1. Wrong material in the wrong tank: A fatal reactive chemistry incident
This process safety moment describes an incident in which the incorrect material was unloaded from a tank truck into a storage tank. The material from the truck reacted with the material in the storage tank to produce highly toxic hydrogen sulfide gas. Two workers were exposed, one of whom died from the hydrogen sulfide exposure. The incident discussed occurred on June 4, 1999 in Whitehall, Michigan, USA, and was investigated by the United States National Transportation Safety Board (NTSB). It was the subject of the March 2009 issue of the Center for Chemical Process Safety “Process Safety Beacon” For more information about the incident, see the NTSB report incident. You can download the full report from the NTSB web site at:

2. Process Safety Moments
It is sincerely hoped that the information presented in this work will lead to even more innovation and advancement for the entire industry; however, neither the American Institute of Chemical Engineers, its consultants, CCPS Technical Steering Committee and Subcommittee members, their employers, their employers' officers and directors, the presenter of this work, the creators of this work, their employers, nor their employers' officers and directors, warrant or represent, expressly or by implication, the correctness or accuracy of the content of the information presented in these Guidelines. As between (1) American Institute of Chemical Engineers, its consultants, CCPS Technical Steering Committee and Subcommittee members, their employers, their employers' officers and directors , and (2) the user/viewer of this work, the user/viewer accepts any legal liability or responsibility whatsoever for the consequence of its use or misuse.
CCPS Process Safety Moment No. 001

3. What happened?
A company operated a location that had several chemicals stored in tanks in various places in the plant.
Plant procedure was for the shift supervisor to authorize the tanker driver to off load chemicals.
Water solutions of sodium hydrosulfide and ferrous sulfate were among the chemicals received by truck and unloaded into storage tanks.
A plant (a tannery) received bulk shipments of ferrous sulfate (FeSO4), sulfuric acid (H2SO4), and sodium hydrosulfide (NaHS). The products were kept in separate storage tanks at the plant. The ferrous sulfate unloading area was on the northeast side of the tannery. The unloading connection was not equipped with a lock or any other safety or protective device. The plant received ferrous sulfate shipments during all shifts. According to company officials, when the chemical was delivered on the first shift, the environmental manager or the superintendent would usually assist the driver. The officials stated that when a shipment was delivered on the second or third shifts, the shift supervisors were allowed to assist drivers. These employees would, according to the company, show the driver where to unload the product and point out the compressed air connection, used to pressurize the truck for unloading. After the cargo transfer was completed, the employees were to sign the invoice noting that the cargo had been delivered. These procedures were not in writing, and the investigation determined that they were not always followed. The company did not have a program to train its employees for unloading bulk cargo.
CCPS Process Safety Moment No. 001

4. Plant Layout
The truck of sodium hydrosulfide was supposed to be unloaded to these tanks
It was actually hooked up and unloaded to the ferrous sulfate tanks at this location
This plant layout shows the location of the sodium hydrosulfide unloading and storage tanks (above) and the ferrous sulfate unloading and storage areas (below). On the day of the incident, no plant employees were in the vicinity of the either unloading and transfer area.
CCPS Process Safety Moment No. 001

5. What happened?
Shift supervisor was expecting a delivery of Ferrous Sulfate.
A tank truck containing Sodium Hydrosulfide arrived at the plant.
The driver had never been to the plant before.
The Shift Supervisor escorted the driver to the Ferrous Sulfate tank and authorized him to off load without inspecting the trucker’s paperwork.
The driver connected to the tank and began offloading Sodium Hydrosulfide through a connection marked “Ferrous Sulfate”.
On the day of the incident, about 3:30 a.m. a truck driver arrived at the tannery to deliver a load of sodium hydrosulfide solution. The truck driver had never been to the plant before. Upon arrival, he asked a tannery employee for assistance. The employee called the shift supervisor, who met the driver at the plant employee’s work station. The shift supervisor stated that the only chemical shipment he had previously received on the third shift was “pickle acid” (ferrous sulfate). He said he had not been told to expect the delivery of another chemical on the shift so he assumed this load was also pickle acid. The supervisor stated that because the driver did not know the plant’s layout and was unfamiliar with where to unload his cargo, he walked the driver through the plant and out to the pickle acid transfer area. The supervisor did not verify what chemical was being delivered. The shipping documents properly identified the cargo as sodium hydrosulfide solution.
The shift supervisor showed the driver the ferrous sulfate connection (the only working transfer connection at that location) so he could unload his truck. The shift supervisor then unlocked a gate to allow the driver to bring his vehicle into the plant. The driver asked the supervisor to sign the shipping documents so he would not have to find the supervisor after the transfer was completed. According to the supervisor, he signed the paperwork without reading it and left the area. The signature block that the supervisor signed stated the following: “I have checked the documents for this shipment and verify that there is adequate storage room to receive this shipment and connection has been made to the proper storage facility.”
CCPS Process Safety Moment No. 001

