Guidelines for Providing Process Conditions for RBI - Part 3: Process Fluids and Consequence Models

In Part 3 of our series on guidelines for providing process conditions for RBI, we move on to discuss process fluids and offer some guidance on how to select a representative fluid that will be effective with respect to your RBI consequence calculations.

Process fluids are one of the most important factors in the consequence model of any RBI risk calculator tool. The process fluid is used to the determine the outcome of a loss of containment, which can range from benign for water to catastrophic for Hydrofluoric Acid. There are four basic process fluid types that are typically used in RBI safety consequence models. These fluid types are Inert, Reactive, Flammable, and Toxic. Reactive fluids are fluids that can cause chemical or thermal burn on contact. Examples are skin irritants and high temperature fluids. Toxic fluids are fluids that are hazardous to your health if inhaled, ingested, or absorbed through the skin. Flammable fluids are fluids that have a flashpoint at or below 199.4°F. Inert fluids are fluids that are not reactive, toxic, or flammable. Examples of inert fluids are air and nitrogen.

Many process streams have multiple process fluids (for example a distillation column may contain various hydrocarbon fractions such as propane, butane, light naphtha, diesel, etc.), however only one of these process fluids will need to be chosen as the representative fluid to determine the consequence for a particular asset or component of an asset being modeled. The process data collection step is the ideal time to choose the representative fluid that will be used to determine the consequence. Process engineers should be integrally involved in choosing the process fluid for any process stream. In addition to the process engineer's knowledge of the process streams for the particular unit, the Safety Data Sheets (SDS) can provide information that will help in determining the correct representative fluid to use when there are multiple fluids in the process stream. The heat and material balance can also be useful in selecting the correct representative fluid since it provides a breakdown of each constituent of a process stream into percentage by composition.

Based on the information gathered from the process engineer, SDS, and heat and material balance, the below guidelines can be used in helping to select the best representative fluids to use in RBI consequence model as well as identifying the toxic fluids that should be included:

• For process streams that have multiple flammable fluids, the fluid with the highest concentration by weight and having the lowest Auto Ignition Temperature (AIT) should be used. The AIT can be found in the process fluid's SDS.
• In cases where the fluid with the highest concentration does not have the lowest AIT, choose the fluid with the highest weight concentration if is an order of magnitude higher in concentration.
• If multiple toxic fluids are present, the fluid with the lowest Toxic Endpoint (ERPG2) or Permissible Exposure Limit (PEL) should be chosen. The ERPG2 can also be found in the toxic fluid's SDS.
• For situations where the fluid is a flammable/toxic mix, consequence models provide an option for using a flammable representative fluid with a percentage toxic fluid. An example of this would be where there is a sour hydrocarbon stream with a small percentage of H2S. The hydrocarbon would be the flammable representative fluid while the H2S would be the toxic fluid with a percentage of composition. The final consequence category would be the higher of the flammable or toxic consequence.

The goal in choosing a representative fluid is to determine a consequence to be used for a risk calculation. Some representative fluids can be determined easily but there are times when the choice of assigning the representative fluid is not easy to make. In those situations, performing a "What if Analysis" or Sensitivity Analysis using the various constituents of a multiple component process stream can be helpful in making the decision. The fluid providing the most conservative consequence should be used for the final risk calculation. Choosing the correct process fluid is an important factor in calculating the consequence in risk models and should not be taken lightly when trying to accurately determine the relative risk of an asset.

Stay tuned for the next entry in this eight-part series covering guidelines on assigning process conditions for RBI efforts:

1. Guidelines for Providing Process Conditions for Risk Based Inspection (RBI) Implementation and Revalidation (Introduction)
2. Corrosion Under Insulation (CUI) and How it Relates to Risk Based Inspection
3. Process Fluids and Consequence Models (this article)
4. High Temperature Damage Mechanisms
5. Low Temperature Damage Mechanisms
6. High Temperature Hydrogen Attack
7. Environmental Cracking Damage Mechanisms
8. Concluding Remarks

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