ASHRAE Journal - April 2011 - 56

If cavitation cannot be eliminated at the design stage, select suitably resistant alloys. For detailed guidelines for pump design and material selection, refer to the U.K. standard in Reference 5.

Stress Corrosion Cracking
Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at an elevated temperature in the case of metals. It is more common among alloys than pure metals. High levels of stress in service, or residual stress from manufacturing may result in selective corrosion of more highly stressed regions of an otherwise corrosion resistant structure. In the aggressive marine environment, even the more resistant alloys may be affected by hydrogen-induced cracking, or by chloride or sulphide stress corrosion cracking. The stresses can be the result of the crevice loads due to stress concentration, or can be caused by the type of assembly or residual stresses from fabrication (e.g., cold working). Annealing can relieve the residual stresses. Although not a widespread problem, failure from stress corrosion can occur where both stress, internal or external, and a corrosive environment are present. The corrosion is specific to the material and its environment, and stress, whether imposed or residual internal, must be tensile. The mechanism differs according to the material and the environment but failure would not occur if either stress or corrosion were absent. Cupronickel has a good resistance to stress corrosion cracking and is not susceptible to chloride or sulphide or ammonia in seawater, which is why it is normally used in seawater piping for marine refrigeration systems. The refrigerant piping can be provided with flexible vibration eliminators to avoid pipe stresses during operation. It is generally understood that stress corrosion cracking occasionally can affect high-pressure vessels in ammonia refrigeration systems. But research suggests that it is not restricted to high-pressure vessels and may affect copper pipe work in fluorocarbon refrigeration systems.6 One method to reduce the probability of stress corrosion cracking is to control the temperature. Using liquid refrigerant injection with screw compressors can control discharge temperature. Removing non-condensable gases by proper purging reduces the condensing pressure and temperature. Titanium and its alloys are resistant to stress corrosion cracking in most media, including marine environments. However, certain titanium alloys are susceptible to stress corrosion when in contact with chlorinated hydrocarbons or fluorinated sealants.7

eration systems. Common locations for crevice corrosion in heat exchangers are at gaps between the tube and tube sheet or at gasket joints. Two factors are important in the initiation of active crevice corrosion. First, the higher concentration of the electrolyte in the crevice. And, second, the differential electrolyte chemistry inside and outside the crevice (a single metal part undergoing corrosion is submerged in two different environments). Both factors are caused by deoxygenation of the crevice. Some of the phenomena occurring within the crevice may be somewhat reminiscent of galvanic corrosion. To prevent crevice corrosion, you need to eliminate the crevices by using welded butt joints instead of riveted or bolted joints, performing continuous welding or soldering, rather than lap joints, as well as using non-absorbent gaskets such as Teflon can reduce crevice corrosion.

Fouling
Fouling is accumulation of unwanted material on solid surfaces, most often in an aquatic environment. In marine refrigeration systems, fouling can occur in heat exchangers, especially in a shell-and-tube condenser and an evaporator and water pipelines. Fouling phenomena are common, complex and diverse. Unlike corrosion, fouling leads to high operational losses by inefficient heat transfer and increased pressure drop. To compensate for fouling, heat exchangers are liberally sized, which results in higher capital cost. There are also increases in inspection and maintenance costs. The indirect damages arise from using biocides and increased energy or fuel consumption. Fouling also causes many additional problems like corrosion damage, flow blockages/ redistribution, flow induced vibrations, etc. The fouling material can consist of either living organisms (biofouling) or a non-living substance (inorganic or organic). Biological Fouling. Biological fouling is an undesirable accumulation of organisms such as algae, bacteria, diatoms, plants, and animals on surfaces. Calcareous organisms attach to the base surface using different types of glues. These are sticky holding mediums for other types of fouling, which otherwise would not adhere to clean surfaces. Biofouling of marine heat exchangers, water pipelines, etc., promote corrosion. Filtration, chlorination and biocides are necessary to prevent frequent shutdowns. Non-Biological Fouling. This fouling may occur by precipitation, sedimentation, coagulation and chemical reaction. At a higher temperature, calcium bicarbonate present in the water decomposes to form calcium carbonate and its precipitates. The scaling is higher at the hotter outlet of a heat exchanger than at the cooler inlet. Maintaining lower discharge and condensing temperature can help delay scaling. A chemical fouling inhibitor can interfere with the crystallization, attachment, or consolidation steps of the fouling process. In addition, additives may alter the structure of the fouling layers so that they can be removed easily. However, chemical cleaning methods are not practical in marine reashrae.org April 2011

Crevice Corrosion
Crevices can develop a local chemistry, which is different from that of the bulk fluid. For example, in boilers, concentration of non-volatile impurities such as sodium, sulfate or chloride may occur in crevices near heat-transfer surfaces because of the continuous water vaporization. Fouling of heat exchangers also can cause crevice corrosion in marine refrig56 ASHRAE Journal

ASHRAE Journal - April 2011

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