Flow switch for dry running protection

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Bob NH

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You could do it with a reverse action pressure switch and a time-delay relay.

Install a pressure switch on the suction of the pump that will open when the pressure drops below the point where you want to operate the pump. Reverse action pressure switches are available from different sources. For example, The FRG52 (links below) can be set to a cut-out pressure (switch opens) as low as 1 psi and a cut-in pressure of 6 psi. The FRG32 has a cut-out pressure as low as 4 psi and a cut-in pressure adjustable from 6 to 20 psi above the cut-out pressure.

http://ecatalog.squared.com/catalog/173/html/sections/20/17320014.html

http://ecatalog.squared.com/catalog/173/html/sections/20/pdf/17320014.pdf

Then install an On-delay relay such as the SSAC TRU Series using the "Delay on make" function where the output contact is closed at a set time up to 1000 minutes (16.67 hours) after voltage is applied.
http://www.ssac.com/standard/ff-tdr1.htm

http://www.ssac.com/catalog/TRU01A01.pdf

When the pressure drops below the set pressure (for example the 1 psi setting of the FRG52) the power is removed from the relay and the pump stops. If the pressure dropped because the pump was drawing water, then the pressure might be immediaely restored, applying power to the relay. However, the relay contact would not close until the delay setting had elapsed.

For example, it would be possible to set the delay for one hour, in which case the system would "try again" every hour to see if there is anough water to supply the pump.
 
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Bob1000

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Please have a look to the flow switch link below
http://www.watts.com/pro/_productsFull.asp?pid=685&ref=1

I think kiking in the pump by a timer activated by the pressure switch is a must because if you wait till the flow switch paddel get pushed by the flowing water you must wait till the city pressure is able to flow to the pump inlet and that would not happen at the cut in pressure of the pressure switch if it is greater than the city pressure ( most of the cases it is bigger) otherwise you would open a faucet and wait till the pressure tank empties ALL its water capacity then the pressure is less than the city pressure for the flow switch to kick in .

A second timer would be needed as well to stop cycling at the city pressure near to flow switch pressure threshold

I do agree with you that the Thermal sensor is a bad idea because the pump would be toast before it cuts out

I am thinking of another idea ..
The new pump would be able to deliver 87 PSI sucking from a bucket of water without any need for city pressure just if the water is just present at the pump inlet , so I am thinking of fixing a sensor that would sense the presence of water in stead of the flow switch

Any comments?
 

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There will still be water in the lines, it will just be at 0 pressure. A loss of prime or reverse acting pressure switch would be the simplest. If you give it a wide bandwidth, such as off at 20 and on at 40, it should not bounce the pump as the pressure in the city line increases.
 

MarkHash

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I agree that the octal base plug-in timers would be the better choice here. Good luck and let us know how it works out.
 

Bob1000

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There will still be water in the lines, it will just be at 0 pressure. A loss of prime or reverse acting pressure switch would be the simplest. If you give it a wide bandwidth, such as off at 20 and on at 40, it should not bounce the pump as the pressure in the city line increases.


I know that the reverse acting pressure switch would be the best and simplist but I can not find it in the local market here as they only sell the Square D normal one no the reverse action one , it seem that no body would be interested in it lol

I will go to some HVAC shops and try to find something similar that can do the job

I think a low pressure cut out with automatic reset would do the job
 

Wet_Boots

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One can always connect a relay to a pressure switch and turn the DPST action into DPDT action.
 

Bob1000

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One can always connect a relay to a pressure switch and turn the DPST action into DPDT action.


I think I know what you mean but still the min pressures values that the nomal pressure switch is designed for are much higher than the rates I would be looking for because I would be looking for a 5 psi threshold ( Cut out) or even less because the pump would suck any water in the pipes and can give 87 psi at the other end

Will search for an HVAC one with an automatic reset and advise back
 

Bob NH

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There are lots of solutions; many of which have been suggested here. I suggest that you pick one and implement it.

First you must define your requirements to yourself.

1. What is the lowest source pressure that will be available when the pump is drawing low flow (maybe 3 to 5 GPM) from the system?

2. What is the required pressure range (low at pump start; high at pump shutoff)?

Then, buy and install a pump that provides the GPM and pressure range that you need.

