This paper 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, as well as 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, critical, and hazardous to the environment due to enclosed and extreme electrolyte conditions of non-ambient temperature, pressure, velocity, and chemical compositions. As these conditions cause adverse effects like high corrosion activity, cathodic protection depolarization, and higher anode consumption, it is important to take special care in design. Another limitation is how the cathodic protection system’s effectiveness cannot be monitored apart from the anode consumption rate. It is therefore difficult to evaluate the cathodic protection performance and life.
By Qurban A. Lashari, Corrosion/Cathodic Protection Engineer, & Muhammad A. Sherwani, Corrosion Engineer, Saudi Aramco
Background
In petroleum industries, the extraction of crude oils goes through various processes to separate the residual impurities (gasses, water, and sludge) and then further procedures to establish the different grades of extracted oil. During these processes, different types of vessels are used that must be protected as their internal surfaces are exposed to extremely corrosive environments.
Although these vessels are protected through internal linings as a first line of defense from corrosion, the application of cathodic protection is still necessary as a secondary protection method.
These linings may not be 100% effective and the possibility of degradation with the passage of time always exists.
About Process Vessels
A process vessel is a pressure container used to complete the production process, such as separating, combining, or breaking of a product or substance. It is a container with an internal differential pressure or temperature compared to the outside and designed to contain products/ substances that cannot be stored in an open atmosphere due to their highly reactive, toxic, and flammable nature.
There are two types of process vessels: those with and without internal com ponents. The first category, referred to as vessels, is provided with substantial internal components for processing like separators, dehydrators, desalters, and hydrocyclones. The second category, referred to as drums, has no internal components, and is used as intermediate storage or surge containment of a process stream like knockout drums, seal drums, surge drums, flash drums, feed drums, and drain drums.
About Well Streams
The well-fluid stream generally constitutes the mixture of crude oil, natural gas, and saltwater. These mixtures are very difficult to handle, transport, and store. Therefore, the first step in processing is the well fluid stream separation into separate streams (oil, gas, and water). This process takes place in separator vessels, also called high-pressure production trap (HPPT), intermediate pressure production trap (IPPT), and low-pressure production trap (LPPT) or somewhere as an HP production separator and LP production separator.
The saltwater in oil creates serious corrosion and scaling problems in transportation and refining operations. Therefore, it is necessary to remove water up to the acceptable limits: less than 0.5%-1% of the total volume. The oil leaving the separator may still contain between 10% and 15% of emulsified water, which requires further treatments like dehydration.
The presence of salt content in the leftover water makes the fluid more corrosive and hence needs to be minimized to acceptable limits; between 28.5 and 42.5 ppm. The treatment required for this purpose is desalting which is similar to dehydration with the addition of wash water to lower the salt concentration.
Some crude oils contain hydrogen sulfide (H2S) and other sulfur products. When it contains more than 400 ppm of H2S, it is classified as a sour crude. Sour crude oil presents serious safety and corrosion problems. The treatment to remove the H2S or reduce the sulfur to an acceptable limit is a dual process of stabilization and sweetening.
The separated natural gas generally contains undesirable components such as H2S, CO2, N2, and water vapors. Field processing of natural gas requires removing or reducing these elements to an acceptable limit through the gas-sweetening process.
Water vapor is also the main impurity in natural gas. It is not unacceptable in the vapor phase; however, it becomes critical in liquid or solid phases when gas is compressed or cooled, and subsequently condensates. This liquid phase of water causes problems in pipelines, such as accelerating corrosion and reducing capacity by accumulating in low-level regions. Glycol dehydration is the pre-treatment to remove the water vapors prior to compressing, storing, or transporting.
The produced water collected from the separation, dehydration, and desalting processes still contains hydrocarbons in the form of droplets and cannot be utilized or disposed of directly. The primary treatment of the produced water is to remove oil droplets through de-oiling units and skimmers.
This produced water is further treated to remove dissolved solids, sulfates, nitrates, contaminants, and scaling agents. This water is contained in different water drums used for multiple utility purposes such as washing, cleaning, or other maintenance reasons.
Sometimes sand and solid particles are also produced with well-stream fluid. This causes many serious problems in the production, processing, and transportation of crude oil, such as erosion, corrosion, accumulation, and equipment reliability and instrumentation errors. Therefore, the effective control, safe removal, and environmentally friendly disposal of these solids are very important. The separation of solids from liquids can be accomplished in de-sanders, which may be situated in multiphase oil, produced water, and sand jetting streams.
All drained hydrocarbon liquids from pressurized vessels are collected into the closed drain drums. The liquid in this vessel contained dissolved gases that can flash and become hazardous if not handled properly. After removing all hazardous gases safely from drained liquids, it is collected into the slop oil drum for recovery and safe disposal.
Similarly, all separated liquids and solids from different processes, as well as from the fluid transportation system, are gathered into one central unit called the Slug Catcher. These collected materials are then cleaned to remove absorbed contaminants and chemicals in order to comply with environmental and disposal regulations.
