Environmental aspects of metal corrosion at oil and gas chemical enterprises

. Metal corrosion of technological equipment not only has a significant impact on industry, but also has serious consequences for the environment, being one of the causes of environmental problems. The object of research in this paper is the issue of corrosion damage to the metal of technological equipment, types of corrosion and corrosion damage, and methods of protection against corrosion. On the example of a shell and tube heat exchanger with the help of nondestructive methods of control (external and internal inspection, ultrasonic thickness measurement) the research of corrosion rate of its main elements has been carried out. According to the results of researches the calculation of corrosion rate of the main elements of the heat exchanger was carried out. The corrosion rate for the heat exchanger shell was found to be 0.37 mm/year, which exceeds the value of 0.1 mm/year, which is accepted in the design of this heat exchanger. The obtained results allowed to identify the most vulnerable places of the research object and to choose the most effective ways to protect the base metal of the heat exchanger from corrosion.


Introduction
Currently, the technosphere is becoming the main source of danger.Significant growth of production volumes, intensification of existing technological processes, location of potentially hazardous objects in close proximity to places of human habitation, use of explosion-and fire-hazardous environments, violations of industrial safety requirements lead to the fact that emerging accidents can be catastrophic, affecting not only the environment, but also humans [1,2].Therefore, in the last decade the problem of reducing the number and scale of man-made accidents is one of the priority tasks of society [3,4].
The problems of industrial safety are most acute at the enterprises of oil and gas chemical complex.At the enterprises of oil and gas chemical complex nowadays a large number of different types of technological equipment is operated under difficult conditions and operating modes [5][6][7][8][9][10].
Operation of worn-out equipment, poor quality of repair and maintenance of equipment, violation of operation mode, corrosion and erosion of metal, product flow through the detachable joints of process equipment, low technological and production discipline, inefficiency of existing industrial safety management systems can lead to degradation of process equipment materials and, consequently, to an increase in the number of accidents at hazardous production facilities.
Degradation is a process that occurs over a long period of time in metal and is expressed in the inability of equipment to resist the impact on it of various conditions and modes of operation, loads, leading ultimately to a decrease in strength and terms of residual life of equipment.

Materials and methods
The main type of technological equipment degradation processes at the enterprises of oil and gas chemical complex is corrosion [1,11,12].
The corrosion process is a chemical interaction of metal with substances in the environment, which leads to its destruction and formation of oxides and salts.As a result, metal corrosion leads to a reduction in the strength and durability of metal equipment and structures.Corroded equipment cannot withstand design loads, which can lead to destruction and deterioration of the mechanical properties of the equipment, resulting in leaks of hazardous substances.Such leaks pose a serious threat to the surrounding ecosystem as they cause accidents and incidents.
Metal damage is only one aspect of this problem.When metals corrode, various chemicals such as oxides, salts, dissolved metals and other wastes are released into the environment, which can have a negative impact on soil, water and air.
Corrosion of metals has a significant impact on the environment and is one of the causes of environmental problems, leading to pollution of ground and surface water, which negatively affects living organisms and ecosystems.Also, waste emissions from corrosion can lead to atmospheric pollution and the formation of harmful substances such as nitrogen and sulphur oxides, affecting air quality and human health.
Another negative aspect of metal corrosion is the huge economic loss that is caused to industry.Corroded equipment and structures are subject to replacement and restoration of damaged areas and elements, which requires expenditures for purchase of new materials, repair works, stoppage and unplanned downtime of production processes of oil and gas chemical complex enterprises, reduction of service life of technological equipment.
As an example, we can recall the 2020 disaster in Norilsk at the thermal power plant, when 20,000 tonnes of diesel fuel leaked and spilled through the tank wall as a result of corrosion.The norm of pollutants in local rivers and water bodies was exceeded tens of thousands of times.About 6,000 tonnes went into the ground and poisoned underground water, some of the toxic substances reached the Kara Sea.
One of the main measures to protect against metal corrosion is the use of protective coatings, which create a barrier between the metal and the aggressive environment, preventing the penetration of moisture and corrosive substances.One of the common types of protective coatings are painting, electroplating, heat treatment, application of anticorrosion coatings and use of special lubricants and impregnations [13,14,15,16].
The limiting condition in the operation of process equipment at the enterprises of oil and gas chemical complex, exposed to corrosive influence, is the reduction of its wall thickness to the rejection value, below which the operation of equipment elements is prohibited due to the fact that the necessary reserve of its load-bearing capacity is not provided.
As it has already been noted corrosion damage, leading to the destruction of process equipment, is caused by the combined effect on the metal of aggressive, fire-explosive environments and formed corrosion deposits during the operation of equipment.
Based on the above, the development of methods to prevent corrosion and manage its consequences becomes an important task in the field of environmental protection.In this regard, there are tasks to determine the corrosion rate of technological equipment and analyse the influence of the environment on its distribution not only at the end of the service life of the equipment during the period of industrial safety expertise, but also during its operation [1,8,12,17,18,19].
The issues of corrosion rate determination arise during the industrial safety expertise in terms of predicting this rate when assigning the further term of safe operation of technological equipment.
As an object of research in this work was chosen shell-and-tube heat exchanger, operating at one of the chemical enterprises since 1981.The material of the main elements of the heat exchanger is St3sp5.
Due to the peculiarities of heat exchangers design it is not always possible to remove pipe grids for internal inspection of the casing.Therefore, in this case, the main methods of determining the corrosion rate of the main elements of the equipment during operation are not only external and internal inspection (visual-measuring control), but also ultrasonic thickness measurement as part of technical inspection and technical diagnostics [18,20,21,22].The period of research on the corrosion rate of the heat exchanger was chosen from 2018 to 2020.
In order to obtain the necessary information before conducting field surveys, the operational and technical documentation of the vessel was analysed, namely:  about the presence of the passport of the established form;  dates of manufacture, installation, commissioning of the heat exchanger;  the name of the manufacturing plant, information on the main structural and technical parameters, material execution of the vessel elements (steel grade, chemical composition, mechanical properties, etc.);  on the scope, methods of non-destructive or destructive control during manufacture and in the process of operation of the vessel;  documents confirming the manufacture of the vessel in accordance with the requirements of regulatory documents (certificates, test reports, etc.);  about deviations from the regulations that occurred during the operation of the vessel (violations of operational modes, accidents, etc.);  on previously conducted inspections and repairs (reason, when they were carried out, defect characteristics, repair technology, etc.).The results of the analysis of technical documentation are taken into account when determining the need, scope and specific areas, zones of external and internal inspection, ultrasonic thickness measurement.
External and internal inspection is a non-destructive method of control, carried out with the naked eye or with the use of simple tools (magnifying glasses of various classes, etc.).This method is mandatory, perfectly fits into the field conditions, does not need consumables and expensive equipment.
External and internal inspection of the research object was carried out to determine the type of corrosion damage, their size and localisation areas, to identify defects that may have occurred during the operation of the vessel.
The welded joints of the heat exchanger were inspected to identify the following defects in them:  a) cracks due to corrosion cracking;  b) thinning of the original thicknesses of the main elements due to general corrosion;  c) localised corrosion lesions in the form of ulcers and sinks, which are stress concentrators and possible sources of crack initiation;  d) technological defects, not detected during the manufacture of the vessel, developed during operation to dangerous dimensions;  e) manufacturing defects such as under-valving, displacement of edges of jointed elements, etc., as well as defects that may have occurred during transport and installation (dents and bulges).The main attention was paid to the areas of the vessel where defects may occur: -zones of stress concentration -places of welding of fittings and supports, necks of fittings to the body, zones of inlet and outlet fittings, etc.;  shell welds, bottom welds and, above all, weld intersections;  the lower part of the shell, which is most susceptible to general corrosion;  ulcerated corrosion areas;  areas subject to repair.
According to the results of external and internal inspection (visual-measuring control) of the heat exchanger except for corrosion, no other defects were found.
Ultrasonic thickness measurement is carried out in order to determine the quantitative characteristics of thinning (reduction) of the walls of the vessel elements in the process of its operation.According to the results of ultrasonic thickness measurement determined the rate of corrosion or corrosion-erosion wear of the walls and establish strength calculations allowable, safe life of the worn elements, the level of reduction of operating parameters.

