Xuedong Chen

Accidents Investigation and Risk Assessment of Chinese Industrial Pressure Pipelines

In recent years personal casualties and fire explosion accidents are often taken place due to pipe failure accidents of petrochemical enterprises in China. For more than a decade years, the authors have conducted extensive investigation and analysis on these accidents and consider that apart from the causes of man-made quality out-of-control and technical level, another major cause is the absence of experience accumulation of some medium environments induced failure accidents for design standards and codes, design units and designers, therefore, it is unable to prevent and control in-service risk factors at the design stage. Since 2003, we have conducted risk-based inspection and analysis of industrial pipelines of all types of units in about eighty large-scale oil refinery plants, chemical plants and chemical fertilizer plants under the jurisdiction of SINOPEC and PetroChina according to API581, API571 etc. in cooperation with Bureau Veritas, France (BV). Through these analyses, we have found out the major failure modes, mechanisms, likelihood and consequences of petrochemical industrial pipelines, and we also consulted the original design and installation documents of all pipelines, it is found that most of the high failure risks of petrochemical industrial pipelines are induced by inadequate consideration to environment induced failure mechanisms at the design stage. In order to avoid repetitive occurrence of these accidents, suggestions on improvement of design and manufacture methods in China are proposed in this paper, that is, the experience of in-service environment-related failures should be fed back to the organizations of design and manufacture standards, design institutions and designers by some effective means, and set up the design and manufacture platform based on risks and life in China, so as to control the risks of pressure pipelines away from accident over the whole life through such measures as reasonable material selection, structural optimization design, selection of reasonable manufacturing and installation process etc. at early stage of design and manufacture.© 2009 ASME

Introduction of the Risk Based Optimization and Risk Criteria Analysis of Spare Inventory in Petrochemical Plant

Spares plays an important role in the asset integrity management. In petrochemical industry, great amount of money is occupied by spares which are lower inventory but higher unit price (critical spares). Some typical inventory optimization models, such as Economic Order Quantity (EOQ) method, which focuses mainly on the optimization of storage and order costs, is unsuited for the optimization of inventory of critical spares. While consume based analysis method can precisely predict the demand of spares only if the historical consuming data is adequate and sufficient. But in petrochemical industry, it is always difficult to obtain such data. In order to quantify the optimal spare inventory and determine stock strategy of critical spares, a risk based inventory analysis methodology was put forwarded in this chapter. The probability of stockout was obtained by systematic consideration of spare parts reliability, configuration of available spare equipments and spare parts and lead time of order. Taking stockout-costed loss into consideration, the risks of all critical spares of pumps in Y Company were obtained. It was found that the quantity of low risk spares whose potential yearly loss are less than 500,000 RMB take up to 80 %, while it account for 83 % of overall stock fund. It is except that great amount of money can be saved if those spares can be optimized by risk based spare optimization method. By comparing of the amount of spares and associated proportion of fund its’ occupied, the determination of the risk criteria in spare inventory optimization was also discussed in the last part of this chapter.

Corrosion and Protection Status in Several Chinese Refineries Processing High-Acid Crude Oil

Naphthenic acid corrosion and corrosion protection status in several Chinese refineries processing high-acid crude oil was summarized and analyzed, including material selecting, corrosion monitoring methods, corrosion case. The processing experiences indicate that the most several corroded parts when processing high-acid crude oil occurred in the vacuum column and transfer line. In one refinery, vacuum tower wall, tower packing and transfer line with SS316L (Mo ≥ 2.5 %), were corroded amazing after 3 years operation, which need to be upgraded to SS317L and with the use of corrosion inhibitor to enhance corrosion protection. In another refinery, many high temperature heat exchangers made of carbon steel in the distillation unit were found corrosion severe. In the third refinery, the atmospheric transfer line with SS321 material had a corrosion rate of 0.3 mm/year, found through the fixed-point thickness measurement, and then upgraded to SS316L. From the processing experiences, it could be concluded that selecting SS18-8, SS316L or even SS317L is necessary when processing high-acid crude oil. Corrosion inhibitor is suggested to be used in the most sever corrosion part, for example in the vacuum column and transfer line. Naphthenic acid corrosion of SS316L has an incubation stage. Corrosion is slower in the initial stage, following by a rapid development once the corrosion forming which made the surface of the metal rough. Therefore, every time a unit shutdown, corrosion inspection should be carefully conducted.

