Watermaker LP pump requirements - any expertise please?

Goldie

Goldie

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The LP pump fitted to my watermaker has failed irreparably, but causes no immediate problem as I've fitted the 'like for like' 20 year old spare which the previous owner had left with the boat and so far, it's working fine - but, it is 20 years old..... I would now like to replace the spare but the pump in question (12v Stuart Turner CH4-14-DC12) is no longer produced. What I'm trying to identify is a currently available, suitable pump. There is a thread on 'Liveaboard Link' regarding watermakers and without wishing to hijack the thread I did raise the question and a couple of Forumites kindly responded that they use Jabsco Water Puppy pumps. Before I order one though, I'd like to make sure that this is indeed suitable for my installation.

The 'old' Stuart Turner is rated at max working pressure of 5 bar and with a flow rate of 62.5 l/min at 0.5 m head and I've found nothing comparable to that spec to date. The question I ask then, is what does the system actually require from its LP pump? What flow rate and pressure? Any other factors to consider? The HP pump is a Cat 277 (engine driven via belts) and has an output (from the manufacturer's website) of 16 l/min. The system runs through 3 pressure vessels/membranes, each of approx. 25 inches in length and fresh water output is on the order of 80 l/hr. The LP pump is mounted below waterline level and the HP pump roughly at waterline level so no significant head is involved.

Any knowledgeable guidance as to what is actually required from the LP pump would be very much appreciated. The company which originally manufactured the system have moved away from watermakers and frankly, aren't interested in advising.

Many thanks in anticipation.
 
have you tried running your system without a LP pump?
mine works fine without a LP pump and as in yourcase HP pump is at around w/l level.
shouldn't really need one tbh!
 
Water is incompressible so whatever the potential for water flow from the LP pump it will actually be delivering only what the HP pump is pumping, ie 16 litres per minute. As for pressure, the LP pump is really only there to assist priming the system (or repriming whenever some air gets in to the inlet - which it will if the water maker is ever run in rough seas and/or when heeled over).

So the spec for the LP pump is:
1. Must prime when dry, hence centrifugal won't do unless it's below the water-line
2. Only needs to pump 16 litres a min, plus a bit of margin
3. Needs to develop only some fraction of a Bar - enough to over come the head between it and the highest part of the pipe to the HP pump. One bar can raise water about 10m, so 5 bar was way ott.
4. Must not develop high pressure or be damaged when it's output is blocked (eg by the engine not yet turning the HP pump). Alternatively you could add a pressure relief valve.
5. Must withstand sea water and not corrode
6. Should tolerate some matter in the water, since I assume it's mounted after the sea-water inlet strainer but before the fine (~10 micron) filter.
7. Must be able to run continuously for some hours at a time.

Impeller driven raw-water engine cooling pumps fits the bill, so an electric equivalent might well do the job. Some at ASAP for instance sold as impeller bilge pumps.
 
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I think all three of us who have build our own Water maker have gone thought it with you in the other posting I like Chris and Geen use a 277 cat pump and as already explained your best bet is a Jabco puppy pump impeller driven ,
That pump will give you enough lift to feed you cat pump , there possibly other pumps on the market that might do the job , but that one for sure will .
 
sailaboutvic said:
unless he use a LP pump his cat pump will starve even if its below water leave .
Click to expand...
is that specific for cat pumps?
I have a WM series General Pump and with a 2540 and a small membrane does 100lt/h in the med, I wonder if I should try a LP pump as well

cheers
V.
 
A puppy pump is self priming , will give you around 30LPM , there have a Bronze body, stainless steel shaftand has a presure up to 6PSI plenty enough to feed 277 cat pump .
they cost around £150 .
when i first build mine I tried other jabsco pump
A they couldnt keep up with the cat pump and i constant getting air problems and
B they wouldnt had last long anyway .
my water make is now going onto its four year running most days for a hour for 9 month each year
 
If you look for 'Cat Pumps - Whiteboard Sessions' on YouTube you will find a series of excellent videos. None exactly cover LP pumps but one looks at tank design and 2 are on cavitation which is probably the biggest risk if your pipe work and LP pump are not up to standard.
Anyone looking to make their own watermaker, and does not have a engineering background, I highly recommend having a look.
 
sailaboutvic said:
I think all three of us who have build our own Water maker have gone thought it with you in the other posting I like Chris and Geen use a 277 cat pump and as already explained your best bet is a Jabco puppy pump impeller driven ,
That pump will give you enough lift to feed you cat pump , there possibly other pumps on the market that might do the job , but that one for sure will .
Click to expand...

