Passive Fire Protection Analysis
Passive fire protection of the following shall be assessed:
- Vessels
- Pipework between vessels
- Shutdown valves and blowdown valves
- Vessel and pipework supports
The approach to determine the required passive fire protection for a particular item of process plant equipment or structural steel work shall be broadly based on that described within the API Publication 2218, Fireproofing Practices in Petroleum and Petrochemical Processing Plants. This approach includes the following steps:
- Hazard evaluation, including the quantification of inventories and potential fuels
- Development of fire scenarios, including release rates and duration, to establish the size of the fire scenario envelopes
- Determining fireproofing needs based on consideration of the probability of an incident impacting that particular item of equipment, the time to failure, and the potential impact of damage that may result in terms of escalation and risk to life
Fire Hazard Evaluation
The first step in evaluating the requirement for passive fire protection is the identification of fuel sources and their location. This includes information relating to the fuel itself, and the inventories stored or processed, together with the process conditions.
Fire Scenario Development
The fire hazard information enables the likely fire scenario to be developed based on a consideration of the events that follow a release of material. For example, a release of gas that undergoes immediate ignition will result in a jet fire. Delayed ignition results in the formation of a gas cloud and subsequent explosion or flash fire, depending on the degree of congestion.
The fire scenario considers the amount of material that may be released and the release duration based on the system pressure and the orifice size. If the release involves a liquid, consideration is given to any local impounding or dike that restricts the size of the liquid pool. In addition, the effect of vaporisation and flashing from the pool is also considered.
Source Term
Source term modelling involves determining the initial release rate and release rate profile as the system depressurised based on the material, operating condition and inventory. This study will consider the effect of the fire and gas system detecting any release and initiating isolation and blowdown. It is assumed that the blowdown shall be designed in accordance with API RP 521, Guide for Pressure Relieving and Depressurisation Systems, i.e. designed to be depressurised to 100psig (6.9barg) or 50% of the design pressure, whichever is less, within 15 minutes. The system inventory and blowdown criteria shall be used to establish the release rates and release durations.
Fire Consequence Modelling
This stage of the fire analysis involves determination of the impact of each of the identified hazardous outcomes end events. Pool and jet fires will be considered. The consequence modelling will determine the jet flame length, pool fire size, and fire duration.
In the event of fire impairment, the onset of damage to structural, equipment or pipework targets is dependent upon the size and location of the flame, the intensity of the fire and its duration. The size and location determines which targets will be hit. The intensity of the fire depends on the type of fire.
Escalation consequence modelling
The consequences of vessel, supports, pipework or structural steel failure shall be considered in the assessment. This shall include:
- The energy and resultant over-pressures following a vessel failure
- The inventory of hydrocarbon at time of failure
- The criticality of the process item, e.g. HP flare knock out drum
- The degree of structural redundancy
Time to failure
The time to failure for the item of process equipment or structural steel shall be evaluated based on methods described in the Scandpower Guidelines for the protection of pressurised systems, and the Institute of Petroleum Guidelines for the design and protection of pressure systems to withstand severe fires.
Needs Analysis
The needs analysis determines if passive fire protection is considered necessary for a particular piece of equipment, pipework or structural member. This analysis starts with consideration of the severity of equipment’s engulfment in the largest credible fire that has a significant duration. The fire must have sufficient duration to still impinge onto the target surface at the time to failure. Consideration is given to the vulnerability of the equipment (for example the possible presence of a liquid to facilitate heat removal from the system), and the impact of failure due to fire impingement. For vessels, the pressure at the time of failure is based on the vessel undergoing blowdown from either relieving condition or operating.
The Active Fire Protection has been shown to be effective in a pool fire but uncertainties remain regarding the degree of protection afforded in a jet fire. Hence, Active Fire Protection is given credit for the pool fire scenarios only in this study.
