Repair of Closed Fermentation Chamber and Its Influence on Strength Properties of the Tank – Case Study

The paper shows research on a closed fermentation chamber (CFC) that is an element of the Biogas Combined Heat and Power Plant located in the Gdańsk East Sewage Treatment Plant. This tank is mounted as an above-ground cylinder-shaped structure covered with a cone-shaped roof. Corrosion and leaks appeared in the tanks during many years of operation. In this work, it was decided to investigate their cause and propose a method of counteracting them. The article presents the research on the influence of individual operational and environmental loads on the stress distributions in the tank structure. Numerical simulations using the Finite Element Method were carried out for shell and solid models. Moreover, the stress distributions in cylindrical steel tanks were compared depending on the shape of the chamber top. The research results allowed us to identify the design errors of the closed fermentation chambers. It finds out that the connection of the cylindrical part of the tank with the conical roof requires reinforcement. A tank repair technology using Belzona composites was proposed to modify the joint geometry. A simulation was performed, taking into account the proposed repair technology. Its results showed a reduction of stresses compared to the state before repair by 19%.


INTRODUCTION
Unstable crude oil prices or ecological aspects related to reducing carbon dioxide emissions into the atmosphere contributed to the search for new energy sources. Biogas CHP plants are one of the technologies of new electricity sources [1].
The production of biogas is a technology used primarily for the production of renewable energy, as well as for the valorization of organic residues. Biogas is the end product of the anaerobic digestion process in which microorganisms decompose organic matter. It consists mainly of methane and carbon dioxide and may contain small amounts of hydrogen sulphide, hydrogen, water, carbon monoxide and siloxanes. Methane, hydrogen and carbon monoxide can be burned or oxidized, releasing energy, which allows the use of biogas as fuel [1][2][3]. This process was widely used in households providing heat and electricity. Nowadays, the biogas sector is growing rapidly and cutting-edge developments are laying the foundations for creating biogas plants as advanced bioenergy plants [4].
One of such bio-power plants is the "Gdańsk-Wschód" sewage treatment plant. As a result of wastewater treatment in the "Gdańsk-Wschód" treatment plant, sewage sludge is produced, which is fed to the fermentation chambers, where the process of methane sludge fermentation takes place. The biogas produced during fermentation, after purification, is used for the energy needs of the sewage treatment plant, i.e. burned in a local gas-oil boiler plant [5].
In 2012, the CHP (Combined Heat and Power Plant) were launched at the Gdańsk East Wastewater Treatment Plant. This investment allowed for the reduction of energy consumption from non-renewable sources and the reduction of greenhouse gas emissions. The construction of CHP was carried out as part of the project entitled "The energy use of biogas at the Gdańsk East Sewage Treatment Plant", co-financed by the Regional Operational Program for the Pomeranian Voivodeship for 2007-2013. The installation is powered by biogas produced in closed fermentation chambers (CFC), fuel for the internal combustion engine that drives the electric current generator [6].
During the modernization, the contractor decided to use a ready-made structure, which is grain silos. The silo should meet not only the requirements of European standards in terms of strength and stability of the structure. It is important at the design stage to also anticipate the impact of nonstandard conditions. However, before considering the "unusual" loads that may be subjected to the structure of the silo, basic strength and stability calculations should be performed [7]. These tanks are not designed to work under pressure. The constructors were probably aware of a stress concentration in the connection between the wall and the roof. Therefore, the structure has been stiffened by using a 300´100 channel section. This procedure disturbed the stiffness proportions due to the combination of a thin steel sheet with a massive profile. Therefore, after several years of operation, leakages caused by corrosion appeared in them. The historical analysis of similar structures has shown that corrosion may not take place if there are cereal products in them. The presented analysis shows the fully rusty tanks, which were excluded from operation [8]. In the case under consideration, corrosion occurred only at the reinforcement of the tanks.. Many methods of diagnosing the causes of corrosion require the collection of appropriate specimens for testing. There are electrochemical tests, voltaperometric methods, electrochemical impedance spectroscopy, salt spray tests and hydrogen degradation. Unfortunately, destructive tests were not allowed as it would have violated the structure of the facility. Therefore, having the knowledge of literature and experience, analyzing the nature of the phenomenon and the environment in which the object works (acidic atmosphere, elevated temperature, electrolyte), it was assumed that it could be the result of electrochemical and stress corrosion. A series of numerical simulations using FEM was carried out to confirm this thesis. Computer simulations are currently the basis of engineering activities, they are used to design structures and select materials [9], optimize existing structures [10] or diagnose the causes of damage. In the case under consideration, it was assumed that the damage to the tank was caused by stress corrosion cracking. It was decided to use the simulation to find the cause of the stress in the studied area.

