Bore Water Pump query.

[alchemysa]

I recently bought this 'Constant pressure' pump for my bore. ($139 at Bunnings). Its just used to water the garden at the shack. It will be replacing an old pump that had a large pressure tank. I understand how the old pump worked: when the air pressure in the tank dropped, it triggered the cut-in switch. But how do these pumps with no pressure tank work? I understand that if it detects a flow rate of 2 litres a minute then it kicks in. But how can there even be a flow to detect if the theres no pressure tank? Just wondering.

 

[Egon]

Line pressure.

 

[J_J]

I recently bought this 'Constant pressure' pump for my bore. ($139 at Bunnings). Its just used to water the garden at the shack. It will be replacing an old pump that had a large pressure tank. I understand how the old pump worked: when the air pressure in the tank dropped, it triggered the cut-in switch. But how do these pumps with no pressure tank work? I understand that if it detects a flow rate of 2 litres a minute then it kicks in. But how can there even be a flow to detect if the theres no pressure tank? Just wondering.

A flapper valve switch mounted in the output line. With no water flow, the flapper is straight and when it senses water flow, by bending, it activates the pump.

In order to have a pressure switch, the pump has to have a compressible vessel of some kind, be it a bladder or pipe with air trapped inside, a spring loaded cylinder, much like an accumulator, and there may be others.

 

[Egon]

A flapper valve switch mounted in the output line. With no water flow, the flapper is straight and when it senses water flow, by bending, it activates the pump.

How might the initial fluid flow be initiated?

 

[alchemysa]

How might the initial fluid flow be initiated?

I guess JJ meant theres something like a spring loaded pressure tank inside the electronic head that initiates the water flow.

 

[J_J]

How might the initial fluid flow be initiated?

I am looking at it this way, if there is no water flow, there is equal pressure/static pressure/ 0 psi on both sides of the flapper valve. Once the output spigot or outlet valve is opened, there will be water movement, and less pressure on one side, and the flapper will bend in the direction of water flow, activating the pump. As soon as you shut off the water, the pressure goes back to the static state.
Maybe some water hydraulic engineer will jump in here and correct us if we are wrong.

An analogy might be a sail switch used in A/C systems, which is a vane activated switch in the path of air flow, which will occur when the A/C cuts on. At rest, the electronic filter is not activated, so when the A/C cuts on, the vane switch is activated and turns on the electronic air filter. When there is no air flow, the vane goes to a static position, and the electronic filter is shut off.

 

[Egon]

Once the output spigot or outlet valve is opened, there will be water movement, and less pressure on one side,

For water movement there will have to be pressure differential across the out spigot will there not.

 

[J_J]

For water movement there will have to be pressure differential across the out spigot will there not.

I am thinking that gravity will try and let the water out and air in, and doing so will cause a differential. That is assuming that the outlet is pointed down You could set up an experiment and try it.

Try this, close off one end of a 6 ft pvc pipe, and 3/4 in ID , and fill with water. Now turn it so the open end is down and see if you get water flow. If there was a flapper valve in that line, I think it would activate.

 

[J_J]

I guess JJ meant theres something like a spring loaded pressure tank inside the electronic head that initiates the water flow.

A device to hold pressure doesn't have to be very big. Look at CO2 cartridges.

Can you visualize a small nitrogen filled cylinder with a switch on the end added to a water line, and set to turn on a pump at 22 psi and shut off at 40 psi.

Air against water will be absorbed in time, so a bladder is used to separate the pressure container from the running water. Some of the old water tank system had to be recharged often to keep things in order.

 

[Egon]

I am thinking that gravity will try and let the water out and air in, and doing so will cause a differential. That is assuming that the outlet is pointed down You could set up an experiment and try it.

This is kinda an iffy thing if you want to rely on it to start a pump is it not.