Hybrid Irrigation and Domestic System Pressure

[cmartinsen]

I have a question regarded a water delivery system I've inherited the maintenance of. The system pulls water out of a very clean irrigation ditch feed by an artisian well. Our pump is a goulds gt20 (2 hp), it pulls water out of the ditch to feed the irrigation system directly and fill/pressurize our steel holding tank. The irrigation system works great, but the domestic water pressure fluxuates dramatically and directly with the pump activity. The two systems (irrigation and domestic) are not run at the same time. The domestic system pressure is controlled by a 30/50 pressure switch wire to the pump. The pressure line runs from the 500 gallon steel tank to the pressure switch. OK, that's basically the system. Now on to the question->

Question: I would like to maintain a constant (or near constant) pressure in the domestic water lines, but am not sure how to go about it. Do I need a pressure tank between the big steel pressurized tank and the domestic water main? An inline booster pump? Are there other creative solutions?

Thank you for your help or direction.
Craig

[jadnashua]

You need a pressure tank mostly to minimize the pump on/off cycles. With the 30/50 switch properly working, the pressure will always be varying between those two values. If the pump (or auxiliary one) always ran when you were drawing water, then you might be able to keep the pressure constant. Some storage tanks are pressure tanks that rely on trapped air instead of a bladder, but the (newer?) versions use a bladder. The size of the pressure tank can determine how much water you can draw prior to noticing a pressure drop, but it will always be dropping with the pump off while drawing water from the system until the pump turns back on. My unprofessional opinion.

[cmartinsen]

Jim, thanks for the input!

After evaluating the system further I am considering installing bladder storage/pressure tank between the existing 500 gallon steel (with no bladder) pressure tank and the domestic water line. The existing tank relies solely on air for pressure (previously supplied by a compressor, but adapted to pressurize from the pump). The pump is cycling more often than I'd like at a 18 gpm draw through the domestic system (2HP pump and comes on every 20 sec.).

Would it help reduce pump cycles to install a bladder storage/pressure tank between the steel tank and line? Would the pressure switch need to be switched to the bladder storage/pressure tank? What size pressure tank would be needed to supply 18 gpm and allow for 1 1/2 minute rest for the pump? Could I tighten the 30/50 pressure switch to say 40/50 (it is adjustable...) with the addition of a bladder storage/pressure tank and still provide a rest for the pump?

Thank you for your input,
Craig

[PEW]

If you are using the 500 gal tank as part of the domestic supply, it sounds to me like it is short on air.

Paul

[Mike Swearingen]

I'm no pump pro, but here are two excellent tutorial sites on the subject that may help:
Ron Peeks' www.peekspump.com
Jess Stryker's www.jessstryker.com
Good Luck!
Mike

[jadnashua]

If your existing pressure tank no longer has an air pocket in it, that is your problem. Since water doesn't compress, the only way you are maintaining pressure in the tank is from the (minute) compression caused by the tank expanding! As soon as you open a valve, the pressure almost immediately (probably after drawing out a cup or less of water) to nearly zero, turning on your pump. You need either the trapped air in that tank, or a bladder type pressure tank to maintain pressure between pump cycles.

[cmartinsen]

Everyone thank you for your input and advice.

I've got our system working with pressures between 40 and 50 psi at a flow of 18gpm. To achieve this I drained our 500 gallon to 3/5 full. This created a much larger air pocket, as recommended. I pressurized the tank to 50 psi using a compressor with the tank at 3/5 full. To test this setup I opened a lawn sprinkler (4 gpm), a hose (5.8 gpm), a drip system (5 gpm), a cellar humification system (.5 gpm), a shower (2.5 gpm), and a mini drip misting system (.5 gpm) for a total draw around 18 gpm. I engaged the pump with the pressure switch and adjusted it to 30/50. With a timer (my wife's kitchen timer) I timed the rest period for the 2 HP pump to 180 seconds (120 seconds recommended for 2 HP pumps). At 180 seconds I raised the tank pressure was 40 psi, so I readjusted the 30/50 pressure switch to 40/50. Thank you for your help, our system is working great!

I'm going to monitor it weekly for proper water level in the tank and pump rest time.

My next project is to tie the irrigation system and domestic system together using a couple of relay switches. The idea is allow a valve to the domestic system to open in response to low pressure (using the existing pressure switch) and close at the high pressure.

Thanks,
Craig

[Gary Slusser]

The 50 psi air pressure is technicaly too high for a switch setting of 40/60. The air should be only 1-2 psi lower than the cut-in settiing (40) of the pump. Of course with air over water the air will be constantly absorbed into the water, decreasing the volume of air. That leads to short cycling.

Look into a cycle stop valve, they allow constant pressure and use of a very small pressure tank. They save on the electric bill and pump motor starts. The use of a 2 hp motor for a house water system is going to be expensive due to the amp draw (meter spinning) at start up of the pump.

www.cyclestopvalves.com

Gary
Quality Water Associates