The head pressure in a canister is balanced. The pressure on the intake side balances the pressure on the outlet except for the small amount that it takes to move water through the filter and hoses. The static head is zero, the dynamic head is affected by media type, hose diameter, hose length, flow rate, etc. The main reason filters advise you not to place the filter too far below the tank is because they are low pressure systems that will leak under excessive pressures. The reason they have a minimum distance below the water line is to ensure enough suction pressure to take care of the pump's needs without that noisy cavitation that can sometimes happen. Cavitation noise means that you are damaging the impeller by allowing it to cavitate
Gph rule different for canisters?
- Thread starter Mr. Convict
- Start date
uhhh, I'm not aware of a suction providing too much pressure. I would think that a canister produces far more pressure when it is on.
I hope this help Cory Keeper but it gets a little complicated in the real world.
What is correctly called NPSH is the amount of pressure that exists at the inlet of the impeller. It is the amount of pressure that exceeds the boiling pressure of the water at the temperature that is being used. At 80F the saturation pressure for water is 0.5068 psia. Since atmospheric pressure is about 14.7 psia you have 14.2 psi of NPSH at a wet suction. If the impeller needs more than that to work properly it will cavitate. If you set the filter impeller 2 feet below the tank surface you will increase the static pressure available to the impeller by 2 ft WC or about 1 psi. If you are moving water through flexible tubing and filter media you probably have much more than 1 psi being lost due to the suction path flow restrictions. Rather than get you involved in calculating the actual head losses in the suction and filter media, the manufacturers take a simple approach and give you a minimum height that should always be enough under any reasonable circumstances for their impellers. It doesn't change the need for a particular suction pressure but it takes the calculations out of your hands by giving you a typical value to go by.
What is correctly called NPSH is the amount of pressure that exists at the inlet of the impeller. It is the amount of pressure that exceeds the boiling pressure of the water at the temperature that is being used. At 80F the saturation pressure for water is 0.5068 psia. Since atmospheric pressure is about 14.7 psia you have 14.2 psi of NPSH at a wet suction. If the impeller needs more than that to work properly it will cavitate. If you set the filter impeller 2 feet below the tank surface you will increase the static pressure available to the impeller by 2 ft WC or about 1 psi. If you are moving water through flexible tubing and filter media you probably have much more than 1 psi being lost due to the suction path flow restrictions. Rather than get you involved in calculating the actual head losses in the suction and filter media, the manufacturers take a simple approach and give you a minimum height that should always be enough under any reasonable circumstances for their impellers. It doesn't change the need for a particular suction pressure but it takes the calculations out of your hands by giving you a typical value to go by.
