Previous in Forum: Pneumatic Control Valve Fail Safe Position (When Instrument Air Fails)   Next in Forum: Ultra Sonic Flow Transmitter
Close

Comments Format:






Change Mode
Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







Add Vote
9 comments
Rate Comments: Nested
Instrument
Participant

Join Date: May 2010
Posts: 3

Use of Level Indicating Transmitter

05/10/2010 6:37 AM

Is it common practice to use the level indicating transmitter(LIT) with Level Switch Low & High(LSH&LSL) on a tank and reasoning that with a statement like the switches acts as backup to your analog reading or just not using the full capability of the transmitter when writing a program on a PLC? I stand to be corrected on this one but i think the only faults u gonna get on an analog transmitter are 1. Improper calibration 2.Open and short circuit and if that is true cant with cater for that when writing a program? Your help on this will be highly appreciated
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Good Answers:

These comments received enough positive votes to make them "good answers".

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
Tornado
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 21017
Good Answers: 795
#1

Re: Use of Level indicating transmitter

05/10/2010 7:20 AM

No, you can't. For instance, what happens if there is a failure in the PLC itself?

__________________
 In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply
Instrument
Participant

Join Date: May 2010
Posts: 3
#2
In reply to #1

Re: Use of Level indicating transmitter

05/10/2010 7:33 AM

But even the switches are powered by the same PLC we r talking abt
Register to Reply
Tornado
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 21017
Good Answers: 795
#3
In reply to #2

Re: Use of Level indicating transmitter

05/10/2010 8:02 AM

Oops; that might be a mistake. I don't think hard-wired switches should not be powered through a PLC.

__________________
 In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply Score 1 for Off Topic
RDGRNR
Guru
Engineering Fields - Instrumentation Engineering - New Member

Join Date: Dec 2009
Location: San Antonio, TX USA
Posts: 844
Good Answers: 29
#4

Re: Use of Level Indicating Transmitter

05/10/2010 8:53 AM

An input by any other name is still an input. You should write in some type of AND function to compare the two.

__________________
 "Do not worry about your difficulties in Mathematics. I can assure you mine are still greater". - Albert Einstein
Register to Reply Score 1 for Off Topic
Instrument
Participant

Join Date: May 2010
Posts: 3
#5

Re: Use of Level Indicating Transmitter

05/10/2010 9:43 AM

Are there any other faults one gonna get on a LIT(Level Indicating Controller) except the ones i mentioned above and if not with the exception of improper calibration what values will one get on your N registers on a PLC(Controllogix5000) when you have short or open circuit on your transmitter on a scale of 0-4095?
Register to Reply
2
JohnDG
Guru
United Kingdom - Member - Not a New Member Hobbies - Musician - New Member Hobbies - Fishing - New Member

Join Date: May 2006
Location: Reading, Berkshire, UK. Going under cover.
Posts: 9684
Good Answers: 468
#6
In reply to #5

Re: Use of Level Indicating Transmitter

05/10/2010 12:37 PM

Without knowing the exact nature of the level sensor (and the transmitter) and all the possible failure modes - it's impossible to say. There may be a fault condition which results in an output within the valid range (i.e. indicating neither short nor open) which is, however, not a true reflection of the liquid level. A simple float-operated level sensor for example (like a car fuel gauge sensor) could just get stuck.

__________________
 "Love justice, you who rule the world" - Dante Alighieri
Register to Reply Good Answer (Score 2)
Go to Next Good Answer
PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#9
In reply to #6

Re: Use of Level Indicating Transmitter

05/11/2010 12:42 PM

Quite. If the material pumped into the tank were water, then it probably wouldn't matter if there were an overflow. However, if it were petrol, a hard-wired trip on the pump would be essential, just in case the PLC failed and some pillock tried to fill the tank on manual!

To the original poster: carry out a formal HazOp Study on the plant to evaluate the need, and the configuration, of these controls!

__________________
 "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
ellisamatt
Member

Join Date: Feb 2010
Posts: 9
#7
In reply to #5

Re: Use of Level Indicating Transmitter

05/10/2010 10:44 PM

The level transmitter could read a false signal...depending on type...i.e. ultrasonic false reflection from dust or build up or waves or capacitance false reading from dirt stuck to probe. These are process issues and depending on the importance of the level you are reading you can deal with these situation in many different ways...but it is this false reading situation that i see as the only other possibility to what you have identified....also normally you can set your level transmitter to fail high or fail low...therefor on failure you can determine what the resultant action will be...i.e. fail low...tank reads low level therefor outlet pump stops
Register to Reply Score 1 for Good Answer
No more "Almost" Good Answers.
3
Carl_E
Power-User

Join Date: Nov 2009
Posts: 335
Good Answers: 63
#8

Re: Use of Level Indicating Transmitter

05/10/2010 11:48 PM

There are other fault conditions that analog continuous level transmitters are subject to that result in erroneous level readings.

Top mounted, non-contact technologies, like radar or ultrasonic can produce 'loss of echo' for reasons from condensation on the emitter/transducer to variations in the atmosphere above the liquid. Reflections from objects in the tank or from dished or flat bottoms can produce erroneous level readings. Laser, ultrasonic or radar can be interrupted by dust or steam. Capacitive technology can be affected by build-up on the probe. Magnetostrictive floats can hang. In short, any level technologies have some condition or mode subject to providing erroneous level measurment(s0.

A typical control strategy is to use a high (and on occasion low) level switch that uses a different sensing technology than the primary continuous level sensor. The use of a different technology ensures that a false reading by the continuous sensor has a lower probability of being replicated by the point level switch.

The point level switch can be an input to the same controller (PLC, DCS, level controller) or wired to a separate pump/valve interlock circuit.
Register to Reply Good Answer (Score 3)
No more Good Answers. Go to first "Almost" Good Answer
Register to Reply
Have a Different Question?
9 comments

Good Answers:

These comments received enough positive votes to make them "good answers".

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
Copy to Clipboard

Users who posted comments:

Carl_E (1); ellisamatt (1); Instrument (2); JohnDG (1); PWSlack (1); RDGRNR (1); Tornado (2)

Previous in Forum: Pneumatic Control Valve Fail Safe Position (When Instrument Air Fails)   Next in Forum: Ultra Sonic Flow Transmitter
You might be interested in: Level Sensors, Radio Transmitter ICs

Advertisement

Got Something to Say?
Ask a Question
Start a Discussion

Search this Forum
Instrumentation:
Go

CR4 Sections
Aerospace
BioMech & BioMed
Chemical & Material Science
Civil Engineering
Communications & Electronics
Education and Engineering Careers
Electrical Engineering
General Discussion
Instrumentation
Manufacturing
Mechanical Engineering
New Technologies & Research
Software & Programming
Sustainable Engineering
Transportation

Site Directories
Forum Directory
Blog Directory
User Group Directory
RSS Feeds

CR4 Rules of Conduct
CR4 FAQ
Site Glossary

Who's Online (59 right now)
Show Members
Advertisement