situation some time ago and rather than a manual throttling valve, we used a back pressure valve to ensure the supply pump delivered a constant supply pressure regardless of the flow through the system. We couldn't afford (read as too cheap) to have a variable frequency drive for the supply pump.

I'm sure a very small backflow valve is available somewhere (too lazy to find one right now). For those not familiar with this rubbish, basically it's a pressure reducing valve that's installed backwards and is normally closed (it opens as the sensed pressure increases) to control the UPSTREAM pressure.

 

[kloudie1]

The Water Puppy is a positive displacement pump which is not going to work as a feed pump unless the speed of the puppy is turned down so it supplies the needed 3.5 gpm. The low output you are seeing is probably telling you that the impeller is shot from not being able to push water through at the "rated" amount... If you want to use this pump, you'll want to either slow it down to the correct flow rate using a speed controller like this:
http://www.amazon.com/Motor-Control-Controller-Professional-Regulator/dp/B00H8GRFU2

OR

Probably a much better option would be to get an accumulator between the puppy and the HP pump. The accumulator should have a pressure switch set to turn off the puppy at about 5 psi. (note that the max rated pressure for thepuppy is 6 psi. much more resistance and it tears up the impeller) Note that the puppy may not be the best choice of pump. A washdown pump something like this :
http://www.amazon.com/Jabsco-326050...qid=1403106342&sr=8-3&keywords=wash+down+pump (has its own pressure switch)
might be better but you'll need an accumulator between it and the HP pump..

 

[capta]

The Water Puppy is a positive displacement pump which is not going to work as a feed pump unless the speed of the puppy is turned down so it supplies the needed 3.5 gpm. The low output you are seeing is probably telling you that the impeller is shot from not being able to push water through at the "rated" amount... If you want to use this pump, you'll want to either slow it down to the correct flow rate using a speed controller like this:
http://www.amazon.com/Motor-Control-Controller-Professional-Regulator/dp/B00H8GRFU2

OR

Probably a much better option would be to get an accumulator between the puppy and the HP pump. The accumulator should have a pressure switch set to turn off the puppy at about 5 psi. (note that the max rated pressure for thepuppy is 6 psi. much more resistance and it tears up the impeller) Note that the puppy may not be the best choice of pump. A washdown pump something like this :
http://www.amazon.com/Jabsco-326050...qid=1403106342&sr=8-3&keywords=wash+down+pump (has its own pressure switch)
might be better but you'll need an accumulator between it and the HP pump..

OK, I'm confused now.
A) It's a brand new water puppy, ie; new impeller
B) the water puppy is the recommended pump, however;
C) the water puppy does not supply ENOUGH water, henceforth I am seeking a bigger pump, ie; maxi puppy (or another CONTINUOUS DUTY pump, if suggested), for this installation.
D) as far as I know, there is no maximum feed (25 to perhaps 30 gpm) to the hp pump that I could achieve with any pump I would run; 3.5 gpm is a minimum needed.

I've never seen a small watermaker system with an accumulator tank, nor the bypass system mentioned above, so I think we are over thinking this a bit.

I'm looking for help figuring out how much flow loss reducing the maxi puppy's 1" intake and output lines to 3/4" intake and 1/2" output lines would create, so I can be sure that the maxi puppy will supply at least (and hopefully a bit more) the minimum 3.5 gpm I need, before I purchase a $300.00 pump and have it shipped to Grenada.
Thanks.

 

[kloudie1]

If you can tell the length of the suction line and discharge lines and the number and type of fittings, that number can be calculated for an ideal pumping situation.. The problem (from the info that I have) is that you are feeding a positive displacement pump with a positive displacement pump.. this is a big no-no in pump engineering. It can be done with an accumulator and a way to "meter" flow to the HP pump.. the Puppy is trying to push ~6 gpm and the HP pump is taking only ~3 gpm.. Positive displacement pumps don't have much "slip" so what has to happen is that the water pressure in the Puppy is distorting and bypassing the impeller; it has no other place to go. Operating like that, my guess wouuld be that the impeller probably would not have a life span of more than about 4-6 hours (maybe a bit longer if the water is cool) before it is toast. Putting in a bigger positive displacement pump without a pressure switch will fail the bigger pump faster than the smaller one. If ya have a 3 gallon bucket and you are pouring into it with a 6 gallon bucket, and ya can't stop pouring (which would be like positive displacement).. ya don't have any place for the water to go.. without an accumulator and/or a pressure switch, ya can't slow the pouring..
A well specified centrifugal pump would be the correct feed pump in this application, but I have not seen a small centrifugal that would work there.. A centrifugal with a best efficiency point (bep) flow rate of ~4 GPM at whatever head (pressure differential) you need would be the correct pump for that service. Just giving you the facts..

