Safety sensors for tank volume high-high sensor?

PreLC · Jul 22, 2020

Hello PLCS.net!

I wanted to ask someone who has experience in this. How does one do fluid level measurements if it's a safety/environmentally critical operation?

Thanks!
-PreLC

qdominique · Jul 22, 2020

We have used ultrasonic sensors when dealing with fluid levels in the waste department. It is on the 4-20ma scale, the higher the reading, the lower the fluid level. Seems to be working fine but if there is foam it will make the measurement fluctuate.

danw · Jul 22, 2020

Use two different technologies, one for continuous level measurement and a different level technology for a point level, high level alarm. The use of two different technologies avoid the problem of a common mode failure of both technologies to detect a high level that can result in a spill. The concept is called 'diversity'.

Typical liquid continuous level technologies are hydrostatic head pressure, non-contacting (top mounted radar) or contacting radar (guided wave radar), ultrasonic, capacitance or magnetostrictive. If you need nuclear technology you probably already know about it.

Typical point level technologies are capacitance, vibrating fork, mechanical float(s), and conductivity.

I_Automation · Jul 22, 2020

For " safety/environmentally critical" I would actually propose 3 different types of sensors to compare results.

Float, ultrasonic, and vibrating fork. Either the ultrasonic or vibrating fork can be changed to radar. Also, if it's a non-metal tank, an external proxy can sense fluid through the walls depending on the fluid.

Using a pressure switch on the tank bottom is OK for sensing low level, but I would not rely on that to determine full or overfull for safety needs.

parky · Jul 22, 2020

Just to add to the above, make your high & High High level signals true when not covered or actuated, or active when not covered, the reason is if power or signal is lost the PLC will see this and assume a high level has been reached.
There is a difference between N/O & N/C, therefore use a sensor that when not covered or detected but powered gives a signal to the controlling system, this way a broken wire or loss of power to the sensor will be seen as a high level (signal lost).
Also, for extra safety, do a check on the low & high sensors so if an illogical combination appears it also shuts the system down. for example, if the High or High High sensor is indicating level and the low is not then there is a problem.

Tom Jenkins · Jul 22, 2020

I always use a float switch for Hi/Hi level alarming. If the switch is in a hazardous environment use a Gems SafePak or similar intrinsic safety barrier in the panel.

janner_10 · Jul 22, 2020

I would have 2 electronic for level and make sure results are similar and a mechanical float type for that HH level.

saultgeorge · Jul 22, 2020

parky said:
Just to add to the above, make your high & High High level signals true when not covered or actuated, or active when not covered, the reason is if power or signal is lost the PLC will see this and assume a high level has been reached.
There is a difference between N/O & N/C, therefore use a sensor that when not covered or detected but powered gives a signal to the controlling system, this way a broken wire or loss of power to the sensor will be seen as a high level (signal lost).
Also, for extra safety, do a check on the low & high sensors so if an illogical combination appears it also shuts the system down. for example, if the High or High High sensor is indicating level and the low is not then there is a problem.

Hi, this is known as being wired as a "failsafe" configuration. I do that with almost all my digital alarms in my projects.

Safety sensors for tank volume high-high sensor?

PreLC

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qdominique

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danw

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I_Automation

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parky

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Tom Jenkins

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janner_10

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saultgeorge

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