Ladder diagram for alarm panel 1

[ktmbruce]

I am just starting out in my on-hands electrical training and have been handed a challenge project. I am to layout a ladder diagram for an alarm panel for our plant. It will involve one central located panel which will be able to monitor nine different fault possibilities throughout the plant ie. low boiler water, low cooling sump levels, power failure etc. It is to indicate the fault with individual relays, lights and a buzzer. The buzzer to have a timer involved with a reset button to silence the alarm momentarily while resolving the cause for the alarm. I also believe having it set up with "fail safe" design would be an asset, as there would be an alarm should current flow be severed in any of the nine lines running throughout the plant. Any help out there would help solidify my new found interest, and be greatly appreciated.

 

[busbar]

Sorry—no delicate way to put this—but from industrial-safety, regulatory and liability standpoints, a non-NRTL-labeled panel wouldn't be worth a plugged nickel.

 

[automatic2]

I performed a similar style of plant sensation with the objective of comparing deviations in plant services to that of manufacturing consistencies (plastic processing). I monitored air pressure in several sub areas, water pressures and temp., chill systems, status of air movement/HVAC equipment, etc. All signals were brought back to my office PC and logged. The system was not designed as an alarm system but as a monitor. The resulting study of plant dynamics was/is fascinating. A typical scenario would be that compressed air supply would begin to oscilate +- 5psi over an 3-4 hour period. Long before it affected production we would investigate and solve the problem. Anytime production experienced deviation, we would check service status over last couple of hours and on numerous occasions, problem that were just beginning were identified. The monitor simply took frequent samplings of analog signals. Off/on states were interrupt driven and time stamped. Every thing was programmed using dbase as the monitor (.bin routines for sampling and crude graphing). Sensors were grouped where possible and driven/resolved using cheapo 286 motherboards that I had assembled device drivers into there rom's. I used RS232 as comm at that time but if I did it again I would go the I2C buss route.

 

[jbartos]

Suggestion: It appears that current trend in monitoring of faults, and alarms is over redundant computers communicating over LAN or WAN.
What about adding some control to it?
Visit

http://www.cwtp.com.sg/DTSS Design Brief/Basis of Design/Section 11.pdf

for more info.

 

[jraef]

ktmbruce,
IMHO, your system is too small to consider using a PC or even a PLC. Although the hardware can be purchased cheaply enough, there is more to the picture than meets the eye. Sticking to relays can help you get familiar with the concept on a smaller system that is easier to understand.

Fail-safe in this instance means having the relays constantly energized and the alarm or trouble condition maintaining that circuit completion. This implies then that your field devices have Normally Closed (NC) contacts which may or may not be the case. Another potential pitfall is that when power is first applied, an alarm is sounded until all the relays pull in, something that can be problematic in and of itself. As simple as this appears, it may be better to record all of your conditions and exceptions, then take it to a professional.

Did you understand the meaning behind Busbar's response? NRTL stands for Nationally Recognized Testing Laboratory and refers to someone like UL or ETL. What he is refering to is that control and or monitoring systems such as this need to be built to certain standards in order to assure that they are not the causes of additional problems themselves. No offense intended, but it sounds as if you company has assigned an important task to someone who may not yet be ready to handle it, as evidenced by the fact that you are asking complete strangers protected by anonymity to assist you in designing it! If, heaven forbid, something goes wrong and catches fire, the insurance company investigator will look at this panel and determine that it was not built by someone with NRTL listing. He can then invalidate the claim entirely! Take your problem to a panel shop who can apply an NRTL label and allow yourself to sleep at night. They will have experience in the requirement and pitfalls involved in "simple" tasks such as this.

Subvert the dominant paradigm... Think first, then act!

 

[jbartos]

Suggestion: If it is the challenge project, it may then be actually challenged. What is wrong with that? If one looks at some prices of the older hard wired technologies meeting various industry standards, and compares them to the coming or up to date technologies meeting similar industry standards, then the choice is often in favor the modern technologies, even in relatively simple applications. In some cases, wireless connections are less expensive than the hard wired installations.

 

[spector]

Am I the only one who understands your question?

You are in training....You are not designing an actual system for install and operation, correct? But your training is expected to help you later when you need to troubleshoot and repair such systems in your plant?

Great. Do like I first did when I got onboard my first of many nuclear submarines and thereafter when I started a new job at every place I went. Go dig out the prints for your install. Using those prints, trace out and locate every component. Take readings and measurements thru the system. Identify and research each component....Then draw it all out-making it work on paper.

Glad your plant insists on such training. Stick with it, and good luck! Come back here with your questions.

