Abstract

High numbers of workplace casualties and injuries are reported each year in Bangladesh due to explosion of the boiler in the rice husking mills, as well as other industries including garments factories. Rice husking mills are using locally made steam boilers, which are fabricated of steel plate obtained from the ship breaking yard and not equipped with standard safety devices and fittings. The motivation of this work is to find the causes of explosion of those boilers and provide some modifications to reduce workplace casualties. In this regard, some rice husking mills in the rural area of Bangladesh are inspected to evaluate the working and operating condition of the boilers. The mechanical properties of steel plate obtained from the ship breaking yard is found by tensile test in UTM and those mechanical properties are used in numerical analysis of the boiler, which is currently operating in the rice husking mills and fabricated locally from same source of steel plate. The FEA tool ANSYS workbench is used in numerical analysis of the boiler to evaluate maximum working stress due to thermal and static pressure load, which is found below the yield strength of the test specimen. From the instantaneous pattern of explosion in all boiler incidents and result of the numerical analysis, it is concluded that strength of steel plates obtained from the ship breaking yards is not the cause of boiler explosion, operational malfunction and lack of safety measures are the main causes of boiler explosion in Bangladesh especially in the rice husking mills. To improve safety measures, modification cost of the boiler is evaluated by modifying a locally fabricated steam boiler, which is also made of same source of steel plate and currently operating in the rice husking mill. The modification cost to install safety devices and fittings is found about Tk. 25,000 ($300), which is easily affordable by rice husking mill. Finally, some steps are recommended to reduce boiler explosion and concluded that steel plates obtained from the ship breaking yards have enough mechanical strength and can be used to fabricate low-pressure boilers.

1. Introduction

Bangladesh is the 8th most populated country of the world with more than 159 million of people ¹. Rice is the main food in Bangladesh and per capita consumption about 400 gm/day, which is the highest in the world. Rice cultivation covers more than 70 percent of the total cropped area in Bangladesh ². After harvesting of the paddy, it needs to be processed for consumption. There are three stages of rice processing which includes parboiling, drying and milling. Rice processing sector creates significant number of jobs for women in the rural area of Bangladesh ². Rice husking mills are using locally fabricated low-pressure steam boiler for parboiling the paddy, and automatic rice mills are using industry standard boiler for both parboiling and drying purposes.

There are three types of rice mills are available in Bangladesh — rice husking mills, semi-automatic and automatic rice mills. There were total 11951 rice mills including 198 automatic, 220 major rice mills and 11533 rice husking mills in all six divisions of Bangladesh in 1999 ³. The number of rice husking mills is increased significantly in the last two decades and Bangladesh rice husking mills association has around 17,000 members now ⁴. Automatic rice mills are using industry standard boiler with necessary accessories and safety devices. Semi-auto and rice husking mills are using locally fabricated tubeless boilers without safety devices and fittings. As a result, risk of boiler explosion is relatively higher in semi-auto and rice husking mills than the automatic rice mills.

Due to lack of understanding of operating principle and properties of steam, boiler becomes one of the most dangerous sources of casualties, properties damage and serious injuries since it is invented. The ASME Boiler and Pressure Vessel Codes are standard providing a wide range of rules, regulation, and guideline to ensure the safety, and security of the boilers operation and other pressure vessels ⁵. However, boiler regulations and safety measurements are less likely to be followed in the rice husking mills and other industries in Bangladesh. As a result, high numbers of accidents and workplace casualties are reported in each year due to explosion of the boiler in the rice husking mills, as well as other industries including garments factories. As per office of chief inspector of the boiler, the ministry of industry of Bangladesh, there are 13 numbers of incidents of boiler explosion in the Bangladesh from the year 2006 to 2017 and 29 people get killed in the workplace. Out of 13 boilers 6 were unregistered, 8 incidents and more than 82% casualties occurred only in the rice mills ⁶. They have inspected and certified 26,046 boilers in the different industry including 2,698 new boilers and 1,118 locally fabricated boilers within year 2011 to 2017 ⁷. According to private organization and newspaper survey (Table 4, Appendix-A), total 250 workers died and 279 injured in the different industries in Bangladesh due to boiler explosion from year 2005 to 2017 including 51 workers died and 57 injured in the rice husking mills. Boiler explosion in the rice mills contributes about 34% of the total casualties and 20% of the total injuries.

