While bronze is ideal for raw water pipes, its cousin brass is susceptible to corrosion.
Steve D'Antonio
I regularly come across design, material and assembly errors in raw water pipe systems on cruise ships. These defects can, and sometimes do, lead to flooding and total loss of vessels. Here you can see the three most common examples of these defects.
The Unseacock
Seacocks allow crews to quickly and easily control water flow, whether for routine maintenance or in the event of a broken hose, fitting or other seawater emergency. These valves must be easily accessible and relatively easy to use. “Easily accessible” means that access to a seacock does not require tools or moving a large amount of equipment.
One of the most common seacock defects is mismatched threads. Throat fittings, sometimes called “skin” fittings, are almost always manufactured with parallel, straight or NPS threads, while most inline ball valves use tapered or NPT threads. The two are completely incompatible, and yet I regularly encounter this dangerous assembly practice, both on new and old ships. The mismatch results in the engagement being barely two to three thread turns compared to a proper seacock with matching threads, which gives eight to ten turns.
Underrated hose
Hoses used for raw water applications must be robust and resistant to abrasion, crushing and kinks.
Steve D'Antonio
The average cruise ship may use more than half a dozen types of hoses, from fuel and waste hoses to drinking water exhaust. Hoses used for raw water, particularly below the waterline, should be specifically designed for the application.
The most common raw water hose carries a SAE J2006 rating (usually it is a black composition called EPDM, but it can also be red or blue silicone). This type of hose is suitable for marine wet exhaust systems. It's sturdy.
With a few exceptions, most clear PVC tubing is typically not suitable for raw water, even if reinforced with nylon filaments or coils (some of which are designed for the food industry).
When I confront builders and shipyards with clearly non-compliant hoses, they often ask me, “What makes this hose a problem?” My answer is simple: “Is this hose approved for an application where a ship could sink if it failed?”
Additionally, if the hose pinches or kinks easily, especially when warm, it is not suitable for raw water and is particularly not suitable for priming or suction applications.
Wrong alloy
Only a handful of metal alloys are suitable for use in raw water pipes. Bronze, composed primarily of copper, tin and usually trace amounts of silicon and other metals, is highly corrosion-resistant for raw water applications. However, there is bronze, and then there is bronze.
Manganese bronze and Tobin bronze, for example, can contain a significant amount of zinc, making them technically part of the brass family. Therefore they are completely unsuitable for raw water installations. Any copper alloy that contains more than 15 percent zinc is technically brass and therefore should not be used for raw water pipes.
Using high-zinc alloys often results in dezincification, where zinc corrodes out of the alloy, leaving a porous, weakened structure with a telltale pink hue.
Stainless steel is not ideal for use in raw water, even alloy 316.
Steve D'Antonio
Some builders use brass through-hulls and seacocks, with the caveat that these must be bonded to anodes and cathodically protected. This approach is flawed and has resulted in flooding and ship losses. Forgetting to replace a zinc should not result in seacock failure. Aside from stray current corrosion, seacocks, hull penetrations and other metal raw water pipes should last the life of the vessel. Stainless steel, even 316, while resistant to corrosion, does not have the corrosion resistance of bronze. While this does not prohibit the use of stainless steel for this application, it is clearly a second choice to bronze.
A non-metallic option such as fiberglass-reinforced nylon for seacocks and thru-hulls that meets American Boat and Yacht Council standards is also an acceptable alternative.
Steve D'Antonio provides services to boat owners and buyers through Steve D'Antonio Marine Consulting.
Comments are closed.