As many of you certainly know there was a fatal accident on 24th April at Silver Trail Distillery in Kentucky. A still exploded and severely hurt two stillmen, one of which later died from his injuries.
The Fire Marshal came to the concluison that the cause for this accident was a pressure build-up inside the still which led to its explosion. The still was propelled away and was found with its bottom “folded back away from the side walls and only remained attached at one narrow point“, so essentially like an opened can of beans. The column part of the still also was found detached from the pot.
Interestingly the distillers reported an incident where the still had a split in a seam three years ago which could be fixed.
Now what exactly caused this pressure build-up? The column contained a good gallon’s worth of glass marbles that were supposed to increase the surface area for condensation inside the still. The most likely reason, according to the Fire Marshal, was that the vapour flow was blocked by these marbles so the pressure could build up.
According to Revenoor, the manufacturer of the still, the welds of the still will start to crack at a pressure of 4 to 5 psi (approx. 0.28 to 0.34 bar). 5 psi is only slightly above atmospheric pressure and only about a tenth of a properly inflated tyre for a car.
The still had no pressure gauge, but it had a relieve valve in the column that was supposed to open in case of excess pressure. Unfortunately the valve used here has been designed for water heaters, and furthermore it has an operating pressure that is much higher than the pressure required to blow up the still, in a range from 75 to 150 psi (approx. 5 to 10 bar). The Revenoor representative was quoted the valves were added “to keep the lawyers away”.
What can we learn?
To be honest, as soon as I saw a picture of the Revenoor still used by Silver Trail Distillery I was shocked. The design looked flimsy, the execution very DIY-like. The still does not really look like a professional product but rather like something a hobbyist welded and soldered together in their garage. I am neither a distiller nor an engineer but I know enough about physics to know that you have to be extra careful with any edges and visible welds. They are potential weak points of the structure because forces concentrate here and excess pressure has a potential point of attack.
My impression was confirmed by looking at the promotional PDF file about the Revenoor product range where the stills are described more closely.
I won’t pick on the fact that these stills were originally designed for farmers to make their own biofuel. Of course they can be used for spirits as well, at least in principle if as a distiller you don’t care how the design of your still affects the taste of your product. As whisky geeks we know how important this is, but technically a specific geometry is not required. All you need is functional hardware that gets the job done.
The wall strength of the 300 gallon (1135 litre) still is only 3/16 inch which is less than 5 millimetres. The shipping weight is only 1200 pounds (544 kg) for the entire still. You need to be aware that operating a copper still wears down the metal because of the reaction of the vapour with the still walls. This reaction is actually desired because it has a benefitial effect on the the spirit. Sulphur is removed here that can be a reason for unwelcome flavours. So if you start with 3/16″, you will sooner or later be down to 2/16 or even less.
Coming back to the glass marbles in the column, the Fire Marshal’s report includes a primer on ethanol distillation issued by Purdue University where the practice of packing columns is discussed. This is commonly done to avoid costly rectifier plates, especially in smaller stills. There are a number of options to achieve this, the best method being specially designed rings or saddles that have a large surface but will not block the flow of the vapour. The paper explicitly states: “Marbles are poor packing.” They block the flow too much and they require a tall column with a large diameter.
Everything here indicates that these stills were designed to be as cheap as possible: A simplistic design using thin sheet metal, welds with only minimal strength, cheap marbles in a narrow column, no pressure gauge, a relieve valve with operating pressure beyond still specifications, rendering it effectively useless.
All this can be subsummed under “You get what you pay for”. Anyone who fancies opening their own distillery should think twice and thrice about their most important piece of equipment. Running a still is potentially dangerous, and you want to be better safe than sorry. If your budget only allows you to shop at the rock bottom of the market, you might need to rethink your plans.
But of course also still makers should be aware of the risks they are dealing with. The remark about putting in the water heater relieve valve “to keep the lawyers away” speaks volumes. You don’t want to offer the cheapest possible product, but the the best possible. A commercial still needs to be able to handle pressure build-ups and it needs a mechanism for an emergency shutdown. Your garage job may have done 1000 distillation runs without any problems, but it still may blow up tomorrow. And evidently you don’t want to be sued by your customers for selling them unsafe equipment.