Watermaker sucking air in

[Lepke]

If you have a boost pump so the line to the filter is under some pressure, you can use an air ejector/eliminator like used in a hydronic system.

[Tellie]

Originally Posted by geoffr

Tellie,

I'm not sure I understand how the boost pump would help (although it sounds interesting!).

I am assuming with my set-up right now that as air enters the system the PowerSurvivor 40 just can't suck up the water when there is too much air in the input side. (Although it is able to eventually overcome this air in calm water if I put the bypass on (not making any fresh water) to clear the air before going back to normal operation.

1. Would the boost pump help the pump overcome this air (that can expand easily)?
2. Would this pump then push the air into the PowerSurvivor? (I assume this wouldn't damage it?)
3. I assume you are thinking the boost pump would only be needed when there is air in the system (or the possibility (ie at sea)) ?
4. Once I have fresh water output I could turn the pump off and water still flows through it?

What pump would you recommend? (I've read the March pump should be at least 1 food below the waterline which will be a challenge..)

Thanks!

The pump that can be used for this is a small Johnson CP mag drive pump 12volts. One that will move about 1-3gpm will work fine. This is mounted below the waterline so its input is constantly flooded. When you start this pump and watermaker it will provide a positive pressure sending the salt water to your system far faster than allowing your unit to suck up the water to it. The air in the line will pass through the watermaker as you start it fairly quickly. This air won't harm the system as it has not had a chance to pressurize much. Once the system starts to make water and is flooded you can turn the pump off if all is well or keep it running the whole time. Your system doesn't require that amount of input 2-3gpm but the pump will be fine only delivering what your system needs. Having this set up, especially if you are getting air ingress to the system from sailing healing etc. during non-use will make your life a whole lot easier. These are pretty small watermakers and if the input line is a long run it can take a good bit if time depending on plumbing to get the intake water up to the unit if it suffers from the above air ingress issues. I like these particular pumps, we have used a good bit of them and they are pretty trouble free. It is rare we ever see one fail.

[Bluechart]

My understanding is that air being sucked into the system will damage the membranes. I’d suggest a tank to continuously fill, and draw from while underway, to prevent air entering your system.

[SV Coronado]

It's normal to suck air in rough seas. I can tell by looking at the sea state if my WM will work well. If not, I'll wait until we're at anchor or motoring in flat seas. The only time it's a hassle is when crossing oceans or long passages when the seas are rough for days. At that point, I'll change course to give the WM a chance to run without sucking air.

[markxengineerin]

I am worried about my feed pump cavitating, not the membrane, but a segmented off section of a bilge seems like a nice place for a "watermaker day tank". It is a major inconvenience when the sea state does not allow watermaking, because of the limited number of days you can go between use without system pickling. If my life schedule aligns with the watermaker's needs and I want to make water, I don't want to be interrupted by mother nature. This is on my long term to-do list!

[jsanton]

Your water filter is essentially a reservoir. Can you mount it upside down? Then the feed to the WM would never have air in it.

[botanybay]

On my Cruising CAL 35 heading downwind from Los Angeles to the Marshall Islands I had exactly this problem with a PUR 40E.

Sitting still the watermaker was wonderful, it was below waterline, installed per directions and produced great water. The intake was as far down on the bottom of the hull as I could get it to stay away from air and oil getting into the system.

However, with 25knts of wind behind us, 9' - 12' slow swell and the watermaker average output was about 1/3 of what it should be and did not sound right. It would take considerable time to push the air through the system. Definitely not good.

It was actually one of the reasons I came up into Honolulu was to do some work on the watermaker (and a failed autopilot drive motor which was brand new, and a squeek up the mast I could not identify and never got settled enough conditions to go aloft to check).

The solution I came up with completely solved the problem, was simple, and I have replicated on several boats over the years.

