Two-stage chemical mixing system

A chemical mixing system for mixing at least first and second chemical constituents to obtain mixed chemical having a desired concentration within a qualification range, including:

• a first constituent inlet for receiving a first chemical constituent;
• a second constituent inlet for receiving a second chemical constituent;
• a mix vessel for mixing mix vessel batches of the first and second chemical constituent;
• a first line for fluidly coupling the first constituent inlet to the mix vessel, and including a first line valve for controlling the flow of the first chemical constituent through the first line;
• a second line for fluidly coupling the second constituent inlet to the mix vessel, and including a second line valve for controlling the flow of the second chemical constituent through the second line;
• first mix vessel level sensing means for providing first vessel level signals when the mix vessel is filled to a first level, wherein the first level corresponds approximately to the volume of the first chemical constituent that will provide mix vessel batches having the desired concentration;
• second mix vessel level sensing means for providing second vessel level signals when the mix vessel is filled to a second level, wherein the second level is a level greater than the first level by an amount which corresponds approximately to the volume of the second chemical constituent that will provide mix vessel batches having the desired concentration;
• third mix vessel level sensing means for providing third vessel level signals when the mix vessel is filled to a third level which is greater than the first level and less than the second level;
• fourth mix vessel level sensing means for providing fourth vessel level signals when the mix vessel is filled to a fourth level which is greater than the second level;
• a mix drum for holding and mixing a plurality of mix vessel batches of the mixed chemical;
• a mix drum line for fluidly coupling the mix vessel to the mix drum, and including a drum line valve for controlling the flow of the mixed chemical through the mix drum line;
• a concentration monitor for providing concentration signals representative of the concentration of mixed chemical within the mix drum;
• and
• a control system coupled to the first line, second line and drum line valves, the first second, third and fourth mix vessel level sensing means and the concentration monitor, for controlling the mixing of the chemical constituents in the mix vessel and the transfer of the mix vessel batches to the mix drum, including:
• first control means for actuating the first line valve to fill the mix vessel to the first level with the first chemical constituent;
• second control means for actuating the second line valve to fill the mix vessel from the first level to the second level with the second chemical constituent if the concentration of the blended chemical within the mix drum is within the qualification range;
• third control means for actuating the second line valve to fill the mix vessel from the first level to the third level with the second chemical constituent if the concentration of the blended chemical within the mix drum is greater than the qualification range;
• fourth control means for actuating the second line valve to fill the mix vessel from the first level to the fourth level with the second chemical constituent if the concentration of the blended chemical within the mix drum is less than the qualification range;
• and
• fifth control means for actuating the drum line valve to transfer the mix vessel batch of mixed chemical to the mix tank.