Criteria for provision of the PFP
The criteria for determining fireproofing shall be based on a set of agreed criteria that may include, for example, the criteria detailed in the Scandpower guidelines:
- Vessels failing before 5 minutes should be provided with PFP; based on vessel wall thickness data to be provided by COMPANY
- Vessel supports shall be provided with passive fire protection if the requirements of NFPA 15 cannot be met
- Vessels failing after completion of blowdown maybe considered to cause minimal damage as blowdown will have de-inventorised / de-pressurised the system
- Structural steel work assessment shall be based on the structural steel redundancy assessment, which is to be provided by COMPANY, and a qualitative assessment of likely consequences of failure
Passive Fire Protection Analysis
Passive fire protection of the following shall be assessed:
- Vessels
- Pipework between vessels
- Shutdown valves and blowdown valves
- Vessel and pipework supports
The approach to determine the required passive fire protection for a particular item of process plant equipment or structural steel work shall be broadly based on that described within the API Publication 2218, Fireproofing Practices in Petroleum and Petrochemical Processing Plants. This approach includes the following steps:
- Hazard evaluation, including the quantification of inventories and potential fuels
- Development of fire scenarios, including release rates and duration, to establish the size of the fire scenario envelopes
- Determining fireproofing needs based on consideration of the probability of an incident impacting that particular item of equipment, the time to failure, and the potential impact of damage that may result in terms of escalation and risk to life
Fire Hazard Evaluation
The first step in evaluating the requirement for passive fire protection is the identification of fuel sources and their location. This includes information relating to the fuel itself, and the inventories stored or processed, together with the process conditions.
Fire Scenario Development
The fire hazard information enables the likely fire scenario to be developed based on a consideration of the events that follow a release of material. For example, a release of gas that undergoes immediate ignition will result in a jet fire. Delayed ignition results in the formation of a gas cloud and subsequent explosion or flash fire, depending on the degree of congestion.
The fire scenario considers the amount of material that may be released and the release duration based on the system pressure and the orifice size. If the release involves a liquid, consideration is given to any local impounding or dike that restricts the size of the liquid pool. In addition, the effect of vaporisation and flashing from the pool is also considered.
Source Term
Source term modelling involves determining the initial release rate and release rate profile as the system depressurised based on the material, operating condition and inventory. This study will consider the effect of the fire and gas system detecting any release and initiating isolation and blowdown. It is assumed that the blowdown shall be designed in accordance with API RP 521, Guide for Pressure Relieving and Depressurisation Systems, i.e. designed to be depressurised to 100psig (6.9barg) or 50% of the design pressure, whichever is less, within 15 minutes. The system inventory and blowdown criteria shall be used to establish the release rates and release durations.
Fire Consequence Modelling
This stage of the fire analysis involves determination of the impact of each of the identified hazardous outcomes end events. Pool and jet fires will be considered. The consequence modelling will determine the jet flame length, pool fire size, and fire duration.
In the event of fire impairment, the onset of damage to structural, equipment or pipework targets is dependent upon the size and location of the flame, the intensity of the fire and its duration. The size and location determines which targets will be hit. The intensity of the fire depends on the type of fire.
Escalation consequence modelling
The consequences of vessel, supports, pipework or structural steel failure shall be considered in the assessment. This shall include:
- The energy and resultant over-pressures following a vessel failure
- The inventory of hydrocarbon at time of failure
- The criticality of the process item, e.g. HP flare knock out drum
- The degree of structural redundancy
Time to failure
The time to failure for the item of process equipment or structural steel shall be evaluated based on methods described in the Scandpower Guidelines for the protection of pressurised systems, and the Institute of Petroleum Guidelines for the design and protection of pressure systems to withstand severe fires.
Needs Analysis
The needs analysis determines if passive fire protection is considered necessary for a particular piece of equipment, pipework or structural member. This analysis starts with consideration of the severity of equipment’s engulfment in the largest credible fire that has a significant duration. The fire must have sufficient duration to still impinge onto the target surface at the time to failure. Consideration is given to the vulnerability of the equipment (for example the possible presence of a liquid to facilitate heat removal from the system), and the impact of failure due to fire impingement. For vessels, the pressure at the time of failure is based on the vessel undergoing blowdown from either relieving condition or operating.
The Active Fire Protection has been shown to be effective in a pool fire but uncertainties remain regarding the degree of protection afforded in a jet fire. Hence, Active Fire Protection is given credit for the pool fire scenarios only in this study.
Criteria for provision of the PFP
The criteria for determining fireproofing shall be based on a set of agreed criteria that may include, for example, the criteria detailed in the Scandpower guidelines:
- Vessels failing before 5 minutes should be provided with PFP; based on vessel wall thickness data to be provided by COMPANY
- Vessel supports shall be provided with passive fire protection if the requirements of NFPA 15 cannot be met
- Vessels failing after completion of blowdown maybe considered to cause minimal damage as blowdown will have de-inventorised / de-pressurised the system
- Structural steel work assessment shall be based on the structural steel redundancy assessment, which is to be provided by COMPANY, and a qualitative assessment of likely consequences of failure