PRAGMATIC ASSESSMENT OF EXPLOSION RISKS TO THE CONTROL ROOM BUILDING OF A VINYL CHLORIDE PLANT L.P. Sluijs 1, B.J. Haitsma 1 and P. Beaufort 2 1 Vectra Group Ltd. 2 Shin-Etsu (contact details: Vectra Group Ltd., Laan Copes van Cattenburch 139, 2585 GA Den Haag, The Netherlands; Tel.: þ31 (0)70 311 0170, e-mail lucassluijs@vectragroup.co.uk) The potential explosion risks of a vinyl chloride plant were investigated with respect to the control room building. The risks of explosion to the control room building were determined by combining the possible effects of explosions with the leak frequency of flammable substances in the plant. The results, in the form of an explosion exceedance curve and a building risk FN-curve, were used to assess the risks and ultimately led to recommendations regarding the use of the control building. This paper presents an overview of the pragmatic approach used to conduct an explosion building risk assessment and illustrates the use of quantitative risk assessment as a practical tool for investigating the risks to onsite personnel. KEYWORDS: explosion, building risk assessment, exceedance curve INTRODUCTION Shin-Etsu PVC B.V. (Botlek, The Netherlands) produces vinyl chloride monomer for the manufacture of PVC products. The plant was built in the seventies with its control room building located adjacent to the plant that processes various toxic and flammable substances. This situation seems, given the increased explosion risk awareness in the industry, unfavourable due to possible explosion risks of the plant. An explosion exceedance study was performed to quantify these risks. The exceedance methodology quantifies the frequency for explosions exceeding a certain value by combining the explosion occurrence frequency distribution, with the probability that the explosion load exceeds this value. This probabilistic approach results in a more realistic estimation of the explosion risks compared with traditional methods using the maximum worst case explosion overpressure only. In the Netherlands quantitative risk assessments tend to focus on risks to offsite people in the surrounding area only. In this study a quantitative risk assessment was used to investigate the risks to onsite personnel. In this paper the methodology of the explosion exceedance study is described and the results, conclusions and follow up from this assessment are discussed. METHODOLOGY Potential explosion hazards are caused by accidental leakages of flammable substances, which are used in the process. Among these are vinyl chloride monomer (VC), ethylene dichloride (EDC), propylene, ethylene and hydrogen. Explosion overpressures are generated when flammable vapours are ignited and the flames propagate through a congested area. The control room building is located next to the plant and explosion overpressures could cause damage to the control room building. The risks to the control room building were determined by combining the possible effects of explosions with the probability of occurrence. Essentially, the following approach was taken: A. Identification of congested areas in the plant; B. Determination of release frequencies of flammable substances; C. Determination of the size and extent of flammable gas or vapour clouds; D. Determination of filling degree of congested areas per release; E. Calculation of the explosion overpressure at the control room building; F. Determination of the explosion probabilities; G. Presentation of the risks for the control room building. The following sections expand on the pragmatic way of performing these steps of the assessment. This approach is also described in the scheme given in figure 1. IDENTIFICATION OF CONGESTED AREAS IN THE PLANT 13 areas were identified in the plant, which provide a means for pressure development during the combustion of a gas or vapour cloud. In order to calculate possible explosion effects using the consequence modelling software FRED [4], these areas are characterised by region dimensions (length, width and height), number of rows of obstacles in each direction (number of grids) and the blockage of the rows of obstacles in each direction (blockage ratio). The presence of a roof is also specified. DETERMINATION OF RELEASE FREQUENCIES OF FLAMMABLE SUBSTANCES A count of main equipment items containing flammable substances of the plant was performed based on the Process Flow Diagrams to determine the release frequencies. 1
