This article is an effort to provide information about the types and structure of the process vessels used in petroleum industries, concepts for the use of cathodic protection and various factors involved in designing galvanic anode protection systems such as surface area for protection, coating breakdown factors, current density considerations, electrolyte resistivity, anode selection criteria, and design life considerations. The Cathodic Protection designing of process vessels is difficult and critical due to enclosed and extreme electrolyte conditions of non-ambient temperature, pressure, velocity, chemical compositions and hazards to the environment. Since these conditions cause adverse effects like high corrosion activity, cathodic protection depolarization and higher anode consumptions, it is important to take special care while designing. Another limitation is the cathodic protection system effectiveness cannot be monitored except for anode consumption rate, therefore it is difficult to evaluate the cathodic protection performance and life.
By Qurban A. Lashari, Corrosion/Cathodic Protection Engineer, & Muhammad A. Sherwani, Corrosion Engineer, Saudi Aramco
CP Concept & Design
Coating Breakdown Factors
Usually there are three different assumptions about the coating breakdown factors:
- Coating breakdown due to ineffective coating application.
- Coating breakdown during the installation phase due to mishandling.
- Coating breakdown due to coating degradation over time.
The first two assumptions can be minimized or almost eliminated by taking actions like proper coating application, inspections, and effective coating repairs after all mechanical works completion.
The third assumption is the potential factor that cannot be controlled. Hence, some possibilities to be considered include the percentage as bare surface area or by adding some allowances in design current density.
There are no specific values determined by any industry standards or specifications, therefore considerations are made based on observation and studies during testing and inspection (T&I) by the users/owners or based on the experiences of CP design engineers. An effective method to estimate this value has been specified in Det Norseky Veritas (DNV) standard RP-B-401.
Current Density Consideration
This is the most important factor through which the structure current demand is calculated, and which determines the re- quired anode quantities; therefore, the approach should be as realistic as possible in determining the values. Many standards and specifi cations exist which specify the current density values depending on their tests, research, and past experiences.
The current density required to protect the structure is very much dependent on the following factors:
- Type of coating and its condition.
- The operating temperature of the vessel.
- The internal fluid movement based on vessel function.
- Oxygen concentration inside the vessel.
- Acidity of internal fluid (presence of H2S).
- Internal fittings & coalescing packing materials.
- Variety of metallic structures.
An increase of these factors causes depolarization of the structure (cathode), which increases the current density requirement for protection, hence these factors should be accounted for during the design stage for current requirement calculations:
The coating condition in new vessels compared to old vessels is better, hence the low current density consideration with provision of expected coating degradation up to the design life.
The operating temperature of the vessels is sometimes high. Since most of the represented data about current densities is based on ambient conditions, the current density should be either considered or calculated with temperature rise effects.
The hydrocyclones vessels are continuously subjected to internal fluid cyclic movements; therefore, some provision should be added in current density values.
The oxygen concentration inside the vessels is increased due to the water flow or turbulence; therefore, all those vessels which handle water in large amounts require consideration of this factor as well.
Low pH value fluids will require additional consideration in current density values. Sour crude and gas require the removal of acid gas (H2S) for which amine solutions are used. The amine solutions themselves are alkaline but become contaminated with acid during the sweetening process. Therefore, vessels handling or processing sour fluids and amine recovery units will require consideration for the extra safe value.
The acidic conditions in process vessels will also have an adverse effect on galvanic anodes by increasing their consumption rates, which is further discussed below in Anode Selection Criteria.
National Association of Corrosion Engineers (NACE) has specified in standard SP0575-2007, the current density consideration range i.e. from 50 to 400mA/m2. These values contain considerations for depolarized state due to factors like H2S, oxygen, high operating temperatures and high flow rates.
Electrolyte Resistivity
The electrolyte resistivity for the consideration of cathodic protection system designing is related to the accumulated water resistivity, since the corrosion characteristics are mostly associated with water. This relation is inversely proportional i.e., water of low electrical resistivity tends to be more corrosive, while water with high resistivity tend to be less corrosive.
Several factors influence water resistivity, but most important are concentration of ions in the water, mobility of the ions in the water, acid forming components (H2S and CO2), and temperature of the water.
Since all the above-discussed processes are associated with either removal of water, lowering the salt concentration, treating the water, and using it as a utility during different process, the resistivity is therefore expected to be different in each phase. This needs to be determined properly and used for the design calculations.
Based on personal experience of last 10 years, working on vessel internal CP designs in GCC countries, following are observations related to accumulated water resistivity values:
- The unprocessed well stream fluid contains water with resistivity less than 100 ohm-cm.
- The processed crude oil contains water with resistivity greater than 1,000 ohm-cm.
- The unprocessed natural gas con- tains water with resistivity greater than 1,000 ohm-cm.