RESEARCH OBJECT
The facilities working on the site of the sewage treatment plant are the product of the English company Permastore. This company produces universal tanks in the Glass-Fused-to-Steel technology, i.e. steel panels covered with cobalt glass embedded in the surface as a modular structure that theoretically allows the tank's design to any user's needs. They are used for the storage of loose and liquid materials in agriculture, industry and municipal services -for example, as process tanks, storage tanks for drinking water, sewage or sludge, generally for applications where there are no requirements for gas tightness and the tank does not work under additional pressure above the liquid surface. This is due to the way the cylindrical shell of the tank is closed through the conical roof. The bottom of the CFC is made of reinforced concrete in the shape of an inverted truncated cone with an inclination angle of 45° and is also the foundation for the steel tank structure. The chamber's ceiling is a truncated cone of sheets as above, with a slope angle of 15°. The tanks in the steel and reinforced concrete part (protruding above the ground) are protected with thermal insulation made of mineral wool covered with a corrugated sheet. Steel structure dimensions [11,12]: The sheathing of the steel part of the tank consists of 11 so-called "carg" (Figure 1). Cargs -these are belts made of steel sheets of varying thickness from 3.8 mm to 9.2 mm in height, connected with overlapping screws. The roof sheathing is made of 7.3 mm thick sheet steel. The connections are sealed with rubber.
The tank's structure at the junction of the cylindrical part with the conical roof is reinforced with 26 sections of C-profi les 300´100 channels with a radius of curvature of the cylindrical part. C-profi les are connected to each other by fl at bars twisted with screws. Twenty mm wide expansion gaps are provided between the C-profi les. The roof structure is radially reinforced with 5 mm thick steel profi les. Details of the tank bindings are shown in Figure 2.
As a result of many years of operation, there is a local loss of tightness on the circumference of the upper cylindrical part at the contact with the conical dome. The cause of the leakage is corrosion pitting that occurs around the entire circumference of the tank in the structure of the tank reinforcing ( Figure 3). In the article, it was decided to investigate the causes of this corrosion and propose appropriate remedial measures.

RESULTS OF THE CALCULATIONS
The immediate cause of tank leakage is stress corrosion cracking, where the tank stresses are concentrated. Therefore it is necessary to establish its causes, i.e. loads causing these concentrations. The loads of the tank structure that act on it F igure 1. Closed Fermentation Chamber CFC for methane production (on left) and its interior (on right). One of the cargs is shown in a red rectangle F igure 2. Connecting the cylindrical and conical parts reinforced with a C-profi le 300, reinforcement of the conical roof and thermal insulation with mineral wool constitute two groups. The fi rst includes continuous static loads, which are sewage's weight and hydrostatic pressure. The second group consists of periodic loads of a variable nature, which are usually not considered at the design stage of a given technical object. They include: • wind; • the temperature diff erence in the elements of the tank structure; • water hammer; • other (soil settlement, shock loads).
To investigate the influence of individual loads on the stress state distributions in the tank structure, several numerical simulations were carried out using the FEM method for shell and solid models with different simplification levels. Depending on the model, 4 nodal shell or hexagonal solid elements were used. To find the cause of stress corrosion cracking, it was necessary to check which of the loads caused the stress concentration in the area of the channel section. For this reason, in the initial phase, the tank was tested as a whole, and in case of doubt, simulations were carried out on a more detailed model. About 15 variants of simulation were carried out until the most probable cause of corrosion was selected. In the article, it was decided to present only the most interesting cases. Since many of these variants had different mesh parameters and finite element types, finite element details were omitted. The following material data was adopted for the simulation and calculation of stress distributions, strains and displacements in the fermentation tank: Steel: • sewage density rsew = 1000 kg/m3 • air density rair = 1.4 kg/m3 • gravity g = 9.81 m/s2 Many simulations were carried out. However, due to their multiplicity, the article presents only selected results for the given loads, i.e.: • simulation of hydrostatic load in sewage; • simulation of constant overpressure gas load; • simulation of the load on the hydrodynamic wind pressure; • simulations for uncontrolled pressure build-up; • simulation of the load resulting from the temperature diff erence.
The simulations of the hydrostatic load with sewage were made on the shell-solid model. The tank was loaded with the hydrostatic pressure of the sewage following the formula: The HMH reduced stress values in such a loaded tank do not exceed 160 MPa and occur above the tank restraint. The maximum displacement at this location is 9.6 mm ( Figure 4). The stress distribution does not indicate where to connect to the roof, which makes it possible to rule out this as a cause of stress corrosion cracking.
