Chemical Process Safety Questions

How do you convert from one set of units to the other if the toxicant concentration in air is low?

(PPM*(MW)*P/RT)=mg/m3

What is relaxation time in the context of static electricity? What categories of materials have long relaxation times? Why is that important?

The relaxation time is the time required for a charge to dissipate through leakage. Liquids and solids typically have long relaxation times. It is important to be aware of it so that static electricity cannot build up and become a potential ignition source - must reduce the charge build-up because it is an electro-static hazard. When pumping fluids into a vessel through a pipe on top of the vessel, a streaming current is crated which is a basis for charge build-up.

All other things being equal, how is the concentration of a passive contaminant at a given location related to the mass release rate?

.Directly proportional for both puff and plume. All other things the same, lower source rates reduce the amount of contaminant moving downwind thereby reducing the concentration at a ll downwind distances.

The AIChE Code of Ethics provides imperatives for members to do in support of the engineering profession (up to date version on aiche.org; search for "code of ethics"). While all are important (and some basic to being a decent human being), there is some overlap in these, so for the purpose of summarizing, focus on 1,4,5,7,9 as particularly relevant to this course. Using the case studies in Ch 1,discuss how each of these have been violated. Be able to define conflict of interest.

1. Hold paramount the safety, health, and welfare of the public in performance of their professional duties. 4. Issue statements or present information only in an objective and truthful manner. 5. Act in professional matters for each employer or client as faithful agents or trustees, and avoid conflicts of interest. Conflict of interest is is a situation in which a person or organization is involved in multiple interests, one of which could possibly corrupt the motivation of the individual or organization. 7. Perform professional services only in areas of their competence. 9. Continue their professional development throughout their careers, and provide opportunities for the professional development of those under their supervision.

What is the difference between a plume and a puff release?

A plume is a constant release rate for a long period of time. A puff is usually considered an instantaneous release.

What is a spirometer? What does it measure?

A spirometer is a tool that measures the amount of air capacity in human lungs.

Name 5 guidelines which are used to identify the need for a relief device.

All vessels: Reactors, storage tanks, towers and drums Blocked-in sections of cool liquid filled lines that are exposed to heat or refrigeration need reliefs Positive displacement pumps, compressors, and turbines Storage vessels need pressure and vacuum reliefs to protect against pumping in or out of a blocked-in vessel or against the generation of a vacuum by condensation Steam jackets

What causes plume meander?

Alterations in atmospheric conditions. Wind at various speeds and directions. Surface roughness. Thermal considerations that cause changes in plume density. Though the mean wind direction may be the same there is variation associated with it that can cause meander. When looking at the averaging time for a plume, meander causes greater variation for longer periods of time.

How are passive atmospheric dispersion coefficients related to atmospheric stability? For increased atmospheric stability, do dispersion coefficients increase or decrease at a given distance? Be clear about what atmospheric stability means.

Atmospheric stability classes help qualify the conditions that exist during a toxic gas release. The more stable the environment, the less mixing occurs because there is less turbulence caused by wind speed and thermal heating of the ground surface. As stability increases, dispersion coefficients decrease causing the concentration to increase at a given distance. For increased atmospheric stability (probably D-F) the dispersion coefficient decreases which causes the concentration of the toxin to increase at a given location. All other things the same for a passive plume, increased atmospheric stability reduces the atmospheric turbulence present thereby increasing the concentration at all downwind distances.

Explain what is meant by a causative variable. Explain how a probit is typically related to the response of a biological organism to some causative variable.

Causative variable - the dosage, impulse, radiation or ect that causes an effect on the organism population. If the probit function is plotted vs ln(V) then the resulting graph will give a linear interpretation of the % of people affected by a given event vs the severity of the causative variable. This is much more useful than try to analyze the % of people affected based on response vs log dosage curve alone because of the sigmodial shape of the resulting curve.

What are the four fire classifications used on fire extinguishers?

Class A: for ordinary combustible materials - wood, paper, etc. Class B: for flammable and combustible liquids and gases Class C: for electrical fires - contains a non-conductive extinguishing substance. Class D: for combustible metals - should ONLY be used for this type of fire.

What are the four pillars of Risk Based Process Safety? What are the elements in each pillar? Be able to explain what they mean and they can be applied to the case studies in Ch 1.

Commitment to Process Safety, Understanding Hazards and Risks, Manage Risk, Learn from Experience. Look at document from homework 4 for specific elements in each pillar.

What is the difference between a continuous and instantaneous release?

