Air bubbles in watermaker

[ancl]

I have an idea about air bubbles I'd like to check across the community;
When sailing, air bubbles run beneath the hull and end up in the scoop to the watermaker intake, which is something that wears out the pump. My idea is to take the water from a T-connection to the saildrive (where the engine's cooling water is taken) instead of a scoop mounted on the hull.

The engine I have is a Penta D2-75 and the watermaker needs an incoming flow of appx 8 ltr/min.

Could this work?

[SailingHarmonie]

Why do you think there are no air bubbles coming up the engine intake? When sailing fast air bubbles are everywhere under a hull, especially a wide flat design like yours.

But aside from that…

Drawing engine water and watermaker feed water from the same thruhull CAN work, but it requires upsizing the thruhull and piping to supply the extra water flow without flow restriction. Watermakers need a lot of water, and do not do well if they have to “fight” the engine for it. Reducing the inlet pressure to your RO feed pump can cause more damage through cavitation than air bubbles do.

Virtually every watermaker manufacturer says to NOT do this because they know from experience that almost no users will get this modification done correctly.

[markxengineerin]

I had the bubble issue. To solve it, post #35 and 36 here:
https://www.cruisersforum.com/forums...in-274448.html

In summary- on the inlet to the pre-filters, install an extra filter canister, mounted upside down. It is normally full of water, as it's below the waterline. When bubbles enter the system, they float to the top of the canister, and the bubble free water is sucked (from the bottom) to the pre-filters. Every 10 minutes or so, shut system off, and a valve attached to the top of the canister opens up. The air is quickly evacuated by the pressure of the ocean, water level rises back to top, good for another 10 minutes. I have an arduino doing the timing and switching of the system and solenoid.

I think any watermaker that's used underway and in heavy weather needs something similar, as air bubbles are a constant issue, unless the pumps used are tolerant of air bubbels. My pump (Procon style) is 100% not tolerant of air bubbles- they cause cavitation and rapid catastrophic failure. I proved this to myself a few times before implementing this system, no problems since.

[SailingHarmonie]

Originally Posted by markxengineerin

I had the bubble issue. To solve it, post #35 and 36 here:
https://www.cruisersforum.com/forums...in-274448.html

In summary- on the inlet to the pre-filters, install an extra filter canister, mounted upside down. It is normally full of water, as it's below the waterline. When bubbles enter the system, they float to the top of the canister, and the bubble free water is sucked (from the bottom) to the pre-filters. Every 10 minutes or so, shut system off, and a valve attached to the top of the canister opens up. The air is quickly evacuated by the pressure of the ocean, water level rises back to top, good for another 10 minutes. I have an arduino doing the timing and switching of the system and solenoid.

I think any watermaker that's used underway and in heavy weather needs something similar, as air bubbles are a constant issue, unless the pumps used are tolerant of air bubbels. My pump (Procon style) is 100% not tolerant of air bubbles- they cause cavitation and rapid catastrophic failure. I proved this to myself a few times before implementing this system, no problems since.

There are a couple things I don’t like about about this, but mainly have a few questions about how you manage two issues.

RO Membranes have a rigorous spec on the rate of pressure rise and fall they can tolerate. Typically this means that to go from 0 Bar to ~50 Bar should take roughly a minute on the way up, and again on the way down. How do you manage this issue when switching the pump on and off every 10 minutes? This seems like it would be very rough on membrane’s support grid.

Also, every RO system I have used typically takes two or three minutes after its initial start up pressurization to actually produce water in spec for TDS. If you are running steady for an hour, this can be ignored without serious consequences, because the excess salt is diluted in the tank, but if you are shutting down every 10 minutes and restarting, the system is going to spend 20 to 30% of the time making water out of spec, significantly reducing the quality of the product water. How do you address this? This issue MIGHT not come up with rapid on/off cycling of the system, since the excess salt in the initial product water comes from diffusion across the membrane during a static hold period. Have you measured to confirm this?