Results and discussion
To measure the thickness of metal ultrasonic thickness gauges are used, according to the requirements of the current regulatory and technical documentation and providing measurement error of no more than ±0.1 mm.Ultrasonic thickness measurement of the main elements of the investigated apparatus was carried out according to the standard method both from the external and internal surface of the vessel.Measurements were carried out on 4 shell formations and bottom radii through 90 0 along the circumference of the element.The vessel shell consists of several kingdoms, so the measurements were carried out on each of them.Thickness measurement of sockets was carried out if their diameter is larger than the diameter of the bores requiring reinforcement in accordance with the normative-technical documentation.The number of measurement points should be sufficient to determine the actual condition of the vessel, namely:  on the shell halves not less than 3 on each of the formers (on the edges and in its middle part), i.e. 12 points on each halves;  on bottoms not less than 5 (one point on the bottom pole and on each of the 4 radii at the points of conjunction of the cylindrical and convex parts);  on necks of fittings not less than 4 (through 90 0 on the circumference of the neck, spigot).When performing thickness measurement of the vessel shell inaccessible for internal inspection, the number of measurement points was increased.On the sections of the heat exchanger surface, where the measured wall thicknesses differ by more than 10 %, additional measurements were made on a grid with a step of 20 mm.
At the thickness measurement points, the heat exchanger surface was cleaned to a metallic sheen.The thickness was determined as the average of the results of three measurements of each element.
The results of ultrasonic thickness measurement of the main elements of the heat exchanger equipment are presented in Table 1.

Conclusion
According to the results of calculations of corrosion rate of the main elements of the heat exchanger it was established that the corrosion rate for the shell of the heat exchanger is 0.37 mm/year, which exceeds the value of 0.1 mm/year, which is accepted in the design of this heat exchanger according to passport and literature data in 3.7 times [1,23].
The obtained results allowed to identify the most vulnerable places of the research object and to choose the most effective ways to protect the main metal of the heat exchanger from corrosion.Measures on anticorrosion protection of metal from corrosion are justified, taking into account the reduction of costs for heat exchanger replacement and increase of service life, and give a good economic effect.

Table 1 .
Results of ultrasonic thickness measurement