Anode Selection Criteria
The galvanic anodes (magnesium, zinc, and aluminum) have been developed in alloys to remain active and to extend life, since the pure form always has the problem to undergo self corrosion and cannot stay active. The efficiency and consumption rate of anodes depend on alloys of anodes and the environment to which it is exposed.
Therefore, for the application of vessel internal special type of anode alloys are used, Aluminum Alloy-III: This anode is alloyed with the addition of indium, most suitable for seawater, brackish water, and mud saline. Since in crude separation process the vessel contained similar environment, therefore these anodes are most suitable for this application. The efficiency and consumption rate of this anode is very much effective with rise in temperature. Its standard potential and consumption rate are based on 250 Cº temperature. As the temperature increases, the consumption rate of the anode also increases.
Zinc ASTM B-418-73 Type-1 and Mil Spec A-18001K: These anodes are also suitable for seawater and brackish water. All zinc anodes have the limitation to undergo intergranular corrosion at temperature above than 500 Cº. Therefore, these anodes can be used only in those vessels, which operate less than the specified temperature.
High Temperature Zinc Anode: This is the special zinc alloy anode with addition of 0.1 to 0.25% of Aluminum, to work at elevated temperature without subjecting to inter-granular corrosion problems. Therefore, for vessels operating temperature in excess of temperature above 500C to 800C, this anode shall be used.
The acidic environments accelerate the anode consumption rates by depolarizing the anode potentials. Therefore, the fast consumption of anodes leads to shorten the required design life of anodes.
Design Life Consideration
The required design life of the anodes generally referenced from the client’s requirements and specification, which are based on their testing and inspection (T&I) and maintenance programs.
The anode design life calculation is mainly dependent on anode current outputs. The size and dimensions of the anodes control the anode current output in the given environments. Sometimes if the anode size with reference to its weight requirements for design life do not match under such circumstances the anode current output shall be controlled by applying coating over some part of the anode.
Anode Installation Methods
Anode Arrangement & Installation
The most typical method for the installation of anodes inside the vessels is to fi x anodes over the welded supports on vessel internal walls. These supports are either brackets, flat headed bolts or internally threaded small pipe (boss). The anodes with mounting straps installed over the brackets or bolts, and anodes with core pipe installed over the boss. Both installations are shown in Figure 2 and 3.
The anode positioning and location is very important for uniform current distribution inside over the vessel internal surface that needs protection. Hence following points need to be considered while designing:
- The total designed or proposed quantity of anodes shall be distributed evenly over the surface required for protection.
- The vessels with the compartments must have at least one anode installed in every compartment.
- The anode should be located as near to the center of compartment as practical.
- Ensure that anodes remain submerged in the electrolyte.
The vertical water vessels containing no internal components and compartments can provided with anode strings suspended from top head. This arrangement provides better current distribution because anodes are parallel to the walls and deteriorated anodes can be replaced without lowering the water level or draining the vessel. Refer to figure 4 for installation details.
Anode Monitoring System
The anode monitoring system is a very effective method to monitor the anode current outputs inside the vessel, through this the anode performance with respect to the exposed environment can be monitored, and the anode life can be estimated.
For this purpose, dedicated nozzles are provided in the vessel, and a test station is installed with current measuring shunt outside the vessel. The position of anode monitoring facility is critical and normally at the end of vessel, but in some cases additional monitoring facilities are required. Refer to figure 5 for installation details.
For monitoring the anode should be electrically isolated from vessel. Therefore, the dedicated nozzle plate should be provided with proper insulation gasket and confirmed through the testing.
Conclusions
The CP designing of process vessels is difficult and critical due to enclosed and extreme electrolyte conditions of non-ambient temperature, pressure, velocity, chemical compositions and immune to environment. Since these conditions causes adverse effects like high corrosion activity, CP depolarization and higher anode consumptions. It is important to take special care while designing. Another limitation is the CP system effectiveness cannot be monitored except for anode consumption rate, therefore difficult to evaluate the CP performance and life.
This paper discussed all possible aspects which can contribute in vessel internal CP system designing. Every discussed factor should be well evaluated during design stage, so that the estimation can be effective for every possible worst condition and match the life requirements.
The periodic testing and inspection (T&I) of process vessel is an effective method of observing the designed CP system effectiveness and deficiencies. This gives the complete track record and feedback about the corrosion, erosion, metal loss and anode consumptions. This database can be an effective tool for the CP system designing of those new vessels which handle electrolyte like other old vessels as well as redesigning of the old ones.
References:
- NACE SP 0575-2007 “Internal Cathodic Protection Systems in Oil-Treating Vessels”.
- DNV Standard RP-B401 (2005) “Cathodic Protection Design”.
- Havard Devold “Oil and Gas Production Handbook”.
- K. Abdel-Aal and Mohmed Aggour “Petroleum and Gas Field Processing”.
- Somnath Chattopadhyay “Pressure Vessels Design and Practice”
