Hi Kate,
Follow this step-by-step from MaG's walkthrough:
The aim of the puzzle is to get enough pressure to weld the pipes to fix the leak.
Increase gas pressure: See that the gauge read 50 on the top release nozzle.
Place the gauge on all other holes and see that the rest are all reading zero pressure.
Place the gauge back on top middle hole of the top horizontal pipe. Right click (red arrow) the wheel valve right of hole to bring the pressure down to zero.
We need to have all the flow to the small vertical pipe at the bottom.
Place the gauge on top right hole of the large right pipe. Left click (green arrow) the wheel valve above it to bring the pressure to 50.
Place the gauge on bottom right hole. Double check that it reads zero. If not turn - right click (red arrow) the valve to make it so.
Now the bottom middle vertical pipe is the only source of gas. Place the gauge on bottom middle hole of that vertical pipe. Double check that it reads 100. If not turn the valves to make it so.
Place the pipes from inventory on the empty spaces. Connect the tip of the blowtorch on the release nozzle that has 100 pressure reading - the bottom middle one.

