Methodology Step 8

Pressure Relief Devices 

• Do CoF per Demand Scenario. CoF is NOT identical to the CoF for the Equipment that the PRV protects.
• CoF represents the consequence of failure on demand.
• Notes for External Fires and Pressure accumulation. 

Corrosion Under Insolation (CUI)

• The CCAM priority is assessed on the Equipment. It is advice to manage CUI over CCAM.
• Notes on Consequence Assessment. 

Stress Corrosion Cracking

Use an equivalent hole size that represents the leak; see also equivalent hole sizes per cracking mechanism in the appendices.

N

Slight damage: < 100 kUSD

No/slight disruption to operation

L

Minor damage: 100k -1 MUSD

Brief disruption

M

Local damage: 1 - 10 MUSD

Partial shutdown

H

Major damage: 10 - 100 MUSD

Partial operation loss (up to 2 weeks shutdown / reduced throughput)

E

Extensive damage: > 100 MUSD

Substantial or total loss of operation

Production Loss

Deferred income (loss or downgrading of product) plus product wasted (e.g., flared or spilled), usually calculated as margin loss plus manpower cost.

Repair Costs

The costs other than labour (but including fixed contractor costs) incurred to have the damage repaired.

Labour Costs

The costs (hours) of own personnel or contractor staff on reimbursable contracts.

(Total) Production Loss

C_TPL = C_prod + C_misc

First, the Production Loss Equation (PLE) should be predefined (see Configuring the Toplevel via Settings). The PLE defines production loss rate in different time intervals.

You can input the Downtime period, T_down, and the period and percentage of Reduced Throughput, T_red, and P_red:

T_lost = T_down + T_red ´ P_red

IMS then uses the PLE to calculate the production losses, C_prod, from T_lost.

Except for downtime and reduced throughput, you can also specify other Miscellaneous costs, C_misc, e.g., reprocessing costs (energy), efficiency losses, etc.

Repair Costs

C_repair=

C_mat+C_contr+C_mrep

These are all costs (excluding labour) required to restore a failed item back to its desired performance. Three elements are considered in the questionnaire.

• Material/Equipment: Materials, Equipment, spare parts, etc. required for repair or replacement (C_mat).
• Contract payments: Fixed contractor costs (lump-sum contracts) for the repair and/or installation (C_contr).
• Miscellaneous costs: Other costs such as secondary damage, costs of making temporary facilities, etc. (C_mrep).

Labour Costs

C_labour = T_craft ´ R_craft + T_ops ´ R_ops + T_staff ´ R_staff + T_contr ´ R_contr.

Variable labour costs of internal and contractor personnel are calculated for a specific failure. The labour rates for each discipline are predefined (see Common Data Settings): Craftsmen, R_craft, Operators, R_ops, Staff, R_staff, and Contractors, R_contr. These are very generic and can probably be used for all units on one site. Failure specific input are the time each discipline needs as a response to the failure: Craftsmen, T_craft, Operators, T_ops, Staff, T_staff, and Contractors, T_contr.

N

Slight injury

• No Treatment Case or First Aid Case
• Illnesses that result in noticeable discomfort, minor irritation or transient effects that are reversible after exposure stops

L

Minor injury or health effect

• Medical Treatment Case
• Lost Workday Case or Restricted Work Case, where either has a duration of up to and including 5 days
• Illnesses with reversible health effects such as food poisoning and dermatitis

M

Major injury or health effect

• Lost Workday Case or Restricted Work Case, where either has a duration exceeding 5 days
• Illnesses with irreversible health effects such as sensitization, noise induced hearing loss, chronic back disorders, or repetitive strain injury
• Mental illness due to stress with reversible health effects

H

Permanent total disability or up to three fatalities

• Including illnesses with irreversible health effects such as corrosive burns, specific occupational or work-related diseases
• Mental illness due to stress with irreversible health effects

E

More than three fatalities

• Including illnesses with irreversible health effects such as corrosive burns, specific occupational or work-related diseases reducing life expectancy

Flammability

The flammability index, Nf, originating from the American NFPA (National Fire Protection Association) standard 704, is used as a factor in flammability. It ranges from 0 to 4. Substances with a Nf > 1 are considered flammable.

Another factor is product temperature, T_product, in relation to the product’s Flash Point (FP)*, and Auto Ignition Temperature (AIT).

A flammable cloud originates from:

• The release of substance with Nf > 1 and T_product > AIT

(if it immediately sets fire, no explosion takes place)

• The release of a substance with Nf > 1 and FP < T_product < AIT

(evaporation should lead to a flammable cloud).

