We have a Sta-Rite Signature 2000 4” ¾ hp submersible water pump. The well is 227 feet deep; the water level is at 110 feet and the pump is down 147 feet. The pump is connected to an Amtrol Well-X-Trol bladder water tank. The Square D pressure switch is set to cut in at 40 PSI and cut out at 60 PSI. The tank pre-charge is 38 PSI.

I have verified that the above pressure settings are correct by following the procedure in the Signature 2000 owner’s manual available here: http://www.onga.com.au/secure/downlo...fileid=1001397

TO CHECK PUMP PRESSURE SWITCH SETTING
  1. To check pressure switch setting, disconnect power to pump at supply panel (but be sure to leave pressure switch connected to power supply wires).
  2. Remove pressure switch cover.
  3. Open a faucet near tank.
  4. Allow water to drain until pressure switch contacts close; immediately close faucet.
  5. Check pressure at valve with standard tire gauge or with pump pressure gauge (if supplied).
  6. Pressure gauge should read at pump cut-in setting (30 PSI for 30-50 switch, 20 PSI for 20-40 switch, etc.) If not:
  • Adjust switch according to switch manufacturer’s instructions.
  • Reconnect power supply to pump and pump up pressure in system.
  • Disconnect power supply to pump again and recheck switch setting.
  • Repeat until pressure switch starts pump within ±1 PSI of proper setting.
  • If cut-in setting is too low, system will rattle or develop water hammer when pump starts.
  • Cut-out setting is not as critical as cut-in setting. Make sure that pump will stop running in a reasonable time. If it does not, cut-out setting may need to be adjusted down slightly. Be sure that after readjustment, system does not rattle or hammer on startup.
  1. Re-check tank air pre-charge to be sure it is 2 PSI below pump pressure switch cut-in setting.

When any upstairs faucet is drawing water and the pump starts, it starts with a bang (water hammer). On the other hand when I open the faucet located below the pressure gauge to draw down the water, the pump starts quietly when the cut-in setting is reached.

What is causing the water hammer at pump startup, and, more importantly, how do I get it to stop? Why would a low cut-in setting cause a water hammer?