Low Level Sensor Work around

RacerGofKGB · Mar 27, 2013

Hello all, I am new here and fairly new to PLC's. We just installed an outside oil tank and are transferring oil from it to an inside oil tank. I wrote the program for it(First one since college 12 years ago) and all is working well. I only have one issue and I am hoping you guys can help me work around it.
The program is set up like this... Once the low level of the inside tank is hit, the outside tank pumps kick on and fill the inside tank until the high level switch is hit. Basic stuff here. If the low level switch is hit on the outside tank, the pumps will not kick on. This was to prevent running the pumps dry or uncovering the heating elements in the bottom of the tank. The problem is that the lowest switch on the outside tank is so high, that I still have enough oil for 3 additional transfers to the inside tank.

Is there a way that I can set up the program so that once the outside low level is hit, I will have 2 or three more pump cycles before the pumps shut down?

Lancie1 · Mar 27, 2013

Is there a way that I can set up the program so that once the outside low level is hit, I will have 2 or three more pump cycles before the pumps shut down?

Yes, there are several ways, from cheap and easy to expensive and difficult.

The easiest cheapest way that first comes to mind is to determine from experiments how long it takes to do an average transfer. Then find out how many transfers can be done AFTER the outside tank low level is reached.

In your PLC program, you can set up a timer or a counter to record the number of "transfers" AFTER low level switch goes ON. A counter may be best, if the number of possiblke transfers is constant. If the possible number is 3, set your "Allowed Transfers Counter" to 3. Notice that this method STILL uses the outside Low Level Switch to determine WHEN to start counting. When this Counter hits 3 (or whatever the maximum number is), set up a Pump Lock-out bit that is ON until the outside tank gets refilled (hits High Level again?).

If you cannot come up with a reliable, constant number of "Transfers After Low Level", then figure out the average seconds per gallon delivered by the pump, then base your Pump Lock-out on the allowed pumping time AFTER outside low level is reached.

That is the cheap easy way. A hard expensive way is to move the low level switch in the outside tank.

widelto · Mar 27, 2013

While you move your low level switch to a lower position, you can use a counter (up to two) to fill up your inside tank. The risk is that you will never know when your outside level switch is hit, so that I prefer to use two instead of three om the counter to assure you will never run pump dry.

Anirban Hazra · Mar 27, 2013

RacerGofKGB said:
Is there a way that I can set up the program so that once the outside low level is hit, I will have 2 or three more pump cycles before the pumps shut down?

That can be done.First determine how many pump cycles you can exactly run safely without running your pumps dry.Then use a counter and code somewhat like this(the number of such cycles is considered as 2 in the shown logic):

|                           C1

| HL-IT    LL-OT        +--------+  

|--|\|------| |---------|  CTU   |

|                       | PV = 2 |

|                       +--------+

|

| LL-IT    HL-IT  C1.DN      Pump 2  

|--| |----+-| |----|\|--------( )

|         | 

| Pump 2  |

|--| |----+

|

| LL-OT                        C1

|--|/|------------------------(RES)

|

LL-IT = Low Level - Inner Tank
HL-IT = High Level - Inner Tank
LL-OT = Low Level - Outer Tank

I have written this ladder assuming that the level switches get closed or get on when the water level falls below them.If they work in a reverse manner just swap the NOs with NCs in the logic accordingly.Also, I have taken the number of cycles as 2

Lancie1 · Mar 27, 2013

Racer,

What brand and model of PLC are you using? There are some differences that might be important in this program. For example, many PLC software programs do not have a timer DONE bit, so the logic would be slightly different for some brands.

rdrast · Mar 28, 2013

Another problem that should not be handled with programming "Work Arounds".

Two options:
1) Move the switch on the outside tank to a position representing tank empty.
2) Install a second switch on the outside tank indicating tank empty.

Mark Buskell · Mar 28, 2013

What happens if the upper switch fails? This is why you have a overflow or hihi switch.

RacerGofKGB · Mar 28, 2013

Thank you all for the input. I should have some time today to sit down and try to work through this.
Lancie- I am using RSLogix 5000 with Compactlogix L32E
rdrast - I agree that the best way to achieve this would be to move the sensor. However, pulling off the insulation, cutting a hole and welding a new flange on the tank is not an option at this point.
Mark - There is a hihi switch on both the inside and outside tanks.

Bit_Bucket_07 · Mar 28, 2013

Assuming that your pump operates at a fixed rate, the safest way to do this would be to determine the amount of time that you can safely pump out of the source tank after the low level switch trips without encountering an actual low level condition.

Then, forget about how many "charge cycles" are involved and operate a retentive timer whenever the low level switch is tripped and the pump is operating. Use that timer's done bit to disable the pump, and don't reset the timer until the low level switch is reset.

This is still a software patch for a hardware insufficiency and is therefore something that I wouldn't, in good conscience, recommend. However, if you're going to install a software patch anyway, it should at least be the best patch that you can use.

TConnolly · Mar 28, 2013

Lets take a different track and discuss a super easy retrofit.

We use this sensor a lot in hydraulic oil tanks.

http://www.ifm.com/ifmus/web/padv/040_040_010_010_025_010.html

Its submersible, you drop it in the tank through an opening in the top and let it lay on the bottom of the tank, so installation couldn't be simpler. It gives you a 4-20mA analog output that is proportional to the height of the fluid. The sensor is calibrated for inches of water, however if you divide the sensor range by the specific gravity of your oil then you'll have inches of oil and can scale accordingly.

dploof23 · Mar 28, 2013

TConnolly said:
Lets take a different track and discuss a super easy retrofit.

We use this sensor a lot in hydraulic oil tanks.

http://www.ifm.com/ifmus/web/padv/040_040_010_010_025_010.html

Its submersible, you drop it in the tank through an opening in the top and let it lay on the bottom of the tank, so installation couldn't be simpler. It gives you a 4-20mA analog output that is proportional to the height of the fluid. The sensor is calibrated for inches of water, however if you divide the sensor range by the specific gravity of your oil then you'll have inches of oil and can scale accordingly.

Holy Smokes!!! I thought this was very cool....until I saw the 500-600 dollar price tag. Ouch! Never seen one of these. We use some ultrasonic sensors to monitor some tanks we have.

So I guess this would be an easy, expensive way to solve this!:nodi:

Dave

dmargineau · Mar 28, 2013

TConnolly said:
Lets take a different track and discuss a super easy retrofit.

We use this sensor a lot in hydraulic oil tanks.

http://www.ifm.com/ifmus/web/padv/040_040_010_010_025_010.html

Its submersible, you drop it in the tank through an opening in the top and let it lay on the bottom of the tank, so installation couldn't be simpler. It gives you a 4-20mA analog output that is proportional to the height of the fluid. The sensor is calibrated for inches of water, however if you divide the sensor range by the specific gravity of your oil then you'll have inches of oil and can scale accordingly.

I too vouch for this one...

It's fairly new (a couple of years or so) and it solved some of our (decades old veterans say!) level control issues.
One thing to be mentioned though...the orifice the sensor is using to read the differential pressure (some 0.125" Dia) is prone to clogging up if used within unfiltered liquid tanks.
If the media is fairly clean though, it works like a charm...

TConnolly · Mar 28, 2013

That's list price. Your distributor will probably have it for a lot less than two or three hours of wages for a couple of union pipe fitters and welders wages.

Low Level Sensor Work around

RacerGofKGB

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Lancie1

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widelto

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Anirban Hazra

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Lancie1

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rdrast

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Mark Buskell

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RacerGofKGB

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Bit_Bucket_07

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TConnolly

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dploof23

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dmargineau

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TConnolly

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