Comparison of variable, constant and fluctuating pressure methods
S M McFarling, P I Morris
The objective of this work was to evaluate potential improvements in pressure treatment arising from the use of a control valve. A constant pressure of 1035 kPa was compared to the conventional variable pressure method where the pressure is allowed to drop to 860 kPa as the solution is absorbed by the wood. A fluctuating pressure method (FPM) where the pressure cycles between 1000 and 1070 kPa was also evaluated. With unincised western hemlock, the results were in the order FPM>constant>variable pressure. Incising acted to moderate differences in preservative penetration among treatments. With incised material, results were in the order constant>FPM>variable pressure. This work has resulted in the installation of control valves as upgrades to several existing treating plants. Further work is required to determine if the FPM process has benefits compared to constant pressure in commercial practice.