# Low water shut off for water pump



## FishinElectrcian (Jul 18, 2019)

If you have a pressure tank.. There's a Gray box on it it sometimes square d.. Get a pressure switch with a deadman safety $40 it's got dpst contacts and a lever on the side, 1/4" pipe thread on bottom. Sometimes called a low water cut off pressure switch


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## Peewee0413 (Oct 18, 2012)

Flow switch or conductivity sensor can work if the piping supports proper install.

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## joebanana (Dec 21, 2010)

If it works, don't fix it. Why make things more complicated than necessary? Replace the old pump with one that can run dry if you're that worried about it. There are numerous way's to accomplish what your asking, depends on your budget, expertise, timeline, environment, access to a PLC, and approval. In most cases, simpler is better.


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## paulengr (Oct 8, 2017)

Use conventional start/stop circuit. Then the pressure switch is in series with the NO holding/sealing contact so it automatically drops out the pump when no one is around. But if it stops for any reason (power loss) someone has to restart.

Another option is a low pressure switch on the suction side or a level switch if there is a tank involved that uses NC contacts like a stop button. That way it cuts off only on low pressure/head loss.

Depending on your piping arrangement especially if you use a foot valve or a bypass priming check valve it can never totally run dry. It might just lose prime, water falls back into it, primes for a few seconds, loses it again, over and over.


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## FishinElectrcian (Jul 18, 2019)

https://www.amazon.ca/Square-Schnei...ocphy=9001444&hvtargid=pla-605065379269&psc=1

One of these.. It's got everything built into it, only drawback is if it's not a pressurized system it won't work. As in if it just dumps into a tank and never builds up to 20 psi to hold it on it won't work.


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## Rora (Jan 31, 2017)

Using a pressure switch could be tricky... don't think you'd want it on the discharge side (to monitor that the pump is putting out pressure, and if it isn't, there must not be water?) it would make the interlock more complicated since you'd be putting yourself in a catch-22 to turn it on. A flow switch on either side would have the same issue.

edit: whoops, didn't read the whole post, you already covered this.

Depending on the head pressure at the suction side, putting a pressure switch there might not leave you with a lot of band to work with... I don't know the details of your process, but if "water present" is going to be atmosphere+head, no water is just atmosphere, there may not be much difference depending on the tank elevation and as it drifts out of spec you could be dealing with some nuisance trips.

There is a very simple solution to this that wouldn't require more expensive instrumentation called an "optical liquid level sensor" which uses the principle of reflection and refraction when liquid is present on the prism surface. There are no moving parts and you wouldn't have to deal with calibrating a pressure switch.










Wire it to fail safe, line high on liquid present to contactor NO. Assuming that you're expecting the plumbing that feeds the pump to be full when it's operating, it should be mounted on the top to ensure the pipe is full, this should also help prevent any sediment from collecting on it. This solution would not require anyone to be present for the pump to start back up again.

Better yet, mount it on the tank if it's nearby so it doesn't trip out due to bubbles or something stupid like that.


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## mburtis (Sep 1, 2018)

So my initial thought was a start/stop push button control with the additional pressure switch contact just like paulengr mentioned. Only problem is that if the power bumps the pump would kick out and have to be restarted manually. We are 15 miles out of town, running a 4 mgd water plant on a line that was never intended to see more than residential service, so power bumps are common. One thought to get around this is, have a simple on/off switch control, then the pressure switch, then put a momentary NO pushbutton in parallel with the pressure switch. To start the pump the momentary pushbutton is held down until the pump builds sufficient pressure to close the pressure switch. 

I found a switch from Dwyer that is very similar to the one FishinElectrician linked but simpler with a lower range. Can get them down to 15 psi actuation with a 15 psi dead band. Meaning i could set the pressure switch at say 20psi and have it kick out at 5psi, or 15 and 0 or whatever. Which would be about perfect. Should be about 25 psi in the line pushing it up the hill back to the plant where its open ended. The pressure shouldn't drop 15psi fast enough to kick the pressure switch out during a power bump, at least i wouldn't think so. 

this pump takes its suction from a tap in a 20 inch effluent line from a 2 million gallon clearwell. typically operating it should have between 7 and 24 foot of head on the suction port. Thought about a low pressure switch in the suction line. Found some with very low ranges (1 psi) which would work, only issue is all the ones i have found that are sensitive enough for the suction side would require the use of a starting relay, where as if i run one on the pressure side i can find switches heavy enough to run the motor directly. 

