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Category → Lab Equipment

To avoid an exploding calorimeter, maintain it

Damaged calorimeter bomb head. Credit: UC Davis

A University of California, Davis, physical chemistry teaching lab had a near miss when a bomb calorimeter exploded last month. Luckily, no one was injured as the lid of the calorimeter jacket hit the lab ceiling and pieces of the bomb head and a mercury thermometer blew into the room. Although a report of the incident (pdf) doesn’t outright say so, the cause of the explosion appears to be equipment failure due to lack of maintenance.

According to the report, the students in the lab were measuring the heat capacity of benzoic acid:

A nominal 1 gram sample of the material to be investigated is pressed into a pellet and placed in the sample cup of the bomb head. A nickel alloy fuse wire is attached to the two electrode leads and bent to contact the sample pellet. The head is assembled into the bomb vessel and sealed hand tight with the screw cap and o-ring. The vessel is purged and charged with oxygen. The bomb vessel is placed in a bucket of water (carefully measured) which is placed inside a jacket. On the underside of the top of the jacket a small mixer blade is installed that is used to stir the water in the bucket. Consistent mixing is important to achieve stable temperature. A thermometer is also built into the jacket top, extending down into the water in the bucket, to measure temperature change. The jacket top is installed on top of the bucket and the mixer engaged. Two electrode leads are plugged into the terminals on top of the bomb and then plugged into the ignition unit.

Once the temperature has equilibrated, the sample is ignited by pressing the switch on the remote ignition unit, located separately (typically 24”) from the calorimeter. Users should not stand over the calorimeter or approach the calorimeter for at least 20 seconds. The temperature is monitored over time until no further change is observed. The bomb is removed from the bucket and combustion gases are vented slowly (over a minute or more) through the exhaust valve into the fume hood.

The calorimeter in question exploded shortly after ignition. The investigation determined that the most likely cause of the explosion was that a valve seat failed and “allowed the combustion products to escape, causing the exhaust valve assembly to be ejected from the bomb head. The oxygen can act as a torch and is hot enough to burn through the bomb head.”

The calorimeter manufacturer, Parr, “recommends that all o-rings and valve seats be replaced annually or after 500 firings. Bomb vessels should be reconditioned after 5000 firings.” But there were no records of routine maintenance for the calorimeter, which was manufactured in 1985. Perhaps UC Davis’s experience can also be a “lesson learned” for other departments with equipment in a similar state.

Check your hydrogen fluoride cylinders

Matheson issued a technical alert last month recommending that its customers check their hydrogen fluoride cylinders to make sure they got what they expected. It seems there may have been some ordering confusion involving cylinders that have 100 psig or 0.6 psig. As Matheson notes in the alert:

the delivery train requirements would be quite different as between the two variations of HF. The use of an incompatible delivery train can, in turn, lead to over-pressurizing the delivery train and/or exposure to HF liquid or vapor.

The cylinder confusion has led to at least one HF exposure incident, according to a source who was not authorized to speak for their institution. A researcher was unknowingly using a high-pressure tank with a set-up designed for low pressures. The person was working in a hood and wearing gloves and a lab coat when a tubing connection burst, and HF got to the researcher’s skin through a gap between the gloves and coat sleeve. The emergency response was “perfect” and the researcher is fine.

Matheson’s technical alert gives new unique item numbers for its various HF products. On a separate-but-related note, Matheson also recommends that HF in cylinders not be stored for longer than two years, because HF can react slowly with the iron in steel cylinders.

Coincidentally, HF made it onto prime time television last week courtesy of the series House. ChemBark critiques the episode at HF Stupidity on House, M.D. and has a video showing the relevant parts of the show, if you don’t want to watch the whole thing (“Nobody’s Fault”). I particularly liked the sideways bunsen burner flame.

Laboratory damage from Japan earthquake

So what happens when a magnitude 9.0 earthquake strikes 80 miles away from your lab?

Yes, that fume hood was in fact torn from the wall. Credit: Masahiro Terada/Tohoku University

Note the bent pipe to the left from the sink being pushed out. Credit: Takahito Watanabe/Tohoku University

And this is in a country with what are likely the most stringent earthquake codes in the world:

Japan has gone much further than the United States in outfitting new buildings with advanced devices called base isolation pads and energy dissipation units to dampen the ground’s shaking during an earthquake.

The isolation devices are essentially giant rubber-and-steel pads that are installed at the very bottom of the excavation for a building, which then simply sits on top of the pads. The dissipation units are built into a building’s structural skeleton. They are hydraulic cylinders that elongate and contract as the building sways, sapping the motion of energy.

More photos from Tohoku University accompany today’s C&EN news story: Japan Fights for its Rising Sun.

Chemicals and elevators

Do you ride in the elevator with chemicals or gas tanks?

In grad school, I regularly rode in a small elevator with big (100 L?–memory is hazy on this point) tanks of liquid helium or liquid nitrogen. In hindsight, that was not the smartest idea. If the elevator had gotten stuck for some reason, I’d have been trapped in a small, poorly ventilated space with the venting tank.

Smaller amounts of liquid nitrogen could have posed a problem, too, as would any compressed gas cylinder or volatile chemical.

Some places  have dumbwaiters that allow for transport of small containers between floors. Best practice when an elevator is involved seems to be to station one person on each floor–one to put the chemical/tank on the elevator and send it off, the other to receive the delivery. One place’s policy states that this is the preferred option, and “if an attendant must accompany the container in the elevator, an escape pack supplemental breathing apparatus must be carried in the elevator.”

Do my trips with the cryogens put me in the minority? What’s the transportation culture at your school or workplace?

A safety belt for reagents

My apologies for the lack of a round-up last week–I was distracted by a shiny new computer to get up and running. To start off this week, we’ve got a guest post by The Haystack‘s Carmen Drahl, who’s out on the West Coast for the Open Science Summit.

This shelf sits in Richmond Sarpong‘s organic chemistry labs at the University of California, Berkeley. I was there for a conference last week and also took the time to visit a couple labs in the area. Sarpong and his students were kind enough to show me around and talk to me about some of the reactivity they’re exploring.

The room where the team keeps the solvent stills has a lovely view of Cal’s Memorial Stadium. “It’s right over the Hayward Fault,” says Alli Hardin Narayan, a fifth year grad student in Sarpong’s group.

Even though I’d read about the typical earthquake-country safety recommendations for chemistry labs here at the Safety Zone, I didn’t have a real feel for what they looked like. As a refresher, here are some of the typical recommendations.

Cabinets are bolted to the walls, shelving that holds glassware has a 2-inch lip to prevent items from sliding off, shelving in classrooms has glass-windowed doors with a latch that must be tripped by your thumb, and lab stools do not have wheels.

Looking at the 2-inch lip, the first thing I thought of was the safety bars that amusement park workers lower onto you when you ride, say, the Musik Express. But humor aside, the simple design works.

Photo: Carmen Drahl/C&EN