Please help an intern design the best liquid level monitoring system.

[Marsbars2319]

Hi everyone, I am an electrical engineering intern who recently received a project that is definitely a bit over my head, and I could use some help. I am tasked with redesigning and implementing a new control system for the main wastewater lift station at our plant. The current system in place is inadequate and failed recently causing an overflow. It currently uses several mechanical float switches and is not wired or programmed to be very fail-safe or redundant.

The goal of this redesign is to replace the mechanical float switches with something that requires less maintenance and is much more reliable/less likely to fail. Along with that I am also supposed to build the system to be much more redundant, whether it be through multiple redundant sensors, wiring, programming or all of the above. This is a very important system, and an overflow is obviously very undesirable, so money is no object in this project.

Some information about the system, the lift station is located outside of the plant, is a circular, concrete pit, 8 feet wide, around 17 feet deep. It has 3 pumps at the bottom. Inflow averages 1200 gallons per minute, and it has a volume of approximately 10,000 gallons. As far as the liquid goes, PH can range from 2 to 12 (mostly stays on the caustic side). It is turbulent, with foam a possibility, I would say less than a foot. Temperature can come in at 112 degrees Fahrenheit. Vapor is also a concern.

My first and main question/task to figure out, is which type or types of level sensors to use to do the monitoring. From my own limited research, based on the conditions of the lift station, the two best options seem to be guided radar, or hydrostatic pressure. If you could provide some guidance on which would be the best and why I would appreciate that.

The next concern is how to make the system redundant, so failure of one sensor does not cause an overflow or cause the pumps to run dry. The main idea now is to perhaps run 2 analog sensors, and program them in the PLC to compare the values and check the tolerance on them. Also perhaps leaving a high-level float in the system as the backup. I would also appreciate some feedback on the best way to do this as well.

Any and all help is welcome, if you have any other advice on this project please do share. Too clarify i won't be doing this by myself, I will have the help of senior electrical engineers, I am just trying to help and do as much as possible myself.

 

[cardosocea]

I think if money's not an issue a variety of techniques is required. Hydrostatic pressure and guided wave are good bets (if you get a chance to install them in a critical system), the other thing I'd add on would be actual level switches, like tuning forks. These would be the last line of defense and even then you'd have several layers to count on.

The important thing here is to bear in mind what the system should do on each situation and which sensor will you believe in should they all disagree.

Edit: for the cost of installing guided wave you may as well install two of them.

 

[parky]

I agree with cardosocea, The things I would add are radar sensors vapour can be an issue, I would choose the extra sensors with care, also as already stated use a different set of sensors for backup, make sure you have fault monitoring on the pumps even if this is tripped, also alarm handling sent either digitally or by communications to some sort of monitoring station i.e. an HMI or PC that can be easily seen as this appears to be a remote system.

 

[jstolaruk]

Whatever you choose, think if terms of fail-safe. If the sensors fail, lose of signal, then the pumps / heaters aren't allowed to run. A positive signal that the level is OK.

I'm currently dealing with a problem of the customer purchased level controls that aren't fail-safe yet he wants the system to protect itself from a low tank level exposing the heating rods and pump intake. He purchased level sensors whose outputs only change when the tank reaches high level (high level alarm). He says use the N/C contacts. I can't get into his head that it won't protect the heater/pump; I won't be able to detect that the tank is low level no matter what contacts are used.

 

[parky]

Think about high level, it should be true or giving a signal when not covered or detecting level the low level should be true or giving a signal when covered, the reasons for this is if the high level loses it's power or a wire break it sees it as being covered, the low level works in reverse so if it sees no signal i.e. wire break or loss of power it sees it as below level, this way a fault generally will stop the pumps or heaters etc. One other thing is illegal signals for example if you have high level but low level is not sensing then there is something wrong.
Other things you could do (depending on how it controls the level) for example if filling it is normal to fill to high level (not High High level that's alarm condition) and it does not get there then there is probably a fault causing no feed. so there are many things you can do (extra probes HH level & LL level), pump monitoring, not seeing level sensors changing state (assume at some time in the process the level probes/sensors should switch).

 

[mylespetro]

Hi everyone, I am an electrical engineering intern who recently received a project that is definitely a bit over my head, and I could use some help. I am tasked with redesigning and implementing a new control system for the main wastewater lift station at our plant. The current system in place is inadequate and failed recently causing an overflow. It currently uses several mechanical float switches and is not wired or programmed to be very fail-safe or redundant.

The goal of this redesign is to replace the mechanical float switches with something that requires less maintenance and is much more reliable/less likely to fail. Along with that I am also supposed to build the system to be much more redundant, whether it be through multiple redundant sensors, wiring, programming or all of the above. This is a very important system, and an overflow is obviously very undesirable, so money is no object in this project.

Some information about the system, the lift station is located outside of the plant, is a circular, concrete pit, 8 feet wide, around 17 feet deep. It has 3 pumps at the bottom. Inflow averages 1200 gallons per minute, and it has a volume of approximately 10,000 gallons. As far as the liquid goes, PH can range from 2 to 12 (mostly stays on the caustic side). It is turbulent, with foam a possibility, I would say less than a foot. Temperature can come in at 112 degrees Fahrenheit. Vapor is also a concern.