So many different ways to handle such as simple task, makes for interesting discussion.


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## mburtis (Sep 1, 2018)

Rora, Thank you for that sensor, I knew something like those existed but couldn't seem to come up with what they were called. Something like that could work well, I will have to do some looking around. Only problem might be finding one that can switch 120 as i don't have 24vdc available.


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## Peewee0413 (Oct 18, 2012)

Rora said:


> Using a pressure switch could be tricky... don't think you'd want it on the discharge side (to monitor that the pump is putting out pressure, and if it isn't, there must not be water?) it would make the interlock more complicated since you'd be putting yourself in a catch-22 to turn it on. A flow switch on either side would have the same issue.
> 
> edit: whoops, didn't read the whole post, you already covered this.
> 
> ...


Not a catch 22. Have you heard of a parallel circuit? Either a timer or manual "prime" with the start button. A sensor upstream of pump for starting, flow switch downstream as a redundancy, you'll never run the pump dry again. Or what I would do is buy a pump that wont smoke when ran dry. Peristaltic is one choice. 

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## Rora (Jan 31, 2017)

Peewee0413 said:


> Not a catch 22. Have you heard of a parallel circuit? Either a timer or manual "prime" with the start button. A sensor upstream of pump for starting, flow switch downstream as a redundancy, you'll never run the pump dry again. Or what I would do is buy a pump that wont smoke when ran dry. Peristaltic is one choice.
> 
> Sent from my SM-G965U using Tapatalk


It's a catch-22 if you're limited to simple on/off control, straying from that means someone either has to be present to restart it or add even more stuff so it tries on its own. Either way it's going to be more expensive and complicated.

OP could say for sure, but this seems like one of those things that nobody cares that much about or wants to spend a lot of money on. If it was a critical system then a more robust system with redundancy and controlled start-up would be appropriate.



mburtis said:


> Rora, Thank you for that sensor, I knew something like those existed but couldn't seem to come up with what they were called. Something like that could work well, I will have to do some looking around. Only problem might be finding one that can switch 120 as i don't have 24vdc available.


Yes I like them as a modern solution for single point level detection, they're pretty reliable in a clean process as far as my experience has been. As you point out, the downside of it involving electronics is the 3 wire VDC. Not sure if you can get around that with a different spec, the ones I've seen use LEDs and are fairly compact, they'd have to fit something to rectify an AC supply. Who knows, maybe there is a manufacturer who makes them.

That's a lot easier to find a workaround for, if you're using a contactor anyways, you can get a low capacity instrument power supply for pretty cheap these days since these are only going to pull like half a watt. I should point out that you'd want to make sure the output satisfies the latching current for your contactor as these are typically wired to a digital input.

Could even use a 24 VDC wall wart (with low power delivery) if you really want something cheap, easy, and highly questionable. If it's wired fail-safe, worst that could happen is someone having to go duct tape it back in the socket.


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## glen1971 (Oct 10, 2012)

mburtis said:


> So my initial thought was a start/stop push button control with the additional pressure switch contact just like paulengr mentioned. Only problem is that if the power bumps the pump would kick out and have to be restarted manually. We are 15 miles out of town, running a 4 mgd water plant on a line that was never intended to see more than residential service, so power bumps are common. One thought to get around this is, have a simple on/off switch control, then the pressure switch, then put a momentary NO pushbutton in parallel with the pressure switch. To start the pump the momentary pushbutton is held down until the pump builds sufficient pressure to close the pressure switch.
> 
> I found a switch from Dwyer that is very similar to the one FishinElectrician linked but simpler with a lower range. Can get them down to 15 psi actuation with a 15 psi dead band. Meaning i could set the pressure switch at say 20psi and have it kick out at 5psi, or 15 and 0 or whatever. Which would be about perfect. Should be about 25 psi in the line pushing it up the hill back to the plant where its open ended. The pressure shouldn't drop 15psi fast enough to kick the pressure switch out during a power bump, at least i wouldn't think so.
> 
> ...


Do you have a PLC on site? Or is it all relay logic? 
If you are trying to restart after a brief power blip, use a time delay relay to maintain your start circuit during a blip. Or an auto-restart after a longer bump, depending on what other logic is in play..