Figure 1. Overview methodology It was assumed that each piece of equipment also consisted of piping, valves, flanges and instrument connections. The length of piping and number of valves, etc. was based on experience and checked against P&ID-s of several important main equipment items. Each equipment item has a probability that a release of the contents will take place. The release frequencies are divided into small (3 9 mm hole size), medium (10 30 mm hole size) and large releases (.30 mm hole size). Very small hole sizes (,3 mm) were considered to lead to a release which would not result in an explosion in the (naturally) ventilated congested areas and are therefore excluded in this assessment. The equipment was specified by location and flammable substance. Release locations were defined within the plant, for which the same process conditions are assumed for each present flammable substance. Apart from the congested areas identified in step A, 19 additional release locations were identified from which a flammable cloud can drift to the congested areas. So in total there are 32 release locations. The final result of step B is that for each release location a release frequency is calculated per flammable substance. Two cases were considered with respect to the failure frequencies. In Case A the determination of release frequencies per equipment item was based on E&P Forum data [3], 2
Table 1. Frequencies of small, medium and large releases Hole size Case A: Oil & gas statistics Frequency (per year) Case B: Shine-Etsu casuistry Small (3 9 mm) 1.07 1.07 Medium (9 30 mm) 0.45 0.15 large (.30 mm) 0.26 0.03 which are based on failure data from the Oil & Gas Industry. These data differ from the experience of Shin-Etsu during 35 years of VC production. Therefore adjusted failure frequencies were also considered in this assessment, Case B. The total frequencies of small, medium and large releases are given in Table 1 for both cases. DETERMINATION OF THE SIZE AND EXTENT OF FLAMMABLE GAS OR VAPOUR CLOUDS Dispersion calculations were performed with the Shell consequence modelling program FRED [4] to determine the dimensions of flammable gas or vapour clouds for each release, based on the Lower Flammable Limit (LFL) of the released material. These calculations were done considering three standard weather conditions. The applied wind speeds were based on a Vectra in-house model, in which the reduced wind speeds in congested areas are taken into account. DETERMINATION OF FILLING DEGREE OF CONGESTED AREAS PER RELEASE In order to calculate the strength of an explosion, the filling degree of congested areas caused by a release is required. The dimensions of a cloud from step C are used to calculate the part of a cloud, which reaches a congested area. The filling degree of congested area is then calculated by assuming a horizontal release leading to an ellipsoid shaped cloud. For releases originating in congested areas the free cloud volume was compared to the congested area volume to determine the filling degree. It should be noted that in the dispersion calculations with FRED [4] the impact of obstacles can only be taken into account via the surface roughness parameter. The impact of large obstacles is considered by determining a probability (yes/no) of a release in a certain location reaching a congested area. This rough approach is based on the location of an area given on the plot plan, showing installations, buildings, etc. This approach is not valid for areas, which contain air coolers, because of the induced airflow rapidly dilutes the flammable gas. For these areas the following simple, conservative approach is applied: if a release (mass per time) could lead to a flammable concentration in the air flow sucked into the air cooler area higher than the LFL, then maximum filling degree is assumed. If the LFL is not reached, an explosion cannot occur. The result of step D is that for each release the filling degree of all congested areas is calculated. The filling degree ranges from 0 to 100%. CALCULATION OF THE EXPLOSION OVERPRESSURE AT THE CONTROL ROOM BUILDING Explosions in the congested areas are modelled with the Congestion Assessment Method (CAM) in FRED [4], using the filling degree calculated in step D as well as explosion parameters of the flammable substance. These calculations result in the potential explosion overpressure, which is reached at the control room building for each release depending on the weather conditions. It should be noted that the FRED explosion modelling is always using stoichiometric mixtures of flammable vapour/gas and oxygen for the calculation of explosion overpressure. DETERMINATION OF THE EXPLOSION PROBABILITIES Beside the probability of accidental releases (step B) the explosion frequency is also determined by the probability that delayed ignition occurs. The ignition probability used in this assessment is based on a Vectra study [5] and is dependent on the release rate. The release direction influences the probability of a release reaching a congested area. In this assessment it is assumed that all releases are horizontal with a uniform release direction orientation and that the probability of release being towards a particular congested area is equal to 1/12 (based on a wind rose of 12 sectors). The dispersion of a release and therefore the explosion scenario depends on the weather conditions. The distribution of probabilities for weather conditions is based on weather data collected [2] and is adapted for the situation on a chemical plant [6]. This has resulted in a probability distribution for the three weather conditions as considered in this assessment and in which congested and open areas are specified. These probabilities are used to calculate the probability of an explosion scenario. A scheme for calculation of the explosion probability for each release of a flammable substance is provided in Figure 2. PRESENTATION OF THE RISKS FOR THE CONTROL ROOM BUILDING Explosion risks are determined by the combination of the probability of explosion scenarios and the consequences of these scenarios. The risks for the control room building are presented as an explosion exceedance curve showing the probability of exceeding an explosion overpressure at the location of the control room building due to leakage scenarios in the plant. Comparison of explosion risk with building explosion resistance has resulted in a building risk FN-curve showing a 3