Thanks very much again Vic. Ironically, Jabsco themselves are the ones advising against it as although their website says it’s rated for continuous running, Jabsco advice is max 20 mins to avoid overheating! I’d like to go Water Puppy as per your own experience and advice if I can.
 
I can only go by mine experance but also Chris Jones as well as Geen so I understand uses the same pump , we run our for a hour at a time and have been for near four years without a problem , the only problem I had was after the first winter when reusing the pump I found the impellor swelled up and wouldnt turn unless I lossen the cover , since then over winter I just run the pump with fresh water one a month and not had a problem.
I guess jobsco may be saying that to cover them self .
Im not googling stuff up and repeating what I found on the internet , my advise comes from using the pump and knowing it will do the job you need it for .
last thing its not the first time I felt the pump after running it and tho if warm to touch which one would expect , its never felt hot
 
sailaboutvic said:
I can only go by mine experance but also Chris Jones as well as Geen so I understand uses the same pump , we run our for a hour at a time and have been for near four years without a problem , the only problem I had was after the first winter when reusing the pump I found the impellor swelled up and wouldnt turn unless I lossen the cover , since then over winter I just run the pump with fresh water one a month and not had a problem.
I guess jobsco may be saying that to cover them self .
Im not googling stuff up and repeating what I found on the internet , my advise comes from using the pump and knowing it will do the job you need it for .
last thing its not the first time I felt the pump after running it and tho if warm to touch which one would expect , its never felt hot
Click to expand...

You're absolutely right Vic, and there's nothing beats first hand experience, much appreciated. I'm having problems with this site too - typing slowly and STILL waiting for it to catch up.. I think a Water Puppy will be the way to go.

Thanks to everyone who took the trouble to respond. It's this sort of knowledge that makes the Forum what it is (or was before they broke it).
 
Give Whale pumps an email and ask what they would suggest replacing the Stuart Turner with. I don't think it is too important, just as long as it supplies the HP pump at positive pressure (10 psi in my system).
 
dgadee said:
Give Whale pumps an email and ask what they would suggest replacing the Stuart Turner with. I don't think it is too important, just as long as it supplies the HP pump at positive pressure (10 psi in my system).
Click to expand...
Whale don’t do continuous duty pumps. I asked them once.
 
Our old watermaker used a HP Cat pump and an LP impeller pump. The LP pump was a Water Puppy (I think 3000 from memory). The watermaker was replaced with a Spectra model and the boat subsequently sold so unfortunately I cannot check.

Some watermakers add adittional cooling to their LP pumps (in the form of “computer” fans and aluminium heatsinks) to facilitate reliable long run times. This was not done on our Water Puppy but the motor did fail after 4-5 years of use. Not a bad life but extra cooling is worth considering especially in hotter climates.

The only other thing to consider is impellor pumps use a lot of electrical power. I think the LP pump on our old watermaker consumed more power than our the total power draw of our Spectra watermaker. If you are trying to run the watermaker on solar or a small petrol generator an impeller pump will add to the load.
 
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Goldie. A suggestion is: Stuart Turner K7-2 centrifugal single stage pump. 46372. I bought mine from PlumbNation two years ago for £142 + VAT to feed my Cat pump, which is a 277 equivalent. Mine is AC, but they will do a DC version I expect, or wire it into your inverter. They don't use a lot of power. It is well made and works well. A similar spec to the original Scot pump I had. It is a totally enclosed, externally fan cooled centrifugal impeller design. Big problem though is it has a brass pump body. I had to make do as I couldn't get a perfect spec in time. If you do freshwater flushes and can leave with fresh in or drain it then it won't be a problem. A Scot 35066 is bronze and probably a better choice, but not easy to find. Others are out there. March may have something suitable. Iwaki are the very good, but rather expensive.
 
if I may reward and ask again a question not clearly answered earlier in this thread:

how do I know if my HP pump is starved for water?
I assume that since it does the specified water output I'm fine.
I don't have a LP pump, and other than starting I don't see any bubbles on the lph vertical sight thing with the steel ball in it (guess there's a proper name for it...)

I do get output jumps (starving/overpressure/resetting) of the pump only when filters are blocked or if I'm doing more than 7kn while w/m is running.

cheers

V.
 
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