6. Unloading arrangement
Incident Scene
Unloading arrangement
This picture shows the unloading area for ferrous sulfate, with a hose connected from the tank truck to the pipe going to the ferrous sulfate storage tank.
When the driver arrived at the transfer area, a transfer hose was already connected to a pipe, marked “FERROUS SULFATE,” on the side of the transfer building. During the post accident investigation, investigators found the other end of the transfer hose connected to the tank truck and determined that sodium hydrosulfide solution had been transferred from the tank truck into the storage tank containing ferrous sulfate.
CCPS Process Safety Moment No. 001

7. Incident Scene Unloading connection
Here is a close up picture of the ferrous sulfate unloading connection, which is clearly labeled. Despite this, a truck containing sodium hydrosulfide was unloaded through this pipe on the day of the incident.
CCPS Process Safety Moment No. 001

8. What happened?
The two chemicals (sodium hydrosulfide and ferrous sulfate) react and produce hydrogen sulfide (H2S), a colorless gas which is highly toxic at 800ppm.
An operator noticed an unusual odor and passed out.
The operator recovered sufficiently to notify others to call 911.
The truck driver was found unconscious and later pronounced dead at the scene.
Sodium hydrosulfide solution reacts with ferrous sulfate solution to produce hydrogen sulfide, a poisonous gas.
Chemistry:
FeSO4 + 2 H2O ---> Fe(OH)2 + 2 H+ + SO4(2-)
2 H+ + 2 NaSH ---> 2 H2S (gas) + 2 Na+
Hydrogen sulfide toxicity: The primary route of exposure is inhalation and the gas is rapidly absorbed by the lungs. Absorption through the skin is minimal. People can smell the “rotten egg” odor of hydrogen sulfide at low concentrations in air. However, with continuous low-level exposure, or at high concentrations, a person loses his/her ability to smell the gas even though it is still present (olfactory fatigue). This can happen very rapidly and at high concentrations, the ability to smell the gas can be lost instantaneously. Therefore, DO NOT rely on your sense of smell to indicate the continuing presence of hydrogen sulfide or to warn of hazardous concentrations. A level of H2S gas at or above 100 ppm is Immediately Dangerous to Life and Health (IDLH).
EXPOSURE LIMITS FOR HYDROGEN SULFIDE:
20 ppm OSHA ceiling
50 ppm OSHA peak 10 minute(s) (once if no other measurable exposure occurs)
10 ppm ACGIH TWA
15 ppm ACGIH STEL
10 ppm (15 mg/m3) NIOSH recommended ceiling 10 minute(s)
For an overview of hydrogen sulfide hazards, see the US OSHA fact sheet at
About 4 a.m., an employee in the basement of the tannery building smelled a pungent odor and lost consciousness. The employee said that after regaining consciousness about 10 minutes later, he made his way out of the tannery to an area adjacent to the south parking lot, where he found other employees on break. One of these employees called 911. The truck driver was found unconscious inside the tannery building approximately 230 feet from the transfer area. He was pronounced dead at the scene and was later determined to have been overcome by hydrogen sulfide gas. No telephone or other means of communication was located near the transfer area that the driver could have used to notify plant personnel of an emergency.
CCPS Process Safety Moment No. 001

9. Plant Layout
The truck driver was found unconscious at this location and died from hydrogen sulfide exposure
An operator passed out at this location, but fortunately recovered in time to get help.
This plant plot plan shows the location where the operator was exposed to hydrogen sulfide, and fortunately recovered in time to seek help. It also shows where the driver was found, and unfortunately he died from exposure to hydrogen sulfide.
The hydrogen sulfide produced by the reaction vented from the storage tank into an area where it flowed into the building and other work areas.
CCPS Process Safety Moment No. 001

