C o n t r o l Ta l k
Hunter: Here are some particularly challenging chemicals
Hunter: Even when you’re aware of the dangers and specify
that come to mind:
the right materials, an error in the order process can occur. I once specified a pressure transmitter for high-pressure ammonia service and selected the proper non-Viton O-rings. The vendor used the correct O-rings inside the transmitter, but used the standard Viton O-rings for the manifold. We installed the transmitter, and I put it in service by opening the isolating valves. The transmitter starting reading, so I turned and had taken three steps toward the control room when I heard an increasingly angry hiss. I looked back and saw an ammonia vapor cloud enveloping the transmitter. Sometimes it isn’t the chemistry that bites you, but the unanticipated process conditions. An engineering firm specified a magmeter for a urea slurry application and the meter worked great—until Production steamed it out and blocked in the pipe. As the steam condensed, the meter was subjected to a full vacuum, which collapsed the liner and destroyed the meter. In other cases, a slight change in the process can have very unexpected results. One plant decided to replace the heating medium of a reactor feed exchanger with high-pressure steam condensate, rather than high-pressure steam, to increase heat transfer and raise plant rate. This worked wonderfully until we began running process safety vent (PSV) sizing calculations, and realized that if a tube failed and water reached the reactor, then the material inside would disassociate into very large amounts of carbon dioxide and ammonia. The PSVs could handle a tube failure with steam, but a tube failure with water would generate several times more vapor. Fortunately, the issue was corrected before a tube leak occurred.
Anhydrous ammonia—It’s explosive at high concentrations, dissolves Viton, reacts explosively with water, and will readily attack copper or any copper-bearing alloy. It can also create enormous water hammer if allowed to vaporize in a long pipe. Hydrogen cyanide (HCN)—It’s flammable and a deadly poison, but it also reacts explosively with almost everything. It will explosively polymerize with high-pH material and low-pH material, and can even explode if just left alone in a dead leg. It enters the pores of Teflon and polymerizes, puffing it up like popcorn, and will attack most other soft goods as well. It also polymerizes in nooks and crannies, freezing up control valves and any moving equipment. The number of explosive incidents involving this chemical is legion. One client had a small storage tank of HCN detonate. They did some testing to determine how big a PSV or rupture disk it would take to adequately protect the vessel, and the results indicated it would take a rupture disc bigger than the tank! They gave up and just eliminated the storage tank, feeding the chemical straight from the producer to the user with a small surge vessel between the plants. Sulfuric acid—This very common chemical poses a number of material selection challenges because it attacks different materials at different temperatures and concentrations. Carbon steel works great at high concentrations, but dissolves as the concentration falls. Similarly, fiberglass is great for dilute sulfuric acid, but is attacked if the concentration is too high. Concentrated sulfuric acid reacts violently with water and generates large amounts of heat. Many exotic alloys (and fiberglass) can easily handle dilute acid at low temperatures, but fail at higher temperatures. Sulfuric acid also liberates hydrogen gas as it reacts with metals, so tanks tend to gather hydrogen in the head space. Needless to say, sulfuric acid reacts explosively with bases. Many processes require dilution of sulfuric acid, but that’s very difficult to do given the heat and wide range of acid concentrations encountered. Oxygen—This gas is just looking for a chance to react with anything and the reaction is usually explosive. A little grease is all it takes to start the reaction, and the temperatures are so high that the oxygen will start burning the metal pipe walls around it.
Greg: Whether a material will fail depends on concentration, temperature, pressure and other process conditions including multiple phases. Velocity also accelerates corrosion and erosion.
Hunter: Control valve selection must consider cavitation, erosion, chemical attack, polymer buildup, etc. We had one valve taking a 1,500 psi drop of urea/carbamate solution. Between the chemical attack, incredible velocities, sound levels and flashing conditions, we couldn’t find a valve that would last a week. We ultimately ended up taking a solid block of Ferralium and machining a valve out of it. It cost $60,000 for a 2-in. x 6-in. valve.
Stan: What are some parting words of wisdom? Hunter: Automation engineers tend to focus on electrical and
Nitric acid—This acid violently reacts with many organics. One client learned that the hard way when a pump exploded and the impeller head was thrown through a cinder block wall. (Thankfully, nobody was hurt.)
Stan: Even if you think you’ve got the application covered, what can go wrong? 48
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software issues because they understand them. However, the enormous hazards associated with chemistry and material selection warrant our attention and diligence. If you’re working with unfamiliar chemicals, take the time to find the process engineer and ask a lot of questions. He’ll be thrilled to expound on his field and you’ll exit the conversation a much wiser (and safer) automation professional.