The basic concept is to choose a material to suit the working conditions as available metal substrates corrode at different rates. A coating can be used to protect material from an environment in which it would normally be serviceable.
Aluminum corrodes at a much slower rate than black steel, but hot dip galvanizing the steel surface can extend its life by providing a sacrificial coating; the zinc corroding rather than the steel due to its properties. The galvanized steel could be protected by a barrier coating to stop the galvanic corrosion and prolong the life of the product. If there is any damage to the coating, however, the corrosion can slowly creep under it due to the rate of corrosion of the galvanized layer. With proper maintenance, this erosion can be minimized.
Since the rate of corrosion of aluminum is slower than steel, the basic product will last longer: However, when it is introduced into a coastal region, this may not be enough for a plant installation with a life expectancy of 20 years or more. In this condition a barrier coating may be applied to the aluminum to stop initial corrosion and produce a long lasting product with adequate maintenance. Some material manufacturers would give up to 15- year guarantees for their barrier coatings in industrial applications.
High initial cost tends to limit fiberglass application to those areas where high corrosion can be anticipated due to the environment in which the cable tray has to operate. A correctly specified coated product may survive for a reasonable time period even in these harsh atmospheres.
When considering the proposal for installing cable trays, a number of factors must be considered in order to select the most appropriate material to be used. These include:1. Load bearing capability 2. Atmospheric conditions 3. Local chemical environment 4. Impact resistance 5. Price and life expectancy of tray or project
1.The load bearing capability can be easily modified to suit a particular project by the correct specification of material thickness and support structure, and as such bears no interest in the choice of material substrate to be used.
2.The atmospheric conditions dramatically alter the choice of material, which should be used. If the product is to be specified in a dry inland area with little or no humidity, then a hot dip galvanized or aluminum substrate can be used quite adequately. The cable tray will corrode slowly but should give a good life expectancy. For those who require only a short life span and can tolerate the poor appearance of corroding cable trays, black steel can be used. However, for a professional installation, this is not a valid option. In humid and salt laden, coastal environments, where corrosion is fact of life, the situation changes dramatically. Protection must be given to ensure long product life. In the Middle East, a cooling effect is frequently experienced at night, which has the effect of condensing salt laden humidity onto exposed materials. Low concentrations of some acids and alkalis may also be found in the atmosphere around certain coastal installations. In these environments, a barrier coating should be chosen to maximize product lifespan, whilst minimizing maintenance. A decision must now be made between coated HOG steel or coated aluminum and what coating should be used. Consideration must be given to the life expectancy of the total installation as inferior products would entail disproportionate use of resources to complete unscheduled repairs and the added cost of a plant shutdown.
3.In an aggressive chemical environment, fiberglass can be used. There are, however, disadvantages to their use that require assessment. Fiberglass trays have a tendency to experience surface erosion until "fiber blooming" occurs. These also fade from the specified color if installed in an exposed site with high U.V. Fiberglass manufacturers state that data exists supporting the premise that fiber blooming does not significantly detract from its performance. It does, however, produce a poor surface appearance. It is possible to coat either HOG steel or aluminum as an alternative to fiberglass, but this requires a competent applicator to ensure that pinholes or surface imperfections are not present on the coating film. The coating must also be resilient to the specific chemicals present in the location.
4. The impact resistance and adhesion of the galvanizing and coating can extend the life of the cable tray as these are subject to high impact loads due to transportation, site handling, and installation. The impact loads are mainly limited to outer corners and edges, which are the most vulnerable areas. If the coating has poor impact resistance or adhesion then it may be damaged or removed, exposing the material substrate to attack. Thicker and softer coatings tend to suffer more from impact damage. SDIP and HDIP products have been specifically chosen to withstand this type of treatment.
5. When assessing the price of a cable tray installation, the life expectancy and cost of replacement should also be considered. A manufacturing installation with a life expectancy of 20 years or more should also use a cable system with a similar expectancy. Appendix 1 gives some indications of life expectancies of different substrates and coatings. These are very general as it is difficult to obtain accurate data in this context. Appendix 2 shows the comparative cost of various substrates and coatings. It also ranks the corrosion resistance of those permutations.
The specification stage for a cable management system will entail a number of choices. Appendix 3 provides the general criteria describing the benefits of each substrate and coating. For simplicity, the coatings have been left as generic or trade names, although it should be noted that there would be some variation between products within a category.
|COMPARISON OF DIFFERENT MATERIALS WITH LIFE EXPECTANCY|
|SUBSTRATE * Coated Aluminum * Coated HDG SteelAluminum HDG STEEL BLACK STEEL||INLAND L/E 20 - 25 Years 15 - 20 Years 15 - 20 Years 10 - 15 Years 7 - 12 Years 3 - 7 Years||COASTAL L/E 20 - 25 Years 10 - 20 Years 10 - 15 Years 8 - 12 Years 5 - 10 Years 0.5 - 3 Years|
|> L/E = Life Expectancy (Average, will depend on location) * = Dependennt on the Coating applied HDG = Hot Dip Galvanized|
|CORROSION RESISTANCE RANKING AND INCREASED COST OVER HDG|
|SUBSTRATE Aluminum HDG Steel HDG Steel HDG Steel Fiberglass Aluminum Aluminum HDG Steel HDG Steel HDG Steel||COATING SDIP HDIP SDIP EVA None Epoxy None Epoxy PVC None||RANKING 1 2 3 4 5 6 7 8 9 10||COST ABOVE HDG 67% 55% 50% 150% 150% 85% 25% 70% 150% Baseline|
|Each system is rated as a percentage higher than the cost of the baseline HDG. The following are generic or trade names: SDIP = Super Durable Industrial Polyester HDIP = Hybrid Durable Industrial Powder PVC = Poly Vinyl Chloride EVA = Ethylene Vinyl Acetate|
|COMPARISON OF BENEFITS FOR DIFFERENT MATERIAL COMBINATIONS|
|Aluminum||SDIP||Relatively cheap option to extend life of Aluminum Cable Tray with 15 year coating guarantee|
|HDG Steel||HDIP||Relatively cheap option to extend life of Galvanized Steel Tray. Guarantee under investigation. New to middle East.|
|HDG Steel||SDIP||Relatively cheap option to extend life of Galvanized Steel Tray. Five-year guarantee Already specified in Saudi Arabia|
|HDG Steel||EVA||Chemical resistant coating for specialized areas. Very expensive for large insttallations.|
|Fiberglass||NONE||Unknown life in this environment. Needs coating at regular intervals to mmaintain tidy surface appearance.|
|Aluminum||Epoxy||Not reccomended due to inability to offer adequate protection against U.V. and suffer chalking and premature loss of integrity.|
|Aluminum||None||Good product for inland areas but has a reduced life in coasttal or corrosive environments.|
|HDG Steel||Epoxy||Not recommended due to inability to offer adequate protection against U.V. and suffer chalking and premature loss of integrity|
|HDG Steel||PVC||Not recommended in NEMA VE1 due to the variability in the surface of galvanizing, creating quality problems.|
|HDG Steel||None||Good where there is no humidity. Limited life in corrosive areas since galvanizing is only a sacrificial layer, exposing bare steel.|
|N.B. The comment relating to relatively cheap is the add-on cost for cable tray to be removed from an installation before a declared downtime.|