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Old 05-10-2014, 08:07   #1
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Freshwater pumps in parallel?

I have a big motor yacht. Originally the fresh water was pumped via a Raz Marine centrifugal pump. There were two problems with this. First, it required an external pressure switch, which was hard to set. If it over-pressured, the fill solenoid on the washing machine permitted water to seep through, causing a nice surprise when opening the front load door. Second, it was not self priming. One day while I was out, a guest ran the tank dry and the centrifugal pump ran dry and burnt out the impeller.
So for now it uses the priming pump, a 4 gpm Shurflo. Adequate but I need higher flow rate for the 10 outlets.
What if I hooked up 2 or 3 of the Shurflos (or equivalents) in parallel? What problems could arise?


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Old 05-10-2014, 10:19   #2
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Re: Freshwater pumps in parallel?

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Originally Posted by dohenyboy View Post
...a guest ran the tank dry and the centrifugal pump ran dry and burnt out the impeller...
Centrifugal pump impellers cannot burn out.

Those small Shurflow pumps are not adequate for large vessels. Get a real pump sized for your application.
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Old 05-10-2014, 10:27   #3
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Re: Freshwater pumps in parallel?

I have two water pumps (fresh water system) in parallel on my boat. I believe that the original rationale of the previous owner was to provide freshwater to the watermaker for a flush when away from the boat without having to leave the whole freshwater system pressurized.

Of course, it doesn't work that way.. if you pressurize part of the system the whole system is pressurized. But I find it is a big advantage for two reasons. First, the primary pump is selectable -- port vs. starboard tanks, but I need to lift up a cabin sole board and flip a valve to change tanks; the backup pump is always port tank. I find that if I leave the primary to always pull from starboard then I can switch tanks at the press of a button (flip one breaker off, the other one). Second, while I carry a spare water system pump, fitting it requires contorting oneself in the back of the engine room for a while -- not something I would want to do underway if I could avoid it. Having the second pump allows an easy "hot-swappable" redundancy.
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Old 05-10-2014, 10:43   #4
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Re: Freshwater pumps in parallel?

Pumping in parallel is just like electricity in parallel, think of two batteries in parallel: same voltage (pressure) more amps (flow). The issue is proper control of pump cycling.
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Old 05-10-2014, 10:52   #5
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Re: Freshwater pumps in parallel?

Okay maybe the impeller did not burn up. Maybe an existential crisis was triggered by running dry. Point is, it ran dry and now it does not pump.
If i repair it, someday it will run dry again leading to another failure.
If i have 2 or 3 little pumps in parallel, and one breaks, I can get a replacement anywhere. Meanwhile I will still have water.
If there is 1big pump, fancy one, and it breaks, I don't think I can drop into Budget Marine and pick up another one.
Of course I would rather have a thrall present to manually pump the water, but I am not sufficiently well-off for that.


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Old 05-10-2014, 11:20   #6
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Re: Freshwater pumps in parallel?

It's a good idea to have redundant systems for getting water out of your tanks. But your proposed solution reads more like a poor-boy work-around instead of a system intended for a larger yacht.

Centrifugal pumps can be run dry without damage. Repairing your existing one seems the wisest approach.
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Old 05-10-2014, 11:22   #7
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Re: Freshwater pumps in parallel?

While I'm not an expert in marine pumps, I do teach pump and hydraulics in the fire service.

Centrifugal pumps can "burn up" if a water flow is not maintained through the system. These pumps rely upon the flow to cool the pump. However, when the flow is insufficient or nonexistent, one of two things can happen as the bearings and housing heat up. If there is enough heat and a bit of water, the water is converted to steam and that expansion and pressure rise can actually crack the pump housing. The other thing that can happen is the heat warps the impeller (most often referred to as "burning up the pump"). This happens more often now as most centrifugal pumps have plastic impellers instead of the older metal (brass) ones.

Hope this clears up some of the confusion here.

Cheers,
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Old 05-10-2014, 11:26   #8
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Re: Freshwater pumps in parallel?

At least some impeller driven pumps cannot be run dry, as I recently discovered. A March raw water circulation (refrigeration) pump ran dry on our boat recently and it no longer pumped. Unfortunately the wet end on the pump was so corroded that disassembly to replace the impeller was not an option.
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Old 05-10-2014, 11:53   #9
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Re: Freshwater pumps in parallel?