[markxengineerin]

My membrane is about a year old, no degradation in performance yet. Don't all boat systems rise and fall too rapidly according to that spec? I don't mind the membrane aging it faster, as long as I can get a few years out of it at my rate of use. They're comparatively cheap compared to other parts of the system. Definitely preferable to rebuilding pumps every other month from cavitation.

For the salinity issue, I have an in-line meter with two different probes, they usually read within 10 or so PPM of each other throughout production, very stable, then I check the "flush water" again at the end, which I draw from one of the tanks into a 5 gallon pail (manually). That is a different handheld meter, and it has also been very close to the other two readings.

So I think they're good points and things I hadn't thought of myself, but, neither has had consequences yet. I was curious about any salinity differences right after startup, but wasn't able to measure any (from my memory of a while back).

[ancl]

Thanks both - you definitely have valid points. Checking for that simple solution again - since this is a common problem - I found this "Automatic Air Vent" that releases unwanted air from any hydronic system, ie radiators. This video is a little wordy but gives some insights on how it works. https://youtu.be/Ogw83f-Tbvg

Not very bulky so it should be easy to install. Might just buy one and try it!

[BobFord]

I run w/maker with a lift pump . Am now installing a small holding tank above pump and filter bank . I expect that air will "filter " , to top of tank and can then be 'tapped off ' , as marhxengineer suggests. What could go wrong ? Comments appreciated. Cheers, Bob

[markxengineerin]

Bob your idea is good! With something like that, I don't think you even need to switch the system on/off, and you could leave it "vented" at all times, as long as you prevent spills. No downsides that I can see, just didn't package as easily into my boat as what I went with.

[BobFord]

Originally Posted by markxengineerin

Bob your idea is good! With something like that, I don't think you even need to switch the system on/off, and you could leave it "vented" at all times, as long as you prevent spills. No downsides that I can see, just didn't package as easily into my boat as what I went with.

Thank you ,Mate-I had not considered 'permanent vent'- nice idea.

[ancl]

My initial question of taking water from the saildrive is not a good one - I buy that!

In search of simple solutions, could an "Automatic Air Vent" as used by plumbers in radiator systems etc, could be of any interest? (Google it, I can't add links here.)

But... It must be placed on the pressure side of the flow, otherwise it would allow air to be sucked in. So, using it will require a feeder pump with higher flow than the high-pressure pump. Comments? :-)

[markxengineerin]

A device that bleeds air after the pump would not help in my case, because my goal is to keep air out of the pump. The other thing to watch out for with these valves, when used, is them getting salted up and stuck. That happened before I started squirting a little fresh water down to rinse after each use. A "permanent vent" gets rid of that problem too, but you still need a good way to keep a good "level" in the day tank. Too much- overflow. Too little- you'll draw air. Not an impossible problem but it will probably involve a sensor, a relay, things that add complication and subject to failure and salt water messes : )

[ancl]

Originally Posted by markxengineerin

A device that bleeds air after the pump would not help in my case, because my goal is to keep air out of the pump. The other thing to watch out for with these valves, when used, is them getting salted up and stuck. That happened before I started squirting a little fresh water down to rinse after each use. A "permanent vent" gets rid of that problem too, but you still need a good way to keep a good "level" in the day tank. Too much- overflow. Too little- you'll draw air. Not an impossible problem but it will probably involve a sensor, a relay, things that add complication and subject to failure and salt water messes : )

You're right Mark, the case will need a feeder pump and the autovent placed after this, but before the high pressure pump. And the risk of the fine mechanic getting salted up is definitely relevant.

If we move to the "permanent solution"... If there isn't room for a day tank, couldn't I just put a small hose (3 mm) between the top of the filter and any oulet? It would decrease the pressure a little but hopefully not too much. Again, circle around a system set up as: feed pump - filters - high-pressure pump - membrane.

[markxengineerin]

Follow up with some measurements- the PPM does go up a little for the first minute, but the effect on a tank of water is negligible, as measured in the tank after it's full.