• Nf < 2 (non-flammable) or Nf >1 and T_product < FP

Released quantity

Available fuel is estimated by the released quantity, or in case of a continuous release the released quantity per hour. Alternatively, the Equipment inventory can be used instead (e.g., rapture case).

IMS combines the Released quantity and Flammability (property) in the Fire Consequence matrix to determine the Consequence score. The highest possible score for fire is “H” (high), which corresponds to “≤3 fatalities”.

Two scenarios can cause impact (blast) effects:

• A Vapour Cloud Explosion (VCE) or
• Another (Pressure) Explosion.

The severity of both incidents must be estimated. The maximum of the two is the overall Consequence.

VCE Possibility

A VCE is only possible if a flammable gas cloud is ignited in a congested or confined area. This cloud can come from released flammable gas or a flammable vapor from flashing or evaporating volatile liquid. Delayed ignition of the cloud in an open area will cause a flash fire (also called cloud fire or deflagration) with a very small or no pressure wave. A congested area shapes the explosion, which may trigger transition from deflagration to detonation (supersonic) with much larger pressure wave.

The Consequence of a vapor cloud explosion is determined by the degree of congestion and /or confinement which is determined by the number of obstacles (e.g., pipe work) that a flame front must pass during combustion. Congestion is difficult to assess, but most process plant areas can be defined as medium congested. Only a few are heavily congested, e.g., all enclosed spaces, dark (no sunlight) areas in the middle of a plant, and units with a lot of Equipment (e.g., Crude Distiller Units). Congestion should not only be assessed at the area of release, but also for areas where the cloud can travel to.

Release vapor mass

The other important parameter, needed to determine VCE Consequence, is the amount of fuel available. This is estimated by the released quantity or, in case of a continuous release, the released quantity per hour. Note: First determine the typical hole size for a particular DM’s failure. Then determine the release rate from the hole size and the pressure.

IMS combines the Released vapor mass (quantity) and VCE Possibility (property) in the Common Consequence matrix to estimate the VCE Consequence.

Other explosion

Other forms of impact effects, particularly rupture (e.g., runaway reactions, or BLEVE (Boiling liquid expanding vapor explosion), severe over pressurization), resulting in failure of high-pressure Equipment, potentially causing flying debris (e.g., pipeline rupture, high speed compressors, brittle fracture). This “high pressure” consequence part is specifically meant to deal with Equipment rupture (and usually dominates for non-flammable, non-toxic (e.g., air and steam) pressure events).

The severity of a rupture depends on the potentially stored energy, which, in turn, depends on pressure and volume (Energy = p x V). For pipes, the effective volume is estimated by taking a section with a length of fifty times the diameter, D, i.e., V=50D ´ ¼pD². Thus, in this case, the Consequence is determined by the energy content. See Release Rate Calculations (High Pressure Consequence of Gas Pipes).

Toxic effects to people may occur when a failure results in the release of a toxic substance into atmosphere.

Typical examples in a refinery, are containment failures of Equipment holding hydrogen sulphide (H₂S) or hydrogen fluoride (HF).

The dose received by a person is determined by the concentration of the toxic agent in atmosphere, and short-term exposure time. Dose and toxicity, together, determine the health effects.

Note: A release of a fluid that is corrosive to the skin is also considered as a toxic effect.

Toxicity

The toxicity index (or health index), Nh, originating from the American NFPA standard 704, is used as a measure of toxicity. It ranges from 0 to 4. Substances with an Nh > 1 are considered toxic.

Concentration

To accurately calculate a toxic dose is outside the scope of this assessment, since it requires a lot of detailed information (leak size, atmospheric conditions, dispersion models, location, exposure, etc.).

Instead, the dose is estimated by the concentration of the toxic substance within the Equipment item (e.g., vessel or pipe). It is a coarse estimate, but at least provides a handle to rule out combinations of toxicity and concentration, which do not lead to health effects.

The released quantity is not considered, but it is assumed that it is large enough to cause harm.

The mitigation analysis can be used to correct the outcome for cases where only a small amount of toxic gas is released.

IMS combines the Concentration (quantity) and Toxicity (property) in the Toxic Consequence matrix to estimate the toxic Consequence.

For most of the contaminations involving liquid “sprays “, where liquid burns (strong acids - corrosive burns) can happen, or other special effects (phenol), the Consequence is max (class High). The same applies for the release of a hot liquid, like steam or heating oil.

A hazardous event will only lead to health & safety consequences if people are present in the hazardous zone and have no means of averting or protecting themselves.

Thus, to be realistic, these two mitigating factors must be considered.