At 7' of level in your well, you would have about 3 psi at your suction port (84"/27.71). As you are replacing the sampling pump, is there other logic in play to shut down the main pump(s) before a certain level? Can you use this logic for this pump? 

IMO.. Get a pressure switch and install it on the suction side of the pump, this way you are protecting it from running dry. Set it at 3 psi falling (for 7'), or whatever level you want to run at a minimum. Just be careful of the deadband on the switch, so that it will reset within a decent level. Use the normally open contacts on the switch in series with an on/off switch to an ALB 509 Size 0 starter (or equivalent). That will eliminate the worry of a power pump, as it will restart if there is a level in the well.


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## mburtis (Sep 1, 2018)

There are plcs in the plant but this pump is in a pit probably 1/4 mile away. And there is conduit from the pit to the plant, as there is a flow meter and a level (pressure sensor) meter in the same pit that feed back to Scada. Its not worth pulling the wire even if there is room, let alone getting the programming done even as simple as it would be.

Discussed the time delay relay to keep the circuit latched in. We have an automatic standby gennie so it would never be without power for maybe more than a minute unless the gennie doesn't start, which case we got way more important stuff down than this pump anyway.


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## glen1971 (Oct 10, 2012)

mburtis said:


> There are plcs in the plant but this pump is in a pit probably 1/4 mile away. And there is conduit from the pit to the plant, as there is a flow meter and a level (pressure sensor) meter in the same pit that feed back to Scada. Its not worth pulling the wire even if there is room, let alone getting the programming done even as simple as it would be.
> 
> Discussed the time delay relay to keep the circuit latched in. We have an automatic standby gennie so it would never be without power for maybe more than a minute unless the gennie doesn't start, which case we got way more important stuff down than this pump anyway.


So for this pump, does it come from a panel in the plant straight to the pit? Or did someone take it's power off a receptacle, light, etc?

I'm assuming this pit is accessible and dry? Where the level transmitter is, can you add a valve for your switch to run this pump? Or is it a PITA?

Just trying to get a better picture in my head of what you're up against.


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## mburtis (Sep 1, 2018)

Well there is a feed to a panel in the pit, then the power for this pump comes out of that panel to an outlet. The level transmitter and the flow meter go directly back to the plant via 4-20 signals i believe. Been a while since I looked at those details as ive been planning all along to just use the outlet power source and make the controls stand alone. I could easily add a pressure switch to the same tap as the level sensor, just have to change an elbow to a T as there is already a dial gauge installed there as well. I don't know as i would call this pit dry, Ive seen it with a couple foot of water in the bottom of it due to various reasons/failures, luckily all the equipment is several feet off the bottom. But typically its dry, or at least no standing water. Pits not a real pain to get into, but it is outside the fence in the middle of the neighbors pasture, which is typically muddy or snowy half the year, plus having to get out the air monitor.


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## micromind (Aug 11, 2007)

Another possibility would be a current switch. This is a C.T. that reads current to the motor and if it falls too low, it'll toggle its contacts. 

Since the current on a basic centrifugal pump will fall if it loses its prime, the switch can be programmed based on the normal operating current and the no-load current.


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## tates1882 (Sep 3, 2010)

Why over think it, add a sump float assembly that has no/nc contacts. NO for pump protection, NC for a small audible alarm. Get a self priming pump. This exact setup is found in tons of sump pits and is pretty reliable. PLC and analog signals for something so simple sounds like maybe the engineers need to step back and let an industrial electrician take over, lol.

EDIT: missed that you are pulling off of a larger pipe. change float to an inline flow switch on the discharge, add a one shot time delay relay, and ice cube to handle to logic, Or a smart relay.


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## mburtis (Sep 1, 2018)

I considered a flow switch. Was having problems finding one small enough and still cheap, only moving 2gpm or so. Im sure they are out there and it would work well I bet. It would end up being basically the same circuit whether a pressure switch or flow switch.

Also looked into the current monitoring. Seems it is a pretty common way to do it. may have to find a cheap switch to play with and test out. Any particular pro or con of this way vs a process (flow or pressure) switch? Guess there would be no moving parts or anything to go wrong.


