In this guest post David Ashcroft talks about what a steam explosion is, it’s causes and its consequences
When we think of explosions, we tend to think more of combustion-type explosions. Certainly, many injuries and deaths, as well as untold equipment damage has been done due to the releases of energy caused by the sudden combustion of a solid or liquid fuel, as it ignites explosively, creating large amounts of gasses, which rupture the container in which they were contained. Looking at boilers in particular, failure to purge the built up fuel before igniting a burner has caused much harm. Thus, today’s modern Flame Safety System, are designed to prevent an accidental or even intentional attempt to light off a fuel in a boiler when the proper steps have not been taken to prevent this. For a recent example of how destructive these explosions can be, look at the not so distant example of Burns Lake, where built up sawdust ignited explosively, levelling the building and taking many lives.
The Smelt/Water Reaction, a Type of Steam Explosion
Today though, I would like to look at a different subject, the smelt/water reaction, a type of steam explosion. In this reaction, no chemical combination or combustion takes place, rather the water coming in contact with molten smelt found in a chemical recovery boiler. This is the energy release caused by a sudden change in state of water, turning into vapor. As water in its liquid form converts to steam, the expansion ratio is 1700 to 1, or higher.
The Dissolving Tank Steam Explosion
Steam explosions can happen in different ways. One way, that has been quite common in such accidents, is in the dissolving tank. Normally, in the dissolving tank, the smelt exiting the boiler is fractured into macroscopic particles that cool instantly upon contact with the surface of the water. However, if this smelt is not properly broken up, devastating reactions can take place. This can be a failure of the steam jet breaking the stream. This may be due to a loss of the steam flow, misdirection of the stream missing the flow, or causing a build up of a “cherry” on a wall, which subsequently drops into the water as a large glob.
When this “bomb” drops into the water, the water around it instantly flashes to vapor, the violence of this sudden expansion causes the bomb to fracture, greatly expanding the surface for reaction, and the force of the energy release multiplies, as each subsequent reaction, opens more surface area to the water. Additional force to cause this fracturing, is also found as the “bomb” is repeatedly seam blanketed by the flashing water, and then these shock waves impact the tank sides and bounce back and impact the “cherry,” further shattering it and again increasing the contact area of heat exchange.
The Furnace Steam Explosion
Steam explosions caused by the same reaction, with a different cause can also be found in the furnace itself. This can be either due to water leaking from a boiler tube, dilution of the solids content of the liquor being fed into the boiler to a point below 58.5% solids, or due to other causes of water entering the furnace. This water drops into the liquid smelt bed, and then explodes outward, as the liquid flashes instantly into vapor, any liquid thrown from the first pop, will be atomized causing accelerated heat transfer, and additional energy release into the furnace.
If this first reaction occurs near a tube of the boiler, this can cause a failure of the tube, triggering an additional, much larger reaction, as the water from the tube is pushed out under extreme pressure and most of this water flashes instantly to steam, giving additional energy to the “explosion,” while the liquid which failed to instantly vaporize, fueling the already accelerating smelt/water reaction, this quickly accelerating reaction can continue to build, causing more leakage, causing further reactions, until the heat energy in the smelt is expended or the water left in the boiler is depleted.
All the time the flashing of the boiler water to steam, due to the sudden release of pressure also continues to vent. Often, due to the viciousness of this reaction, the walls of the boiler will be unable to contain the rapid expansion, and the seams connecting the walls will rupture, releasing all this vapor and molten smelt into the area around the boiler and often overcoming the envelope of the boiler-house itself, blowing the walls apart also. It has also often occurred that the explosion that starts in the dissolving tank is responsible for opening the boiler causing a subsequent furnace explosion.
Safeguards to Prevent Steam Explosions
Many engineering safeguards exist to prevent or limit these steam explosions, some examples are explosion dampers on dissolving tanks, giving the explosion somewhere to go, boiler leak detectors, either linked to the steam and water flow indications, or audiometric devices which are tuned to the frequency of a boiler leak, and of course the ESP system which when activated shuts all fuel and air to the boiler, and drains most of the water out of the furnace to prevent there being the water to propagate the explosion.
However, the best preventative plan is still operator attention, and the most important part, the regular round performed by trained operators. Through good listening, looking, and believe it or not, smelling, most causes of this reaction can be controlled. Yes, incomplete combustion gives a acrid sulfur smell, which is a trigger to look deeper for the cause. With today’s trend towards smaller maintenance and operations crews, do we still have somebody whose main task is to ensure the integrity of our plant? With DCS technology, automatic port rodders, automatic control valves, there is a lot less people out in the plant where the leak can be detected.
B. David Ashcroft has worked in the pulp & paper Industry, in the Pacific North-West, for the past 30+ years, and is currently employed by Howe Sound Pulp & Paper, as an Assistant Chief Engineer, and has also recently been on assignment by the Parent Company, Paper Excellence, as Chief Engineer at their Prince Albert location, while a permanent Chief was located for this location. Connect with David on LinkedIn