Estimating Consequence and Severity
TATA VENKATA SURYA PRAKASH, PMP?
A Chemical Engineer with Project Management Professional (PMP?) & 26 Years of experience as Project Manager with expertise in Oil & Gas Major Projects Management from Conceptualization, FEED, EPC.
A material weather it is a liquid, gas, or solid form, or a combination of these and is flammable, ignition may occur resulting in an explosion and/or fire. In the event of an immediate ignition of a pressurized gas or two-phase release, jet fires may occur. If immediate ignition is absent, material may dispersion leading to form a vapor cloud with delayed ignition causing a flash fire or explosion. LOC Liquid spills which will burn as pool fires if ignited. If toxic material is released, plant personnel or the public may be exposed to unhealthy concentrations(BHOPAL MIC Release) and also leading to physical effects like radiation flux from fires, overpressures from explosions, and toxic concentrations from toxic releases. These in turn will have “impact” on personnel, environment and property, and may result in losses such as injuries, fatalities, environmental harm, and property damage. In addition to these initial effects, there could be follow-on losses due to business interruption, loss of quality of product, demolition requirements, and loss of credibility with the public, regulators, customers, and stockholders. The range of consequence endpoints for a loss of containment scenario include the release of the hazardous material, the dispersion of the hazardous material, physical effects from fires, explosions and toxic releases; and the losses from the impact of physical effects. Consequence endpoints are quantifiable as a release can be measured in terms of the released quantity; the dispersion in terms of dispersion distance/area (for specific concentrations); and the losses in terms of number of injuries and fatalities, property damage, financial losses or indirect losses.
CONSEQUENCE EVALUATION
The different types of consequence evaluations are:
DEVELOPING SCENARIOS
A scenario is an unplanned event or sequence of events that results in an undesirable consequence. Each scenario consists of:
? An initiating event (e.g., loss of cooling) that starts the chain of events and
? A consequence (the potential for over pressuring the system, release of toxic or flammable material to the atmosphere, fatality, etc.) that results if the chain of events continues without interruption.
Elimination of scenarios through an ISD (Inherently Safer Design) approach is an widely used option for RISK REDUCTION.
As an example if a process is designed or modified in such a way such that the inventory of released toxic material is reduced to level where consequence is minimal and associated risk is reduced.
If a vessel is designed to resist an internal explosion, or the shut-off head of a pump, or a relief flow is passed to a flare rather than directly to the atmosphere, the risk associated with scenarios with these consequences may be reduced or eliminated.
Scenario shall have a unique initiating event/consequence pair and same initiating event can result in different consequences, additional scenarios should be developed. In some cases many scenarios may spring from a common initiating event (e.g., loss of a utility to a facility) and separate scenarios should be developed for individual sections of the plant.
Scenario also includes:
? Enabling events or conditions that have to occur or be present before the initiating event can result in a consequence.
? The failure of safeguards (IPL'S) - Thumb Rule is "Not all safeguards are IPLs, but all IPLs are safeguards".
The Scenario Modifier include but not limited to:
? The probability of ignition of a flammable material (liquid or vapor release).
? The probability of a person being present in the area affected by the event.
? The probability that a fatal injury will result from exposure to the effects of the fire, explosion, or toxic release—includes evacuation or protective action, or
? The probability that an estimated financial loss to the facility of a certain magnitude will result.
I conclude with a classic example of:
Loss of cooling (the initiating event) can result in a runaway exothermic reaction in a batch reactor and overpressure, but only during a portion of the reaction (the enabling condition) when the system is in the reaction exotherm phase and thus vulnerable to loss of cooling.
There are many safeguards in place against overpressure (alarms, operator interaction, manual venting, SIFs, relief devices, etc.) that may have been identified by a hazard evaluation team. Here, these safeguards might determine that only two of these might be considered as meeting the requirements of an IPL for LOPA.
? BPCS (basic process control system) function (i.e., interlock) designed to detect high temperature/pressure and take action to prevent the runaway exothermic reaction; and
? correctly sized and maintained relief valve to prevent the overpressure of the system following an exotherm.
The above scenario for loss of cooling leading to overpressure of the reactor:
1. Loss of cooling (Initiating Event).
2. Reactor in a condition where exotherm can occur if cooling is lost (Enabling Condition)
3. BPCS fails to act correctly (Failure of IPL)
4. Relief valve fails to act correctly (Failure of IPL) RESULTING IN:
5. Overpressure of reactor system (Consequence—flange leakage and/or potential rupture with large release of energy and/or hazardous material and potential for fatalities, injuries, or property or environmental damage).
TATA VENKATA SURYA PRAKASH
"MIC is an Intermediate product whose storage is convenient but not essential" Trevor Kletz on BHOPAL MIC leak