Journal of Occupational

Safety assessment of glycol recovery unit in a gas refinery by failure mode and effects analysis technique

Mahdavi S, MSc¹, Rasti Pisheh P, BSc², Jozekanaani M, MSc³*

1- Faculty Member, Dept. of Occupational Health Engineering, School of Health, Lorestan University of Medical Sciences, Khorramabad, Iran. 2- MSc Student of Occupational Hygiene Engineering, Tehran University of Medical Sciences, Tehran, Iran. 3- MSc in Occupational Hygienist, Governing Health Section, HSE Main Office, National Iranian Gas Company (NIGC), Tehran, Iran

Abstract                                                                             Received: November 2016, Accepted: February 2017

Keywords: Safety, Risk Assessment, Failure Mode and Effect Analysis

Introduction

The performance of a system and installation, especially complex systems, requires a high level of safety and reliability that is created by safety instructions and strong safety culture. Ignoring these instructions may lead to adverse consequences for a system, installation, human and environment. So providing the system in a safe state by personnel and management is important not only for processing units of that system but also for the whole system and installation (1). According to safety definition that states the level of escaping from hazard, the first stage in installing and improving safety level in a workplace is identifying the hazard (2) and the next step is hazard assessment (3). There are different methods for assessment such as HAZOP (Hazard and Operability Study), LOPA (Layer of Protection Analysis) and failure mode and effects analysis (FMEA). It should be noticed that selection of appropriate method for assessment and identification of hazards has important role, so that use of HAZOP in chemical processes and FMEA for equipment systems are considered (3). In other words, one of the important components of a safety system management is the identification of potential process failure modes and their associated consequences. Hence, high reliability and safety of factories, installations and systems such as petrochemical industries, oil and gas refineries should be assured continuously (4).

In order to maintain safety of systems and installations by identifying potential failure modes, FMEA technique could be considered as an efficient method. This technique is an organized methodology for analysis of failure causes and its effects. The method was first introduced by the US military forces (MIL-P-1629) in 1949 that recently has got an inseparable part of ISO-9000 and QS-9000 (5). Also, this technique was applied with obvious requirements of reliability and safety by airplane manufacturing enterprise in 1960 and then developed in other industries such as automobile industries, oil and gas refineries. The aim of FMEA is identification and prioritization of possible failures in products and processes (6).

A study performed using FMEA method by Arabian et al. (5) in 2010 to identify the current failure modes in power producing systems, presented that FMEA method has the ability to improve the reliability of wind turbine systems so that the reliability has a role in anticipation and cost efficiency. Moreover, FMEA method has an important role in development and better design of wind turbines. This technique could be a useful instrument for designers to identify the weak points in designing the turbine. In the study of a turbine, 16 failure modes in 107 parts of studied system were identified (7).

Also, Wang et al. (3) studied development of risk management by FMEA for continuous catalyst correction to evaluate the human damage severity and environmental pollution. The results illustrated that FMEA is useful in identifying the crisis condition (4).

In other study, Dudek et al. (8) used FMEA as a method to monitor the process of production in an organization. He illustrated that this method can help to identify the potential failure and other failures that are effective in the process of production.

The study of Lipol et al. showed that although FMEA has been designed to identify the current potential failure in a product or a process and to assess the risk before occurrence, this method is practical in designing process and development of a system or product. Conducting this method in the stages of process activity or existing products contributes to productivity too (9).

Carazas et al. applied FMEA to determine the performance of the most critical parts of turbine and gained the valuable qualified information about designing and performance of system. Based on this method, 24 failures were found for one of the turbines with 5-years life time activity that 12 failures were related to the first 2 years of turbine life which resulted from the problems of calibration for pressure gauge that was in exhausting collector, and other failures were for deficiency of cleaning the fuel filters that resulted in making the filter gluey and heavy and weakening the quality of natural gas (10).

Alinezhad et al. used FMEA and ETBA (Energy Trace & Barrier Analysis) to assess the safety of circle liquid gas vessels. According to FMEA method, 29 potential failures were identified of which the highest risk priority number (RPN) was 294 and the lowest was 48 with unacceptable failure as PRN > 100 (11).

Alimohammadi and Adl surveyed the safety of 2 gypsum factories by FMEA method. The results showed that the highest rate of failure in studied furnaces was related to split and deformation of hooves the head and bottom of the furnace (12).

In other study that was performed by Heidari et al. in a pharmacy factory, it was discussed that when using the FMEA method, the accurate understanding of system and also accurate recognition of hazard are necessary in order to consider the accurate figures for occurrence possibility and risk severity, because inaccurate consideration of these figures would result in error in accident control (13). So applying this technique is required to complete knowledge of system.