A flare or vent disposal system collects and discharges gases from pressurized process components to the atmosphere or to a safe location for final release during normal operations and abnormal conditions (emergency relief). In a vent system, the exiting gas is dispersed into the atmosphere, however in a flare system, the gas exits by burning due to its toxicity or flammable nature. The gases fed to the flare system are from Knockout Drums. A Water Seal Drum is provided at the flare header to provide a positive seal to isolate the flare, which is an ignition source from the header to the process unit.
There are some chemicals used in a closed-loop system during the processing of crude oil and natural gas, which contaminate gradually. Continuous recovery of these chemicals is required for smooth and cost-effective operations. Examples of these processes include glycol and amine recovery.
Further to the above petroleum process vessels, some storage drums are used between units to dampen fluctuations in flow rate, composition, or temperature. They allow one unit to be shut down for maintenance without shutting down the entire plant.
Flash evaporation is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling device. This process takes place in the vessel which is known as the flash drum.
Accumulators are used to store water under pressure and to take up expansion or contraction within the hot water system. These vessels are charged with water from a pump system and can deliver an extremely high flow rate until empty.
CP Concept & Design
Structural Details
Generally, the process vessels are composed of three main parts: shells, heads, and nozzles.
The shell is the primary component that contains pressure. Pressure vessel shells are welded together to form a structure that has a common rotational axis. Most pressure vessel shells are cylindrical, spherical, or conical in shape.
All pressure vessel sheets must be closed at the ends by heads. Heads are typically curved rather than flat. Curved configurations are stronger and allow the heads to be thinner, lighter, and less expensive than flat heads.
A nozzle is a cylindrical component that penetrates the shell or heads of a pressure vessel. The nozzle ends are typically flanged to allow for the necessary connections and to permit easy disassembly for maintenance or access. Nozzles are used for the following applications:
- To attach piping for flow into or out of the vessel.
- To attach instrument connections, (e.g. level gauges, thermowells, or pressure gauges).
- To provide access to the vessel interior at manways.
- To provide for direct attachment of other equipment items (e.g. heat exchanger or mixer).
Nozzles are also sometimes extended into the vessel interior for some applications, such as for inlet flow distribution or to permit the entry of thermowells.
The main factors influencing material selection for vessels are strength, corrosion resistance, hydrogen attack resistance, fracture toughness, and fabricability. Generally, process vessels are composed of three main parts: shells, heads, and nozzles. Their attachments are constructed with the following materials grades:
Design Overview
The process vessels are typically designed in accordance with the American Society of Mechanical Engineers (ASME) Code Section VIII. This section is further divided into three divisions:
- Division 1: Applies for pressures greater than 15 psig and lower than 3,000 psig.
- Division 2: Identical to Division-1, except for a few conditions related to stress, design, quality, fabrication, and inspection.
3. Division 3: Applies for pressures greater than 10,000 psig
Division-1 is used most often as it contains enough requirements for most pressure vessel applications.
The mechanical design of a pressure vessel begins with the specification of design pressure and temperature. Pressure imposes loads that must be withstood by the individual components. Temperature affects material strength and allowable stress. The design conditions are the most severe conditions of coincident pressure and temperature that are expected during normal service.
Operating conditions are associated with the normal use of process vessels. The operating pressure and temperature are based on the maximum internal or external pressure and temperature that the vessel may encounter.
The first line of defense for any structure’s protection from corrosion is the coatings. Similarly, the vessel’s internal surfaces and components are electrically continuous with the vessel body and should be treated with an effective coating system.
The selection of coating type is dependent on the following factors:
- Compatible with extreme temperature requirements.
- Compatible with extreme pH levels.
- Compatible with immersion, wet/ dry cycles.
- Resistant to service environment.
- Meets applicable regulatory requirements.
- Effective protection for the required duration.
- Compatible with substrate.
Surface Area Consideration
The cathodic protection for the process vessels is only required in areas where water is expected to be accumulated in its liquid phase. Hydrocarbon fluids not only contain free water but also emulsified water in the form of water droplets. These droplets also cause corrosion when they break upon hitting the vessel walls. Therefore, for protection, it is safe to consider the surface area up to the highest liquid level.
In natural gas processing, the wet gases contain water in the vapor phase, which condenses and settles in the bottom of the vessels. Thus, the cathodic protection requirement is limited to the portions where water accumulates. In produced water treatment process as well as other vessels that contain water as a utility for main crude and gas treatments, the cathodic protection system requirement consideration should be for the entire surface area of the vessels.
Since all the vessels contain internal components there should be a consideration of the protection of the additional surface area. This is generally considered by taking a safe percentage over the calculated surface area of the vessel body. This can be assessed by knowing the vessel type and function.
Part Two of this article can be found in the August issue, to further explore how to prevent corrosion.
