Calculations showed that monomer wasn't in the flammable range? Calculations are under ideal conditions, with good mixing, and assume pure components. But that's not always the real world. The torch heat on the metal could have made a local flammable mixture. Or, there could have been decomposed material on the inside, another component in the tank or "lights" were present.
Operating the other connected vessels made the lockout procedure a sham. Could this have been done safely? Possibly. Fully isolating the tank would have been the first choice. Even if there were no valves in the vent line, a pancake blind between flanges would have done the job. If that wasn't feasible (and "why not?" should be answered), they could have erected a barrier with ventilation to blow any fumes from the mixer opening away from the torch area, with continuous monitoring on the welder's side. Or they could have used a wet cutoff wheel, or water jet cutter, to avoid any heat or sparks. Or they could have temporarily blocked the opening with rags (with a place for the tank to breathe elsewhere, of course).
And the greatest insult is blaming the welder. Did the welder do the vapor pressure calculations, or sign off on the hot work order, or develop the lockout procedure? This is the kind of incident that gives the industry a self-inflicted black eye.
Alex Smith, process platform lead
M+W Group U.S., Boston, Mass.
FAULT THE WORK PROCESS
Forget about flogging the dead welder, fire the maintenance manager, then fire the safety manager who blamed the welder! Welders do as they are told. They rely on us, those with college degrees, to get them home safe. There are so many things wrong with this work process it's hard to begin.
First, no work should ever be permitted, covered process or not, that deviates from a well-critiqued plan. Ad hoc plans should not be allowed! The foreman overseeing the work should have foreseen what would happen if the bolts were frozen. There are safer ways to cut a bolt loose that don't involve open flame. Even a grinder would have been a safer choice, though not as fast.
Second, where was the fire watch? A good fire watch moves around the work site looking for potential problems. A fire watch could have seen where the sparks were going, especially if they fell into the vessel through the agitator shaft hole.
Third, couldn't somebody find a way to slip a spectacle blind behind a tank flange? Although this maneuver is dangerous in its own right, operators equipped with respirators could manage it. Another idea would be to flow purge nitrogen into the vapor space of the adjoining tank to force the vapor down into the liquid.
Fourth, did anyone verify that the monomer temperature was, in fact, normal? Perhaps the temperatures were hot. If they were, then maybe it would have been possible to cool the tank liquid or the gases above the tank. Another option would have been to use spark-resistant blowers to pull the vapors out of a manway door and not allow them to accumulate above the liquid.
And, lastly, the vapor content was "calculated." I see too much reliance on simulations in place of laboratory tests or measurements. Who can say the composition is as expected in a simulation?
Dirk Willard, lead process engineer
Fluor Global Services, Inver Grove Heights, Minn.
The horizontal counter-current-flow induced-draft finned air condenser on our debutanizer column can't keep up in the summer whenever the air temperature rises above 105°F. Instead of an outlet temperature of 115°F, which would ensure the exiting liquefied petroleum gas (LPG) is a saturated liquid, we get 125°F and the LPG leaves as a mixed gas/liquid. We're looking for ways to limp through until the next turnaround. We must get as much as possible out of the condenser because we are limited by space and load on top of the column. The column takes a feed from the fluid catalytic cracker at about 260°F. Naphtha is withdrawn from the bottom at about 360°F. The LPG — a mixture of propylene, propane, butanes, butenes and trace C5+ — leaves the top of the tower as a saturated gas at 155 psig and 142°F and goes to the air condenser. Under design conditions, the condenser should provide condensed liquid at 120°F to a water cooler, which then should discharge the condensate at 90°F. Our water cooler is designed for liquid LPG and is stressed at high temperature. Do you have any ideas on how we can improve the performance of this condenser?
Send us your comments, suggestions or solutions for this question by November 9, 2012. We'll include as many of them as possible in the December 2012 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, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
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