10. Why did this happen ?
The Shift Supervisor assumed the contents of the delivery based on experience, and what he was expecting to receive.
Plant procedure for oversight of the delivery of chemicals was not followed.
The truck receiving and unloading procedure was not written.
No plant employee observed the off-loading.
Probable Cause: The National Transportation Safety Board determined that the probable cause of this incident was the failure of the company to have adequate unloading procedures, practices, and management controls in place to ensure the safe delivery of chemicals to storage tanks.
The shift supervisor assumed that the truck contained the same material which he had frequently received in the past and did not check the shipping papers to confirm the actual content of the truck. This was said to be required by plant procedures, but these procedures were not written down. Also, the truck driver was not familiar with the plant, but was left to unload the truck by himself, with no plant employees in the area during unloading.
CCPS Process Safety Moment No. 001

11. Why did this happen ?
There was no method of communication at the unloading area.
Piping at the off-loading point was properly labeled (ferrous sulfate) but did not match the material listed on the shipping papers (sodium hydrosulfide).
Nobody checked that the material in the truck was the same as the material marked on the piping where the truck was connected.
Additional factors include the lack of communication capability at the unloading area. If the truck driver had realized that he was in danger, he had no way to immediately call for help.
The shipping papers for the truck clearly identified the material in the truck, and the piping at the unloading station was clearly labeled for a different material. But nobody checked that the material in the truck was the same as what was indicated on the labeling of the unloading pipes.
CCPS Process Safety Moment No. 001

12. Can this happen at your plant?
Do you have a written procedure for receiving chemicals?
Does it include safety instructions and a step to verify that the correct materials are being unloaded to the correct tanks?
How do you confirm that tank trucks, railroad tank cars, or other containers being unloaded actually contain the material that you think they contain?
Do you know the consequences of improperly mixing two of the materials you receive by truck or other bulk container by pumping one of them into the wrong tank?
Are you certain that tank vents are directed to a safe place so people will not be exposed to vapors vented from the tank?
Could an incident like this one occur at your plant? Unloading the wrong material into a tank can have serious consequences. In this incident, the two materials mixed in the tank reacted to create a highly toxic gas which caused a fatality and nearly caused another. Other mixing errors might have different serious consequences – for example a chemical reaction which generates a lot of heat or gas causing a storage tank to overpressurize and possibly explode. At the very least, a mixing error would contaminate a large storage tank of material, ruining it for use, costing a lot of money, and creating a problem for disposal of the contaminated material.
Here are some questions to think about for your plant:
Do you have a written procedure for receiving chemicals? If so, do you actually use it?
Does it include safety instructions and a step to verify that the correct materials are being unloaded to the correct tanks?
How do you confirm that tank trucks, railroad tank cars, or other containers being unloaded actually contain the material that you think they contain?
Do you know the consequences of improperly mixing two of the materials you receive by truck or other bulk container by pumping one of them into the wrong tank?
Are you certain that tank vents are directed to a safe place so people will not be exposed to vapors vented from the tank?
CCPS Process Safety Moment No. 001

13. Can this happen at your plant?
Are unloading connections and piping clearly labeled with the material name and flow direction?
Is the unloading connection secured to prevent unauthorized off-loading?
If your plant allows truck drivers to unload materials, how do you know that they are properly qualified, and understand your plant’s safety rules and procedures?
What is your plant’s procedure for how to deal with the arrival of a shipment of any chemical at an unexpected time?
Can you communicate to get help if there is an emergency at the places where you unload or transfer chemicals?
Here are some more questions to consider:
Are unloading connections and piping clearly labeled with the material name and flow direction?
Is the unloading connection secured to prevent unauthorized off-loading?
If your plant allows truck drivers to unload materials, how do you know that they are properly qualified, and understand your plant’s safety rules and procedures? Does your operating procedure or driver safety information instruct the driver to remain with his vehicle and not wander around the plant?
What is your plant’s procedure for how to deal with the arrival of a shipment of any chemical at an unexpected time?
Can you communicate to get help if there is an emergency at the places where you unload or transfer chemicals?
CCPS Process Safety Moment No. 001

