Relay Logic for Ovens

Just got finished with a plant upgrade for some ovens, and the control cabinet was full of icecube type relays, and no PLC. Anyone know why they wouldn't use a plc to control all of the gas valves ? There must have been over 50 small relays in the cabinet.

Controls guy told me it was the cost, but I can't imagine this would be cheaper.

It would be cheaper if there were nobody in the company that was capeable of installing and programming such a device. Most control electricians would be able to do the relays with a wiring diagram or schematic.

PLC Pie Guy said:

It would be cheaper if there were nobody in the company that was capable of installing and programming such a device. Most control electricians would be able to do the relays with a wiring diagram or schematic.

Click to expand...

Generally that is the case. Someone posted a relay logic control panel on reddit a coupe of months ago that was built for a prison so that an electrician or someone in maintenance could open the doors manually.

Software cost and accessory prices are another reason. Businesses will spec out relay logic because they do not realize that there are PLCs that do not require $5k software and a $1k adapter to program. At least some businesses think (albeit incorrectly) before they buy machines.

A lot of places will cut corners and buy a pre-programmed machine but will not purchase the software to program the PLC or even get a copy of the program. They get all up in arms when they are told they should have bought the $3k programming software if they wanted to do that. /Rant

I know of a company that was in business through the early part of the 21st century that was still building pill presses using relay controls and hydraulics designed in the 70' and delivering them with blueprints made from the original hand drawn vellum sheets. I doubt they had anyone on staff that knew anything about controls, but they could follow a schematic and hook up what they had on paper.

There may be some safety concerns. I'm assuming there is some sort of safeties that will prevent the ovens from blowing up. Back in the day relay were reliable and often used. Going to a PLC many not seem to compicated but when you are talking safety circuits it can become a liability nightmare.

A lot of combustion OEMs will PLC the motors (conveyors, fans, etc.) but leave the combustion control such as gas valves, temperature control, high limits and purge timers to hardware devices the like of Honeywell to insure safety and limit liability. That's been my experience although I have seen completely PLCed systems and have to say I don't prefer them nor will I design them like that for the same reasons as the OEMs.

maybe for gaz system safety and ease of use but basicaly a plc logic won't worn out like relay so if it is something protected with a reliable plc brand, it will be more reliable than relays...
Some complex relay and timer logic would becomme very difficult to repair unless someone can read the drawing and understand the drawing or replace all relay 1 by 1 until it work again VS you can online monitor a plc project to find the clue in a minute. In other hands, it is harmless to permit relay replacement but could be dangerous to let a hand user messing with the plc program....
But like other said, some electrician preffer what they could repair themself without costly soft and using a screwdriver instead of a RS-232 cable...

In some other way, engeneer companny working for (Or against at some point) their end user will use standard quoting spec for all their customers and when we need to use a plc, we need to use another spec calling for high end level plc costing a lot more than a entry level model even if it is for a basic relay logic. So what everyone will do is to try making it work without using the plc spec and come out with a 50 relays control logic.
A S7-1200/ML1100 could be cheaper than 50 relay+wiring+time but if you need to compare it to a Contrologix/s7-300, it would need a lot of relay to overcome the price of it...

At the end, the end user technician or electrician should lokk at the spec to see if what they ask is really what they want.

I see a lot of old relais furnaces on board of ships, just because the temperature is very high a normal PLC will stop working for one.
They must have a certificate for operation safely, and that costs money so very reluctant to make another approval round. lloyds DNV or any.

TConnolly said:

I know of a company that was in business through the early part of the 21st century that was still building pill presses using relay controls and hydraulics designed in the 70' and delivering them with blueprints made from the original hand drawn vellum sheets. I doubt they had anyone on staff that knew anything about controls, but they could follow a schematic and hook up what they had on paper.

Click to expand...

This. I have so often received a simple machine or system from an OEM... and the electrical and controls showed clearly that the last time anyone with a clue was involved was 1990 or so. Actually, I once worked for an OEM where that was the case... LOL. Sometimes, you can see the degradation in logic from the original programming to the hack that you've received in your brand new machine.

Paul T said:

This. I have so often received a simple machine or system from an OEM... and the electrical and controls showed clearly that the last time anyone with a clue was involved was 1990 or so. Actually, I once worked for an OEM where that was the case... LOL. Sometimes, you can see the degradation in logic from the original programming to the hack that you've received in your brand new machine.

Click to expand...

Oem are usually more mechanical skilled and oriented than controls. For them it's like a dangerous black box and the least they can work on it, the better they will feel. If they could just install a large switch to make it ''on'' and ''off'' like the very old motor starter they would feel very happy!

I don't know if someone found a way to interest mechanical people about the added value of more complex controls???