INTRODUCTION
Material selection for corrosive chemical services should be based on life cycle cost for the plant equipment. Often corrosion resistant alloys and non-metallic materials have suitable corrosion resistance for a given application. Equipment design, fabrication, operating, maintenance and salvage expenses are essential elements in life cycle costs calculations. The risks associated with equipment operation, maintenance, inspections and salvage expenses are frequently neglected in calculating life cycle costs yet they often are the major portion of total life cycle costs. Risks arising from maintenance, inspection and salvage actions are different for equipment constructed of non-metallic materials of construction than those constructed of corrosion resistant alloys; these differences that affect life cycle costs are discussed.
Petrochemical industries, pulp & paper industries and petroleum production experiences indicates that the major component of life cycle costs for process equipment are the maintenance costs occurred after placing the equipment in service. Equipment constructed of highly corrosion resistant alloys, such as high alloy stainless steels, nickel based alloys and reactive metal generally are typically more costly to fabricate and install than equivalent equipment constructed of non-metallic materials. However material selection should not be based on initial installed cost but instead on life cycle costs. In general the maintenance, risk and salvage yearly expenditures are greater for non-metallic equipment compared to those for metallic equipment. A comparison of the likely life cycle costs occurred by both metallic and non-metallic equipment after installation will assist those selecting materials of construction.
The purpose of this paper is to highlight those maintenance and inspection practices required during the service lives of non-metallic process equipment that differ from such practices required for metallic equipment. These differences should be considered when choosing between various materials of construction using life cycle cost considerations. Depending upon service conditions and economics non-metallic equipment may or may not be preferred over equivalent metallic process equipment.
Life cycle costs include fabrication and installation cost plus those operating, maintenance, risk and salvage expenses likely to be experienced by equipment throughout its service. Included in life cycle costs are the maintenance and other outlays associated with loss of containment (leakage), loss of functionality and insuring equipment integrity. When all expenses are considered, equipment constructed of corrosion resistant alloys frequently is less expensive on a life cycle cost bases than equivalent equipment constructed of non-metallic materials even though the non-metallic equipment had a lower installed cost.
Non-metallic process equipment as described in this paper refers to fixed equipment (vessels, tanks and piping) that has a suitable corrosion resistant non-metallic surface exposed to the process. Non-metallic process equipment can be constructed of a homogenous nonmetallic material or can consist of a non-metallic corrosion resistant layer of suitable thickness supported on a non-metal or metallic structural frame. A metallic structural substrate is almost always carbon steel. Failure of the protective non-metallic coating or lining over a structural substrate in aggressive corrosive service can result in extremely rapid substrate corrosion leading to leakage. Fiber Reinforced Plastic (FRP) equipment as a rule consists of a FRP structural substrate with less corrosion resistance than the more corrosion resistance inner la