Study on the Relief Capacity and Safety Integrity of Multiple Relief Valves of Charge Gas Pipe in Ethylene-Cracking Plant

The shutdown of charge gas compressor in large-scale ethylene-cracking plant always involves emergency pressure relief of charge gas through multiple safety valves. The emergency relief capacity plays an important role on the safety of the overall plant. In this paper, by studying the difference between the configuration of the pressure relief system of two 1000 KTA ethylene-cracking plants (the inner diameters of the charge gas pipeline in both plants are 2m, while the number of same-sized relief valves are 28 and 19, respectively), the relief capacity of multiple relief valves is studied and compared with empirical calculation and numerical analysis. It is found that, due to the interruption of fluid flow when compressor is emergency shutdown, the upstream pressure of each relief valve increase steadily with the continuously make-up of the charge gas, but the difference between the inlet pressure of all relief valves can be neglected. With the increase of the upstream pressure, the opening of relief valves is determined mainly by the set pressure. In multiple valves pressure relief scenario, normally the downstream valves have greater relief capacity than those upstream valves if both relief valves have the same back pressure. Also, by analysis it is noted that the pressure relief capacities of multiple relief valves in both plants are sufficient. The minimum number of relief valves required for process safety is obtained. The maximum achievable Safety Integrity Level (SIL) of pressure relief system is determined by calculation of the reliability of the redundant relief valves. The analysis is used for determination of the SIL of the pressure relief system. The finding is also significant for determination of the required capacity of multiple relief valves.Copyright

Failure and Prevention of Pressure Vessels due to Instability of Process System

From the analysis of engineering risks of petrochemical plants in recent years, it is found that the change and instability of the process systems have significant effect on the failure of pressure vessels, and the failure of pressure vessels takes place repeatedly due to the inadequate prediction of dynamic risk of the system in the design of pressure vessels. Starting from the analysis of systematic engineering risk methodology, several typical failure cases in petrochemical process systems due to the inadequacy of process design and interlock control are introduced and analyzed in this paper, and the corresponding measures for preventing the failure of pressure vessels caused by system instability at the design and the in-service stage are proposed. It is believed that the basic measures for preventing occurrence of such failure accidents are the analyzing the dynamic risk of the system during the determination of design parameters of pressure vessels at early design stage, verifying process changes and adjusting the configuration of interlock control system according to the design and operating parameters of pressure vessels at the in-service stage.Copyright

Assessment of Safety Integrity of LDPE Tubular Reactor With Emergency Pressure Relief Requirement Consideration

The production of low density polyethylene (LDPE) with tubular reactor process always involves ultra high pressure (up to 310 MPa (3059 atm)) and medium temperature (normally less than 310°C (590 F)). As polymerization is highly sensitive to temperature and pressure fluctuation, emergency relief valves are required to prevent the runaway of a reaction (C2 H4 decomposition, a strongly exothermal reaction). The pressure increase during decomposition develops rapidly (normally in several second). The pressure relief speed plays an important role in the safety integrity of reactor. The purpose of this paper is to demonstrate the evaluation of safety integrity of tubular reactor by taking process deviation and pressure relief requirement into consideration, and to determine the safety requirement specification of the emergency relief. A state of the art analysis is proposed in this paper. The time-dependent pressure fluctuation and relief capacity, as well as various decomposition scenarios during emergency pressure relief are studied. The analysis is then used to determine the required redundancy of the emergency relief valves. The probability of failure on demand (PFD) of multi-valve relief operation is calculated with the classic reliability model. The pressure fluctuation due to emergency pressure relief is calculated with a proven-in-use relief model. Necessary assumptions are made about the relief operation. It is concluded that 2 out of 5 (2005) emergency relief valves are sufficient for the emergency pressure relief in order to prevent the overpressurization of the tubular reactor under a given condition that the emergency relief valve fully opens in less than 3 seconds.Copyright

Several Failure Analysis Cases of Pressure Equipment Under the Conditions of Complex Medium Environment

