Off-Grid water, pump fail safe help

Industrial Toys · Aug 1, 2019

Personally, if I did it, and I automate stuff like that all the time, I would want a heads up BEFORE it runs out of water. But still the out of water limit switch as well.

I like the small stainless float switches. They are low current and need to run a solid state relay or go into a low level input. What I used would also depend on the hardness of the water. If it was too hard, I might just go with the large plastic float that has a contact in it. I used one of those into a high quality (supervised) INNOVONICS transmitter to go about five hundred feet to a friends house, as a low level warning in a hillside spring fed well that he used for his heat pump. It was just a low level warning but could have controlled anything you wanted.

gary49 · Aug 1, 2019

I assume the 300 gallon tank is basically a springbox and is gravity fed by the spring, so there is some water flowing into it all the time. If you have an average sized pressure tank, say a 40 gallon nominal tank the actual drawdown between switch off and switch on is roughly 7 gallons. So if the switch is on, even a 1 gallon/minute flow into the system with no water being drawn from said system would result in the pump running 7 minutes before it reached cut out pressure. Not great for the pump,but they're pretty tough. More likely scenario is somebody leaves the hose on, the pump pumps out the springbox, pressure falls, and the switch cuts out. The Pumptec is safer, but considerably more money, I think the switch will do the job for you, but you pays your money and you takes yer choice.

Industrial Toys · Aug 1, 2019

If the level switch controls the pump, yes, the pump and cycle too frequenly. I bought some junky little logic board from E-Bay that has a upper and lower limit/float switch input to address just that. The pump can't start again until the high limit is activated. Board probably cost two and a half bucks!

CobyRupert · Aug 2, 2019

gary49 said:
I assume the 300 gallon tank is basically a springbox and is gravity fed by the spring, so there is some water flowing into it all the time. If you have an average sized pressure tank, say a 40 gallon nominal tank the actual drawdown between switch off and switch on is roughly 7 gallons. So if the switch is on, even a 1 gallon/minute flow into the system with no water being drawn from said system would result in the pump running 7 minutes before it reached cut out pressure. Not great for the pump,but they're pretty tough. More likely scenario is somebody leaves the hose on, the pump pumps out the springbox, pressure falls, and the switch cuts out. The Pumptec is safer, but considerably more money, I think the switch will do the job for you, but you pays your money and you takes yer choice.

Lets look at another scenario:
Say switch normally turns pump on at 40 psi, pumps ballast tank to 60 psi and turns pump off. My understanding is there is a check valve between pump and ballast tank.
What happens if source tank runs out of water when pump has raised ballast tank to 50 psi? Assume nobody is drawing water (or pressure) off of ballast tank.
Wouldn't the pump continuously run until it burns itself up because the pressure switch never sees any pressure less than 50 psi even though pump is dry? Or run until enough spring water enters source tank that pump can fill ballast tank to 60 psi, whichever comes first.
Is this a "perfect storm" / low probability scenario?

Industrial Toys · Aug 2, 2019

I both like and don't like designing such things. It's rarely as straightforward as you had first thought. And later, you won't be able to figure out what you did, or why.

Fallon · Aug 2, 2019

Industrial Toys said:
I both like and don't like designing such things. It's rarely as straightforward as you had first thought. And later, you won't be able to figure out what you did, or why.

Yup. It's even worse for somebody trying to figure out what it why you designed & built it that way.

gary49 · Aug 2, 2019

CobyRupert said:
Lets look at another scenario:
Say switch normally turns pump on at 40 psi, pumps ballast tank to 60 psi and turns pump off. My understanding is there is a check valve between pump and ballast tank.
What happens if source tank runs out of water when pump has raised ballast tank to 50 psi? Assume nobody is drawing water (or pressure) off of ballast tank.
Wouldn't the pump continuously run until it burns itself up because the pressure switch never sees any pressure less than 50 psi even though pump is dry? Or run until enough spring water enters source tank that pump can fill ballast tank to 60 psi, whichever comes first.
Is this a "perfect storm" / low probability scenario?

I haven't seen the OP's set-up, I'm making some assumptions such as, it's a springbox with a continuous flow of water coming in, it has a submersible pump with a check valve built into the top of the pump section, and the pump is likely lying on it's side in the springbox. If that's the way it is, the motor section will still be mostly underwater when the pump starts to suck air, which will keep the motor cool enough to not burn up, and it will pump whatever water comes into the springbox. If there's no water flowing out of the pressure tank, the system will pump up to 60# and the switch will cut out. If there is water flowing out of the pressure tank faster than the flow into the springbox, the low pressure cut off switch will soon cut the pump off. The check valve is to keep the pressurized water in tank from flowing back into the springbox. If it is a jet pump, it will lose it's prime as soon as it sucks air, that's a bit different, that might burn up the impeller in your scenario. I'm not a fan of jet pumps because they aren't self priming, the pumptec would cut it off, but possibly not the low pressure switch. Pump would need to be re-primed, even with a pumptec. Jet pumps are a PITA.