Materials for valve design for service in sea water

Valves are a costly part of marine system however the price of valve is based on the design followed. Besides of any kind of production problems, it is seen that a globe valve will be costlier than a butterfly valve due to its higher weight. It is often become more satisfactory to choose consistent materials for valve production like a butterfly valve instead using a costly design of valve, a globe valve and use economical materials. While using the globe valve, for instance for its suitable flow control properties, the additional price of corrosion resistant materials suitable to the design should be accepted.

Few designers tend to prevent the corrosion issues by valve linings. It is associated with the valve design such simple valve’s shape and structure can be offered with dense rubber lining that can be clamped firmly among the flange welding the valve to the pipes and is not based on the suitable adhesion among the rubber and structure. The complex shaped globe vale and lining should adhere perfectly to the metal surface. Test describes that this adhesion is tough to get and linings generally have limited life.

The valves have specific characteristics that should be taken into account to prevent the corrosion issues. For instance, the shafts on the butterfly turns, enter the lining and it is essential to offer a positive seal among the lining and the stem to avoid contact of seawater to cast iron component. Damages to this have caused valve failure due to the corrosion materials accumulated on the lining causing valve penetration.

Second widely used seawater valve is the membrane valve that comprises of a versatile membrane that separates the valve internals from interaction with contact with marine water. While in use, the membrane is elongated into the stream limiting or ceasing the flow. The shape is simple and rubber linings on the body are suitable. Cavitation damage to the rubber memberane is noticed with extreme throttling.

It is considerable to observe the complex flow path inside the globe valve. Various sharp alterations of the direction happens inside the valve that develops extreme turbulence that results into impingement attack on the valve body. Alloys offering good impingement resistance like Copper-Nickel alloy 70/30 are preferred for choosing this kind of valve.

Galvanic factors in valves

It is seen that most of copper base alloys offer equivalent potential and can be utilized without the considering the severe galvanic effects. The stainless steel 310 installed with brass trim offer excellent galvanic security from the wide area of ferrous material and have a significant life.

It is a good idea to use Monel alloy 400, K-500 and stainless steel grades for cathodic protection of the valves. The copper alloys are used in valves to maintain the galvanic compatibility. Do not use the unsecured iron valves in non-iron units.

The systems made from stainless steel grades are free from the galvanic compatibility issues however various alls are used for the various components.

Precautions should be followed, even if stainless steel grade is used with copper alloy based heat exchangers as they can have a significant galvanic influence. It can be taken care by fitting anodes in the water boxes, As iron ions in marine water are advantageous for copper alloys, their implementation is recommended.

Marine pumps

Centrifugal pumps are usually used in the marine systems and are operated by stable speed electric motors. With the standard rotation speed, the speed tip of the pump impeller reaches to 20m/s at this pace, several copper based alloys are attacked in the fast running seawater. Although only specific parts of the pump interact with the high velocities, besides from these parts, the copper alloys can often be successfully utilized in these pumps.

Pump skins

 For the pumps made from copper alloys there is often adequate gap made between the impeller and skins to secure the pump skin get attacked by the running water through the impeller however it is absorbed in the sluggish water flow through the metal surface towards the pipe. Preventing the direct impingement, the materials such as Cu-Ni alloy 70/30 serve efficiently.

Although in few cases, early damages in the pump casing have been seen that resulted into impingement. It can be caused by increased pump speed. In case of such damages, the casing life becomes very small such as only 1.5 years. To prevent damage of this kind, the design should be improved, to decrease the marine velocity at the metal surface, or materials offering greater resistance should be used. Test shows that copper-nickel alloy 70/30 offer greater resistance as compare to tin bronze. They even offer better resistance than various other copper containing alloys in the running seawater.

For the pumps made from welded nickel bronze sheet, a severe phase corrosion is noticed in the heat affected region of the weld that damages when stressed such as by water stroke.

Pump Impellers

It is in connection with the quickly running, turbulent marine water and for distribution pumps that are in service for a long time, they must be constructed from a alloy that prevent attack in such media. Using Nickel- Copper alloy Monel 400 and K500 or stainless steel grade 316 offer excellent resistance to running marine water, their cast forms are recommended for constructing pump impellers. Use of cast iron or low steel can damage the pumps.