There are several hundred of failure cases of pressure vessels and piping in China every year. The causes for part of accidents have been clearly analyzed, and preventive measures have been taken making the similar accidents substantially reduced, but the causes for quite a few failure accidents are still not found effectively, the similar accidents is still taking place. Through study, the authors find that the major reason for deviation of failure analysis lies in that equipments are mostly operating in complex medium environment, and mutual competition may exist among multiple failure mechanisms. Sometimes changes of some influential factors may cause the dominant failure mechanism change, even leads to totally different analysis conclusions. Combining with the analysis and verification of several failure cases of pressure vessels and piping in petrochemical enterprises, the judgment method for the dominant failure mechanism under multiple failure mechanisms is discussed in this paper, which may be helpful to provide some effective means for failure prevention of pressure equipment under complex environment.Copyright

Analysis Method of Failure Likelihood on Pressure Equipment With Combined Action of Multi-Failure Mechanism

Since 2003, we have conducted a lot of researches on risk based assessment technology in China according to standards API 581, API 571 etc., mastered the risk distribution conditions of in-service pressure equipments covering all kinds of units of about 80 large-scale oil refinery plants, chemical plants and chemical fertilizer plants in China, and formed the failure trees and technical guide documents for RBI of typical petrochemical units fit to Chinese national conditions. However, we find there are interactions of various failure modes and mechanisms of many petrochemical equipment during their actual running due to the diversity of corrosive media and the complexity of influential factors in the process of risk assessment implementation process, which has not been taken into account in the existing API 581, if we cannot determine the possible dominant failure modes and failure mechanisms as well as influential factors of secondary mechanisms in the actual service environment of pressure equipment, the risk assessment analysis results obtained tend to have big difference from actual conditions. In this paper, researches are conducted with respect to the judgment methods of dominant mechanisms in case of joint action of multiple failure mechanisms of pressure equipment and influential law of secondary mechanisms in several typical complicated medium environment of petrochemical enterprises based on the survey and analysis of failure accidents, risk assessment and experimental research of pressure equipment, and suggestions on improvement with respect to failure likelihood calculation methods in risk assessment in API 581 are given.Copyright

Time-Dependent Risk Control of Crucial Tube Furnace With SIL Requirement Considered

Hydrogenator feed tube furnace is of great safety concern due to its rigorous operation condition (up to 550°C, 17MPa) and flammable/explosive materials (H2, crude oil) involved in refining installations. Unlike instruments where the failure may not necessarily cause direct hazards, the failure of tubes in high temperature furnace always cause severe damage and downtime-caused economic loss. Normally the main cause of tube failure is time-dependent material degradation mechanism (such as creep, fatigue and oxidation). The occurrence of two tube rupture accidents gives birth to the consideration whether it is necessary to add a new isolation/mitigation layer in tube furnace in order to control time dependent risks. Taking advantage of the progress of life and probability prediction techniques (such as API581), the time-dependent risk of high pressure tube furnace was studied by taking into account of contributions provided by control system and safety instrumented functions (SIF). The frequency of severe initial events (leakage or rupture) of furnace tube was studied and fault tree analysis method was used to analyze the combined failure probability of time-dependent tube failure rate, basic process control system (BPCS) as well as safety instrumented system (SIS). The necessity of adding a mitigation function to reduce consequence caused by tube failure (leakage or rupture) was studied in order to control time dependent risks. A mitigation SIF at tube’s late service stage to reduce risks was proposed, which is of great significance for furnace safety.© 2010 ASME

Guaranteeing Long-Cycle Safe Operation of Ethylene Plants by RBI Technology

Ethylene plant is the critical plant in petrochemical industry, the output of its major product - ethylene directly reflects the development situation of a country’s petrochemical industry, therefore, the long-cycle safe running of the ethylene plant is of important significance. At present, the operation cycle of ethylene plants in developed countries is generally 4∼6 years, whereas that in China is mostly 3 years, which seriously restricts the production capacity, cost and international competitive power of Chinese ethylene production enterprises. With a great number of new or expansion projects of 1,000,000 tons/year ethylene plants put into production successively in China, it is extremely urgent to guarantee long-cycle running of ethylene plants. Starting from 2003, we initially adopted risk-based inspection (RBI) technology in Chinese ethylene plants, and achieved the objective of extending the inspection interval of ethylene plants from the original 3 years to 6 years in combination with technical means like online inspection etc. Up to the present, we have completed RBI of 13 ethylene plants, and the running cycle of most of them has been extended. In this paper the application situation of RBI technology in ethylene plants is presented, by means of RBI, major factors that influence long-cycle safe running of ethylene plants are identified effectively, and feasible risk reduction measures have been developed, which provide reliable assurance for long-cycle safe running of the ethylene plants.Copyright