liquid level control to proportional valve

[grobert]

I have an OEM customer that wants to open / close a proportional valve based directly from a liquid level in a tank. We are proposing an ultra sonic level transmitter giving 4-20 into a PLC or converter card with direct PWM 600ma proportional output to valve.

We are proposing the use of a tube to allow the sensor to see a more stable rise / fall of liquid. The liquid is water with small amounts of oil, typically keeped at >15ppm. I quess the basic question is if "US" is best for this app, it sounds like it is. Would you use a plc? Could I get 4-20ma AI with a PWM 0-600ma AO? Would you look for a converter to do this off the shelf since this is all of the logic? Thanks in advance.

 

[Barry1961]

It sounds to me like you would not need a PLC. Mabey just an pot to control the fill level. Most proporional valves have enough electronics onboard or in the controller to handle what you have descibed. You might want to check the valve mfg.

Barry

 

[DougMSOE]

I would use a "bubble bell" for this application. This is old tech. but it will work on just about any type of level control and very dirity/messy apps.
A bell is mounted or in some apps lowered into the liquid. Attached is the lowering chain or cable along with a tube which supplies an inert gas or air just enough to cause a slight bubble to be discharged from the bottom of the bell.
Above the surface where the supply of air/gas goes to the bell, a "T" connection is installed. The side open goes to your pressure xducer. The only drawback to this system is that it requires a small amount of air/gas (just enough for a bubble once in a while)at a steady pressure. The output is linear, easy to clean, and simple.
I've used this in a number of liquid apps and it has worked every time.

 

[quark]

Do you want to open/close the valve or proportionately control it? If you want open/close operation, just use a swithc instead of a transmitter (check with Dwyer, they have pressure gauge switches, actually a pressure gauge with two set points and electrical contacts, which are cheap and good). Endress+Hauser manufactures vibrating fork pressure switches which are cheap and can take care of foaming.

If you want proportionate control, buy a valve which can directly take 4-20mA input (why to waste a PLC for one valve?) as suggested by Barry. Ultrasonic level transmitters can't take care of foaming.

Regards,


Believe it or not : A Neutrino is so mass less(and electrically neutral) that, a conservative estimate states, it can pass through a 50 light year thick lead block.

 

[jojot]

TRY USING A DIFFERENTIAL PRESSURE TRANSDUCER TO MEASURE
LEVEL. TRANSDUCER ANALOG SIGNAL TO A UNIVERSAL
CONTROLLER W/ 1 ANALOG INPUT AND 4-20mA ANALOG OUTPUT
TO YOUR PROPORTIONAL VALVE.ADD AN ALARM OUTPUT FOR
SAFETY. THIS ALARM MAY BE USED TO OVERIDE PROPORTIONAL
CONTROL SIGNAL TO VALVE.

 

[jbartos]

Suggestion: Visit

http://www.tpub.com/content/doe/h1013v1/css/h1013v1_127.htm

for: Linear Variable Differential Transformer (LVDT) details
(it is extremely reliable)

http://www.tpub.com/doeinstrument/instrumentationandcontrol40.htm

http://www.sec-america.com/Pdf/Ls-c.pdf

http://www.memagazine.org/backissues/aug98/features/flowcont/flowcont.html

etc. for more info