Safety Precaution for heated acid tanks question

[ceilingwalker]

I sketched up a drawing of my suggested circuit and I think Calistodwt. It gives the diagnostics you were looking for without loosing the diversity. It adds diagnostics for the relay operation, if you have a spare contact on the relay.

What you have drawn is exactly what I wanted to do however, I would need to replace 20 such relays and install a bigger DC power supply as this would tap-out what I have in the control now. Because it was end of the quarter, they weren't going to approve it right now. I just wanted to make sure I wasn't overlooking something, and I should have stated in my original statement that I inteneded to do this as a temporary, until I could get approval to change all of my mag relays.

 

[Calistodwt]

Exactly! This is why I was asking for input about what I had done. If the PLC fails (in theory) there should be no way the heater would energize, there are too many conditions that wouldn't be met to permit it.

The problem is that the PLC could fail to danger or fail safe (which is why the fusible link should not be wired to the PLC input) causing the relay to be energised all of the time. Rather just re-wire the original set-up in 24V wiring to help operator safety.

 

[jdbrandt]

Good day all. I have a dip-tank that contains acid and this tank is heated by a 12000watt heater. The way the control was wired was: when SP is less than PV, SLC500 Relay Output Module sends 120 VAC to a one-shot fuse (thermal fuse) and then returns to the control cabinet where it energizes a relay coil that pulls-in contacts, closing the 3-phase power, energizing the 3-phase heater. I had two objectives: 1. Remove the more dangerous 120 VAC in the operator’s path and use a lower 24 VDC circuit and 2. Create a more specific alarm for the one-shot fuse to aide maintenance techs while troubleshooting. I accomplished this by adding a DC-Sink input module and put 24VDC to the one-shot fuse and then connected the coil for the heater relay directly to the SLC500 Relay Output Module. Now this circuit works like this: 24 volts goes across the thermal one-shot fuse and lands on the DC-Sink module. As long as the processor sees a “1” in the bit box for that one-shot, and if the SP is less than the PV, the Output Module will energize the coil, pulling-in contacts and provide 3-phase power to heaters.
Here is my question: Can anyone think of a reason why would NOT be a good idea? I believe I have considered every possible safety precaution and have determined that the system will be just as safe however, I would like to hear what you experienced folks have to say. A second, third, fourth, etc…. pair of eyes on this will make me more comfortable with this change. Thank you

A few code cycles ago, I think they added that any electrically heated tank over 1200 watts needed to have the electrical supply protected by a Ground Fault Protection for Equipment (GFPE) breaker. The supply of those, years ago, was a little scarce, but I think that's changed.

 

[ceilingwalker]

A few code cycles ago, I think they added that any electrically heated tank over 1200 watts needed to have the electrical supply protected by a Ground Fault Protection for Equipment (GFPE) breaker. The supply of those, years ago, was a little scarce, but I think that's changed.

Yes sir, I am aware of that however, this was an existing system. We purchased a new clean line a few years ago and it was all fitted with GFCI protection.