Plant Operations from the Perspective of an HSE Professional
At production sites, workers often interact with production processes and equipment such as pumps, furnaces and others. These pose numerous HSE hazards and a lack of basic understanding of their operations could lead to mishaps—sometimes with major consequence.
In the “old” days, it was not uncommon to hear: “It’s not my job. I don’t need to know that stuff.” This is known as the silo mentality. Departments tended to protect their turf—even top management supported that work pattern. Today, however, we know this type of paradigm could contribute to safety, environmental, and hygiene risks. It is widely agreed that the cross-training of workers enhances overall safety and environmental protection.
Consider this example on Confined Space Entry permit (CSE): A safety professional issued a CSE for a reactor which was in a dirty liquid/slurry service. Reactor was emptied and vapor-freed. Lower Explosive Limit (LEL) detectors showed that it was okay to issue the permit. However, the top of the reactor contained residual slurry deposits, left over after the vapor-freeing operation even though plant operators knew about hazards posed by slurry deposits. However, the safety professional did not know or realize the slurry can release hydrocarbons in an unpredictable way. Shortly after workers began to enter the reactor, LEL detectors pegged-out and alarmed. All workers had to exit the reactor quickly. Fortunately, no one was hurt. The reactor was closed again, and water washed and vapor-freed for eight additional hours. The moral is: had he known a little bit about hazards posed by residual slurry, this near-miss could have been avoided.
Risk perspective of several major plant operations is discussed below. Although, it is not feasible to discuss all operations, an HSE professional still can get a quick perspective of hazards of any operation with a number of queries, including:
- Which chemicals are being processed? (Need to refer to their SDS)
- Range of operating temperatures and pressures
- Safeguards (e.g., relief valves, alarms, interlocks, training manuals and training)
- Upkeep and redundancy of safeguards (For instance, in corrosive services, inlet pipes to relief valve may get clogged, and could disable over-pressure protection.)
- Systems required for regulatory compliance
What follows is a discussion of a number of plant operations and hazards they pose.
Distillation
This is a common operation at refineries and chemical plants. Briefly put, distillation separates a liquid mixture into its component chemicals, based on their boiling points, by using heat. Depending on the chemicals handled by distillation, it could operate at high, medium, or low pressures, including vacuum. A distillation system consists of a distillation column, an overhead condenser/reflux drum, and a reboiler. The overhead condenser could be water-cooled, or air cooled depending on the environmental constraints. Reboiler supplies heat to the column and could be a furnace (fired heater) or could use steam or hot oil. Distillation systems also include control and safety systems—such as pressure, level, temperature alarms and interlocks to halt the process should an unsafe condition develop. As you can imagine, malfunction of any safety-critical instruments could result in a major mishap.
Hazards
- H2S, mercaptans, SO x, amines, and CO are common hazards at oil refinery distillation.
- Because of vicinity of other refining operations such as alkylation, FCC (Fluid Catalytic Cracking) and others, their hazards must be taken into consideration. They include criteria pollutants, hazardous air pollutants, HF, and greenhouse gases.
- Mal-operation of distillation could lead water hammer (high liquid level in the bottom of the column), which could loosen flange connections, eventually causing loss of containment and potential fire/explosion.
- Tube failures of the overhead air cooler could cause hydrocarbon spill with a potential to cause fire. Similarly, tube failures in water-cooled condensers could release flammable hydrocarbons to the cooling tower (if water side is at a lower pressure than the process side) and could create an environmental compliance problem or a possibility of a fire.
- Distillations involving dirty liquids (say, heavy crude) require extensive precautions in emptying and vapor-freeing of the column before issuing a CSE.
Mitigation
- Operator training/testing, regular checks of the safeguards/ interlocks —especially safety critical safeguards
- MOC (Management of Change) in the event of major changes in feed composition.
- Worker chemical exposure monitoring
Pumps
Pumps impart pressure or potential energy to move/transport liquids. In refineries and chemical plants, centrifugal pumps are common (generally used for liquid viscosities below 650 centistokes or approximately 3000 SSU -Saybolt Universal Second). For viscous fluids or slurries, positive displacement pumps are preferred.
Hazards
- Low flow or no flow operation could cause the pump to overheat and release liquid to the environment. Low flow alarms or spillback recirculation (low flow recycle) are provided on many sensitive pumps.
- No flow operation of positive displacement pumps can cause rapid pressure rise and eventual loss of containment.
Mitigation
- Ensure all safeguards are in proper working condition.
- The spillback flow valve must be Air-Fail-Open and should preferably not contain block valves on the spillback line. Reason: If block valve is closed inadvertently, it will defeat the purpose of spillback altogether.
- Never allow operation with a closed discharge valve on a positive displacement pump.
- For piston type positive displacement pumps, use suction and discharge snubbers to minimize pressure surges and hence resulting vibration. Pressure surges are so severe in some cases that they have damaged piping and released hazardous liquids.
- Avoid operation under cavitation. One common audio-visual symptom of cavitation is sound like pumping of marbles. Operators deal with cavitation by raising liquid level in the suction tank or cooling the liquid going to the pump.
Furnaces
Furnaces are used to heat fluids at high temperatures—typically above 800F). Fluid to be heated enters convection section and it flows successively through the shock bank and radiant sections. Generally, a temperature controller at the outlet of liquid flow controls the fuel flow to the furnace along with air flow. Combustion of fuel provides heat to fluid. Flue gases or products of combustion exit the furnace from the stack. Some heaters are required to be periodically stack tested for air emissions and these tests follow EPA or state agency’s testing procedures.
Hazards
- Flameout condition and deficient air operation should be discontinued quickly, as they could cause an explosion.
- Normally, flames from burners are NOT supposed to touch or impinge on a tube. However, if a burner is improperly installed or damaged, its flame could impinge on tubes. Flame impingement could be tolerated for a short time and does not require an immediate shutdown of the furnace. Generally, operators pull out the defective burner, clean it or replace with a new burner.
- Stack thermocouples are a good indicator of furnace operation. With some sulfur containing fuels, stack temperatures below 360F-375 F could cause severe corrosion of convection tubes and/or stack lining. An abrupt rise in stack temperature could indicate a tube rupture.
Mitigation
- Proper training and periodic check of safety critical instruments and interlocks and BMS (Burner Management System) is highly recommended. Operators also check flame shape and color as a part of the shift rounds.
- Conduct an MOC when there is a major change in feed to the furnace.
- Each startup requires furnace be thoroughly purged of flammable vapors (generally with steam). The process is called purge cycle.
Working knowledge of operating systems is helpful in achieving our common goal of minimizing risk. This familiarity of operating systems by HSE professionals could help foster discussions with operators. The bottom line is that this collective work will help you achieve impressive levels of safety and environmental compliance.