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## tates1882 (Sep 3, 2010)

mburtis said:


> I considered a flow switch. Was having problems finding one small enough and still cheap, only moving 2gpm or so. Im sure they are out there and it would work well I bet. It would end up being basically the same circuit whether a pressure switch or flow switch.
> 
> Also looked into the current monitoring. Seems it is a pretty common way to do it. may have to find a cheap switch to play with and test out. Any particular pro or con of this way vs a process (flow or pressure) switch? Guess there would be no moving parts or anything to go wrong.


Assuming you are working with potable water than current monitoring would probably be cheaper considering the flow/pressure switch would need to be drinking water safe. For cheap prototype stuff I use automaton direct to build the shop version. The ct switch is under 100 i think. The flow switch I was thinking is the mechanical type with a flapper that can be trimmed down to adjust for pipe size and flow rates.


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## tates1882 (Sep 3, 2010)

mburtis said:


> I considered a flow switch. Was having problems finding one small enough and still cheap, only moving 2gpm or so. Im sure they are out there and it would work well I bet. It would end up being basically the same circuit whether a pressure switch or flow switch.
> 
> Also looked into the current monitoring. Seems it is a pretty common way to do it. may have to find a cheap switch to play with and test out. Any particular pro or con of this way vs a process (flow or pressure) switch? Guess there would be no moving parts or anything to go wrong.


Are you planning to deadhead the pump between samples or? That's how it sounds like the previous setup was, if so you'll need something other than ct or flow. Would probably need to be pressure, unless you pipe in a return and change to a 3 way valve, basically surfing the line till they need a sample.


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## mburtis (Sep 1, 2018)

no this pump flows all the time. It pumps up to the plant where it feeds a continuous chlorine monitor and a sample sink. Both the monitor and the sample sink flow constantly. Should be flowing about 2gpm 24/7. It is potable water, but with a check valve and running to drains on the other end its not as big of concern.


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## Cow (Jan 16, 2008)

paulengr said:


> Use conventional start/stop circuit. Then the pressure switch is in series with the NO holding/sealing contact so it automatically drops out the pump when no one is around. But if it stops for any reason (power loss) someone has to restart.
> 
> Another option is a low pressure switch on the suction side or a level switch if there is a tank involved that uses NC contacts like a stop button. That way it cuts off only on low pressure/head loss.


Like Paul mentioned, we've done a lot of low pressure switches in series with holding contacts for this.

It works just fine.

But, there are lots of different ways of doing this that will get the job done.

Our typical install is a factory built pump panel with an HOA and start button running an irrigation pump for crop circles.

We use the adjustable 0-250 psi Barksdale E1H-B250 pressure switches. They have Form c contacts.


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## mburtis (Sep 1, 2018)

Cow, Thanks for the switch details, hadn't heard of the company before will have to give them a look. On your setups is the pressure switch typically on the suction side or discharge side of the pump? Is the start button typically wired in parallel with the pressure switch and in series with the latching relay contact?


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## micromind (Aug 11, 2007)

mburtis said:


> Cow, Thanks for the switch details, hadn't heard of the company before will have to give them a look. On your setups is the pressure switch typically on the suction side or discharge side of the pump? Is the start button typically wired in parallel with the pressure switch and in series with the latching relay contact?


I usually put the switch on the discharge. Unless there's some suction lift, it's nearly impossible to tell if it's lost its prime on the suction side.

The switch contacts need to be in series with the stop button and open on loss of pressure.Typically, this is called reverse action.


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## oliquir (Jan 13, 2011)

i would use a pressure switch (or pressure transmitter) with a tiny plc (or progrmmable relay) locally (there are some under 100$!) and program it the way you want (ie adding timers before shut-down, auto-restart,...)


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## gpop (May 14, 2018)

From what i am reading it sounds like a tank in a containment area that has a level sensor. The pump is simply wired to a receptacle which sends water to a sampler. 

option 1: Ask the scada guy for a output based on tank level. Add a relay on the feed wire to receptacle so if the tank goes low the relay opens. (you said the receptacle was feed from mcc).

option 2: they sell a float that is pre-wired with a male/female feed through plug. Simply throw the float in the tank then plug it into the receptacle and plug the pump into it. (normally used on water aeration tanks so its drinking water safe. There are 2 versions (no/nc) so you have to get the one that suits your need)


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## Peewee0413 (Oct 18, 2012)

Why am I getting notifications for this old thread lol...

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