A lot of rice husking mills and some other industries are using locally fabricated boilers, which are not equipped with standard safety devices and fittings. The steel plates used to fabricate those boilers are obtained from the ship breaking yard. Ships are normally scraped after 25 to 30 years of service ⁸ and sometimes those ships are left abandon in the ship breaking yard for long time in an adverse loading and environmental condition. As a result, the possibility of reduction of strength and increase of brittleness due to mechanical creep is high, and strength of steel plate can't be ignored to investigate the cause of boiler explosion.

The motivation of this research is to find the causes of boilers explosion, and provided some modifications to the boiler to improve safety and reduce workplace casualties in the rice husking mills. In this regard, some rice husking mills in the rural area of Bangladesh are inspected to evaluate working and operating condition of the boilers. The mechanical properties of the steel plate obtained from the ship breaking yard are found by tensile test. To find the maximum working stress of the boiler, those properties and operating conditions are applied in the numerical analysis of a boiler — which is currently operating in the rice husking mill, and fabricated from same source of steel plate. The Finite Element Analysis (FEA) tool ANSYS workbench is used for numerical analysis of the boiler for both thermal and static pressure load.

Numerous works are reported regarding on analysis of the boiler, boiler tube and pressure vessels ^{14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25}. In our best knowledge, none of work is reported on strength analysis of the boiler fabricated from the ship breaking yard steel plate, and found the cause of boiler explosion in the rice husking mills and other industries in Bangladesh. Finite Element Analysis (FEA) of a particular boiler is performed with actual operating conditions, thermal and static pressure loads to find the maximum working stress; and check whether the strength of steel plate obtained from the ship breaking yard is the cause of boiler explosion. From the numerical analysis, it is found that working stress in the boiler is below the yield strength of steel plate. Finally, the boiler is modified by installing pressure Gauge, temperature Gauge, water level indicator and pressure relief valve. The methodology and all the steps of this work are described in the subsequent paragraphs.

2. Methodology

Some rice husking mills located in the most paddy growing area — north-west region of Bangladesh are inspected to evaluate working and operating condition of the boilers. It is found that auto rice mills are using industry standard fire tube boilers equipped with standard safety devices and fittings. Some of those boilers are fabricated locally from the steel plate obtained from the ship breaking yard. The pictorial view of the boilers is shown in Figure 13 to Figure 18 (Appendix-A). Most of the boilers are running in the rice husking mill for more than 15 years. Through discussion with the owners of rice husking mills, boiler operators and checking maintenance records, it is realized that they are not concern about the rust, scaling, cause, and consequence of boiler explosion. The safety devices and fittings are left without maintenance for long time, and feed water is feeding to the boiler without treatment. As a result, safety devices and fittings become corroded and not working properly. During visiting of the Jamuna Auto Rice Mill, Raniganj, Dinajpur on May 2016, it is found that pressure gauge of the boiler is not showing actual reading, difficult to take reading of water level indicator due to the rust and rust deposited around the pressure relief valve (Figure 16 to Figure 18, Appendix-A). All of these are the indication of improper maintenance and malfunction, as well as boiler remained uninspected and uncertified for long time. The mill owners were unaware of it; and it was exploded on April 19, 2017 causing death of 13 workers including women and injured fourteen workers ⁹. This is a common example of boiler explosion in the rice processing industry in Bangladesh due to lack of proper maintenance and awareness.

The rice husking mills are using tubeless steam boiler fabricated locally from the ship breaking yard steel plate. The pictorial view of the boilers is shown in Figure 19 to Figure 24 (Appendix-A). Boilers using in rice husking mill are generally circular or semi-circular shape without standard safety devices and fittings installed, except a rubber hose attached on top of the boiler. The rubber hose acts as a pressure safety valve by bursting itself if working pressure inside the boiler is running high. The water level in the boiler is checked manually by inserting stick through a small tube, which is attached at the side of the boiler (Figure 21, Appendix-A) and the tube has a plastic cap at the top. Feed water is feeding to the boiler without treatment, and maintaining water level at 60% of total height of the boiler. The boilers are using manually feed rice husks as source of thermal energy. Most of the boilers are operating at least for more than 6 years without certification, modification, maintenance, and inspection. Although the government of Bangladesh has established boiler produce, testing and setup by laws and regulations; but it is less likely followed in the rice husking mills and other industries in Bangladesh.

