Engineer’s Angle: by JD Anson
“Water, water everywhere, nor any drop to rinse the salt off the hull,” to paraphrase Coleridge’s “Rime of the Ancient Mariner” in a way more appropriate to yachting today.
If anything can irk the boss as much as bad A/C, it’s having to ration the fresh water. We are all used to just turning on the tap for near limitless clean water. But when off the dock for more than a day, the water supply can become tight.
A typical 40m yacht can use an average of 100 gallons an hour, every hour of every day. This water does not just appear from nowhere. A simple process using basic equipment keeps the spa full and the laundry stews happy. But because this is machinery, it requires attention and proper maintenance.
This maintenance is not difficult, but must be performed correctly to have a good effect. The system is mostly mechanical, with some sensors to monitor water quality. Any engineer worth his salt should be able to keep a water maker in good condition.
A low-pressure pump pulls seawater through a strainer and feeds it to the machine, usually through a canister filter similar to that used for swimming pools. This first-stage cleaning will remove most of the dirt and sediment. Periodic back flushing will keep this filter in good order.
The water then passes through a third filter stage, which removes particulates down to 30 microns. These are usually bulky, costly cartridge filters. Replacing them with sock-type filters allows cost savings and the ability to store dozens of spares in the space of a couple of cartridge filters.
Only then is the water clean enough to use in the machine. This water is then pressurized to about 800 psi and passed through the membranes. Briny water is discharged overboard and the fresh water is sent further down the line. The high-pressure pumps are multi-stage piston pumps with oil-filled crankcases. This oil should be monitored for water contamination indicating a failed internal seal, and regular oil changes are mandatory.
Gauges are installed at every point where there is a change in pressure or flow. Monitoring these readings will indicate when it is time to perform maintenance. Low flow at the feed pump will indicate a clogged strainer. A high pressure differential between the input and output of filters will show it is time to change them. A high-pressure gauge reading lower than normal can be due to rough weather causing air to be sucked into the thru-hull or, if suddenly low and coinciding with a high product output, a blown o-ring on the membrane piping.
It is important to know that in cold water, production will be greatly affected. Seawater at 40 degrees F will result in about half the fresh water production of seawater at 85 degrees F. High pressure should not exceed manufacturers’ recommendations, and low production at cold temperatures should be expected.
Water makers also can be used in fresh water to produce clean product. But because the salinity is lower, high pressure will be much lower, and the product rate should be used to determine the pressure. Attempting to exceed designed rate will cause the aforementioned blown seal. This can also occur where there are changes in salinity, such as river discharges into the sea. Moving from salt to fresh without lowering pressure will cause a failure. Moving from fresh to salt without raising pressure will result in cessation of production. When traveling through a salinity or temperature change, best practice is to turn off the water makers until the change has occurred, then bring them back online.
Most water makers incorporate a fresh water rinse system that will flush the plumbing circuit after each use. This will help prevent fouling of the membranes and other components. Chlorine is detrimental to membrane material, so a carbon filter must be installed in the freshwater feed line to remove the chlorine present in dock water. This cartridge should be changed regularly.
Using the fresh water rinse will reduce the need for much of the chemical cleaning, which is a pain to do. The chemicals used to clean membranes are harsh and cause some damage each time they are used. For this reason, chemical cleaning should be performed only when necessary: when production has fallen below specifications or before placing the system in long-term storage.
Water makers use a process called reverse osmosis to produce high quality fresh water from seawater. Basically, using high pressure, fresh water molecules are squeezed through tiny holes where the salts cannot fit and are washed away. This product water is actually purer than it needs to be.
Just as drinking distilled water is bad for one’s body, putting this pure water into a steel – or especially, aluminum – tank can be bad for the boat. This pure water will try to balance itself by leaching metals out of the tanks, including the hull plating. Even though tanks are coated, a microscopic break in that coating can be where the attack can occur.
For this reason, it is imperative that it first pass through a rehardener to allow it to raise its dissolved solids to a point where it is happy. This is usually a small chamber filled with calcium that the product water passes through before going to the storage tanks. A bonus of this process is getting rid of that constant soapy feeling when showering.
Before the water is sent to the tanks, it should pass through a UV sterilizer to kill any microorganisms that may be present. The UV bulb and quartz tube should be replaced about every 4,000 hours of use.
JD Anson has over 20 years of experience as a chief engineer on megayachts. He is currently project manager at Fine Line Marine Electric (finelinemarineelectric.com) in Fort Lauderdale. Comments are welcomebelow.