 

[ktmbruce]

Thanks for the advice everyone. Had not considered liability consequences. And jraef, thanks for clarifying the meaning of NRTL and the importance it has in protecting myself from a catastraphe someday. To spector I am afraid it is being made for installation, and the enclosure and din bar racks are installed. I am in the position where laying out the ladder logic is needed, then once double checked by my supervisor, parts ordered and installed. I do feel a need to tell them about your advice concerning if a fire failure occured. But I also may have not made it clear in my original question, that this alarm cabinet is going to alert maintenance personnel of a failure in sump levels, boiler water levels, compressor power failures for our ammonia refrigeration systems etc. Not a fire detection system, or burglary system which would definitely involve insurance concerns. Keep the advice coming, I am very impressed with the responses, and hope to learn a lot more about ladder logic. ktmbruce

 

[spector]

Sir,

With all due respect, you are in way over your head. Pray that your supervisor will catch your mistakes.

I worked for several years as a loss prevention inspector for Hartford Steam Boiler Insurance and Inspection Co. So your statement "Not a fire detection system, or burglary system which would definitely involve insurance concerns" is incorrect. If you have boiler/machinery coverage, your insurance Co. will demand any such install be properly coded and approved. Investigate if you have such coverage. If so, work with your boiler/machinery insurance provider on your planned install. They will assist you all the way. If you have either FM, HSB or Travelers ins, you can expect decent support from them....The rest only play the numbers game.

But yes, you are correct to desire it be 'fail safe', as they will insist on that too. Boilers and Ammonia refrigeration plants have the potential for tremendous losses and injuries. Once your deductable is satisfied, they make up for business loss and equipment repair/replacement up to the policy limits. So they are very careful when evaulating risks to limit their exposure and are quick to exclude any claim when safety devices or systems are not installed or maintained to code.

Good Luck

 

[ktmbruce]

spector, I am taking your recommendation to heart. I have tried to convince my supervisor that this is risky, and he continues to blow me off, as if I don't want the responsibilty. They don't or won't understand the principal of "fail safe" design. I will take a copy of your response in to try one more time. Thanks for your response. ktmbruce

 

[PUMPDESIGNER]

If possible do it with PLC.
This involves learning that PLCs programming, but you can still learn ladder logic if you purchase a PLC with a ladder programming language.
Here are the reasons you want a PLC.

1 - Flexibility - Modify and improve the system with little or no re-wiring. Even if your first design works, you will inevitably come up with improvements at 2:00 am in the morning and want to make the improvement.
2 - Design Error Checking - If your original design fails you get to catch and repair it before wiring.
3 - Less expensive - You need not purchase some great big PLC system, you can order one today for $100.00 plus the software and cable for about $100.00. We call these small guys "relay replacers".
4 - No back feeds - Standard relay systems can have back feeds that require locating and correcting, PLC ladder programs do not have the problem.
5 - Reliability - At first we used to think that relay systems were more reliable than PLCs, but exactly the opposite is true. Relay systems have far more wire connections and devices like time delays that go bad.
6 - A PLC makes a truly refined system with features you cannot possibly get on a relay system: contact de-bouncing, short 1-5 second time delays to prevent nuisance alarms, plus greater flexibility of "components" such as counters, high speed counters, on delays, off delays, interval delays, alternators, etc., and almost unlimited amounts of them too.

Now is the time to "get your feet wet" with PLCs, not relays and time delays building a system un-refined and error prone.

Richard Neff Richard Neff
Irrigation Craft

 

[ktmbruce]

Richard Neff,thanks for your response. We are not going to be allowed to use a PLC. We have several AB PLC's at work so I have knowledge of these, and suggested that we use one. But my employer is very cheap and insisted on relays. And what we have in stock. I have my hands tied. We have the cabinet set up, timer and relays (9), lights on the front etc. We just can't seem to get the logic correct yet, to get the end result wanted. I want to use "fail safe" logic but they feel it is silly. So I am frustrated. I am enjoying the correspondence on this sight though, keep it coming.
ktmbruce

 

[jbartos]

Suggestion to the previous posting. Perhaps, if you could accentuate some savings from the "fail safe concept", you might get a better response. I used the following reasoning: If the automatic soil cleaning station did not have the fail safe logic with a remote annunciation via telephone communicator, it could be there idle for some time and the time totalizer would show fewer hours. That could mean smaller pay from the Client.

 

[ktmbruce]

Hello everyone who has posted responses to my question. I have subsequently washed my hands of this project. Reason being, the other mechanic who started this project told me to drop the issue of designing with "fail safe" considerations, and told me to drop the issue of liability because if I take it to the manager they will just laugh and blow me off. I guess in conclusion, I work for a company that doesn't care to do this or any project correctly. Thanks again, I will close this subject as of today.

ktmbruce