My first and main question/task to figure out, is which type or types of level sensors to use to do the monitoring. From my own limited research, based on the conditions of the lift station, the two best options seem to be guided radar, or hydrostatic pressure. If you could provide some guidance on which would be the best and why I would appreciate that.

The next concern is how to make the system redundant, so failure of one sensor does not cause an overflow or cause the pumps to run dry. The main idea now is to perhaps run 2 analog sensors, and program them in the PLC to compare the values and check the tolerance on them. Also perhaps leaving a high-level float in the system as the backup. I would also appreciate some feedback on the best way to do this as well.

Any and all help is welcome, if you have any other advice on this project please do share. Too clarify i won't be doing this by myself, I will have the help of senior electrical engineers, I am just trying to help and do as much as possible myself.

I've been working on a lot of smaller municipal wastewater lift stations lately, and they all use Siemens ultrasonic level sensors connected to an LUT400 series transmitter. We use mechanical float switches as backup for the High-High and Low-Low levels. The normal operation is based on setpoints entered on the local HMI, reading off of the ultrasonic sensor, and if either water hits either the high float or drops out the low float, the system enters "float bypass" mode, where it operates strictly off of the floats, bypassing the PLC. Operations must then flip the HOA switches away from Auto and then back again to return control to the PLC.

 

[busarider29]

I've been working on a lot of smaller municipal wastewater lift stations lately, and they all use Siemens ultrasonic level sensors connected to an LUT400 series transmitter. We use mechanical float switches as backup for the High-High and Low-Low levels. The normal operation is based on setpoints entered on the local HMI, reading off of the ultrasonic sensor, and if either water hits either the high float or drops out the low float, the system enters "float bypass" mode, where it operates strictly off of the floats, bypassing the PLC. Operations must then flip the HOA switches away from Auto and then back again to return control to the PLC.

+1

Ultrasonic sensors, yes. However, make sure they are mounted on something rigid that doesn't pick up any potential vibration from another source.
I would try to steer clear of mechanical float switches if I could though. I'd opt for an analog level sensor. Obviously on a much smaller tank, but we use an analog level sensor like this one in our fluid tanks. Check out Deeter Electronics. They most likely will have something you need. They can customize to your needs as well. They custom made the level probe to the length we required. I would call them and describe your project and needs. They most likely will have provided something similar and will know right away what you require. Also, if $$ is not an option, I would at least double up the analog level sensors. We do that with our fluid temperature probes.

 

[Saffa]

As others have said, independent controls based on a different technology is the way to go. For your application, this could be:

Main PLC running on analog level sensors (I'd recommend 3 sensors so you can more easily implement voting systems. With just two, you might know there's a difference between them, but which one do you believe? With three, the system is tolerant to one sensor failing in a normal span value mode, or low or high).

Backup float switch controls (stop / start) for each pump. Use a smart relay for each, they're cheap as chips but you can add some simple features like short cycle prevention, faulty float switch detection, and signaling to the main PLC.

Don't forget redundancy when it comes to power supplies. If everything runs off one 24VDC PSU, well you're back to a single point of failure.

I like to keep the backup float control levels above normal operating levels so the floats aren't continuously submerged. Being banged around in turbulent wastewater screws them up quicker, and it's always the low level / stop float that fails.

The last big sized station we upgraded controls for had two wells, 5 x 250kW pumps in each well, 3 level sensors per well (2 hydrostatic, one ultrasonic) and 6 float switches per well.

We implemented a Schneider M580 hot-standy PLC and spread the IO across 4 ethernet racks, so losing one rack would not take out all instruments, or all pumps etc. We also had two 400V main incomers and a standby generator so there was extra redundancy there.

Half the work was just figuring out what happened in likely failure scenarios, and had we covered it.

 

[Nova5]

Whatever you choose, think if terms of fail-safe. If the sensors fail, lose of signal, then the pumps / heaters aren't allowed to run. A positive signal that the level is OK.

I'm currently dealing with a problem of the customer purchased level controls that aren't fail-safe yet he wants the system to protect itself from a low tank level exposing the heating rods and pump intake. He purchased level sensors whose outputs only change when the tank reaches high level (high level alarm). He says use the N/C contacts. I can't get into his head that it won't protect the heater/pump; I won't be able to detect that the tank is low level no matter what contacts are used.

Absolutely true. Fail Safe wiring and logic.

When the tank is happy, at its operating level, all discrete level sensors should be signaling. This way of something happens to a wire, the system will alarm and react as if that level was reached.

