Burner Safety and E-Stops


Thread Starter

John Monroe

Do Emergency Stops wired to an oven need to kill the purge in the event of activation, or do the E-Stops just kill all of the mechanical movement? I
have had multiple opinions to this one. Some say that everything should be killed including the gas valves and purge cycle. This loses all product
in the oven esp. after re-purging. However, I have also heard that if the mechanical movement is immediately stopped (obviously through hard-wiring etc) that the purge cycle remains unaffected as the exhaust fan is still running and there would have been no build up of gases etc. As all the gas and combustion safeties remain unaffected and fully functional, I do not
see why the oven purge should be killed. Gas valves, maybe but purge cycle?
Anybody shine any light on this one?

Bob Peterson

I have done a number of these type of systems. All of them operate with 2 different estops.

The first estop (usually located at the main panel) stops everything, including conveyor(s), burners, purge, whatever. This estop is hardwired to a control relay that drops all control power and disables the VFDs.

The others, (one at main panel and others located remotely throughout the area of the conveyor) only stops the conveyor, and does not affect the oven at all. I usually wire these pushbuttons into the enable circuit of the VFD. These are usually labeled something like "Conveyor Stop", rather than estop.

I see no reason whatsover to stop a purge cycle, or shut off the gas valves because someone needed to stop the conveyor for a few minutes.

Bob Peterson
Depends on the situation.
If no purge would mean that there is an explosive atmosphere then purge would be necessary.
If mechanical movement means static electricity and explosion, then the design is poor.


Johan Bengtsson

This seems somewhat similar to other topics recently discussed. Should an Estop be used? I say no, it should never have to be, but be there the day it should be needed. And that day it should stop everything having any remote possibility to be dangerous.

If you want buttons to stop a part of the mashine located nearby the Estop button, fine but don't make it look like an Estop, and definitely mark it as something like you suggest below "Conveyor
stop". This can even be put to an input to your control system and handled there. Make it easy enough to control the machine without using the Estop so it is never used unless someone actually is in danger, and better make the Estop stop ten things unnecesary than one necesary. (Some
things might be considered better left running for increased safety but that is another point).

This is my opinion anyway....

/Johan Bengtsson

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Zan Von Flue

I have 2 buttons 1 ABORT and 1 E-stop. When the Abort is pushed the machine will go in a Idle status. Movement stopped, Oven Temperature is not affected, but the oven is purged. However, when the E-stop is push, I cut everything away, movement, oven power, Clorine but I do purge. I have to purge (N2) or the tubes will rust. When the E-stop is pulled, an extra 'DC on' power button has to be pushed to reactivate the machine. Since the N2 is not a poison gas, plus this is the 'No Power' status of the machine, it's safe.

(everybody has an opinion)

The important thing to remember when dealing with safety issues is not to make assumptions on spec. What works (is safe) in one application or even
installation (of the same plant/process/machine), is not necessarily safe in another. Each must be judged on its own merits.

Each project must have a HAZOP study and Risk Assesment (actually it must have several).

In many cases the assumed safe action may give rise to additional hazards, which may be more dangerous than the initial problem. For example, an e.stop which stops conveyors moving product through an oven may create a fire hazard.

Each risk reduction action must also be assessed. IEC standards state that each machine must have a category 0 stop (removal of power) except where
this stop would give rise to additional hazards. For example, a printing press uses regen braking to stop in 5 to 30 seconds, if power was removed
inertia might keep the press moving for a number of minutes, mechanical brakes run the risk of breaking shear pins and causing the cylinders to
continue rotating for many minutes (there is a mechanical break in case of power failure to the main drives, operation of this brake is usual
catastrophic, it has to be rebuilt after use).

In the case outlined below, if stopping the purge would allow the possibility of a gas build up then the risk of this possibility should be
compared to the risk provided by NOT stopping the purge fans. My guess is that the risk of an explosion is 'worse' than the risk to life and limb of maintaining the purge. It is easier to design out the hazards with a fan (extended ducting, fixed guards/finger proof grills, Ex motors).

Mark Hutton
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