Boilers used in the rice husking mills in Bangladesh are fabricated locally from the steel plate obtained from the ship breaking yard. Those steel plates are three decades old as average economic service life of water vessels are 25 to 30 years ¹⁰. During the 30 years of service life, ship structures are subject to various types of repeated stresses from the static and dynamic load, as well as thermal load due to variation of temperature in different global regions of the sea ¹¹. Also, structural steel plate of the ship hulls is exposed to salty seawater and highly humid environment for long time ¹². Structural steel is highly vulnerable to localized and different types of corrosion, hydrogen embrittlement, stress corrosion cracking, and fatigue ¹³, as a result the strength of steel plate of ship hull is not remained same as of initial strength after 30 years of service. To evaluate mechanical strength of steel plate, two test specimens shown in Figure 1 are collected from the ship breaking yard and tested in 1,000 kN capacities Universal Testing Machine (UTM). The stress-strain curves generated from the tensile test of two specimens are shown in Figure 3.

From the tensile test of the specimens, the mechanical properties of steel plate are found close to the properties of AISI 1010 mild steel as shown in Table 1.

Finite Element Analysis (FEA) tool ANSYS Workbench is used for numerical analysis of the boiler. Designing and analysis of boiler by using Finite Element Method (FEM) is a recent development of the ASME Code ¹⁴. FEA is the most powerful tool in the engineering community to design and analysis of pressure vessels and boilers. The displacement and stresses at the geometric discontinuity of the boiler can be easily evaluated by Finite Element Analysis (FEA) tool ¹⁵. This is the most suitable means for obtaining quick and accurate solution of deformation and stresses at each location of the boiler, which is impossible to obtain by conventional techniques without significant simplification ¹⁶.

Numerous works on stress analysis of the boiler and boiler tube are reported [15-25] ¹⁵. In our best knowledge, no work is being reported on strength analysis of the boiler fabricated locally from ship breaking yard steel plates and operating in the rice husking mills. Kondayya ¹⁷ worked on structural and thermal analysis of boiler by using Finite Element Analysis to validate the design of the boiler. Drastiwati et al. ¹⁸ worked on study of failure analysis of boiler tube based on pressure aspect. They suggested that corrosion, overheating, pitting, and improper maintenance are the main causes of failure of the boiler; however only pressure is not the main cause. In their work, this conclusion was not supported by Finite Element Analysis or by stress analysis. Zivkovic et al. ¹⁹ worked on effect of scale on thermal stresses and strains of hot water boiler structure by Finite Element Method (FEM). Gaćeša et al. ²⁰ worked on strength analysis of a fire tube boiler experimentally and numerically. Gradziel ²¹ worked on temperature and thermal stresses distribution in boiler shell with inverse heat conduction method and used Finite Element Method to calculate stresses due to thermal and pressure load. Parit et al. ²² worked on thermal stress and creep analysis of the secondary super heater failed tube boiler by Finite Element Analysis. Braimah et al. ²³ worked on thermal and static stress analysis of boiler by using FEA tool ANSYS to validate the design without mentioning the operating pressure, temperature, and boundary conditions. Babu et al. ²⁴ worked on Finite Element Analysis of steam boiler used in power plants by using ANSYS. Islam et al. ²⁵ worked on optimization of design parameter of the boiler used in rice mills by adding nine additional water tubes in a staggered arrangement, which helped to improve thermal efficiency of water tube boiler by 4% without mentioning of strength analysis of the boiler shell and tubes.

