Sunday, April 5, 2009

Watermaking 101

Since we arrived in Bimini last Sunday, water at the dock has stopped being free.  There's a meter at the dock and we're charged for every gallon we use.  In Bimini Bay Resort the charge was 65c / gallon, which can add up pretty quickly.  Here at Atlantis, its 25c / gallon.  With an average daily use of 30 gallons, this adds up pretty quickly.  That's before any boat washdowns as well!  So it was time to use the water maker in earnest for the first time.  We'd used it a couple of times at both anchorages, but we didn't really need to.  We could have managed if we waited.  Not so in the Bahamas.

When I turned the water maker on in Bimini, I got a "Replace pre-filter" error on the control panel pretty quickly.  That was only the start of the problems.  I'll go through the sorry tale in my next post, but for the problems to make sense, I'm going to give a little insight into how a water maker works.  For the uninterested, here's the elevator car summary:  Water makers take salt water and push it through a filter, removing everything but the water.  What's left is pretty pure drinking water.

Those of you who wish to continue riding up with me to conference room, please stay.  For the rest of you, I believe "TGIF" is just about to start down the hall.  Save me a cold one!

The water maker we have is a Spectra Newport Mk II.  There are several brands of water maker that are highly regarded out there and Spectra is one of them.  I chose Spectra as they're a local company, based in San Raphael, just north of San Francisco.  All the comments about water makers agreed on one thing.  Get the highest capacity one you can.  You'll need to run it less.  After measuring all the spaces in the engine bay, the Newport Mk II was the largest I could easily fit.  At 17 gallons / hour with an average current draw of 15 - 20 amps, it was plenty for our needs.

Through hulls

The first thing a water maker needs is access to raw water.  Raw water is the stuff the boat floats in and is obtained by sucking it up through a hole in the hull.  Of course, you don't just go putting holes in boats.  Not without sinking them, anyway.  Into the hole goes a fitting called a through-hull.  This has a ball-valve in it that can be closed if the hose attached inside the boat comes off (or springs a leak).  I didn't want to put a new hole in the boat, so decided to tap into an existing raw water line.  I chose one of the air conditioner raw water intakes.  You can see the new hose I put in below, it's the clear hose with a white spiral.  There was a black hose originally.  You can still see the original hose for the other A/C raw water pump.  On the upper right, you can see the raw water washdown pump through hull.  This through hull is a smaller diameter and not large enough for the water maker's needs.  In the middle of the clear hose you can see the bronze fitting that goes to the water maker.  Note all the double hose clamps.  Very important when you're dealing with hose connections below the waterline.

Raw water strainers

From the through hull, the raw water passes through a strainer.  This is a very coarse filter that stops the big stuff from going any further.  Not much in the strainer, despite 30 hours of running.  You can see the strainers for the A/C raw water just above and to either side of the water maker strainer.  The one on the left is labeled "ARG-750-P".  Directly down stream are the two A/C raw water pumps.  raw water pumps should always be downstream of the strainers.

Boost pump

The first water maker pump is the boost pump.  It's job is to move the raw water through the strainer and the two pre-filters and to the feed pump.  It needs to be placed below the water line.  The waterline, by the way, is about where that big white conduit above the boost pump is.  The yellow switch allows me to change the mode of the boost pump.  In the left position, marked "run", the pump takes water from the through hull.  You can see the clear hose from the strainer attached on the upper left.  When switched to "service", the pump takes water from the fitting on the right.  This allows me to run chemicals through the system when cleaning or storing.  You attach a standard hose to the fitting and lead it into a bucket containing the chemicals.  I'll get into this in my next post.  The middle position, "off", isolates the pump and allows me to remove hoses and filters without water coming in through the through hull.  I usually close the through hull ass well.  The raw water is pumped into the clear hose with a red stripe and sent to the feed pump.

