Ladder for time scheduled tank control

[font='Arial','sans-serif']I need to design a LAD program on a LOGO! Logic unit for time scheduled tank control. The tank has two inputs consisting of a chemical A and chemical B. The chemicals need to be added to the tank in a certain sequence. This needs to be done in order to create a final product of chemical C.[/font][font='Times New Roman','serif'] [/font]

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[font='Times New Roman','serif'][font='Arial','sans-serif']Sequence of the process is as follow.[/font]

ü [font='Arial','sans-serif']If the start button is pressed valve V1 must open and start two timers of 10 seconds and 20 seconds respectively.[/font]

ü [font='Arial','sans-serif']After the 10 second timer elapsed valve V2 must open.[/font]

ü [font='Arial','sans-serif']After the 20 second timer elapsed valve V1 and V2 must close.[/font]

ü [font='Arial','sans-serif']Valve V3 must then open for 20 seconds and close.[/font]

ü [font='Arial','sans-serif']The sequence can only start by pressing the Start Button again.[/font]



I have started the ladder diagram.
My problem is with step 3, after 20sec only V1 closes and I dont know how to close V2.[/font]

Have you drawn up a flow chart yet? If you have done this the coding will be very clear.