A Goulds HSJ07N will deliver a shutoff pressure exceeding 70 psi at zero psi suction pressure, and it will work even if the suction pressure is 50 psi.

At the "zero pressure suction" condition it will deliver about 15 GPM at 50 psi.

You can set the pressure switch at 50/70 or anything else you like as long as the shutoff pressure is 70 psi or less.

You must have a relief valve set at maybe 75 to 80 psi.

So, all you need is the pump and pressure switch that delivers what you want, and a relief valve to keep from exceeding the safe pressure limit if the pressure switch fails.

If you want even more pressure there are pumps that will deliver what you want.

No relays. No reverse action pressure switches. No worries about varying water pressure as long as it is greater than zero.

If you want it fancier you can get a pump control that will automatically shut things off if the source fails completely.
 

Bob1000

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Have city water cut offs often in summer

There are lots of solutions; many of which have been suggested here. I suggest that you pick one and implement it.

First you must define your requirements to yourself.

1. What is the lowest source pressure that will be available when the pump is drawing low flow (maybe 3 to 5 GPM) from the system?

2. What is the required pressure range (low at pump start; high at pump shutoff)?

Then, buy and install a pump that provides the GPM and pressure range that you need.

A Goulds HSJ07N will deliver a shutoff pressure exceeding 70 psi at zero psi suction pressure, and it will work even if the suction pressure is 50 psi.

At the "zero pressure suction" condition it will deliver about 15 GPM at 50 psi.

You can set the pressure switch at 50/70 or anything else you like as long as the shutoff pressure is 70 psi or less.

You must have a relief valve set at maybe 75 to 80 psi.

So, all you need is the pump and pressure switch that delivers what you want, and a relief valve to keep from exceeding the safe pressure limit if the pressure switch fails.

If you want even more pressure there are pumps that will deliver what you want.

No relays. No reverse action pressure switches. No worries about varying water pressure as long as it is greater than zero.

If you want it fancier you can get a pump control that will automatically shut things off if the source fails completely.


Thank you BobNH
I have bought already a new 2HP expensive pump that I am going to install to rplace my 1HP pump that i have it running right now that is why I am keen on some sort of a suitable but simple dry run protection
The pump I have can deliver 87 psi at zero city pressure and I will set up my pressure switch to 60/80 so now I got my cut out pressure problem solved

I was thinking of a protection method works on either pressure or flow , from the views given here I think the most simple is the pressure switch reverse action but what I am worried about is that may be my new pump when it starts would make a sudden momentary drop of pressure at the upstream side that would make the low pressure switch opens and ut off the power specially if it is set to a low value like 1 psi for example

any comments?

I am planning to go buy it tomorrow so I would appreciate your comments soon

You said if you like it fancy , there is a pump control , what is that ?
of course I am VERY keen on having that sort of protection because in summer I always have frequent city water cut offs in the peak hours and one of them is enough to burn my new pump
 
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Valveman

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A couple of years ago there was a city in Texas that had a big fire. The fire trucks were sucking on the fire hydrants faster than the city and water lines could supply. The suction collapsed the city water lines in many places. If you suck it down below 0 pressure, you can collapse your lines. A flow switch doesn't care about the pressure and could still be closed as you were creating the negative pressure. A low suction pressure switch will catch before negative pressure.

You can give it a wide bandwidth to stop the cycling from the city pressure coming and going. Set it to shut down at 10 PSI and not back on until the city pressure gets back up to 30 or even 40 PSI. You can also install this switch with a small 1 gallon size bladder tank. If you restrict the size of the opening to this tank and switch, it makes a mechanical timer. ½ a gallon of water has to come in or go out of this tank for the pressure to change from 10 to 30. So the slower you let this water in an out, the more time you have between off and on, and the switch does not bounce the pump on and off. I have one set up this way with a 1/4" poly line running from the main line to the little tank and switch. No switch bouncing!!
 

Bob NH

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You could get a thermal switch that would shut off the pump if it gets too hot, which it will if there is no flow.

You could use a reverse acting pressure switch with a wide range setting. You would need to verify the characteristics of the supply to be sure that it would not cycle rapidly by restarting immediately after the pressure dropped. If the pressure returns quickly because the flow stops, then you would need a time-delay relay to protect the pump.

If you can find a pressure switch with a low pressure cutoff it will automatically shut off the pump if the pressure drops about 10 psi below the cut-in pressure. It must be manually reset before it starts again.

