Treat Tanks with Care

A variety of easily avoided problems can cause vessel failure.

By Roy E. Sanders, chemical process safety consultant

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Tanks must be engineered to provide protection — via alarms and high-high level interlocks — against overfilling of hazardous materials and the resulting spillage.

Over-pressure and under-pressure. It's crucial to maintain the integrity of tank venting systems. Otherwise, catastrophic damage may result.

Over-pressure caused a sudden drastic failure at the base of a 12-ft-dia., 24-ft-high fiber-glass acid tank (Figure 1). The tank was equipped with a separate vent line, an overflow line and a vacuum breaker.

As a safety precaution when repairing an underground sewer line that would receive acid if the tank overflowed, supervision had the overflow line blinded and instructed operators to run the vessel well below the overflow line. The thought was that the vent line was sized sufficiently for filling purposes.

Unfortunately, a blind from a previous job had been left within the vent line and wasn't detected. As operators started filling the fiber-glass tank, the inerts had no place to go and the tank was pressurized to destruction. Fortunately, no one was injured [4].

Under-pressure led to a well-maintained low-pressure 20-ft-dia., about-30-ft-tall carbon-steel solvent tank with ¼-in. walls ending up as scrap metal after "improvements" to the vent system. To reduce emissions, vent recovery compressors and more-sophisticated instrumentation were replacing old-style conservation vents.

While the first batch of material after the tank conversion was being pumped out, the roof and two courses of vertical walls were sucked in due to the lack of the nitrogen padding and the vacuum protection system being inadvertently isolated by a small block valve.

A simple hinged vent lid had served well for decades. The lid was replaced with a much more complex system involving a vent compressor to recover the vapors and a nearly zero leakage pressure/vacuum device. The operators were well trained on the new compressor but not on the new piping arrangement. Worse yet, closure of a single small-diameter impulse valve rendered all the well-conceived improvements worthless. Oops, a $100,000 mistake [5].

Plant designers must strive to develop user-friendly piping, layout and control schemes, and must clearly label equipment safety systems to reduce opportunities for failure. Venting systems should ensure proper protection during all phases of operations.

Tank venting systems mustn't be altered or tampered with without a management-of-change review.

AN OLD STORY
None of this is new. In an ICI Safety Newsletter published in the 1970s, Kletz predicted a storage tank would be sucked in each year. Experienced process safety people hear of such situations every so often.

CCPS has pointed out tanks' vulnerability to vacuum in two issues of the Beacon. Vivid photos of failed tanks demonstrated the importance of maintaining proper vacuum protection.

The February 2002 Beacon, titled "A Little 'Nothing' Can Really be Deflating — VACUUM is a Powerful Force!," showed a rail car sucked in and a tank that collapsed while being painted. One main message was: "Whenever vacuum relief systems are removed, covered, modified, etc., special precautions are needed to prevent an incident."

"Vacuum Hazards — Collapsed Tanks" in the February 2007 Beacon stressed three key points:
1. Well-intentioned people can easily block vents.
2. Never cover or block the atmospheric vent of an operating tank.
3. Routinely check for plugging of vents on tanks in fouling service.

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