This Month’s Puzzler
Our ethylene oxide (EtO) batch reactor (view figure) suffered an explosion one Saturday morning. The reactor was filled with glycerol before EtO feeding began. A steam-heated exchanger raised the temperature of the reactor to 240°F. The pressure spiked suddenly when the EtO addition started; the operator had been taught that meant the EtO wasn’t reacting. The reactor had been cleaned the night before with water and caustic.
The shift supervisor authorized the operator to raise the trip point to 400°F and to continue to heat the reactor. The supervisor, who had experience in a similar process where the temperature sensors became fouled and insulated, didn’t trust the temperature reading. Besides, the night foreman had steamed the purge nitrogen in the insulated reactor a half hour before glycerol was added; this isn’t part of the cleaning procedure, just something operators have done for years.
Recent changes in the control system faceplate weren’t fully understood yet. The operator, confused by the labels for the cooling and heating block valves (that feed the temperature control valve), decided to take a walk outside. The faceplate didn’t indicate the pump was circulating through the heat exchanger and catalyst bed all along, which the operator confirmed in the field. However, instead of finding the cooling water valve open and the steam valve closed as expected, both valves were closed. The supervisor ordered the operator to open the steam block valve; the reactor relief valve blew but not before two flanges on the reactor split, damaging the reactor and sending parts of the top reactor pipe flying.
After the accident, the new supervisor blamed the operator for opening the valve. An inspection of the reactor also showed suspicious holes near the top of the reactor in the dip tubes for the caustic, glycerol and EtO.
The superintendent, who wasn’t aware the reactor contained an abandoned refrigeration coil, blamed it for the incident, saying a hole allowed caustic to collect in the coil and that the caustic leaked out and led to a pressure spike when the EtO reacted with it. The superintendent asked a retired operator who knew about the coil and the dip tubes why the caustic never caused this problem before. That person responded that in the past the reactor was flushed by hot water before being purged and dried. Cutback in inert gas usage and limits on water consumption led to recent changes in the cleaning sequence.
What do you think the root cause of the incident was? Could we have prevented this accident?
The accident occurred because the reactor was heated instead of cooled. Superficially, the accident was caused by poor understanding of the new controls. However, the real root cause is much broader; you won’t get at that cause until you better understand the relationship between people and process.
What’s apparent here is the importance of having fresh eyes review the accident as soon as possible and then documenting everything. This minimizes the chance of people in power sanitizing the narrative, which may hurt efforts to fix the problem. Improving safety and operations requires employees involved to understand what they did, both right and wrong. However, don’t focus on assigning blame and meting out punishment; remember, you’ll probably need to rely on the people who got you into the mess to forestall future problems. So, instead, concentrate on identifying the root cause and appropriate steps to avoid the incident happening again.
It’s clear to these old eyes that management doesn’t know the process as well as the operators. However, the operators don’t understand the chemistry and simply are following procedures without comprehending why they do them. I’ve found that involving management in equipment teardowns and inspections and in preparing operating instructions and procedures is an excellent way for them to understand an operation. Also, I suggest they play a role in putting together and prioritizing maintenance lists.
Another problem is poor coordination between shifts, as indicated by the night foreman’s activities. The best approach I’ve found for handling this problem is structured shift change meetings at the beginning and end of every shift. Document these so something can be learned from them. By structure, I mean by topic: safety, production, quality, maintenance and other topics as required. However, allow the meeting moderator to table any discussion that goes more than a few minutes.
The project group doesn’t get off the hook for this accident. New equipment can either simplify operations or complicate them with additional problems. Use a senior operator to review distributed control system faceplates for ease of use.
Don’t let operators off easy during training for new or changed processes. Working through a faceplate helps but walking operators outside the control room to identify each step in a new process also is critical. In addition, explain what’s happening in each step and what can go wrong — an alarm trip summary table helps; add flow charts for complicated troubleshooting. Create a checklist for new and old processes and require operators to tick off each entry.
Ensure everything is labeled and flow arrows are added. Review process and instrumentation drawings and process flow diagrams to verify the operator can walk through them.
Don’t let the senior operators walk out of training; clearly they’re just doing things from rote, not from an understanding of the process — you’ve got to get these senior people involved and asking questions.
Management also requires some changes. Don’t let supervisors give permission for raising temperature limits. Until supervisors understand the process, they shouldn’t be allowed to increase limits.
Dirk Willard, consultant
Our 25-year-old vacuum scrubber system (Figure 1) had worked flawless for two years; now, it’s responsible for a toxic release. We increased the vent volume about 20% last year. At the time, the safety and environmental engineer complained bitterly that the capacity already was overtaxed, noting that we had added more demand over the years and the material balance wasn’t well understood. Still, the system worked fine, although operators complained of fumes.
Our maintenance manager looked at the equipment and found the following:
• The blower has been rebuilt several times; its capacity was increased once, five years ago.
• The pH probes were abandoned — the system is on the roof and the probes fouled often — and replaced by a titrator; the lab services the titrator because maintenance and operations forgot to keep up on reagents.
• The dike floor has cracked and heaved; equipment additions mean the dike now is slightly undersized (58,000-gal required versus 56,000-gal available).
• The mist eliminator was eliminated years ago because maintenance was annoyed by its fouling.
The project group says it will cost a fortune to replace the system and repair the secondary containment because this system is integral to plant operations. In addition, there’s a dispute over containment volume: operations says we should count the blower containment, which is elevated above the scrubber area and drains into it but the safety engineer disagrees. The project group proposes putting a roof over the area to eliminate stormwater concerns. The maintenance engineer says that’s impossible because the diluted caustic in the mixing shack is cooled by an airfoil cooler.
How should we proceed with this project? Do you see any other concerns?
Send us your comments, suggestions or solutions for this question by November 15, 2019. We’ll include as many of them as possible in the December 2019 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.