The nominal working pressure of the tank is 3 kPa. Assuming a safety factor of 1.25, it was decided to load the tank with a pressure of 4 kPa. This imitates the situation during the leak test. The maximum value of the reduced stresses (HMH) is 20 MPa. The stress distribution does not indicate where to connect to the roof, which makes it possible to rule out this as a cause of stress corrosion cracking.
In another simulation, the tank was loaded with hydrodynamic air pressure during a hurricane that occurs on the Polish coast several times a year, assuming a wind speed of 120 km/h (33.3 m/s). Depending on humidity, atmospheric pressure and temperature, air density was adopted for extreme conditions. At pressure patm=100 kPa and temperature q = -25°C, it is approx r =1.4 kg/m3. The stagnation pressure for such assumptions is [13] The value of this pressure is relatively small. Its direction was projected to the direction normal to the elements. In the wind-loaded tank, the stresses did not exceed 2 MPa. Their distribution also did not concentrate on the area of the roof sheathing, so it can't be assumed that this load caused corrosion stresses. The stress distribution does not indicate where to connect to the roof, which makes it possible to rule out this as a cause of stress corrosion cracking.
Calculations were also made, taking into account the uncontrolled increase in pressure, which is possible as a result of overfi lling the tank or as a result of the failure of monitoring devices and the human factor. In the simulation, an increase in pressure of only 10 kPa was assumed, corresponding to 1 m of water column height. The obtained results ( Figure 5) for this case indicate that the weakest design point of the tank is the area where a part of the cylindrical shell is connected to the conical roof. According to the simplifi ed fi lm theory of shells, the stress state in thin-walled shells is infl uenced by the radii of curvature [14]: for example, for a cylindrical vessel with a diameter D, thickness g, where the gas pressure is p, According to this theory, for the construction of CFC's, the place of connection of a part of a cylindrical plating with a conical roof is a peculiarity, where the curvature radii change abruptly and are undefi ned, which results in stress concentration. In the above task, for the adopted calculation models, these stresses reach the value of 400 MPa, but in the actual tank, they may be lower.
In pressurized tanks, even with relatively low values, at the closure point of the cylindrical part of the so-called roof, sharp notches, i.e. abrupt changes in the radius of curvature, are avoided by using spherical and elliptical covers ( Figure 6). In the case of a tank closed with a spherical roof, the maximum Huber reduced stresses are just 10 MPa (Fig. 7c) and are located at the base of the tank. For a tank with an elliptical top, they are already 55 MPa (Fig. 7b) and occur in joining both parts, and for a conical roof in the same place, they reach the value of 433 MPa (Fig. 7a).
The infl uence of temperature change on the strength of the tanks was also considered.
During operation, mesophilic fermentation takes place inside the tank. Then the temperature inside ranges between 32-38°C [15]. On the other hand, in the area where the tank is located, frosts can reach -20°C, which causes even a 50-degree temperature diff erence. Thermal loads are caused by inadequate tank insulation, especially on the roof. There are many thermal bridges in the tank's geometrically "weak" elements, which causes the unfavourable distribution of thermal stresses. Apart from the C-profi le, the outer surface of Figure 5. Stress distribution sHMH caused by a pressure increase of 10 kPa. View from the outside Figure 6. Closing of the tank which is in contact with the surroundings, the place where temperature diff erences may cause cracks, is the roof, radially reinforced with steel profi les, 5 mm thick. The thermal insulation is arranged between the reinforcements, while the upper part of the profi le has the ambient temperature, and in winter conditions, the roof sheathing will bend. The consequence of incorrect isolation is shown in Figure 8. It can be seen that the roof reinforcement is a thermal bridge between the tank interior and the environment. The diff erences can be seen more precisely compared to other roofs in the neighbourhood and on the top of the tanks excluded from the operation.
Due to the greater thickness of the roof and, in this case, maintaining the stiff ness ratio in relation to the stiff ening profi les, the values of thermal stresses amount to about 60 MPa, at a temperature diff erence of Dq = 30°C, and occur in the reinforcing profi le, not in the roof sheathing.
At a temperature diff erence of Dq = 50°C, they increase to about 100 MPa in the reinforcement profi les, and in the plating, they reach values of the order of 20 MPa (Figure 9). These seemingly small values, due to the linear concentration, can cause corrosion and increase with the material loss. Moreover, in the places of joining the roof profi le, additional local stress concentrations arise, which appear at the point of corrosion. The simulation results indicate that stresses up to 400 MPa may occur in the tested case.
Temperature changes have a signifi cant impact on the stress state in the shell of the tank. The length of the ring made of channels is 69.7 m, so for Dq = 30°C, its shortening is 25 mm, for Dq = 40°C, its shortening is 33 mm, and for Dq = 50°C its shortening is 41 mm.