 

[Allan12210]

OK, I'm confused now.
I'm looking for help figuring out how much flow loss reducing the maxi puppy's 1" intake and output lines to 3/4" intake and 1/2" output lines would create, so I can be sure that the maxi puppy will supply at least (and hopefully a bit more) the minimum 3.5 gpm I need, before I purchase a $300.00 pump and have it shipped to Grenada.
Thanks.

Just looking at the output dimensions, if the pump rating is based on 1" then reducing to 1/2" is approximately 25% of the cross-sectional area (.25" radius^2/.5" radius^2 = .25). So you might expect the Maxi to only deliver .25 x 9.5 gpm = 2.375 gpm.

 

[Scott T-Bird]

Capta, you say that you want to increase the flow rate...

to the hp pump at the watermaker. Then you should use the 1" outlet, not reduce it. And I think Kloudie is right ... if you are limiting the flow on the Maxi you risk burning it out.

There is a formula to calculate friction loss in a pipe. Hazen-Williams equation is used and you can even find a calculator on the site "engineeringtoolbox.com" if you search for "hazen-williams" or "friction loss formula". You will need to know the variables which include the roughness coefficient (probably something like C=140) and the "hydraulic diameter" I think the links and calculator will walk you thru it. Since you know flow in gpm, then use English units obviously. One caveat is that this equation is typically used for long runs of pipe, in the hundreds of feet. I doubt there is any accuracy to it when you are dealing with just a few feet. This is more like an orifice condition. The Bernoulli Equation for head loss might be useful, but probably not easily applied. I think I would have to go back to college to use it.

It sounds like you are anticipating that friction loss (or head loss) will compensate the flow to coincidentally match the flow of the water-maker, but there is a reason for the outlet to be 1" for the larger pump instead of 1/2" for the smaller pump. I think it is not a good idea to reduce the outlet to 1/2" in an attempt to compensate for the lesser flow demand. You may end up burning out the pump faster than you can accept. If anything, I would keep the outlet supply to the hp pump/watermaker at 1". If the water-maker manufacturer is suggesting that the Maxi is the correct size, I would bet that they mean with the 1" outlet, not a reduced outlet.

But I agree with all, it doesn't seem right to expect this pump to function in the manner intended.

 

[capta]

Thank you all for your help and suggestions.
I believe I will be able to manage with the information you all have supplied.
Thank you again.

 

[kloudie1]

More info; the pressure loss in the discharge line doubles at the higher flow rate.. The difference in the intake line loss is negligible. The Maxi Puppy has a minimum output of 5.5 gpm which it can do at a discharge pressure of about 16 psi.. more pressure than that (caused by the 3.5 gpm intake of the HP pump) and the impeller starts distorting.. The minimum flow rate for the Puppy is also 5.5 gpm, which it can do if the pressure it is pumping into is about 8 psi. Above 8 and the impeller is distorting.. You might be much better off specifying the Junior Puppy
http://www.xylemflowcontrol.com/files/23670.pdf which would much more closely match the HP pump. If you are pumping through a filter (not a screen) with the charge pump (feeding the HP) the Junior probably cannot supply enough pressure to flow enough.. Maybe they were recommending the Junior? remember these are bilge pumps and not really designed to see a high discharge pressure.

 

[Allan12210]

The minimum flow rate for the Puppy is also 5.5 gpm, which it can do if the pressure it is pumping into is about 8 psi. Above 8 and the impeller is distorting.. You might be much better off specifying the Junior Puppy

I still think its worth trying the recirculation loop based on the information Claude is presenting as it will mitigate the high pressure buildup and subsequent impeller distortion by keeping pressure low and flow rate high. Once the impeller distorts, the pump is in bypass mode internally and little if anything will come out the output. In fact you could use some flexible hose t-eed to the inlet of the HP pump (output of the Junior Puppy) and a c-clamp to squeeze it down as a throttle. The ideal situation is as Ralph describes controlling the pressure, but this should work to some degree. Run the hose overboard or into a bucket initially and just see if the demand requirement of the HP pump is being met.