Operating a pump with two float switches, relay and solenoid valve

Thread Starter

Jerimy.Frailey

Joined Jun 16, 2017
6
I am designing a way to pump ethanol from one tank to the top of another. The best way I can think of is running two float switches, one to let me know when liquid enters the tank, and the second sending the signal to open a valve. Once the level drops to below the second float switch, the valve is closed, stopping the flow. I am using 2 Madison M5910 switches, and an SSR as my relay, what would be the best controller for the valve?
 

crutschow

Joined Mar 14, 2008
24,963
It's not clear what you are trying to do. :confused:
Can the value and pump both be controlled from the same SSR?
At exactly what levels in the two tanks do you want the pump to start/valve-open, and what levels do you want the pump to stop/valve-closed?
 

Alec_t

Joined Sep 17, 2013
11,285
Welcome to AAC!
I'm unclear how your switches are arranged, but won't you need 4 level sensors/switches, to know when each tank is empty/full?
 

Thread Starter

Jerimy.Frailey

Joined Jun 16, 2017
6
The pump is an air operated chemical duty pump, and the valve is used to control air to the pump. When the level gets to the top switch the valve would open, allowing the tank to be pumped down. Once the level falls below the bottom switch, the valve would be closed. The idea I have is to use a relay between the valve and the lower switch, so that the circuit remains closed until the lower switch is opened, deactivating the relay, and closing the valve.
 

Alec_t

Joined Sep 17, 2013
11,285
You will need a latching arrangement to keep the relay energised during pump-down. A spare pole on the relay can do the latching, or you could use a double-coil latching relay.
What is the valve spec? How long does pump-down take?
 

Thread Starter

Jerimy.Frailey

Joined Jun 16, 2017
6
I will use a Fotek SSR-40 for the relay, and the available power is 220v three phase. The switch I have is rated to handle 240V AC. We ran the pump without switches and using a simple air regulator check valve, but we were running a one to one where output liquid was at the same rate as input liquid. Now we want to fill a tank, then after we get to a set point, re introduce the ethyl alcohol into the system to attain a higher proof. I was looking at using this valve: http://www.electricsolenoidvalves.com/1-4-110-120v-ac-stainless-electric-solenoid-valve/ from Jegs. The valve will be inline with the air going to the pump, not in contact with the ethanol pumping through the pump.
 

drc_567

Joined Dec 29, 2008
843
Tank Empty-Fill.jpg

The Madison switches are shown as
Sinking= open circuit
Floating = closed circuit

Install both float switches in the first tank ... one upper and one lower.
Put a SCR ... silicon controlled rectifier ... in series with the SSR 'on' terminal. The lower float switch goes in series with the SCR output current. The upper float switch is used to turn on the SCR control gate, when the tank reaches the full level. The SCR turns off when the lower switch sinks and opens, causing the SCR current to cease. (This is the way SCRs work.) As new fluid enters the tank, the lower switch again closes.
Flow confirmation will require additional circuitry.
 
Last edited:

Thread Starter

Jerimy.Frailey

Joined Jun 16, 2017
6
I have been thinking about using SCR's in some other processes I am working on, but hadn't considered it for this because I figured the over voltage might damage the switch.
 

drc_567

Joined Dec 29, 2008
843
I have been thinking about using SCR's in some other processes I am working on, but hadn't considered it for this because I figured the over voltage might damage the switch.
It looks like the the Fotek SSR has a trigger current of 7.5 ma/12volt ... max. So adjust a series resistor accordingly ... R= 2k ohms should be sufficient.
 

Reloadron

Joined Jan 15, 2015
5,454
Nice switches and a good choice for a hazardous location. Since you are working with three phase (be it wye or delta) I would likely run with a low voltage control system. I would just use a contactor for the pump which I assume is three phase but stay with low voltage control all in an approved NEMA enclosure. I would use a latching circuit where you have one N/O (Normally Open) and one N/C (Normally Closed) switch. As was mentioned the N/O switch is the upper switch and the N/C is the lower switch. When the tank level reaches a High Limit the N/O switch closes and latches a relay. This relay uses a set of contacts to latch itself as well as turn on the pump contactor. The tank pumps down until the N/C switch opens and the system is reset. The pump contactor will have any needed over current thermal protection. This would address a single high / low limit for a single tank. I would use a small control transformer to get 24 VAC control voltage.

Ron
 

crutschow

Joined Mar 14, 2008
24,963
The SCR circuit proposed in post #12 should work but it should be run from a low voltage DC supply, say 12Vdc from a wall-wort.
The SCR won't work with AC in this circuit.
That will also minimize dangerous voltages running around.
 

Thread Starter

Jerimy.Frailey

Joined Jun 16, 2017
6
View attachment 129092

The Madison switches are shown as
Sinking= open circuit
Floating = closed circuit

Install both float switches in the first tank ... one upper and one lower.
Put a SCR ... silicon controlled rectifier ... in series with the SSR 'on' terminal. The lower float switch goes in series with the SCR output current. The upper float switch is used to turn on the SCR control gate, when the tank reaches the full level. The SCR turns off when the lower switch sinks and opens, causing the SCR current to cease. (This is the way SCRs work.) As new fluid enters the tank, the lower switch again closes.
Flow confirmation will require additional circuitry.
That's the way I had thought about the circuit, but instead of controlling the pump, a Graco chemsafe pump, I am controlling an on/off solenoid valve to let the air to the pump.
 
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