Numerical simulations have shown that the design of the CFC tanks is not very well suited to the conditions in which they operate. Large  temperature diff erences with existing thermal bridges cause signifi cant stresses in the roof structure. This is due to the adaptation of the tank for other purposes to the fermentation tank. The connection of the cylindrical part with the conical roof is the place of stress concentration. Another problem is the inappropriate selection of the stiff ness of the cooperating elements, i.e. connection of a rigid C-profi le 300 with a thin sheet of 4 mm thickness. The most dangerous conditions for the tank are the thermal loads induced in the conical connection of the roof with the cylindrical part. Due to the tank's structure, it is unacceptable to load the internal part of the conical roof with pressures above 4 kPa. Such a situation may be caused by an increase in gas pressure above the liquid surface and (which is much more dangerous) an increase in the liquid level above the cylindrical part -overfi lling the tank. As the nominal pressure in the tank is 3 kPa, an overfi lling situation is rather rare. Hence, the most likely cause of tank failure is stress corrosion cracking due to temperature diff erential. The solution to the described problems may be the appropriate repair technology.

THE PROPOSED REPAIR TECHNOLOGY
When analyzing the causes of stress corrosion cracking, it was noticed that the roof joint should be strengthened and the transition between the roof and the tank walls softened. The repair contractor proposed a technology according to which it was necessary to remove the eff ects of corrosion, corrosion pits, supplement (fi ll) the corrosion cavities with composite materials, to strengthen the connection of the cylindrical part with the conical roof with Belzona composite materials [16], the purpose of which is to increase the strength of this joint, tightness and protection against electrochemical corrosion. The proposed repair technology will change the geometry of the connection of the cylindrical part with the conical roof. It seems reasonable to check how this change will aff ect the stress distribution in the tank structure. The geometric changes are presented in Figure 10.
The structure fragment repaired according to the proposed technology was subjected to numerical FEM simulation to determine the stress state. The simulation aims to compare the results of the stress state after repair with the state of the structure before repair. Only a structure fragment, i.e. the connection of the cylindrical part with the conical roof, was analyzed. The object's symmetry was used, and the axisymmetric problem was solved ( Figure 11). The cut-off of the analyzed fragment was completed with appropriate boundary conditions ensuring the continuity and symmetry of stresses. All dimensions were adopted following the documentation of the tank. The appropriate material properties have been assigned to individual elements. The material constants for the fi lling are [17]: • Young's modulus: 1200 MPa; • Poisson's ratio 0.3.
For the barrier coating, the data for the laminate [14] was adopted and presented in Table 1.
The analyzed cross-section of the structure was discretized with fl at, axisymmetric fournode elements and loaded from the inside with a Figure 10. Visualization of tank repair Figure 11. Reduced stresses HMH in the cross-section of the tank plating before and after repair working gas pressure of 4 kPa. Figure 11 shows the reduced stresses of the HMH. In the structure before the repair, they reach the values of 31.8 MPa. In the structure after repair, they drop to 25.8 MPa, which is about a 19% decrease for the analyzed task. The stress value decreases the load on the channel section and, thus, the entire roof joint structure. The pressure distribution at the joint also changes, which avoids local stress concentration. Strengthening and rounding the interior allows for an increase in the inter-repair period but does not entirely solve the problem.

CONCLUSIONS
Biogas is an interesting alternative in the world of more expensive non-renewable energy sources. Its production, however, is costly and not consistently profitable. It also turns out that its collection can be troublesome under unfavourable conditions. Closed fermentation chambers are exposed to many loads. Factors such as wind, hydrostatic pressure and gas pressure are crucial for the durability of such chambers. Moreover, during biogas formation, significant temperature differences arise between the inside of the chamber and the atmosphere, leading to thermal stresses and stress corrosion cracking.
The conversion of a grain tank to a fermentation tank is economically viable. It seems, however, that the designers did not anticipate all the risks of such a modification. Grain products by themselves do not emit heat like the fermentation process. In addition, the chamber is located in a place of severe frost, which puts it under considerable stress due to the temperature difference. The internal shape of the tank is also essential. Sharp roof-wall connections can be destructive for pressure tanks, even with a slight overpressure (3 kPa).
The article assessed that temperature has the most significant influence on the formation of corrosion losses. However, it should be borne in mind that the synergy of all the described loads should be considered for the overall strength of the chambers. The proposed repair technology allows for reducing stresses by about 19%. This reduction occurs due to the formation of an oval shape between the roof and the tank's walls. Due to the good adhesion to steel, the composites used ensure the tightness of the connection of the cylindrical part with the conical roof. It should be borne in mind that the tanks operate under load conditions close to the permissible loads. Therefore they should be protected against the possibility of exceeding these loads. It should consider changing the roof insulation, which will reduce the impact of thermal stresses in the winter.