Online corrosion monitoring

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in the oil and gas industry

Measurement & Instrumentation

Online corrosion monitoring
Online corrosion monitoring

Real-time corrosion monitoring replaces conventional lab-based analysis, allowing plant operators to react to corrosion before it can lead to far reaching consequences, providing an effective means for cost reductions for example in the refining process in the Oil- and Gas industry.

Petroleum refining begins with the desalting (dehydration) of feedstock followed by distillation, or fractionation, of crude oils into separate hydrocarbon groups. The resultant products are directly related to the characteristics of the crude oil processed. Most distillation products are further converted into more usable products by changing the size and structure of the hydrocarbon molecules through cracking, reforming, and other conversion processes these converted products are then subjected to various treatment and separation processes, such as extraction, hydrotreating, and sweetening to remove undesirable constituents and improve product quality. Integrated refineries incorporate fractionation, conversion, treatment, and blending operations, and may also include petrochemical processing.

Crude oil often contains water, inorganic salts, suspended solids, and water-soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning the catalysts in processing units, these contaminants must be removed by desalting (dehydration). The two most typical methods of crude oil desalting – chemical and electrostatic separation – use hot water as the extraction agent. In chemical desalting, water and chemical surfactants (demulsifiers) are added to the crude and heated so that salts and other impurities dissolve into the water or attach to the water, and are then held in a tank where they settle out. Electrical desalting is the application of high-voltage electrostatic charges to concentrate-suspended water globules in the bottom of the settling tank. Surfactants are added only when the crude has a large amount of suspended solids. Both methods of desalting are continuous. A third and less common process involves filtering heated crude using diatomaceous earth.

After desalting, crude oil is continuously drawn from the top of the settling tanks and sent to the crude distillation (fractionating) tower. Fractionation (distillation) is the separation of crude oil in atmospheric and vacuum distillation towers into groups of hydrocarbon compounds of differing boiling-point ranges called fractions or cuts. Conversion processes change the size and/or structure of hydrocarbon molecules. These processes include decomposition (dividing) by thermal and catalytic cracking, unification (combining) through alkylation and polymerization, and alteration (rearranging) with isomerization and catalytic reforming.

Treatment processes are intended to prepare hydrocarbon streams for additional processing and to prepare finished products. Treatment may include the removal or separation of aromatics and naphthenes, as well as impurities and undesirable contaminants. Treatment may involve chemical or physical separation such as dissolving, absorption, or precipitation using a variety and combination of processes, including desalting, drying, hydrodesulfurizing, solvent refining, sweetening, solvent extraction, and solvent dewaxing.

Formulating and blending is the process of mixing and combining hydrocarbon fractions, additives, and other components to produce finished products with specific performance properties.

Other refinery operations include light-end recovery, sour-water stripping, solid waste and wastewater treatment, process-water treatment and cooling, storage and handling, product movement, hydrogen production, acid and tail-gas treatment, and sulfur recovery. Auxiliary operations and facilities include steam and power generation; process and fire water systems; flares and relief systems; furnaces and heaters; pumps and valves; supply of steam, air, nitrogen, and other plant gases; alarms and sensors; noise and pollution controls; sampling, testing, and inspecting; and laboratory, control room, maintenance, and administrative facilities.

Using the 4…20 mA HART signal of the CorrTran MV transmitter, plant operators are able to compare historical corrosion data with up-to-date measurements. In this way, they can determine instantly whether the Oil / Water quality has changed, whether there are changes in the chemical setup of the Oil/Gas or whether the corrosion inhibitors perform correctly. All these conditions have an effect on the corrosion of the piping and can be detected and monitored efficiently with help of CorrTran MV. Following the principles of proactive maintenance, the plant operator is able to prepare and schedule the exchange of components affected by corrosion before the effect of corrosion leads to costly damage. The performance of CorrTran MV assists the operator in monitoring both general and localized corrosion. Especially local corrosion can lead to severe damage if it is not detected at an early stage. This type of corrosion is able to actually puncture a pipe within a short time. Yet, it can be counteracted effectively if corrective measures are taken before it is too late. In other words: CorrTran MV makes it possible to monitor and control corrosion just like any other process parameter.

At the core of CorrTran MV are modern and patented algorithms and data analysis techniques to provide exact measurement of corrosion rate and local corrosion (pitting). To measure the general corrosion rate, the system determines the linear polarization resistance (LPR), thereby using a generally accepted industry standard. This method is further optimized by an additional harmonic distortion analysis (HDA). During the measuring cycle, the corrosion detector also performs an electrochemical noise (ECN) measurement which allows a dependable determination of pitting. At the end of each measuring cycle, the respective corrosion rate and the pitting value are calculated and provided via HART and/or as a 4…20 mA signal.

CorrTran MV combines both procedures in order to gain dependable and fast measuring results. This includes determination of the B value. In order to further increase precision, conductivity measurements are also included into the calculation. The resulting value provides additional valuable information concerning the state of the electrodes. Additionally, the ECN method is used to measure the intensity of local corrosion. ECN describes the measurement of spontaneous potential fluctuations which are generated randomly at the corroding interface between metal and solution. Statistic analysis of the measured current allows the determination of a pitting factor which is an indicator of the speed and intensity of local corrosion.

The standard probes used by CorrTran MV for corrosion detection, consist of three electrodes. One of them induces a low-power signal, while the others measure the resulting potential and current. In order to gain precise measuring results, these electrodes need to be made of the same material as the piping or container to be monitored. The victim electrodes are placed immediately within the flow of the corrosive media and are induced with a weak signal. Within only a few minutes, this signal is monitored and analyzed by the transmitter in order to gain an exact impression of existing corrosion. As a result, service technicians are provided with the needed information to schedule repair and service work according to the actual need, putting them into a position to react before corrosion has gone too far and degrades the ongoing process. In this way, CorrTranTM MV not only contributes to saving time and costs. It also is the basis for proactive maintenance and makes corrosion monitoring part of the daily routine in the Oil and Gas Industry.

Posted on November 8, 2012 - (1171 views)
Pepperl + Fuchs SE
Lilienthalstrasse 200
68307 Mannheim - Germany
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Pepperl+Fuchs is a leading developer and manufacturer of electronic sensors and components for the global automation market. Continuous innovation, enduring quality, and steady growth guarantees continued success, since more than 60 years. The Process Automation Division is the undisputed market leader in intrinsically safe explosion protection components and protection of hazardous area applications. Our products enable you to combine a wide range of electronic equipment into complete, application-oriented system solutions.

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