Continuous release: modeled by a plume and happens over an extended period of time. Instantaneous release: modeled by a puff and happens over a very short period of time.

What are the advantages and disadvantages of using a conventional spring-operated relief valve compared to a balanced bellows relief valve?

Conventional - very reliable, can be used in many services, but affected by backpressure Bellows - not affected by backpressure, protects spring from corrosion, but may fatigue and may release flammables/toxics to atmosphere.

What are the main considerations which would typically indicate a rupture disk should be used as opposed to a spring-operated valve only?

Corrosive surface. Highly toxic material where spring loaded may weep.

What do the categories ED, TD, and LD represent? What does LD50 mean?

ED - Effective Dose. The log-dose curve for an agent if the response has only minor or reversible effects. TD - Toxic Dose. The log-dose curve for an agent if the response has non lethal but has irreversible effects. LD - Lethal Dose. The log-dose curve for an agent if the response is death. LD50 - Lethal Dose for 50% tested

What are the three most common ignition sources in major fires according to the text's table?

Electrical, smoking, friction.

What does ERPG stand for? What are the three levels of ERPG and how are they defined (time and exposure response)?

Emergency Response Planning Guidelines. Max exposure based on 1 hour without: ERPG 1 - experiencing mild health issues, smell odor. ERPG 2 - experience irreversible health issues. ERPG 3 - experience life threating effects.

Give an example of a design change which had unintended consequences. Be able to explain the lessons learned from the case studies summarized in Ch 1.

Flixborough, England incident. Line was switched to bypass reactor 5 in a series of 6 reactors. The design change was a bellows type piping system that wasn't the correct diameter. This led to a leak of roughly 20 tons of cyclohexane, and an explosion killing multiple people.

Explain the effect of meander on the concentration of a puff release at a given downwind distance.

For a puff still use a short averaging time, so at a given downwind distance just changes the location of the puff (20 seconds). Concentrations are not affected for a puff.

What does IDLH mean? What exposure time period is associated with IDLH? What exposure responses are used to determine the IDLH concentrations?

Immediately Dangerous to Life and Health. The exposure time basis is 30 min. This exposure would not allow for the person exposed to escape from immediate dangers.

What are the four pathways by which toxicants typically enter biological organisms? Which pathways are typically the fastest? Which pathways are considered most important in an industrial setting

Ingestion, Injection (fastest), Skin Absorption, and Inhalation (2nd fastest, most important in industrial setting).

Accident events can be categorized into a three step sequence. Name and define each step. Illustrate the application of these steps with an example.

Initiation - The event that starts the accident. Ex: Grounding/bonding, Explosion proof electrical, guardrails, hot work permits, etc. Propagation - The event or events that maintain or expand the accident. Ex: Emergency material transfer, reducing inventories of flammable materials, etc. Termination - The event or evens that stop the accident or diminish it in size. Ex: firefighting equipment and procedures, relief systems, check and safety valves, sprinkler systems, etc.

All other things, being equal how is the concentration of a passive contaminant at a given location related to the atmospheric stability?

Inversely proportional. Because σy and σz decrease with increased atmospheric stability, concentration at a given location increases with increased atmospheric stability.

For pressures below ambient, how do the LFL and UFL typically change with pressure?

LFL: Pressure has little affect on the LFL except for at extremely low temperatures. UFL: not changes significantly until reaches extremely low pressures. Below a certain pressure, there will be no combustion

For pressures above ambient, how do the LFL and UFL typically change with pressure?

LFL: Pressure has very little effect on the LFL except for at extremely low pressures where it can slightly increase the LFL. UFL: As pressure increases, the UFL increases significantly, broadening the flammability range.

For temperatures above ambient, how do the LFL and UFL typically change with temperature?

LFL: decreases with increased temperature. UFL: increases with increased temperature.

Explain the effect of meander on the concentration of a passive plume at a given downwind distance.

Meander changes the mean concentration that we are looking for. Spreads out a plume over time. Meander reduces the concentration at a given distance. For a plume the concentration downwind at a given point is affected by the averaging time (10min)—larger range of where the plume could be so the concentration changes more over that time. For very short averaging times the concentration is not changed because the meander doesn't have a chance to have an effect.

Are aerosols of flammable materials generally more flammable or less flammable than the liquid of the same material?

More flammable - kind of like a vapor. Typically a liquid doesn't catch fire, but the vapors above it catch fire, and aerosols are tiny droplets of water that can act like a vapor.

What does PEL stand for? What organization uses the PEL standard? PEL values are closely related to one of the TLV categories; which one?

