Watermaker feed pump - which one?

[Hoghead]

I need a feed pump for a HP watermaker pump at 4.2 USG/Min. This is the pump that supplies positive head to the HP pump

I am thinking 12V for simplicity and perhaps I could use the feed pump electrical supply to control the 12V clutch on the watermaker pump so that a failure of the feed pump would turn the pump clutch off? I am not smart enough to figure out how to do this however!!
There needs to be some method to protect the HP pump in the event of fed pump failure, a feature sadly lacking on my current set up.

I see systems with both Jabsco Water Puppy impeller style and standard diaphragm pumps like you would have in your fresh water system. I imagine that a magnetic drive pump would work fine as well but I have not yet found a suitable one

The output of this feed pump will be greater than the HP pump so how does one deal with this? The diaphragm pump will switch on and off but I have never used an impeller or mag pump in this situation.

I am a bit leery of a diaphragm pump as they seem to have a short lifespan

Which type?

[Hoghead]

Anyone?

[canadian cat]

here is an article about a diy watermaker. If I can remember correctly he uses a magnetic pump. Take it for what it is worth.
Watermaker: Doing It Yourself | | PassageMaker

[Hoghead]

I saw that too and he used a 110V pump - I am 230V but I could use a similar aquarium pump. However, I would rather use a 12V pump

He does not talk about how any of these pumps cope with having a greater flow rate than the HP pump. The mag drive 12V pumps that I did find at Defender have rather impressive flow rates.
Perhaps I am worrying about nothing?

[phorvati]

Some thoughts on feed flow, Hp pump vibrations and flow. Im not sure how in-tune you are with your watermaker, but i keep an eye on the pressure gauge placed between micron filters and HP pump inlet. If feed pump cuts off I hear and observe immediate difference and shut the watermaker down before serious damage (manifold cracks due to vibrations or fiberglass vessel endcap fails due to pressure spikes). I have observed better brine flow(lower vibration) performance with higher PSI at the inlet of HP pump. I've tried anywhere from small vacuum to 45 PSI. HP pump specs will show recommended inlet PSI. Usually in the range of small vacuum to 60PSI.
I am currently using flojet 4125 series. these pups are heavy duty and have integral overheat/over-current protection that will cut off if they run too hot. Prior to this I was given for free a flojet 4325 which was an overkill. I have an engine driven cat 290 (2.5gal/min) with 2 filmtec 2540 membranes in parallel. Few things i learned from this feed pump:
If feed pump is too powerful it will run too hot, with plenty of inlet pressure, but protection will cut power off, and you might have to wait for pump to cool off. If there is no protection pump could get damaged. Look for words like continuous duty to ensure pump can run for long times. In my case I tried cooling the pump but never got the fan and cooling just right.
What worked better is adding a Tee with a valve after the feed pump so i could lower and adjust feed pump pressure. Keeping 5-10 PSI prevented the 4325 from further overheating and diverted large portion of the feed water overboard prior to micron filters.
After converting to lower flow pump (flojet 4125), I kept the bypass and I have had two seasons of use with anywhere from 5PSI to 30PSI of inlet pressure and 4125 never overheating. I have not tried increasing the inlet pressure(shutting the bypass) due to prior experience of overheating. 4125 feels noticeably cooler to the touch. Ive been meaning to add a fan so i can increase the inlet PSI possibly eliminating the bypass, but have not gotten around to it. Also i have a rather large WM inlet . Its an 1.5" with scoop strainer. it was the old engine intake so I just kept it instead of reducing. What i've learned is that sometimes I get significant vibrations on the high pressure side. Root cause was inadequate inlet flow(could be clogged filter, sometimes bioluminescents and other crap in harbors, or just maxed out inlet flow given the brine flow/pressure settings). As a result of using such a large inlet, I get much better performance if boat is moving forward while making water. It allows me to improve brine flow by rising engine RPMs and still avoid vibrations. While at anchor same Brine flow/engine RPMs would result in much higher vibrations. So i just got used to making water while under way. It keeps me away from debris-filled harbors, and allows me to keep brine flow higher which helps with membrane clogging and growth accumulation.

[Hoghead]

I have been looking at magnetic drive pumps and while no Dealer has yet seen fit to answer my email trying to buy one, Marsh Pump did respond saying that:

"It will not injure our pumps if the flow is throttled back.
However deadheading a pump will damage it as some flow is required for pump lubrication"

Rather than deadheading, I am causing a restriction due to having a higher flow rate than the HP pump so it should be OK, however I will clarify with Marsh

[Hoghead]

I have been looking at magnetic drive pumps and while no Dealer has yet seen fit to answer my email trying to buy one, Marsh Pump did respond saying that:

"It will not injure our pumps if the flow is throttled back.
However deadheading a pump will damage it as some flow is required for pump lubrication"

Rather than deadheading, I am causing a restriction due to having a higher flow rate than the HP pump so it should be OK, however I will clarify with Marsh