Feed pump module

This is the electronic heart of the water maker.  Not only does it contain the feed pump - the large pump that ups the water pressure to around 100 psi - but it also contains the gizmos that determine whether the water is drinkable or not.  First things first, though, and the raw water passes through the pre-filters.  These are two filters similar to your car's air filter, that remove particulate matter from the raw water.  The water enters on the right - you can see the other end of the hose coming form the boost pump - passes through a 20 micron filter and then a 5 micron filter.  At this point nothing larger than 5 microns across is going any further.  A micron is 0.00004 inches.  By comparison, the diameter of the finest human hair is around 17 microns.  One of the many things the water maker measures is the pressure drop across the pre-filters.  If this is too high then the water maker stops and I have to replace them.  The filters are made of some kind of plastic and can be easily washed using my raw water washdown hose.  From the feed pump, the now pressurized water is sent to the clark pump and membrane.

Membrane and Clark pump

Here's where all the real work is done.  The final filter that turns clear salt water into clear drinking water.  The black tube with "Spectra" on the side contains a semipermeable membrane filter.  This is the ultra fine filter that allows water through but prevents the salt from passing.  This is known as reverse osmosis and requires pressures in excess of 350 psi for salt water.  This is where the Clark pump comes in.  The Clark pump is a pressure intensifier.  Using a sort of leverage system, it uses ~90% of the feed water to increase the pressure of the remaining 10% to the 1000 psi required to force the water through the membrane.  The water forced through the membrane returns to the feed pump module for testing.  The remainder is discharged overboard.  If the water has been desalinated enough, less than 750 parts-per-million of salt, it's sent to the fresh water tanks.  Otherwise it goes back to the membrane with more water from the feed pump.  You can see the discharge hose leading off to the left.  This can be removed and another hose attached that leads into a bucket when servicing.  By putting both the discharge hose and the service hose into the same bucket, a closed loop is formed that allows the same liquid to be circulated through the whole system.  This is how it can be cleaned or stored.  I'm not quite sure why, but the membrane filter gets slowly better at filtering out the salt, so there's always a little lag before fresh water is being made.

Collection point for discharge

Here's a nice picture showing you where all the raw water taken into the system eventually ends up.  It's either drinkable, in which case it goes through the blue tube from the feed pump module into the fresh water tank, or it's not, in which case it goes overboard.  The large cylinder with all the tubes is a collection vessel that allows drainage from all over the back of the boat to go out of one hole in the back of the boat.  The clear tube on the lower right of it is the discharge tube from the membrane filter.

Carbon filter

After the water maker has finished running, there's one last task to be done.  A fresh water flush takes place.  Sea water has organisms living in it.  Sitting at the bottom of the marine food chain, these organisms allow for the amazing variety of life in the sea.  If left to themselves and absent the presence of their natural predators - say, if in a nice water maker - they grow.  Voraciously.  So to minimize this growth, the water maker replaces all the water in the system with fresh water.  Water it's just made.  9 gallons of it.  So you can see why a larger water maker is better.  With smaller water makers, you need to run it for over an hour just to make the water its going to use during the flush.  In our case it's the first 30 minutes of water production.  The membrane has one deadly enemy.  Chlorine.  Even the small quantities found in normal tap water will destroy the membrane in a few minutes.  To protect the membrane, all the fresh water goes through a carbon filter to remove the chlorine.  This filter needs to be replaced every 6 months, hence the date of replacement on the filter case.

Fresh water pumps

These three pumps are the fresh water pumps for Lucky.  The two nearest provide pressurized fresh water to all the taps and showers in the boat.  When I moved them here (they were originally where the membrane and clark pump are now) I added a third that is used only for the fresh water flush.  Thus way I can turn off the fresh water in the boat, relieving the pressure in all those hoses, while still leaving pressure for the fresh water flush.  It minimizes the number of potential leaks in the pressurized system when we leave Lucky for any period of time.  When Lucky was trucked across the States, only the water maker and this third pump where on.  All other systems had been turned off.  As delivered, the water maker is programmed for a flush every 5 days.  As it turned out, this isn't long enough.  When I went to use the system in Bimini, I discovered significant biological fouling in the pre-filters which had spread to the membrane as well.  This has caused issues with quality I'll go into in my next post.  I need to do something, but in the meantime I've reduced the period of fresh water flushes to 3 days.  When we leave for home, I'm going to fill the system with a growth inhibiting storage chemical to prevent this from happening while we're away.

So that's it.  Fresh water making as I understand it.  The system works well and is easy to operate but, as I have come to discover, it does require more maintenance than the documentation states.  Feel free to add any additional information you have in the comments.

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