Release frequency Weather Probability of a Probability probability release reaching a or delayed = congested area ignition Explosion probability per flammable substance, per area - divided in small, medium and large releases divided in three types of weather conditions taking account for release direction and obstacles per release, per weather condition, per congested area Figure 2. Explosion probability calculation scheme graphic view of damage (fatalities in the building) versus the probability of occurrence. In this graph the probability of personnel being inside the control room has been taken into account in the frequency of scenarios with a given number of fatalities. The vulnerability of occupants in the building to explosion overpressure was based on API 752 [1]. Note that in this approach the probability of serious injury/fatality is assumed to be one when building collapse takes place. This is considered to be conservative, because survival voids are likely to be present after collapse of the building and personnel may survive. RESULTS An excel-spreadsheet was made for processing the steps described above: For each flammable substance in a particular area, three release scenarios (small, medium and large) were considered for three different weather conditions, leading to a probability of delayed ignition and an explosion loading to the control room building for each congested area on site. The results of the risk calculations are shown in Figures 3 and 4. In each figure the results of Case A (failure frequencies derived from Oil & Gas statistics) and Figure 3. Explosion overpressure exceedance curves for two release frequency data Figure 4. In building fatality exceedance curves for two release frequency data Case B (failure frequencies based on Shin-Etsu casuistry) are shown. The two lines in the exceedance curves are considered as indicators of the range of the explosion risk. From the calculations it was deducted that there are only three congested areas close to the control room, which could lead to an overpressure higher than 0.2 barg. These areas present the main contributors to the risks. It was also concluded that the releases from large hole sizes are dominating the explosion risks to the control room building. The results have been used to assess risk acceptance and to identify mitigation measures with respect to the use and location of the control room. The level of risks calculated indicate that according to the acceptance criteria used by Shin-Etsu, risk reduction measures need to be considered. As a result of this study, the following actions have been taken:. The control room building is no longer used as a muster area during emergency situations. The control room is still considered a safe place for personnel present during normal operations.. A detailed survey of explosion resistance of the building has been initiated and measures have been proposed to improve the pressure durability of the control room building. 4
Shin-Etsu have used this explosion exceedance study to comply with Environmental Permit requirements, in which an assessment of the explosion risks was required. The study has been communicated with the Dutch authorities and they have not raised any objection to this study. CONCLUSIONS The explosion risks of a vinyl chloride plant were investigated with respect to the control room building. The results in the form of an explosion exceedance curve and a building risk FN-curve were used to assess the risks and ultimately led to recommendations regarding the use of the control building. This study illustrates the use of quantitative risk assessment as a practical tool for investigating the risks to onsite personnel. REFERENCES 1. American Petroleum Institute, 1995, API Recommended Practice 752: Management of hazards associated with location of process plant buildings, first edition 2. Committee for the Prevention of Disasters, 1999, CPR 18 E : Guidelines for Quantitative Risk Assessment, first edition 3. E&P Forum, 1996, E&P Forum QRA Datasheet Directory, Rev. 0 4. Shell Global Solutions, 2004, Shell FRED 4.0 User & Technical Manuals 5. Vectra Group Limited, 2005, EPE QRA Ignition Model, Document no. 103-3206-R03 6. Vectra Group Limited, 2005, Calculation of wind speed exceedance probability Exceedance study Shin Etsu 5