Consulting

Fire & Gas Safety System Review

Fire, and Explosion Consequence Analysis

The consequence analysis and modelling is carried out using the software tools. The effects of fire and explosions, i.e. thermal radiation, over-pressures, LFL and UFL dispersion contours are superimposed onto site plans.

The consequence modelling and impairment criteria due to fires and explosions shall be in accordance with the hole size and modelling parameters as detailed in the relevant COMPANY Technical Guidance.

The impact onto the following shall be studied:

• Adjacent equipment / facilities
• Buildings (occupied and functionally significant)
• Onsite and offsite (where applicable)

The methodology is given below:

Step-1: Assumptions Register

Prior to undertaking any consequence modelling all assumptions used in the study shall be documented on an Assumptions Register.

Typically, the register will address the following issues and assumptions:

• Detection, isolation, and blowdown assumptions
• Hole size data
• Composition Data
• Release Data
• Atmospheric Conditions
• Impact criteria for over-pressure, thermal radiation, flash fire, etc.

Step-2: Consequence Analysis

The consequence analysis is undertaken in a series of steps:

• Identification of all fuel types, as determined by their operating conditions and components. A list of typical fuels inventories will be generated as the basis for consequence analysis
• Assessment of available inventories for release, as defined by isolatable sections of plant, and times to isolation of ESD systems, and failure on demand of ESD systems
• Calculation of release rates
• Estimation of potential pool sizes from liquid releases, based upon release rates and times, evaporation rates and local topography
• Calculation of potential flammable vapour cloud sizes
• Calculation of the extent of thermal, flammable gas dispersion and overpressure effects distances
• Time dependant analysis based upon time criteria derived from ESD systems response time
• Suitable sensitivity analyses, including meteorological conditions, topography, surface roughness, operating conditions, isolation times/regimes etc. The sensitivities to be undertaken will be formalised during the data gathering stage and early part of the consequence analysis stage of the project

The impact criteria stated in the COMPANY Technical Guidance shall be used.

The calculated impacts shall be presented as contours overlaid on plot plans.

Step-3: Inventory Analysis

The inventory analysis is undertaken in a number of steps, specifically:

• Review of the Process Flow Diagrams (PFD) to determine where streams are in the process
• Review of the material balance sheets to gain an understanding of the properties of each stream (specifically pressure, volatility, flammable limits)
• Review the Piping and Instrumentation Diagrams (P&ID) to determine the isolatable sections
• Review of the plot plans to estimate pipe route length
• Review of the main plant equipment list for dimensions of equipment items
• Review of the normal liquid levels maintained within the main plant items, if applicable
• Estimation of the total isolatable system volume and the liquid volume
• Estimation of the system depressurisation based on mass balance data and the normal material inflow, taking into consideration successful isolation and blowdown following leak detection

The main objective of this inventory analysis is to:

• Identify the material properties and compositions of the hazardous streams based on the heat and material balances which will be further used for consequence modelling
• Identify the list of hazardous scenarios which has Major Accident Potential
• As an input to the consequence modelling

Step- 4: Isolation and Blowdown

Following the identification of isolatable sections and the inventory analysis, the Fire Safety Assessment evaluates the consequence of the fire and explosion events.  The FSA considers successful isolation and blowdown in evaluating the fire hazards as the system depressurises and the approximate duration of the fire.

Step-5: Fire Safety Critical Systems

The FSA shall review the following safety critical systems:

1.    Fire and Gas detection

Fire and gas detection system review shall be based on consequence modelling output, detector layout drawings, and detector vendor data. The scope excludes the use of fire and gas mapping software in the assessment of detector coverage.

2.    Emergency Shutdown

The emergency shutdown philosophy and emergency shutdown valve locations shall be reviewed to assess that they provide adequate protection against fire, explosion and toxic hazards.

3.    Emergency Blowdown

The emergency blowdown system shall be reviewed against the project philosophy or appropriate international standards, such as ISO 23251 petroleum, petrochemical and natural gas industries – pressure relieving and depressurising systems. The impact of successful blowdown shall be incorporated into the fire and explosion consequence modelling. The scope of supply excludes the calculation of the blowdown orifice size, or an assessment of flare system capacity.

4.    Passive Fire Protection

The requirement for passive fire protection is determined based on the international guidelines for the design and protection of pressure systems to withstand severe fires, and the Scandpower guidelines for the protection of pressure systems exposed to fire.

5.    Active Fire Protection

The FSA shall review and develop the active fire protection system requirements based on the project fire and explosion protection philosophy. The assessment shall establish the fire water application rate, which shall be in accordance with the latest edition of NFPA 15 Standard for water spray fixed systems for fire protection, API 2030 Application of fixed water spray systems for fire protection in the petroleum and petrochemical industries.