 

[busarider29]

+1

Ultrasonic sensors, yes. However, make sure they are mounted on something rigid that doesn't pick up any potential vibration from another source.
I would try to steer clear of mechanical float switches if I could though. I'd opt for an analog level sensor. Obviously on a much smaller tank, but we use an analog level sensor like this one in our fluid tanks. Check out Deeter Electronics. They most likely will have something you need. They can customize to your needs as well. They custom made the level probe to the length we required. I would call them and describe your project and needs. They most likely will have provided something similar and will know right away what you require. Also, if $$ is not an option, I would at least double up the analog level sensors. We do that with our fluid temperature probes.

Also, for even further fail-safe measures, opt for a PLC platform that has I/O terminals that can detect a broken wire. I don't know about others, but Beckhoff is one that has this feature in a lot of their terminals.

 

[danw]

As much as I like hydrostatic head pressure measurements for reliability, hydrostatic pressure only works at or below datum, the low level sensing point. With a hole in the ground there's no feasible way to sense level from a low point other than a bubbler dip tube inserted from the top. A bubbler requires clean, dry air on a continuous basis. If the air gets dirty or oily, the bubbler's constant flow regulator will gum up and fail. And a bubbler dip tube in dirty service needs periodic purging to keep buildup from closing off the opening at bottom, but that can be done by pneumatically bypassing the regulator and blowing supplying air directly down the dip tube (level reading goes wild so needs to be 'locked' just before and during purge)

Ultrasonic sensing is likely to 'see' the top of the foam, not the liquid level. Non-contact radar might penetrate the foam or it might not, depending on the foam density.

Redundant OR'd Point level switches for high-high level shutdown would be prudent.

 

[mylespetro]

I would try to steer clear of mechanical float switches if I could though. I'd opt for an analog level sensor. Obviously on a much smaller tank, but we use an analog level sensor

Yes, I don't love mechanical floats, but that's what the customer specifies for every lift station. I would elect to use tuning fork switches normally, but they wouldn't be suitable in this situation because of the debris that ends up in the tanks that could get hung up on them.

 

[Saffa]

As much as I like hydrostatic head pressure measurements for reliability, hydrostatic pressure only works at or below datum, the low level sensing point. With a hole in the ground there's no feasible way to sense level from a low point other than a bubbler dip tube inserted from the top. .

Check out the Endress FMX-21. They do a heavier duty wastewater option as well. I have installed hundreds of these in water and waste wells and tanks. The main thing is to keep the breather tube terminated properly with the little filter in tact

 

[JLand]

If you go with a tuning fork or conductance level switch, check if you can find one that gives an analog value. I know IFM makes some for IOLink that do this.

You can tinker with the analog input value to ignore foam (or to include foam, if that is desirable) where a purely discrete input level switch won't give you that flexibility.

Multiple redundant level readings are nice, and Rockwell (if you use predominantly Rockwell PLCs) include a redundant analog input instruction with PAx. I am sure other PLCs do this too, or you could of course come up with your own solution.

IMHO a good system would be redundant hydrostatic level transmitters with redundant tuning fork level switches (if there are no significant solids in the water) for high high, high low, and lowlow. You can set up 2 control schemes then - primary scheme controls based on analog input values. If the analog input values is in a fault condition (one or both sensors fail or are giving different values), run on on/off control based on the low and high switches. Alarm/system halt on the lowlow and highhigh switches.

The comms standard de jour is IOLink, which can handle this pretty easily. Even for discrete devices, if they are IOL enabled, they can read comms and sensor status. It's finnicky, but if you can get it working it's pretty slick and gives good insight into the quality of the data you are reading.

So if I had to design this, I would set up a single IOLink system with 5 sensors: a pressure level transmitter, a highhigh level switch, a high level switch, a low level switch, and a lowlow level switch. Then duplicate the whole thing, including the IOLink master. Run under normal conditions according to the pressure level transmitter. If the analog values fail (discrepancy or IOLink shows a connection and/or sensor fault) then go to on/off control based on the high and low level switches. Look for both low or high level switches for control, but if one of these redundant switches fail then just look for one switch. If either highhigh is made, shut down the system.

This is probably not the best way to do it, but it's just an idea. It nice because you only have to run two redundant ethernet cables to the tank from your plant network (of course, I know nothing of your plant, so you will probably have to modify this scheme to get it congruent with the existing equipment) and has a high degree of redundancy. You see similar operation in oil and gas in the US (with the exception that O&G uses redundant PLCs too).

 

[danw]

Check out the Endress FMX-21. They do a heavier duty wastewater option as well. I have installed hundreds of these in water and waste wells and tanks. The main thing is to keep the breather tube terminated properly with the little filter in tact

Submersibles can do a dynamite job IF you keep the vent tube free of humidity.



When humidity gets into the vent tube, the cool water surrounding the vent tube is generally below the dewpoint on muggy summer days. At temperatures lower than dewpoint, the humidity will condense out to water droplets, and the droplets will drain to the bottom of the vent tube.



The collected water at the bottom of the vent tube will exert a hydrostatic pressure on the 'low side' of the differential pressure sensor which should be vented to atmosphere and with no applied hydrostatic head on it. A hydrostatic pressure on the low side will produce an error where the level is reported as lower than the actual level, by whatever the height of the collected water is.