Page 45: of Marine News Magazine (October 2016)
Salvage & Spill Response
ci? c, the coating is a hard anodize. This aluminum oxide ferric metals become brittle at low temperatures, so gears coating is dif? cult to get into small holes, and the thick- and other mechanical assemblies subject to shock and high ness of the ceramic layer is such that putting anodize over loads have to be signi? cantly oversized to avoid breaking threads in a hole will typically make it impossible to install when temperatures plummet to -55 C.
the bolts. In addition, the actions of tightening the bolts Similarly, elastomeric materials become much less ? ex- will crack the protective layer exposing the aluminum to ible, so dynamic frictional seals, typically used for shaft seals seawater corrosion. on motors are less reliable. Due to lower chemical activity
The corrosive effect of sea water is accelerated when at lower temperatures, battery output is degraded, and this there is a mixture of metals. Dissimilar metals in contact can happen just as the demand for power is increasing. with seawater can form a kind of battery, resulting in rapid The end user or design engineer has to come to grips metal erosion known at galvanic corrosion. When neces- with general performance degradation at low tempera- sary, such equipment can be protected by intentionally tures. The choice is to specify performance when the adding sacri? cial metal blocks such as zinc. If one checks system is cold – this signi? cantly reduces performance at the anodic index of metals, it is possible to appraise the normal temperatures – or to provide performance versus likelihood of such corrosion. For example, brass has an in- temperature ranges.
dex of .4 while galvanized steel has an index of 1.2 – the
ETTING TARTED 0.8 difference represents the voltage that will be generated G S between the two metals if they are in contact while sub- Building an oil-? lled, pressure compensated, waterproof merged in an electrolyte such as sea water. In this example, motor is a complex task with several factors to consider. To the combination of brass and galvanized steel is almost cer- do it right, one ? rst needs to know the amount of oil in the tain to result in rapid erosion of one of the metals. assembly, the expected temperature range, the temperature
While corrosion to the exterior of a motor is to be miti- at which the motor is to be ? lled, the ? nal position of the gated, internal to the motor it is best to be avoided. The cylinder after ? lling, and the total volume of displacement copper wire that is typically used for motor winding cor- of the cylinder over its travel.
rodes quickly, but more immediate is the fact that salt If the unit is completely ? lled with oil at room tem- water is conductive, so it will short out the electrical cir- perature, when the motor runs and the oil warms up, the cuit. Sure, there are coatings, varnishes, and potting mate- oil will expand. If there is no pressure compensator (in this rials that are protective, however for long term submersion case, a piston in a cylinder), the hydraulic pressure will nearly all the organic insulation materials will absorb wa- blow the seals out of the assembly.
ter. Hence, it is far better to avoid the problem by exclud- On the other hand if the motor assembly is under ? lled ing the water from the assembly in the ? rst place using oil at room temperature, when the unit gets cold, as in the ? lled and pressure equalized assemblies. Arctic Ocean, the piston will hit bottom, the oil will con- tinue to shrink and the sea water under pressure will get
NIQUE SSUES FOR RCTIC ONDITIONS
U I A C inside the unit.
After taking all other variables into consideration, motors As industry and governments alike awaken to the new that perform in extreme cold do have unique issues. The vis- reality of expanding Arctic operations, many variables – cosity of oil and grease increases at low temperatures. Since unique conditions for properly built electric motors, for ex- drag increases with viscosity or grease stiffness, it takes more ample – will come into play. In this case, ‘sweating the small power from the motors to overcome the stiff grease in the stuff’ will pay off for stakeholders who hope to successes in bearings or the viscous drag of an oil ? lled assembly. a rapidly expanding, but still unfamiliar ocean environment.
A designer must consider the coef? cient of thermal ex- pansion (CTE) more carefully when making equipment for low temperatures. As materials get cold they shrink at dif- ferent rates. If the CTE is not considered in the design, fail-
Richard Halstead has nearly four decades of experience ures can result. A simple example is a bronze bushing being in the automation industry. He is the President and used as a bearing around a steel shaft. As the temperature
Chairman of Empire Magnetics. He is the named drops, the bronze shrinks faster than the steel so the bush- inventor on several active patents, with more in process. ing clamps on to the shaft and prevents it from moving.
He has also authored a number of technical articles.
Another issue facing Arctic operators is that steel and 45 www.marinelink.com MN
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