With summer upon us, maybe you’re planning a cruise or itching to take your boat out on the lake. But did you know neither of those would be possible without valves?
Valves in boats are similar to those used on dry land, but they're made from different materials to better resist corrosion. Never use a regular valve in a marine application because it’s unlikely to last.
Valves are used in ships and boats for the same reasons as on dry land. They are incorporated into systems for drinking and waste water handling, as well as heating and steam systems. Engines need cooling systems, and larger vessels also have fire suppression systems, just like buildings. Plus, tankers use many different valves to load and offload their liquid cargoes.
Two applications unique to the marine environment are ballast control and bilge systems.
Larger ships take on water ballast when empty to make them more stable. Then, as the hold is filled with cargo, the ballast is pumped off. Valves are a core component of these systems.
Bilge systems are installed in ships as a way to pump out water that gets into the vessel. These will include check valves to ensure one-way flow, as well as gate or globe valves and pumps.
Most metals oxidize, which is when they attract oxygen atoms to their surface. In some, such as stainless steel, that forms a corrosion-resistant protective skin. In carbon steels though, it causes flakes of metal oxide to fall off in a process better known as rusting.
Water encourages corrosion, and salt water makes it happen faster still. Even stainless suffers from pitting and stress corrosion in the presence of saltwater, caused by chlorine from the salt. Any valves that will handle water must be selected with corrosion-resistance in mind, especially valves that come into contact with seawater.
Corrosion-resistant valve materials
Bronze, an alloy of copper and tin, has long been a favorite metal for marine applications. It has excellent corrosion-resistance and, as a bonus, resists biofouling (growth of algae on the surface). Unfortunately, it's also quite soft and damages easily, which makes it a poor choice for valve seats.
Alloying bronze with around 10 percent aluminum results in a much stronger, though also more expensive, metal. This has the additional advantage of resisting attack by many common acids and alkalis.
In recent years, many seawater valves have moved to copper-nickel or nickel-chrome alloys. Although costlier than aluminum-bronze, these offer an attractive combination of corrosion resistance and strength.
For the ultimate in durability, some boat-builders, especially those in the defense sector, are turning now to titanium. While very expensive, valves made from this metal last almost forever. A second advantage of titanium in marine applications is that it’s quite lightweight.
Weight reduction is beneficial in marine applications because it allows for an increased payload or more cargo. Another issue is space. Submarines are the most extreme example, but in civilian watercraft as well, space is at a premium.
Gate valves have long been the valve of choice in marine applications. They are robust, reliable and create very little pressure drop when fully open. Their disadvantage is that the stem needs space in which to rise up (a problem avoided by non-rising stem designs). Alternatively, ball or butterfly valves need less space and are faster acting, needing only a 90° turn to operate.
A tough environment
Marine valves do the same kind of jobs as valves used on land but have one significant difference: they are produced from materials that will resist corrosion. This allows them to handle seawater and survive in salty conditions.