The motivation of this work is to find the working stress of the boiler, which is fabricated from the steel plate obtained from the ship breaking yard and investigate whether the strength of steel plate is responsible for boiler explosion. The FEA tool ANSYS Workbench is used to find the maximum working stress by applying operating pressure, thermal load and boundary condition of the boiler, which is fabricated locally from the same source of steel plate and currently operating in the rice husking mill. In this regard, the tubeless boiler of Manik Kha’s Rice Mill, Hemayetpur, BISIC, Pabna is analyzed, which is also fabricated locally from the ship breaking yards steel plate and currently in operating condition. The pictorial view of the boiler is shown in Figure 21 and Figure 22 (Appendix-A). The geometric characteristic and Finite Element Analysis (FEA) of the boiler are shown in the subsequent paragraphs.

The boiler using in the Manik Kha’s Rice Mill, Hemayetpur, BISIC, Pabna is a tubeless steam boiler, which is shown in Figure 21 (Appendix-A); and it is running for more than 6 years without any safety fittings and devices. Feed water is feeding to the boiler at room temperature without treatment and maintaining water level at about 60% of total height of the boiler. The geometry and shape of the boiler are shown in Table 2 and Figure 21 (Appendix-A) respectively. The boiler is operating at a pressure of 22 psi (151.624 kPa), which is found in the pressure Gage reading after modification.

Three mild steel pipes are attached in between the upper shell and bottom plate of the boiler to enhance the rigidity of the boiler shell. The steel angles around the periphery of the bottom plate are attached with boiler shell, side wall and end plates by welding. Four steel plates are attached to the angles just below the bottom plate as shown in Figure 4. The frames comprising four angles, four plates and four steel anchors on each longitudinal side by welded together; and support the whole boiler on a brick wall of the furnace.

3. Finite Element Analysis of Boiler

The solid modeling tool CATIA V5 is used to make 3D model of the boiler and imported in ANSYS Workbench for Finite Element Analysis (FEA). The 3D model of the boiler is meshed with advance curvature function; and meshing of one half of the boiler which is shown in Figure 5.

The translational motion of the boiler at the base is restricted along X, Y and Z direction to ensure 6 degrees of freedom (DOF) are constrained; and make sure that whole structure is not over constraint. The vertical translation is constrained by bottom portions of four angles around the periphery and translation in other two directions are constrained by vertical portion of two side angles. The boundary conditions of the boiler are shown in Figure 6.

The low pressure tubeless boilers are used in the rice husking mills for parboiling purpose only ²⁵, except auto rice mill where industry standard boilers are used for both parboiling and drying purpose. The rice husking mills are using traditional method of parboiling, which consists of steeping paddy in water at room temperature followed by steaming at 100°C by saturated water vapor and then dry it in the sunlight ^{26, 27}.

During inspection, it is found that top surface of boiler shell is remained at a temperature of about 35°C and 800 kg rice husks is feeding manually to the boiler furnace for 6 hours of operation. The calorific value of rice husk varies from 3,000-3,542 kcal/kg depending on moisture content ²⁸. In that respect, 4.65 x 10⁵ watt of thermal energy is added to the bottom plate of the boiler during operation. The thermal analysis of the boiler is done simply by applying convective heat transfer coefficient in the different zone of the boiler for steady state condition; and by avoiding complex simulation of pool and film boiling. The convective heat transfer coefficient in different surfaces of the boiler is shown in Table 3. The purpose of thermal analysis is to determine the realistic distribution of temperature in the boiler shell during operation and use it with static pressure load to evaluate working stress. The boiling temperature of water at 22 psi is 111.7°C ²⁹.

For simplification, assuming that the inner surface of the lower portion of the boiler shell is remained at constant boiling temperature of 111.7°C. The upper and lower outer surface of the boiler shells are subjected to convective heat transfer coefficient of 14.2 and 11.3 W/m²K respectively. The upper surface of bottom plate is subjected to convective heat transfer coefficient of 1000 W/m²K and 4.65 x 10⁵ Watts constant heat flow from the bottom surface. Heat transfer from the boiler by radiation is neglected. For steady-state condition, thermal load and boundary condition is shown in Figure 7.

The influence of thermal stress on the boiler is significant and must be taken into consideration during design and analysis ²⁰. The temperature distribution in the boiler at steady-state condition is evaluated by Finite Element Analysis (FEA) and shown in the Figure 8.