14. Points to discuss
If the Supervisor was too busy, what should have been done?
How well trained on chemical hazards can you expect a truck driver to be?
In particular, think about potential reactions with other chemicals in YOUR plant.
How are chemical deliveries managed at your plant?
How is this kind of incident addressed in your Process Hazard Analyses (PHA)?
How are the hazards and necessary controls communicated to the operators and other plant staff following the PHA?
Here are some other things to think about from this incident:
If the Supervisor was too busy, what should have been done?
How well trained on chemical hazards can you expect a truck driver to be?
In particular, think about potential reactions with other chemicals in YOUR plant. Can you expect a truck driver, who may visit many different plants, to be familiar with, and remember, the potential reaction hazards in all of the plants that he visits?
How are chemical deliveries managed at your plant?
Do you have adequate and consistent controls in place all the time, for deliveries on all shifts?
How is this kind of incident addressed in your Process Hazard Analyses?
How are the hazards and necessary controls communicated to the operators and other plant staff following the PHA?
CCPS Process Safety Moment No. 001

15. Points to discuss
What safeguards could be implemented to prevent a reoccurrence?
Should we make further improvements to prevent a similar event?
Do you know of any incidents in your plant or somewhere else in your company which were similar to this one?
An incident which resulted in a serious incident, chemical reaction, material release, injury, or other significant consequence?
A “near miss” incident which did not have any consequence other than material contamination or financial loss?
Here are some other things to think about and discuss.
What safeguards could be implemented to prevent a reoccurrence?
Should we make further improvements to prevent a similar event?
Do you know of any incidents in your plant or somewhere else in your company which were similar to this one?
An incident which resulted in a serious incident, chemical reaction, material release, injury, or other significant consequence?
A “near miss” incident which did not have any consequence other than material contamination or financial loss?
CCPS Process Safety Moment No. 001

16. For more information: References:
CCPS Process Safety Beacon, March 2009 (“Read only” copy available for download at
United States National Transportation Safety Board (NTSB) Hazardous Materials Accident Brief, Accident No. DCA99MZ006, Whitehall, Michigan, June 4, 1999.
You can learn more about this incident, and other process safety incidents from these resources:
An archive of CCPS Process Safety Beacons available at This archive is “read only” – the Beacons which can be downloaded at no charge cannot be printed. If you want to have access to printable copies of all Beacons in the archive, you can join SACHE (Safety and Chemical Engineering Education). Information on SACHE membership can be found at
You can subscribe to the Process Safety Beacon from CCPS free of charge, and the new issues which are distributed can be printed and freely circulated to your colleagues. The Beacon is normally available in English, and is also translated into more than 20 other languages. When you register to receive the Beacon, you will get a single note with Internet links to all available translations, and you can download all of the current Beacon translations that you want. When registering for the Beacon, we ask that you still indicate which languages that you want to receive. That way we will be able to send an to a limited distribution list if a translation is delayed and becomes available later. Information on how to subscribe to the Beacon can be found at:
For more information about CCPS, please visit the CCPS web site:
A complete report on this incident is available for download from the United States National Transportation Safety Board -
CCPS Process Safety Moment No. 001

17. Get more information:
CCPS books and resources on reactive chemistry hazards:
Essential Practices for Managing Chemical Reactivity Hazards (2003) Individual Hardcopy Book CCPS Electronic Library (Knovel)
Guidelines for Process Safety Fundamentals in General Plant Operations (1995) Individual Hardcopy Book CCPS Electronic Library (Knovel)
CCPS Member Companies: Volunteer to Serve on a CCPS Committee
Not a CCPS Member Company? Learn about the benefits
CCPS Web Site:
Here are some links to CCPS publications which are related to the incident discussed in this Process Safety Moment. These publications are available as hard copy books, and also electronically through Knovel Corporation.
The Center for Chemical Process Safety (CCPS) is a not-for-profit, corporate membership organization within the American Institute of Chemical Engineers (AIChE) that identifies and addresses process safety needs within the chemical, pharmaceutical, and petroleum industries. CCPS brings together manufacturers, government agencies, consultants, academia and insurers to lead the way in improving industrial process safety. More information about CCPS and the benefits of membership can be found on the Internet. For member companies, you can learn about active projects and committees, and volunteer to participate in current project teams.
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