Closed Head Drum Processors
By Rick Da Tech
55 gallon drums are available in both plastic and steel. Some have removable lids and some do not. A drum without a removable lid is called a Closed Head Drum or Tight Head Drum. They have one 2″ opening and one 3/4″ opening in the top. Unmodified Closed Head Drums are not suitable for use as a biodiesel processor.
They're not pressure vessels and will come apart at the seams if too much pressure is applied. The photos to the left show what is left of a closed head drum after being filled with compressed air to 75 psi. The drum launched into the air and landed 70 feet away after one bounce. The missing lid launched into the neighboring woods like a Frisbee and was never seen again. The same thing can happen to a drum with a removable lid if the lid is bolted or secured to the drum.
Dry firing is when an electric heating element, like the ones used in water heaters, burn out after being exposed to air. When a heating element is submerged in water or oil, the heat is dissipated in the liquid. Air acts like an insulator, trapping the heat inside the element until it literally melts.
When we're making biodiesel in a drum the air over the top if the biodiesel is literally explosive. A melting electric heating element is definately hotter than the 851 deg F needed to ignite mehanol vapors. And when those vapors ignite, the pressure inside the drum instantly rises to above 100psi, blowing the drum apart at the seams.
I’ve checked into a number of these bursting drums. They all have a common element. The lid is launched off the drum like a razor edged Frisbee of death. Several have cut right through wooden and steel roofs. They are not often found after launching. The one exception was the lid that went half way through a brick wall. Getting the picture? When drums explode they do so with potentially lethal results. Take a cutting torch to a drum that held flammable liquids and you will see the same thing. One welder lost both of his leg cutting open a drum that still had enough vapors in it to ignite. Many have died cutting open drums that contained explosive vapors. It’s the same explosion that occurs in a sealed drum when the heating element is uncovered.
Enough Doom and Gloom! The solution is really simple. Use a lightweight loosely fitted lid on an open head drum. If you uncover the element in a processor with a plywood lid, the expanding gasses will push the plywood up a few inches and let the burning gasses escape without catastrophic damages. There are plastic lids and wooden lids, it would even be OK to use a metal lid as long as it was loose fitting. Just cutting a hole in the top of the drum is not enough to prevent dangerous pressure build up. Remove the lid completely. Or if you want to flip your drum upside down to take advantage of bungs on the bottom, then cut out the bottom (now top) completely. Whatever you make your processor from, please consider what would happen if it suddenly became pressurized to 150 or 200 psi.
This whole exploding drum thing is one really good reason to use an appleseed design. Water heaters are pressure tested to 200 psi and can survive without launcing parts if the methanol inside the processor accidentally ignites.