7.2 Sea water properties
Sea water properties for pump applications
Sea water properties for pump applications describe the different aspects one need to consider to select a proper pump when pumping sea water. Pumping sea water is normally done with a centrifugal pump but to select the proper material, design and motor size, there are a number of properties that needs to be specified.
Sea water contains a mixture of inorganic salts. Cooking salt, i.e. sodium chloride, forms about 3/4 of the salt content. The salt content at great depths varies insignificantly between the oceans of the world. At a depth of 500 m the salt content is about 3.5%. At the surface, however, the salinity is affected by the climatic and other factors. In the Northern Baltic, the salinity is almost 0%, and in the Red Sea it is about 4%. Evaporation, rain, polar ice, rivers and industrial chemical effluents are all factors which affect the properties of sea water.
The temperature at the surface of the sea varies from about -2°C in the Arctic Ocean and all around Antarctica to about +37°C in the Persian Gulf. The sea-bed temperature is generally between +2°C to +5°C.
Some typical sea water data:
- Density 1020 kg/m³
- pH value 8
- Freezing point -2°C
- Boiling point 100°C
The physical properties of sea water vary little from chemically pure water.
The qualities of sea water and corrosion
Operational experience has shown, especially in the field of ship’s pumps, that the quality of sea water is of great significance as regards resistance to corrosion. There are examples of identical ships where the one vessel has had serious problems with its sea water pumps whilst the pumps in a sister ship have remained intact. This phenomenon has usually been explained by the fact that the ships have been plying in different types of sea water. Ships engaged in coastal and river traffic have a tendency to be attacked more by corrosion than are those which sail the open seas. As regards coast-based industries and power stations, the quality of the water may vary greatly depending upon where the installation is located. In such cases however, the quality of water to be dealt with is usually already known at the design stage, enabling the choice of pump materials to be made. This is not usually possible in the case of marine pump installations.
The variations in the properties of sea water and the effects these variations have on corrosion rates can be summarized as follows:
- Brackish water usually contains a lot of contamination which increases the risk of corrosion.
- The temperature of sea water is of relatively great importance with regard to corrosion rates. In general, higher temperatures increase the corrosion risks.
- Solid contaminants damage the protective-film on the material, thus giving corrosion a chance to attack. The rate of mechanical wear can of course, also be increased.
- Chemical contaminants of various types can also lead to increased rate of corrosion.
- The oxygen content of the water is a requirement for the build-up of protective films. The pH value of sea water is normally about 8.
The dosing of additives may perhaps be regarded as a deliberate chemical contamination of sea waters. Properly conducted, such dosing should not bring any problems from the corrosion point of view. Too strong a dose of sodium hypochlorite, sometimes used to inhibit algae, can on the other hand cause serious pin-point corrosion.
Summarising, it can be confirmed that clean and cold sea water with normal oxygen content gives the least corrosion problems.
Choice of material for sea water pumping
Various types of copper alloy have been used for a long time as a material in sea water pumps. Apart from a relatively good corrosion resistivity, these alloys also have advantages from the point of view of manufacturing technology. Pump casings made from alloys of copper have a fully acceptable life in normal use. Stainless Steel components Swedish Standard SIS 2324 (EN 1.4460), for example have been shown to have good corrosion resistivity in all types of sea water. SIS 2324 has few direct equivalent standards outside Scandinavia, it contains 25% Cr, 4.5-5% Ni and 1.5-1.8% Mo, is ferritic-austenitic in structure and is used throughout Europe. lts superior hardness is particularly beneficial in the erosive conditions of high-velocity pumping.
Pump impellers and wear rings made of Stainless Steel in Cu-alloy pump casings have given good operational service.
The ever increasing pollution of waters has made us move over to more and more corrosion-resistant materials. This means that copper alloys with higher corrosive resistance are now being used in pumps. Stainless Steel has also been coming into wider use in sea water pumps, particularly for pump impellers and wear rings. Looking at development for the future, it seems probable that Stainless Steel will be used to a greater extent. The reason is partly the demand for increased resistance to the increasing pollution of the seas, and partly because the difference in price between a Stainless Steel pump and a corresponding one in copper alloys has decreased. See also Materials for pumps.