There are electronic devices like the Pumptec but they must be matched to the pump and may not work with the one you have.
 
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Bob1000

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A couple of years ago there was a city in Texas that had a big fire. The fire trucks were sucking on the fire hydrants faster than the city and water lines could supply. The suction collapsed the city water lines in many places. If you suck it down below 0 pressure, you can collapse your lines. A flow switch doesn't care about the pressure and could still be closed as you were creating the negative pressure. A low suction pressure switch will catch before negative pressure.

You can give it a wide bandwidth to stop the cycling from the city pressure coming and going. Set it to shut down at 10 PSI and not back on until the city pressure gets back up to 30 or even 40 PSI. You can also install this switch with a small 1 gallon size bladder tank. If you restrict the size of the opening to this tank and switch, it makes a mechanical timer. ½ a gallon of water has to come in or go out of this tank for the pressure to change from 10 to 30. So the slower you let this water in an out, the more time you have between off and on, and the switch does not bounce the pump on and off. I have one set up this way with a 1/4" poly line running from the main line to the little tank and switch. No switch bouncing!!

I think the reverse acting pressure switch together with that small bladder tank is the best idea
But how you say that I should set it up to shut down the pump at 10psi and restart it at 30 or 40? the city pressure is around 20 psi most of the time that is why I need a booster pump
I was intending to set it up cut out at 5psi and cut in at 10 psi , does that make sense?
Because the new pump would give me the pressure I want at any city pressure I want to make full use of that and I want to be able to keep my system working at zero city pressure but in the same time I want it to shut down at dry running when actually there is no water to suck or a negative pressure would happen.

I want to make full use of the new pump but in the same time protect it from burning in case the city water is completely cut off
 

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You can't pump water unless it is there to be pumped. If the city drops below 5 PSI, there is no water left to pump. You will need to wait until the city is back up to maybe 20 PSI before restarting the pump. 5/20 is a range your pump may be able to live with. 5/10 is not.

Otherwise you need to use a cistern tank that can fill when the city does have water, so you can have water to use during the time the City is below 5 PSI. The booster pump can then give you all the pressure you want as long as the cistern has water. Then the cistern can refill later when the city has water again.
 

MarkHash

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Hey valveman, what is the control scheme on the cistern then? It couldn't be a pressurized tank could it? Then it would be essentially another ballast pressure tank right? Would it have to be isolated when it becomes full?
 

Wet_Boots

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Just think how much simpler this guy's life would be if he ponied up for a good thermostatic shower valve.
 

Bob1000

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Just think how much simpler this guy's life would be if he ponied up for a good thermostatic shower valve.

That valve might solve the problem of the pressure fluctuations but would not help if the city pressure is too low to let the pump work and deliver water

Their ideas are smart and deserve to think about , in fact last summer I was thinking about the cestern issue and I would do it if I became sure that the city low pressure periods are too long to avoid that complication

I think a level control switch together with a mechanical float valve would be the control system for that cestern , And I think it would be an open tank , any comments?
 

Valveman

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Toilet type float valve on the incoming city line to feed the cistern. Then suck out of the cistern with the 2 HP pump with pressure tank and pressure switch for control. A float switch in the cistern connected with the pressure switch would keep the pump from running if the cistern were dry. This way you still have a cistern full of water when the city is out, and your pump makes sure you always have enough pressure.
 

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Valveman, can you point me to a link for such an apparatus? I've installed (previous job) sewage wetwells with hi/low/emer floats, dual pumps, and pump cycling timers, but i've never heard of a mechanical "toilet type" combination level control and shutoff valve on the scale that would fill a cistern! I would have thought it had to be an electric valve of some sort.
 

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I can get you a mechanical float valve as big as you want. I think a 6" is the largest I have done but, they make larger ones. Cla-Val, Watts, Bermad, and others make these kind. I have also seen 1" and larger at Farm and Ranch supply houses and Country Feed Stores. I can get you some names if you need them.

You don’t want to use a toilet type float coming from a pump. The closer the float gets to the top, the slower the flow and the pump will cycle on and off. These are called modulating type float valves. You can get non-modulating float valves to use coming from a pump. These are either fully open or fully closed.

The regular modulating type will work fine for the line coming from a city supply.
 
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