The pressure in the selected boiler is found about 22 psi (1.52 x 10⁵ Pa) after modifying it by installing a pressure Gauge. The boiler is analyzed for static pressure of 1.52 x 10⁵ Pa in all surfaces except the floor panel of the boiler, which is subjected to additional 609.6 mm hydrostatic pressure of water. The induced Von-Misses stress and total deformation in the boiler is shown in Figure 9 and Figure 10 respectively.

For evaluation of combined effect of the thermal and static pressure load, the temperature distribution from the thermal analysis is coupled with static pressure analysis. To evaluate structural integrity accurately, the effect of thermal stress cannot be ignored ²⁰. The Von-Mises stress and total deformation of the boiler structure are shown in Figure 11 and Figure 12 respectively.

4. Result and Discussion

The maximum Von-Mises stress and total deformation of the boiler due to thermal and pressure load is found 292 MPa and 5.97 mm respectively. However, due to static pressure load of 22 psi (1.52 x 10⁵Pa), the maximum Von-Mises stress and total deformation is found 196 MPa and 2.34 mm respectively. Thermal stress contributes almost 51% of total increase of stress in the boiler, which can't be ignored for safe design of the boiler. The yield strength of steel plate obtained from the ship breaking yard is found 355 MPa and the maximum working stress in the boiler is found 292 MPa, which is below the yield strength of the test specimen. During visiting of rice husking mills, the temperature of the top surface of the boiler is found about 35°C; and it is found 32.37°C in numerical analysis, which is within an acceptable limit. It is assumed that the variation is due to the change of thermal conductivity of shell materials and lack of availability of exact value of the convective heat transfer coefficient. The main objective of this work is to evaluate whether the strength of boiler shell made of ship breaking yard steel plate is responsible for boiler explosion. In this work, the working stress in the boiler is found below the yield strength of the ship breaking yard steel plate. From the result of numerical analysis, it is concluded that strength of steel plate obtained from the ship breaking yard is not the reason of boiler explosion. Although, the fatigue life of the steel plate obtained from the ship breaking yard is not evaluated. Most of the boiler incidents are reported in the situation, where boiler is being operated for at least more than 15 years. It could be investigated further by doing fatigue analysis of the steel plate obtained from the ship breaking yards. The ultimate strength of test specimen of steel plate is found 513 MPa in a tensile test; and respect to ultimate strength factor of safety of the boiler is found 1.76.

In the most cases, the boilers are failed in a fashion of instantaneous explosion with damaging properties around it; and debris is found flying far away from the location of the boiler. It is caused by instant increasing of hoop stress beyond ultimate strength of the shell material due to instant increase of pressure. By visiting of rice husking mills, it is also found that most of the boilers are old, running without essential safety devices such as pressure relief valve, temperature and pressure Gauge, and water level indicator, as well as running without proper maintenance. Untreated feed water is feeding to the boiler without concerned of scale and rust on the boiler shell. The hot surfaces of the boiler are highly sensitive to thermal strain and deposits of scale leads to cracks in the welds cause water leakage, which could lead ultimate failure of the boiler ¹⁹. The scale deposited on the outlet steam pipes could contribute certain increase of pressure in the boiler and could explode due to high hoop stress. The effect of thermal strain on the boiler is significant ²⁰; and certain decrease of water level in the boiler could lead to spontaneous increase of temperature and ultimate failure of boiler by high thermal strain. Most of the boilers are not equipment with temperature Gage and water level indicator, so there is high possibility of failure of the boiler due to certain decrease of water level. Also, the rubber hose is using an alternative of pressure relief valve which is unreliable, since there is possibility of change of elastic properties of rubber due to the cyclic thermal load. Due to increase of temperature, the wall of the rubber hose could become soft and stick together just before a shutdown of the boiler. As a result, it will not function properly for next subsequent startup of the boiler. Although, it depends on the effect of temperature on properties of hyper elastic material, which could be research further.

