Tags: Risk Based Inspection
In our fifth article in our series on providing process conditions for Risk Based Inspection, one of our senior corrosion specialists discusses several damage mechanisms that are often associated with equipment operating at low temperatures. As with the other topics of this series, we point out additional factors, beyond the standard inputs, that need to be considered when dealing with equipment in this class.
Low temperature damage is often associated with brittle fracture resulting from low toughness properties of a material. It can also refer to a wide range of wet corrosion damage mechanisms that occur at relatively lower temperatures in unit operations. Wet corrosion occurs when a liquid is present and usually involves an aqueous solution or electrolytes. Wet corrosion damage mechanisms include dew point corrosion, corrosion due to salt formation, sour water corrosion, corrosion by acids, and corrosion in various aqueous streams in facilities that treat and/or use water (e.g. raw water, cooling water, steam condensate, etc.). Part 5 of our series focuses on guidelines for providing process conditions for RBI for the wet corrosion damage mechanisms and how they influence corrosion potentials.
The typical process data that is collected and fed into RBI software models includes representative fluid, initial fluid phase, operating pressure, operating temperature, toxics present, and toxic content. This process data is generally considered to be sufficient for the consequence calculation when determining relative risk. Since the consequence values increase as the operating pressure, operating temperature, and toxic fluid content increases, the use of sustained high values for these process parameters is recommended for the first pass. The general rule for temperature is to have an accuracy within 25°F, estimating on the high side and the general rule for operating pressure is to have an accuracy within 10% estimating on the high side.
The process data that typically resides in commercially available RBI software does not directly impact the probability calculation. The impact is more complex and is dependent on how the process parameters affect the corrosion rates and damage mechanisms. In many cases the basic process information collected for the RBI software is not enough to determine the corrosion and damage potentials and additional information that was not captured during the routine RBI data collection stage is required. Additional data fields such as water present and pH may be available in the software but are rarely used since additional effort is required to collect the information and the values do not directly impact the risk calculation. Therefore, the additional process information required and impact on corrosion and other damage mechanisms is managed outside of the RBI software and typically addressed by a damage mechanism review (DMR).
The DMR process includes a systematic review of the process information and materials of construction along with available design, inspection, monitoring, and sampling data. Some of the process parameters required for the DMR and the impact these parameters have on corrosion rates and damage mechanisms is discussed below for some of the more common low temperature or wet corrosion damage mechanisms.
Dew Point Corrosion occurs when sulfur and chlorine species present in fossil fuels break down to form sulfur dioxide, sulfur trioxide and hydrogen chloride within the combustion products of flue gas streams. At low enough temperatures, the acid flue gases and the water vapor present in the flue gas condense to form sulfuric acid, sulfurous acid, and hydrochloric acid. The hydrochloric acid dew point temperature is typically below the water dew point (may be in the range of 130°F). However the sulfuric acid dew point can be significantly above the water dew point temperature. For example, at an SO3 content of 10 ppm and 10% moisture present, the water dew point temperature is 115°F, however the sulfuric acid dew point temperature is 280°F. Since corrosion rates are expected to be higher for metal surfaces that are cooled below the dew point temperature, acid gas concentration, water content, and metal temperature are critical process parameters required for the DMR.
Corrosion due to salt formation occurs when process temperature drops below the salt point in the presence of moisture. The most common issues are related with salts that are formed when amine or ammonia combine with H2S or chlorides to form amine and ammonium salts. The ammonium chloride and ammonium bisulfide salt deposition temperatures can be calculated using industry available curves if the concentration of ammonia, H2S, and chlorides present are known. However amine salt point curves depend on the types of amine present and proprietary models are required to predict amine salt deposition temperatures. In general, chloride salts tend to form at higher temperatures than salts formed by H2S. Depending on the amount of amine, ammonium, and chloride species that are present for the reaction, chloride salt points can be upwards of 300°F. Once the salt point temperature is determined, the information can be used to determine if a corrosive environment exists. Dry salts cause fouling, however wet salts are corrosive to carbon steel. The salts are hygroscopic and will absorb moisture from the environment, resulting in corrosion problems even if a free water phase is not present. Critical process data that is required for the DMR includes amine, ammonia, H2S, chloride, and moisture content along with process temperature; velocity can also play an important role in accelerating corrosion.
Sour water corrosion refers to corrosion due to water containing H2S. Other species such as CO2, ammonia, chlorides, or cyanides may also be present. The main process parameters required to determine corrosivity of sour water streams are the concentrations of H2S, ammonia, pH, and velocity. Additional process data such as chloride, cyanide, and CO2 content are also usually collected as they can enhance corrosion and influence environmental damage mechanisms such as Wet H2S and Carbonate Stress Corrosion Cracking.
Corrosion rates in acids depend on the type of acid present along with acid concentration, temperature, and contaminants. Therefore this information is required to determine corrosion rates. In many cases, very low corrosion rates are expected in acid service unless a free water phase is present and this is the case with sulfuric acid, hydrofluoric acid, phosphoric acid, and hydrogen chloride, among others. However the presence of water can cause severe corrosion in carbon steel materials containing these acids. In general corrosion rates in acid service tend to increase with increasing temperature as long as a free water phase is still present; therefore the acid concentration and temperature are important process parameters required to determine corrosivity in acid service.
Several different types of aqueous process streams may be present in facilities that treat and / or use water. This includes raw water, cooling water, and wastewater as well as demineralized water, boiler feed water, and steam condensate. While each stream has different properties, there are several common factors that influence corrosivity. Corrosion of carbon steel by oxygen and carbon dioxide is an electrochemical reaction and often results in localized pitting of a metal surface. The rate of reaction and severity depends on the level of dissolved gases, temperature, and pH. Deposits formed on the steel surface due to a presence of solids and low flowrates can cause oxygen concentration cells and lead to under deposit corrosion. Microbially Influenced Corrosion (MIC) may also develop under deposits and further accelerate pitting corrosion. The effectiveness of water treatment programs to maintain a neutral to alkaline pH is an important factor in managing corrosion. Process parameters that influence corrosion in various water streams are discussed below.
A wide range of wet corrosion damage mechanisms can be present in processing facilities. The impact that process data has on the probability calculation in commercially available RBI software is complex and dependent on how the information affects the corrosion rates and damage mechanisms. Therefore, the impact of process conditions on corrosion and other damage mechanisms is managed outside of commercially available RBI software and typically addressed by a DMR. In many cases the basic process data collected for the RBI effort is not enough to determine the corrosion and damage potentials and additional information is required for the DMR. This blog provides information on the critical process data required to perform a DMR for several low temperature or wet corrosion damage mechanisms and the impact of the process conditions on corrosion.
Stay tuned for the next entry in this eight-part series covering guidelines on assigning process conditions for RBI efforts:
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