Some other info for you:
That manifold that everything screws into, including that pressure relief valve, is called a tank tee.
The drain valve is there mainly to let you drain off accumulated sediment. You can hook a garden hose to that, and run the water with sediment elsewhere. You should flush out sediment on periodically. The period varies. Annually is good for most wells, but adjust the period based on what you see. I do it maybe every 3 years.
To flush the sediment,
- Turn off the power to the pump. Attach a hose to run water to a good place. Run the pressure tank empty using the drain valve near the input of the pressure tank. Usually you want to use a hose to direct water and sediment to a good place. Note colors and solids coming out toward the end of the flow.
- Close the drain valve, Turn the power to the pump back on for maybe 6 seconds. Turn the power back off. Again drain the water and sediment.
- Repeat #2 until no sediment comes out.
- Check/adjust the air precharge on the pressure tank with the water pressure zero. With a submersible pump, usually you want 2 psi below the actual pressure switch cut-on pressure for a submersible pump.
- Close the drain valve, and turn the power to pump back on.