It is necessary to remind that FMEA technique surveys the system or subsystems in order to identify the possible failures of all the components. It tries to assess the effects of the possible failures on other components of the system. The most important results of this technique are determination of vulnerable items of the process and critical zones of the system. These results will have significant effects on hazard reduction and cost of operation and maintenance with considering quantified index of each fault. The hardware view point of FMEA to the system provides the early recognition of potential failure modes and their elimination in order to decrease the possible losses and also improve the safety level and process reliability (11).

The reliability of glycol unit is very important for the performance of a refinery. Glycol is injected to separation centers and dew point regulating unit in order to absorb the water from the gas and to prevent creating hydrate. So this unit has an essential role in gas treatment and refinement in gas refinery. According to this subject, the failure and damage of equipment in this unit such as transformers and boilers not only could result in damages to the equipment and human injuries, but also reduction of proficiency. Thus FMEA method is a deductive method that is handled for assessing the effects of potential modes of systems, parts and functional duties. In addition, FMEA has the ability to identify the failure modes that may affect the capability of the whole system. Engineering technique to identify the possible failure in the system is studied and applied to assess the safety statues of glycol recovery unit in gas refinery. For this purpose, after careful study of the system process, the functions and system components were identified, failure modes of portions and causes of them were determined, the effects of identified failures were surveyed and controlling recommendations and solutions were presented.

Material and Methods

The current study was a descriptive-analytical research performed in glycol recovery unit of a gas refinery in one of the southern areas of Iran. This unit has lines to separate glycol from liquid hydrocarbon and a glycol condensation machine. The machine includes two condensation towers which one of them is spare and the other works as usual one. Glycol storage unit has condensate glycol storage vessel. In this unit, failure mode of different parts of the system such as aspirator, transformer, coil boiler, pumps, storage vessels and fans were assessed by FMEA method. The process of performing this study was as following:

1. The researchers made a research and assessment team that included Health, Safety and Environment (HSE) supervisor, process and product engineer, mechanic engineer and researchers.

2. The research team studied the system carefully in the aspects of the main function of the process, the function limitation of the whole system and its components by means of observation, interviewing with experts and surveying the guidelines.

3. Then, they identified the critical, sensitive points of the system and those with high accident potential in each section of the vessel body including safety valve, gage, pumps, transformers, boilers and separator.

4. Also, the weakness of a component or an activity about doing an early determined task was surveyed.

5. The effect of a component fault on other parts was assessed.

6. The severity of the effect of fault on system or people was surveyed.

7. The real reasons of fault for a component or an activity was studied.

8. The probability of fault in a component or an activity during a specific period was determined.

9. Also, all controls and actions that had been performed for prevention of fault occurrence or reduction of its severity and probability were studied.

10. After studying the important points, critical components, sensitive and high potential points, failure modes of components, the reasons and effects of identified faults, the severity and probability and recovering of each case were surveyed. RPN was calculated by multiplying three mentioned factors.

11. Then controlling and corrective recommendations and solutions were offered and entered in FMEA worksheet.

Figure 1 illustrates the stages of FMEA method as following:

• Studying the potential failure modes of the system
• Surveying the potential effects of failures
• Surveying the reasons of failures
• Studying and assessing the process of existing controls
• Calculating the RPN

Figure 1: Failure mode and effects analysis (FMEA) process (6)

*RPN: Risk priority number

Table 1: Severity of effect

RPN was obtained from multiplying the severity, occurrence and recovering of the risk. The ranges of these three factors are between 0 to 10 (Tables 1, 2 and 3).

Table 2: Probability ranking

RPN score is calculated by multiplying three mentioned factors according to table 4, so that RPN more than 100 are unacceptable according to similar studies and decision of expert team and then the required corrective actions were prioritized. In this study, after calculating RPN, proper corrective actions were advised to reduce the risk of possible accidents.

Table 3: Detection

Table 4: Priority levels of corrective actions according to RPN

RPN: Risk priority number

Results

After completing FMEA work sheets, 105 failure modes were surveyed as a whole and risk prioritization were conducted based on RPN and also corrective actions were presented so that the highest value of RPN was 150 and the lowest value was 24 and the RPN ranged between 24 and 150. Because of high numbers of FMEA work sheets, one example of work sheets is shown in table 5. As shown in table 5, high value of calculated RPN were related to the risk of not enough air ventilation inside the vessel in anticorrosion and welding operations of TK601A/B vessel with RPN 150, gasket installation with RPN 120, and valves carriage by crane from the workshop to Unit600 with RPN 108, consecutively. So 11.43% of studied failure mode had RPN more than 100.

Table 5: Failure mode and effects analysis (FMEA) work sheet

RPN: Risk priority number