INTRODUCTION
This paper provides an introduction on carbonate cracking and Los Angeles refinery?s recent experience with this corrosion mechanism. Also included in this paper is the methodology used for developing a risk assessment for equipment and piping susceptible to carbonate stress corrosion cracking in a fluid catalytic cracking unit (FCCU) gas plant. The results of the risk assessment were used to identify which equipment and piping should be replaced during the 2003 Los Angeles refinery FCCU turnaround.
In petroleum refining, carbonate stress corrosion cracking (SCC) can occur in fluid catalytic cracking (FCCU) gas plants where the process environment contains a significant amount of carbonate/bicarbonate ions, H2S, free water and ammonia. The equipment most likely to be affected in the FCCU gas plant are the main fractionator?s overhead condensers, fractionator overhead accumulator, wet gas compressor knockout drums and condenser, deethanizer (or other similar light end fractionators) and all associated piping including the sour water streams originating from these areas.
Typically carbonate SCC occurs in non-stressed relieved carbon steel piping/equipment around areas with high levels of residual stresses and in alkaline sour waters. Specifically the conditions are pH?s greater than or equal to 8.5 with a carbonate concentration of 1000 ppm or greater. Carbonate cracks have been found to be mostly intergranular and oxide-filled with cracks having lattice like appearance of very fine tight cracks that usually grow parallel to the weld in residual stress zones of soft base metal, but can also occur in the heat-affected zones of the metal. (See Figures 1, 2 & 3) Several studies have determined that carbonate cracking occurs in a very narrow range of electrochemical potential, which is dependent upon the chemical composition of the sour water. (3,4,5) When free water is present in the process stream, four key elements can be used to assess the susceptibility of carbon steel piping/equipment to carbonate cracking. The four elements are:
1. The pH of the sour water (due to ammonia).
2. The carbonate concentration of the sour water.
3. The electrochemical potential of CS exposed to the sour water.
4. The residual stresses of the piping/equipment. While all four elements can be monitored to determine susceptibility, monitoring the pH of the sour water is considered to be the fastest, most practical and cost effective method to locate areas that are exposed to conditions that can promote carbonate cracking.
It has been determined that post weld heat treatment (PWHT) of carbon steel piping and equipment will reduce the probability of carbonate SCC. However, the standard PWHT procedures used in the refining industry (e.g. 625°C/1150°F) may not be totally effective in preventing carbonate cracking. There have been several cases of cracking occurring in another refinery in PWHT?d piping that was installed as a replacement for pipe that cracked. The company?s internal experience suggested that a higher heat treatment temperature is more effective in preventing carbonate cracking. There is little historic information available on this higher heat treatment temperature, but the Los Angeles refinery?s limited information indicates that it is a successful method to mitigate this type of cracking.
THE REFINERY?S EXPERIENCE WITH CARBONATE CRACKING
In a span of 7 months (June to December 2002) the Los Angeles Refinery experienced 23 leaks in piping located in the Fractionator Overhead and the Wet Gas Compression Sections of the FCCU. Follow-up with intensive Ultrasonic (UT) shear wave inspection locate