Permissible Exposure Limit. OSHA. TLV-TWA or Threshold Limit Value - Time Weighted Average

All other things being equal, how is the concentration of a passive contaminant at a given location related to the wind speed? Consider limiting behavior of both puffs and plumes.

Puffs: Inversely proportional. Higher wind speeds have more atmospheric turbulent energy available to mix air into the cloud, and move the contaminant downwind at a faster rate thereby reducing the concentration at all downwind distances. Not necessarily for denser-than-air contaminants due to competing effects. Plume: Wind speed is not directly present in the equation so effects will only appear in the dispersion coefficients.

What dust size particle range is typically considered as most potentially harmful to the lungs? What does PPE stand for? Give two examples of PPE. Be able to fill in the NFPA diamond (which rating goes in which area, ranges of rankings, etc.) What are three common leading indicators of "best practices" related to process safety?

Respirable particle size range of 0.2 - 5 μm. PPE is personal protective equipment, and two examples are hard hats and safety glasses. Look at NFPA diamond. 1. Regular, documented inspections of safety critical items. 2. Documentation of keeping up with PHA/safety audit action items. 3. Number of demands on safety system (including PRD's)

Discuss examples of where the addition of safeguards decreased the overall safety of the system.

Safeguards can sometimes be a lot of work, or almost annoying, for some employees; therefore, these safeguards get ignored to make the job easier. A good example where a safeguard decreased the overall safety of a process is the Port Wentworth, Georgia, dust explosion. The steel cover panel was added over a process to decrease the sugar dust accumulation in an area, although, this added panel increased the dust accumulation inside the area it covered, and increased the risk because all that was needed was a ignition source and a overheating of a bearing caused this to explode.

What does SDS stand for, and what organization requires their use? List the minimum information categories that it contains.

Safety Data Sheet. OSHA requires their use as a part of the Hazard Communication Standard. The categories are as follows: 1. Identification 2. Hazardous Identities. 3. Composition/Information on Ingredients 4. First Aid Measures 5. Fire-Fighting Measures 6. Accidental Release Measures 7. Handling and Storage 8. Exposure Controls/Personal Protection. 9. Physical and Chemical Properties 10. Stability and Reactivity. 11. Toxicological Information 12. Other Information

Explain how a deflagration and a detonation are related along with their differences.

Similarities: Detonations and deflagrations are both types of explosions with a reaction front and a pressure front. In both situations the reaction front provides the energy for the shock front. Differences: Detonations produces a shock front with a very abrupt pressure rise that can be over 10 atm and a very short duration (about 1 millisecond). A detonation's reaction front propagates at a speed greater than the speed of sound. Deflagrations typically last for a longer amount of time and result in a more gradual pressure rise (no abrupt pressure rise) and typically have a max pressure of about 1 - 2 atm. A deflagration's reaction front propagates at a speed less than that of the speed of sound.

What does stability class indicate? Why is it important?

Stability class indicates the atmospheric conditions during a release. It is important because the level of mixing due to the atmospheric conditions (including wind speed and quantity of sunlight) can be changed significantly due to time of day and cloud cover. The more stable the atmospheric conditions, the less mixing, and the more likely the concentration of toxic flumes will accumulate and cause more damage.

Explain the term minimization with regard to inherently safer design. Give an example of a design change which exemplifies minimization. (Also substitution, moderation, and simplification and error tolerance)

Substitution - Safer materials replacing more hazardous materials. Ex: Use mechanical pump seals vs. packing, using welded pipes rather than flanged. Minimization - reduces the amount of hazards by using smaller quantities of hazardous materials in the process. Ex: change from a large batch reactor to a smaller CSTR reactor, reducing storage inventory of raw materials, etc. Moderation - Using a hazardous chemical under less hazardous conditions. Ex: reducing temperature and pressure, use vacuum to reduce the boiling point, etc Simplification - reduces the opportunities for error and misoperation. Ex: Make piping diagrams easy to read and follow, make design work clean and easy to follow as well as designed with safety in mind, etc.

What are the four categories of inherently safer design techniques? Give brief descriptions of each.

Substitution, Minimization, Moderation, and Simplification. (for definitions look at definitions cards)

What is the surface roughness in the context of atmospheric flow? How is surface roughness related to the mean obstacle height?