In this context, mitigation has a more limited meaning than in general HSE guidelines, where it also includes hardware measures to reduce consequences (e.g., water screens, foam systems, safety relief valves etc.).

Exposure

The frequency of presence and duration of exposure of people in the hazardous zone is one mitigating factor. This should be estimated with the failure mode scenario in mind rather than just calculating an average exposure. In the questionnaire the figures for average exposure are given between brackets but should only be used for loss of containment with a short duration which occur purely randomly.

Some examples to clarify this concept:

• A failure that leads to a hazardous event with a short duration (explosion, small “puff” release of toxic gas) in a confined space (e.g., furnace, compressor house) without effects outside that space and where people are normally not present within that space (Exposure =1, very rare).
• Continuous release of toxic gas in an area where people are not normally present, but it is almost certain that people will enter the hazardous zone during time of release (Exposure =3).

Possibility to avert danger

In estimating the possibility for averting, or self-protection against, the hazardous situation due to a failure mode, the following should be considered:

• The time there is available to react (depends on the leak rate, the nature of the Hazard e.g., Gas/Liquid);
• Detection of the leakage (e.g., seen immediately, detected by technical means or detected without technical means);
• Avoidance of exposure to hazardous event (e.g., escape routes and specific means such as escape masks, etc., available or only available under certain conditions);
• Use of personal protective Equipment (PPE) (e.g., H2S personal detectors); and
• Actual safety experience (with similar Equipment and/or failures).

Both Exposure and Possibility to avert danger may decrease the People Consequence. They are expressed as numbers (1, 2 or 3) and their combination leads to either no Consequence reduction or a reduction by one to two classes, see below.

N

Slight effect

Slight environmental damage - contained within the premises. Example:

• Small spill in process area or tank farm area that readily evaporates

L & G

L

Minor effect

Minor environmental damage, but no lasting effect. Examples:

• Small spill off-site that seeps into the ground.
• On-site groundwater contamination.
• Complaints from up to 10 individuals.
• Single exceedance of statutory or another prescribed limit

L & G

L

L

L & G

L & G

M

Moderate effect

Limited environmental damage that will persist or require cleaning up. Examples:

• Spill from a pipeline into soil/sand that requires removal and disposal of a large quantity of soil/sand.
• Observed off-site effects or damage, e.g., fish kill or damaged vegetation.
• Off-site groundwater contamination.
• Complaints from community organizations (or more than 10 complaints from individuals).
• Frequent exceedance of statutory or another prescribed limit, with potential long-term effect

L & G

L

L

L

L & G

L & G

H

Major effect

Severe environmental damage that will require extensive measures to restore beneficial uses of the environment. Examples:

• Oil spill at a jetty during tanker (off) loading that ends up on local beaches, requiring clean-up operations.
• Off-site groundwater contamination over an extensive area.
• Many complaints from community organisations or local authorities.
• Extended exceedances of statutory or other prescribed limits, with potential long-term effects.

L & G

L

L

L & G

L & G

E

Massive effect

Persistent severe environmental damage that will lead to loss of commercial, recreational use or loss of natural resources over a wide area. Example:

• Crude oil spillage resulting in pollution of a large part of a river estuary and extensive clean-up and remediation measures.

L

N

Slight effect

• Infrequent slight nuisance.
• No observable adverse or perceived effect on livelihood, social and cultural assets, community security, community health, vulnerable or Indigenous People.
• Local public awareness but no discernible concern.

L

Minor effect

• Limited short-term nuisance.
• Limited effects on livelihood and / or social or cultural assets, community health.
• No observable adverse effect on community security, vulnerable or Indigenous Peoples.
• Local public concern.

M

Moderate effect

• Persistent nuisance.
• Effects on livelihood and / or social and cultural assets, community health.
• Limited observable effects on community security, vulnerable or Indigenous Peoples.
• Local or Regional public concern.
• Local stakeholders, e.g., community, non-government organization (NGO), industry and government, are aware.

H

Major effect

• Persistent effects on livelihood and / or social and cultural assets, community health.
• Effects on community security, vulnerable or Indigenous Peoples and / or human rights infringements, that are serious and / or at a community level.
• Mitigation is complex or protracted.
• National public concern.
• Impact on local and national stakeholder relations.
• National government and NGO involvement with potential for international NGO action.

E

Massive effect

• Persistent, severe impact on livelihood, social and cultural assets, community security, community health, vulnerable or Indigenous Peoples and / or human rights infringements.
• Impact may affect a large geographic area or population.
• Mitigation is complex or protracted, and of limited effectiveness.
• International public concern.
• High level of concern and action(s) by governments and / or by international NGOs.