Basically you need a way to let the air out of the system WITHOUT it having to go through the watermaker. There are two variations I have used depending on if the watermaker is above or below the waterline (or if a boost pump is needed even without being above waterline).

Basically coming right off the seacock I put an UPSIDE DOWN house style water filter in line with no filter element installed. I drill a hole for a hose barb (and a small seacock if desired - I generally do just to be able to turn it off when not needed) A hose comes off this barb to WELL ABOVE waterline and left open to the air. The size of the opening to the air can be extremely small. This allows the bubbles from the intake line to rise to the top of the filter element cavity and come off the top while allowing the water with no bubbles to continue on to the watermaker.

If a boost pump is required then the boost pump goes AFTER the "air separator"

This completely solved the problem on my CAL Cruising 35 which liked to surf a bit and tended to have lots of bubbles around the hull going downwind at speed.

I have the same system installed on the Oyster 55, the hull does not end up with nearly as much air around it and the intake is further below waterline but I still see bubbles coming up.

Plumbing Diagram:

seacock -> inverted filter with vent -> Filters for watermaker -> watermaker

There are many variations on the theme, a rise in the tubing but still below waterline and a largish "T" to allow air to vent to a small tube well above waterline. A capped tube which you only open from time to time to allow the accumulated air out. Any solution which will get the air out will solve the problem

Good luck and happy watermaking.

[markxengineerin]

I like it, pretty much the same idea as a sectioned off area of the bilge as a day tank (it would of course be vented too), but way easier to implement. I imagine the volume of the filter cartridge determines for how long you can suck "air" and get away with it. I'm going to try this, I think it should be standard installation recommendation from the manufacturers if it works. Thank you!

[botanybay]

Glad to help!

Let me know if there are any questions during the implementation

[markxengineerin]

Here's a good price on a housing for anyone else interested in this upgrade
https://www.amazon.com/dp/B01CFOGE3K...roduct_details

What do you think about putting a filter in there too? My procon pump currently has just a strainer on the inlet, and it's not enough to keep out any abrasive particles when operating in less than perfect waters. I'm going to test without a filter first, but am I missing a reason why it wouldn't be fine with say a 50 micron in there?

[newhaul]

Originally Posted by markxengineerin

Here's a good price on a housing for anyone else interested in this upgrade
https://www.amazon.com/dp/B01CFOGE3K...roduct_details

What do you think about putting a filter in there too? My procon pump currently has just a strainer on the inlet, and it's not enough to keep out any abrasive particles when operating in less than perfect waters. I'm going to test without a filter first, but am I missing a reason why it wouldn't be fine with say a 50 micron in there?

What is the actual prefilter for your watermaker?
Personally I would be more likely to go with a 30 micron but my final prefilter is a 5 micron on my powersurvivor35. A 50micron on the end of the pickup that is just stuck Iinto the former intake for the marine toilet I replaced with composting head.

[Tupaia]

You should consider a non return valve in the inlet pipe. I had a similar problem inlet about 9inch below water line, I have a very shallow draft. The NR valve is sufficient to hold water in the line and allow the pump to work without sucking air. It is not perfect especially in very rough conditions but it is a cheap and easy fix that works the majority of the time.

[markxengineerin]

I use a 20 micron and a 5 micron before the spectra clark pump, but both are downstream of the procon feed pump. The potential filter here would be additional.

You should consider a non return valve in the inlet pipe.

Thanks, I'll go there if necessary, to be determined by testing.

[newhaul]

Originally Posted by markxengineerin

I use a 20 micron and a 5 micron before the spectra clark pump, but both are downstream of the procon feed pump. The potential filter here would be additional.


Thanks, I'll go there if necessary, to be determined by testing.

A 50 micron would not be a bad idea . Can should considerably extend the life of the 20 micron unit .
( 50 micron are about half the cost of 20 micron )

[geoffr]

botanybay - thanks so much for your input.. I think I'll try what you are suggesting.. (I may even have the parts on board!)