From the Finite Element Analysis of the boiler, it is agreed with Drastiwati et al. ¹⁸ that corrosion, overheating, pitting, and improper maintenance are the main causes of failure of the boiler; and strength of steel plate obtained ship breaking yard is not the cause of failure. The steel plate obtained from the ship breaking yard has enough strength; and it could be used to fabricate low-pressure steam boiler.

As per ASME code if input heat exceeds the limit 200,000 BTU per hour or 58.61 x 10³ Watts, the boiler has to be furnished with pressure relief valve, which must constructed as per ASME code and boiler has to be registered and certified ³². Though rich husking mills in Bangladesh are using low pressure steam boiler, the amount of input heat is about 4.81 x 10⁵ Watts. In this respect, the owner of Manik Kha’s Rice Mill, Hemayetpur, BISIC, Pabna was convinced to modify the safety feature of the boiler by installing some basic safety devices such as temperature and pressure Gage, a water level indicator, and a 1/2 inch pressure relief valve. After explaining the benefit of safety features and consequence of boiler explosion, the owner was agreed to modify the boiler. Total cost of the equipment is about Tk. 15,000 ($200 US dollar) including cost of installation. According to boiler safety guidelines, minimum of two pressure relief valves, two water level indicators and two pressure gauges are required to install to ensure a safety feature. In that case, the total cost of modification is around Tk. 25,000 ($300 US dollars), which is way below the affording limit. The process of modification of the boiler to reduce workplace casualties is shown in Figure 25 to Figure 28 (Appendix-A).

5. Conclusion

From the finite element analysis of the boiler and the simulation presented in this paper the following conclusions are made:

(1) In the view of structural integrity, the strength of steel plate obtained from the ship breaking yard is good enough to use it as a fabrication material of low-pressure steam boiler, which can be used in the rice husking mills, as well as in other industries. Although, the effect of creep and fatigue analysis are not performed in this work. The strength of the boiler which is fabricated from the steel plate obtained from ship breaking yard can be increased further by cladding; and safety of the boiler can be ensured further by maintaining 3.5 factor of safety as per ASME Boiler and Pressure Vessel code.

(2) Most of the boilers incidents are occurred in the Bangladesh due to lack of inspection, certification, maintenance, and significant lack of understanding of consequence and causes of boiler explosion among owners of the boilers and operators.

(3) The database of the boiler incidents and investigations are not maintained by the Department of Boiler Inspection, Ministry of Industries of Bangladesh nor by any other organization. To improve the quality of boiler inspection and workplace environment, it is required to maintain a database of boiler maintenance and incidents, which can be analyzed further to reduce workplace injuries due to boiler explosion.

(4) The Government of Bangladesh and regarding branch of government is failed to take appropriate initiative/action to reduce boiler explosion in the rice husking mills and other industries. The non-profit organization (NGO) and international donors (World Bank, Asian Development Bank etc) could come forward to help rice husking mills community to improve quality of workplace by reducing casualties due to boiler explosion.

(5) The existing steam boilers using in the rice husking mills can be modified easily by adding some basic safety devices such as pressure and temperature Gauge, water level indicator and pressure relief valve. The estimated modification cost is about Tk. 25,000 ($300 US dollars) including installation cost, which is easily affordable. Only needs to motivate the husking mills owner's community by explaining the benefit of safety feature, cause, and consequence of boiler explosion. It will improve the quality of workplace by reducing workplace injuries and casualties, as well as quality of life of the workers especially females who are working in the rice husking mills in the rural area of Bangladesh.

Acknowledgements

The authors would like to thank for supporting this work in all respect from the Department of Mechanical and Production Engineering (MPE) of Ahsanullah University of Science and Technology (AUST), Tejgaon Industrial Area, Dhaka, Bangladesh.

Appendix-A:

Casualties in Workplace Due to Boiler Explosion:

According to private organization and newspaper survey, workplace deaths and injuries due to boiler explosion in Bangladesh from 2005 to 2017 are shown in Table 4.

Pictorial View of Auto Rice Mills (Figure 13 to 18):

Pictorial View of Rice Husking Mills (Figure 19 to 24):

Pictorial View of Modification of Manik Kha’s Rice Mill Boiler (Figure 25 to 28):

References