Choosing relay and PLC output card

Adenitz

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Feb 2010
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Hello,

I asked this question on another forum, but didn't get answer. It seems there are here a lot of hardware people. I hope you'll help.

I need your advice about a problem of choosing appropriate relay that can influence a choice of appropriate SM output module. (I use S7 300 PLC)
I need to have a relay energized in normal operation. The relay is energized from SM output PLC module. This way, in case of power failure, PLC in STOP mode or wire brike, the relay will become de energized and thus ensure safe state of the plant. This relay is in a control circuit of a shut off valve on propan gas.
Now, possible problem is the fact that relay will remain energized for a long time (this can take months) and that is not good (may fail) but I have no other choice. A colleague advices me to use 24V DC relay since it is much more robust and unlikely to fail. On the other hand 230 VAC relays will fail faster if energized for a long time.
I don't know if this is the case. I need your help on this. I have two choices:
1. Use 24VDC DO card and 24 V DC relay for this "fail safe" application.
2. Use 230 VAC DO module and 230 VAC relay

What would you do?
 
I would go 24VDC, not because its more robust ( never heard of that) but because 24VDC is safer to work with.

Get a relay that is rated for continuous duty and it will last for years.

Like all things critical it has to be tested on a regular bases.
 
This sounds like a very important system. Something that could kill people if it does not work.

I would bet that people feel if you don't already know how to do this then you should be working with a supplier that does.

As far as opinions go, you need to use a safety PLC. Or at least a safety relay.

The system should look at two seperate relays that have seperate controls/inputs.

Under normal conditions, one would be normally closed and one would be normally open.

When the system is triggered, the relays would both switch.

So if you ever saw relay A and relay B both ON or both OFF, then you know one of the relays has failed and you can shut down.

I would also program the system to do a self-check at some interval, maybe once a day.

You would have the system cycle on and off and verify that both of the relays respond as expected.
 
Thank you for your help.
It is actually burner application. We're reconstructing it. In thi new solution, old valve is replaced with two solenoid valves connected in series on the pipe. When the plant is running these valves are opened (there are two DO channels on two separate cards), because relays are energized which keep valves in open state. when there is no voltage, relay coils are deenergized and springs keep valves in closed position. That is why I think it is pretty much safe solution.
In case of wires break or PLC is in stop outputs are zeroed and both relays are deenergized. In case there is an error in one valve, other will stop the gas flow.
>----valve1----valve2---->gas
I don't want to make things complicated and didn't describe system in details. I think this is safe solution. Since relays will be energized most of the time I need to have reliable relays in order to prevent accidental shutdowns. I thought that 230VAC relay coil is more likely to fail.
How to check if relay is continuous duty? Just want to have reliable relays that will be energized most of the time.
(I can order 24V or 230 VAC solenoid valves and PLC modules).

Thanks
 
From your question I've got an idea you are pretty new in burner control, and probably not familiar with all requirements in this field. Unless you company is certified, you cannot provide PLC based flame safety control!!! You have to use special flame safety controllers in order to control the burner, including gas/oil valves, Honeywell 7800 series for example. Ofcource you may build such PLC control yourself, but your customer's insurance company may cancel a coverage on a base the flame safety system is not FM approved.
 
Val,
that is interesting point. I have no experience with burner control. We're not implementing logic for the system at all. We're not prepared to do so. Our customer wants to replace existing old solenoid valve (230 VAC) with two new valves in series on the same pipe. We're not changing anything else. Customer wants to make system more reliable and safer because there were few accidental trips (relay failure) that caused downtime. So instead of one valve, there will be two valves (fail safe with springs). When valves open, output channels will be ON and relays will be energized. In case of error at least one valve will shut down the gas (safe state). Since new solution must be reliable also, then I need reliable relay, and wasn't sure whether to use 24VDC or 230 VAC.
So, we are adding two outputs on existing program command to open and two valves instead of one. Nothing more. I don't see how this solution can be more dangerous than previous one. I think that is better and safer since any error will shut down the gas.​
 
I would agree with val_99, and suggest that you have the burner control relay control both of the valves, and simply let the PLC apply a command to the burner control relay...

The burner control relay is much more than a relay. It will include a flame monitor input and possibly an airflow proving input as well as time limits for flame proving and will be certified (if installed according to requirements) to handle the job.
 
I understand what you are doing and I agree the new control would be more safe then old one. I just pointing you to potential problem from FM regulations stand point. By the way, based on the burner BTUs, you may have to use special solenoid valves.
Give me your BTU rating and I'll see what you may need as far as valves.
 
Val,

that is interesting point. I have no experience with burner control. We're not implementing logic for the system at all. We're not prepared to do so. Our customer wants to replace existing old solenoid valve (230 VAC) with two new valves in series on the same pipe. We're not changing anything else. Customer wants to make system more reliable and safer because there were few accidental trips (relay failure) that caused downtime. So instead of one valve, there will be two valves (fail safe with springs). When valves open, output channels will be ON and relays will be energized. In case of error at least one valve will shut down the gas (safe state). Since new solution must be reliable also, then I need reliable relay, and wasn't sure whether to use 24VDC or 230 VAC.

So, we are adding two outputs on existing program command to open and two valves instead of one. Nothing more. I don't see how this solution can be more dangerous than previous one. I think that is better and safer since any error will shut down the gas.​

So your putting your vavles "in front" of the burner control valves? IE this is just supplying gas to the burner control system?
 
IS barriers have nothing to do with the problem. Plus you have to use IS rated solenoids with it because IS barrier have limited power output.
Adenitz, what is the gas line size? is it less or equal then 1/2" ?
 
I would also think that the main gas valves would also need to have proof of closure contacts? You also need the purge cycles I believe minimum 5 air exchanges are needed before reignition and a cool down timer is also needed after a failure. The proof of closure is usually monitored by the flame safe gaurd controller just some additional things to consider when you add a valve to the overall system
 
I didn't expect so much replies. Thank you all for that.

Pipe is 1/2". However, I don't know anything about burners. These solenoid valves are on main gas supply line. There are some control valves on gas, air and I don't know what else which are controlledseparately by some other system. This has indirect link with burners. It just supply gas to the burner and they are not located near the actual burner but closer to the main tank.
We don't need to go into details of boiler operation, since we're just hired to replace the vale with two new ones. My colleague will order valves, I neeed to decide whether to go with 24V relay or 230V.
I decided to use 24VDC continuous duty relays.
 
The others have commented on the key points, but let me add that in a 'previous life', the company I worked for had a standard practice of using a pair of solid-state timers, one time ON, one time OFF. The operating coils of these timers would be energized by a toggling output of the PLC, and the time base would be chosen in such a way that if the PLC stopped toggling the output for a time, at least one of the timers would trip.
Some call these ONE SHOT timers, or retriggerable one shots...no matter. The key is that the two timers form a simple watchdog circuit, and are easily implemented in a control panel.
Just FYI, not a ploy to start a debate on the naming of timer functions.
:)
 

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