The H-O-A terminals I'm talking about have small switches on them that allow you to manually control the output like you would a normal HOA switch. I'm not sure how they would be used except for the pumps & fans. The solenoid valve groups would be very tricky, or at least a PITA.
You could use the built in 0 to 10v
I hadn't thought about that. I will investigate using the 0-10 rather than the 4-20mA output.
you could use a 1/4 din temp controller as a backup alarm and if the site is not maned 24/7 just hook up an auto dialer to it. also email or text is possible for the alarms
I dont think the heat will be a problem if you put it in the shade! good luck
We're going to check to see what we can do with the existing auto dialer. They have an existing AB SLC 5/0X (not sure which model) that they use for monitoring (not sure if they monitor temp, but I'm sure they monitor for ammonia) and it's hooked to an autodialer.
I did an alarm system for a pharma, who has beau coup bucks in inventory that has to be temperature maintained or it's garbage, it can't be sold. At $5M in inventory, no wonder your client has concerns.
If it were me, I'd be selling them a PLC based alarm system in addition to whatever control they want. Any and all mechanical devices AND controls can potentially fail. The key is knowing about failures to be able to take remedial action in time.
We had several meetings brainstorming failure modes. Obviously a pump can fail, its motor can fail or the power to the motor. They were concerned about 'human error' faults too, the "I can't believe someone did that" problems. I'm guessing that a produce warehouse is not employing the upper strata of the work force. They had stories where someone propped a freezer door open and then left the site. No refrigeration system in the world can cool an open system. As ridiculous as that sounds, it happened there.
The customer is concerned with the "human factor" as well. Currently they have to manually change over to high compressor head pressure so they get 150 psi to their ice house so that it will defrost correctly. This will often get left on for a few days needlessly wasting power running the compressors that hard. It is one of the issues they are looking at. I'm hoping to convince them that the savings they'll get by putting some controls in will override their reluctance on using a PLC.
In the system I was involved with the controls that deal with alarms are battery backed up with a large UPS.
All major alarms get emailed or texted to a cell phone and critical alarms dial out on a standard POTS dedicated phone line and the dialer only stops dialing when it gets acknowledgement. (auto dialers are primitive, I thought I was back in the 1976 era when I configured it; but we refused to rely on a PC based autodialer, too much risk of Windows hanging).
For what your system sounds like, I'd be monitoring for
- proof of flows, water (pump) and air (fan)
- temp in the storage area
- controls enclosure temperature (need to know if AC fails)
- loss of site AC power
- loss of a AC phase
- watchdog: the controller has an internal watchdog circuit that trips a dedicated relay if it detects a fault. That same relay is fail-safe, so it 'alarms' if power to the controller drops out.
I will see what is possible for proof of flow. I can see the condensers not being an issue, too much. I'm planning on monitoring the overloads to see if something kicks out. I figure if they are not tripped then they SHOULD be running, but then assuming anything isn't the best route.
I'm planning on a temp transducer in the cabinet to make sure things don't get too toasty in there. I know many of the panels close by became nearly too hot to even touch in the middle of the afternoon without gloves on.
They have some existing power monitoring on the switchboards. I'll see if that's still active.
I'd insist on proving flow with flow switches because I've seen systems that can be valved off where one would still get a pressure indication, but have zero flow available.
Understood. The problem with assuming there's flow just based on an overload not being tripped.
There's a critical alarm test circuit, a momentary pushbutton switch mounted behind the panel door. When it's held in for more than 5 seconds, it trips the critical alarm circuit, emails the list and starts dialing out (dialer has its own battery back-up). Their protocol calls for testing the alarm circuit and its outputs weekly.
I will discuss this with them.
Although I don't know the full scope of your project, I would hope it is still possible to manually operate the machine to at least salvage the product. You may not get all the 'bells and whistles' that the PLC provides, but minimal control is better than none.
This is what the HOA terminal blocks I was talking about would do. At least for some of the things. Pumps and fans.[/QUOTE]
Think of how the ECU in a car works (or is SUPPOSED to work). When a sensor is determined to be 'bad', it uses a default value for that sensor to at least allow the car to be driven. Sometimes called 'Limp Home' mode.
Although, if the ECU itself dies, you're out of luck...
Another thought... Do you really need ALL the I/O on the main PLC to operate the basic functions? Critical I/O could be paralleled to a second, smaller PLC that would run the system, in a limited fashion, when necessary. Years ago, I did a material handling system with a backup PLC. This just ran a default program that cycled the system on a timed basis. Crude, but it would keep them running in the event that the main PLC failed. I don't know if they ever needed to use it though...
-Eric[/QUOTE]
The issue I think that would come up with paralleling the outputs going to the SOVs for the refrigeration. Maybe running these through a 4 pole relay would solve it. When the secondary PLC takes over it would activate the relay so that it's outputs went to the SOVs. Having the switch over happen during defrost might cause some issues. Liquid ammonia into the compressor would be a Bad Thing^TM.
I'd strongly recommend some sort of trend/alarm data logging on alarm points and using an autodialer with a state log. The trends and the autodialer log have both been pulled out to prove that the alarms were sensed and output accomplished, the failure to respond was on the human side. When large dollar inventories are lost, fingers will point in any and all directions looking for a scapegoat. The logs are vindication.
The plus side is they would really like to have data logging and that helps the PLC proposal. I need to discuss how often and what they want to log. They'd like to be able to keey a whole season (six months) in one place. There current system requires them to manually download data every couple days. I think this is the ammonia and possibly room temp data.
The reliability depends on the environment. Warrior did say it was hot, and I would guess that there is not much reliable maintenance.
I know they hired a new maintenance guy last year. He at least is honest and says he isn't an electrician and doesn't like to mess with it. Considering some of the plants I've been in (including this one) that's a good thing. The boss told the customer last year "Your code violations and code violations." At least they understood they had a problem and want to address it.
Luckily the only physical contaminants there are dust (LOTS of dust), a tiny bit of water if it condenses and makes it's way there and of course, heat. LOT's of heat.
What local support will be given the system. Can the locals spell PLC.
They have an existing PLC so there is someone at least locally to deal with it if the need does arise.
How will this system be monitored. Is it in the corner and ignored until everything goes south or does it have scheduled checks by a operator. Is it a 24/7 operation or does it run and stop.
I have a feeling they want the least amount of interaction as possible.
I trust a PLC a lot more than relay logic and I am old enough (thanks for reminding me PCkid ) to have done both. Looking at your spec I would rather poke myself in the eye with a blunt stick than do that with relay logic.
The boss doesn't even want to attempt this with relays, defrost time clocks and what not.[/QUOTE]
Some customers are pennywise and pound foolish. They want something your opinion on what to do, then reject your recommendations due to cost. Then if your ML1400 solution fails in some way, they'll blame you for not providing a more reliable system. If downtime or the safety of their product is paramount, they need to put thing into place to guarentee it. A redundant system would be where I'd start.
I understand that. West coast farmers are the cheapest collective group.
Thanks for everyone's responses. I'll try to keep you guys updated.