Those March pumps use a magnetic drive to couple the motor to the impeller, so can be run dry. If the impeller gets stuck, the motor can't burn out. But the impeller is still stuck.

It is the vane type rubber impellers that can't run dry.
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Old 05-10-2014, 12:42   #10
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Re: Freshwater pumps in parallel?

RAZ does not build PUMPS, they re-manufacture Gally Maid pumps. These pumps are not self priming, so if it doesn't work after running dry you must prime the pump, for it to work again.

The Galley Maid pumps use a rotor and stator design. If left to run dry for long period, you can ruin the stator, this is a replaceable part. The rotor does not get damaged.

Stator Rotor


As to pressure switch, all pumps need a pressure cut in/cut out they are easy to do right, you could also introduce a pressure reducing valve prior to the washer.

As to running dry, a simple solution is a vacuum switch and time delay relay on the suction side.

Lloyd
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Old 05-10-2014, 13:03   #11
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Re: Freshwater pumps in parallel?

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Those March pumps use a magnetic drive to couple the motor to the impeller, so can be run dry. If the impeller gets stuck, the motor can't burn out. But the impeller is still stuck.

It is the vane type rubber impellers that can't run dry.
Yes, magnetic drive. I was able to get the wet-end off. I couldn't get the wet-end apart, though. Water simply flowed from the input to the output (or visa-versa) as if the impeller was gone! I assumed that this was because it had been run dry for a while before I noticed the sea-chest was filled with air.
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Old 05-10-2014, 15:24   #12
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Re: Freshwater pumps in parallel?

While I do not have a monster pump, I do carry a spare. Why not get yours fixed, replace it with a new one and carry it as a spare?
Running multiple pumps should work, if you just use the pressure switch on the pump, some will come on a little earlier, but shouldn't matter. It is sort of a kluge though, and with say three pumps, you have three times opportunity for failure.
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Old 05-10-2014, 21:14   #13
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Re: Freshwater pumps in parallel?

That would make a lot of sense if I lived in South Florida. But I do not. The seal and the stator together run about $100 and then another $100 minimum to ship to me. And hassle with customs. A pump on an exchange basis is $420. So I would guess a new one is about $600. So, looking at about $800 to $900 to do what you counsel and the result is I have to hopefully rebuild it myself and then I have a 7 gpm pump that needs to be primed and if it runs dry again, there goes a few hundred $$ out the window.
If i buy two more shurflo 4 gph pumps, I can buy them tomorrow morning and by noon have it all hooked up. It will cost me $350. Maybe $400. I will have total 12 gpm, almost 2 times the flow. I have a 200 gallon bathtub.It gets filled in 16 minutes instead of 30. And if one of the pumps fails, i still have two going. My life goes on without mishap.
400 or 900? 400 or 900?
Of course I could put a few 50 gallon plastic drums on the flybridge and fill them with water.. would solve all these problems. but when I take the boat out they would probably slosh around a lot and make the boat top heavy.
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Old 05-10-2014, 23:00   #14
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Re: Freshwater pumps in parallel?

As accomplice indicated, centrifugal pumps can fail if they are run dry. Generally bearing failure and or seal failure.

For the large boat owner, take two marine pumps and tie both to a common pressure switch (Honeywell/ Granger) and common header, or get fancy and install a small logic board and pressure sensor to stage the second pump on.

There is nothing wrong with tying pumps in parallel. Done all the time on land.
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Old 06-10-2014, 10:42   #15
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Re: Freshwater pumps in parallel?

I found a solution for those little pressure switches on the buggers where the switch fails after 2 or 3 years...a friend and i were going thru the delima of having a good working pump, that the pressure switch gave up on! my thought was to try one of those square D ac well pump switches...found one that was on @20Lbs, off @40Lbs...put a tee in the hose after the pump so the switch senses the pressure, cut the red wire [on most pumps] to bypass the sw, and voila!! that was in about 1995, and the sw still works! expensive too...i paid $12 in 95, now when i got a spare, they`re up to $14!! and new points are about $5! i know the cost is prohibitive, but they sure work, and keep on working...and for a larger multiple system, buy a washdown pump, and put the square D sw on it...clyde
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