Surface roughness is geographical obstacle either natural or man-made that effects the behavior of gas dispersion. Surface roughness causes more turbulence in atmospheric flow because the features impede smooth air flow. The greater the mean obstacle height the greater the surface roughness, and the more mixing is facilitated. Rural areas have less mixing because it contains more open fields with low surface roughness where urban areas contain more buildings and are more turbulent, and therefore provide better dispersion.

Discuss the importance of timeliness of communication in the management of risk.

The importance in communication can be stemmed towards providing the community with transparency, and ultimately to the AIChE principle #1 (Hold paramount the safety, health, and welfare of the public in performance of their professional duties). An example of poor communication would be the Seveso, Italy disaster, in which many people were harmed by TDCC gas exposure due to a lack of communicating to local authorities. This delayed the time of evacuation in the city and ultimately effected the general population as well as the workers.

Explain the difference between FAR and fatality rate.

The FAR (fatal accident rate) is a measure of the number of fatalities x10^8 per total hours worked by all employees during this time period. This statistic is good for measuring how many fatalities there are in a certain time period, whereas the Fatality Rate measures the number of fatalities per year per number of people who worked. This statistic would be good for measuring the percent of people affected in a certain year.

Explain the difference between bonding and grounding.

The difference between bonding and grounding is that bonding occurs between two conductive materials reducing the voltage difference to zero. Grounding involves decreasing the voltage difference between the bonded materials to ground level or zero to the ground which eliminates charge build up Bonding drops two objects voltage to zero by connecting the two materials, grounding takes two bonded objects' voltage to zero by discharging to the ground.

What is the difference between unstable, neutral and stable atmospheric conditions?

The difference between them is the amount of sunlight and the wind speeds that are caused by the close the surface density of the air. A-C are the most unstable, and therefore have the greatest mixing. These conditions typically occur during the day and can increase mechanical turbulence. D is neutrally stable and can occur both during the day and at night depending on wind speed and insolation. D is the best case scenario at night. Neither increases or diminished the mechanical turbulence. E-F are most stable conditions and occur primarily at night and diminishes mechanical turbulence. The effect of atmospheric conditions is amount of mixing that occurs. The more stable, the less mixing, and the more unstable the more mixing (better for dispersion).

Give an example of a situation where economic issues affected a situation (for good or bad) that we discussed in class. Provide a brief explanation of your example.

The video watched on 1st day of class was a clear indicator of an economic issue affected a situation negatively. The example showed a lack of resources at one facility due to building up another facility somewhere else. This led to poor corrosion reports, improper MOC, and eventually the death of some workers.

What time of day can stable atmospheric conditions typically occur?

These are the E-F classes. These typically occur at night when it is cloudy and there are low wind speeds. D is the most stable during the day.

What does TLV mean? What are the different categories of TLV? Explain what each category means.

Threshold limit value (look at definitions cards for more specific answers for TLV-TWA, TLV-STEL, and TLV-C)

Explain why a pressure gauge is often put between a relief disk and a relief valve where the relief disk is exposed to a process condition.

To make sure there isn't pinhole leaks in the rupture disk

Which will generate higher static voltages and energies given all other factors are unchanged: laminar or turbulent flow?

Turbulent flow - more electrons moving quicker, so charge is more likely to build up.

In what two sets of units are airborne toxic gas concentrations typically expressed?

ppm and mg/m3

Explain the important aspects of the various purging techniques including vacuum, pressure, vacuum/pressure, and sweep-through.

• Vacuum Purging: Vessel must be rated for vacuum. Uses the least nitrogen of the choices because oxygen concentration is reduced primarily by vacuum. Most common, but not used for large storage vessels because they are typically not rated for vacuums. Common procedure for reactors. o drawing a vacuum on the vessel until the desired vacuum is reached o relieving the vacuum with an inert gas, such as nitrogen or carbon dioxide to atmospheric pressure o repeating above steps until the desired oxidant concentration is reached. • Pressure Purging: Vessel must be rated for high pressures. Faster because can reduce cycle time (pressurization process is quicker than drawing a vacuum - pressure differentials are greater), but uses more inert than vacuum o One practical advantage of pressure purging versus vacuum purging is the potential for cycle time reductions. The pressurization process is much more rapid compared to the relatively slow process of developing a vacuum • Vacuum/Pressure Purging: Vessel must be able to withstand both vacuum and high-pressure conditions. Requires fewer cycles than doing only one or the other. o When combining vacuum and pressure purging, vacuum first because less inert will be needed. • Sweep-Through Purging: Vessel does not have to be pressure rated. Uses the most nitrogen of the choices. o Assumes perfect mixing, constant temperature, and constant pressure