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I had quite a few jobs during the 1960's but spent lots of time
working in the wood preservation business. I first went to work
for Northwest Wood Preserving Company in 1961 at the age of
twenty-one. By the early 1970's, I was working for a consulting
firm in wood technology as a inspector-buyer of treated wood
products. My travels took me to many wood preservation plants.
I am now sorry that I did not take many more pictures. The eight
wood treating plants represented here are only a fracion of the
many wood treating plants all around the U.S. where I inspected
and bought wood products.
E-mail to 'mathison (aatt) sdf-eu.org'
If the wood is freshly cut (or 'green'), it must have the moisture in the sap removed to allow space within the individual wood cells for the preservative. The wood is loaded onto rail tram cars and pushed into a horizontal pressure treating cylinder which is a large pressure cooker six feet in diameter and one hundred feet long at The Northwest Wood Preserving Company. I have seen pressure treating cylinders that were eight feet in diameter and one hunderd fifty feet long at other plants. Live steam is then used to heat the wood to approx 245 degrees farenheit with a resulting pressure of approx 15 pounds per square inch. After the wood is thoroughly heated, the heating stops and the pressure is released. As the temperature of the wood is above the atmospheric boiling point, moisture is boiled out of the wood. As the atmospheric process slows, a strong vacuum is used to farther lower the pressure within the treating cylinder and allow the moisture in the wood to continue boiling away until the wood temperature is well below 212 degrees.
The next stage of the Rueping Process is to pressurize the treating cylinder and the wood with compressed air from approx 20 to 60 pounds per square inch depending on how efficient was the above drying process. If the wood had not been freshly cut and was air dryed, the pressure treating starts with this compressed air stage. After the wood will no longer absorb compressed air, preservative is pumped into the treating cylinder and air is bleed out of the cylinder to maintain the pressure equilibrium until the treating cylinder is completly full of preservative. Pressure pumps then continue to force preservative into the treating cylinder and the wood up to a pressure of 150 pounds per square inch. This pressure is held until experience says the wood should retain the required amount of preservative.
As the pressure within the treating cylinder and the wood is released, the compressed air within the wood cells expands pushing out most of the preservative from the empty portion of the cell but leaving the preservative in the wood fiber. After the preservative is pumped from the treating cylinder to it's storage tank, another high vacuum is created within the treating cylinder and the wood to extract more of the excess wood preservative. After that vacuum is released, atmospheric pressure forces the preservative residue on the surface of the wood back into the wood cells leaving the surface of the wood quite clean. The Rueping Process is also known as the "empty cell" process as it leaves the wood clean and pleasant to handle.
Optionally, another short steaming cycle can be used to farther clean the surface of the wood.
Core samples 0.2 inches in diameter are taken from the wood and analyzed in the quality control laboratory to check if preservative penetration and retention is up to specifications.
DANGER - DANGER
The large volumes of HOT preservative oil, the presence of water, and
very large volumes of compressed air, make wood treating plants VERY
dangerous.
A preservative storege tank may be twelve feet in diameter and forty feet tall. If full, it is holding approximately 33,000 gallons of preservative and moisture (water) from the treated wood settles to the bottom of the tank. The tanks are heated with steam coils near the bottom of each tank and adgitated (stirred) with a small amount of compressed air bubbling up from the bottom.
If the temperature of the presevative in the tank is allowed to go higher that 212 degrees before the adgitation is started, the water is blown up into the HOT presevative and immediately turns to steam.
The sudden large volume expansion within the tank causes the HOT presevative to be blown out the top of the tank causing the HOT presevative to rain down over the nearby surroundings.
If the temperature of the presevative in the storage tank is allowed to go above 212 degrees while the treating cycle is in progress, the recovery of presevative from the treating cylinder to the storage tank will also cause the water at the bottom of the tank to be mixed with the HOT presevative. This causes the same result as above.
Even if the temperature of the preservative within the storage tank is under 212 degrees AND the large volume of compressed air within the treating cylinder is allowed to enter into the bottom of the storage tank, the sudden expansion of the compressed air causes the HOT presevative to be blown out the top of the tank and to rain down over the nearby surroundings.
The potential for a fire OR someone to be scalded to death is great.
NOTE: virtually all these old wood treating plants have become hazardous waste clean up sites!
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