E-stop implementation

Folks,

I am installing a lime slaker in a water treatment plant.

The unit has no "factory" e-stop provisions.

The project manager is directing me to put a mushroom type switch on each slaker to interrupt the control voltage in the control panel.

The panel is remotely located so I would be running control power (x-former secondary) out to the push button and back to the panel.

Does this sound typical?

Thanks for your advice in advance...

Von

Yes.

Just make sure to follow any local and national electrical codes when doing so.

I can't tell for sure but it sounds like you are going to be running the secondary from your control transformer through the E-Stop. That is not very typical.

What I usually see done is there is a string of E-Stops that feed into a relay sometimes called the MCR for Master Control Relay or E-Stop Relay. This relay is used either directly or through other safety relays and contactors to break power to outputs that can cause injury such as power to pumps, valves, heaters, motors, etc.

Most people leave power on to things such as PLC's, HMI's, cabinet lights, and sometimes inputs so that the state of the machine can be known even if the power to all moving elements is physically broken by the MCR.

As stated the best advice is to follow the NEC and codes for your industry. I could imagine cutting power to certain elements in a water treatment facility could cause unwanted buildup of dangerous gases or could result in unsanitary conditions.

As Norm aluded to, be VERY sure that the e-stop changes you make don't actually make the environment more dangerous than not having an e-stop in the fist place. The road to perdition is paved with good intentions.


Keith

Wire it to your control power via a relay; run the e-stop to the relay contacts using a lower voltage. You'll stay safe that way.

A Lime Slaker, although a predominantly sealed unit, looks like the kind of machinery I'd use a Category 3 system on. Perhaps Category 2 if you can guarantee that you meet the correct criteria, and you're confident in the environment the machine has been installed in.

Field Safety Device (E-Stops/Guard Switches)-> Safety Relay -> Redundant Safety Contactors and Pneumatic Dump Valves.

I'm not entirely sure why, but I'm not a fan of dropping control voltage from I/O cards as safety, which I put on par with DC- or GND switching 'safty systems'. Usually I drop the phase(s) themselves through safety contactors. Also, don't forget the layout of the machine itself, if not law, it's atleast good practice to place an E-Stop on each accessible side of the machinery that wouldn't allow quick access to any of the other E-stops.

Von said:

Folks,

I am installing a lime slaker in a water treatment plant.

The unit has no "factory" e-stop provisions.

The project manager is directing me to put a mushroom type switch on each slaker to interrupt the control voltage in the control panel.

The panel is remotely located so I would be running control power (x-former secondary) out to the push button and back to the panel.

Does this sound typical?

Thanks for your advice in advance...

Von

Click to expand...

Sounds like an application for a Category 2 Safety Relay. The Estops act as an input to the safety relay in your control panel, and the safety relay has contacts that can power MCRs.

Do a search on Safety Relays

I don't know what a "lime slaker" is, but if it has a motor and you need to add an E-stop for that motor, then isn't the e-stop required to be hardwired in the motor's control circuit as fed by and in the MCC breaker bucket?

kent

In the UK it is normal to use a safety relay as this is designed with dual relays & they re positive guided so that it will open the contacts, also dependant on the catagory, it is usual to feed back the status of the motor contactors (N/C when de-energised) to the safety relay which disables the safety relay from pulling in if one of the contactors has welded.
Of course this only works if a main contactor is fitted feeding all others that require this type of catagory.
i.e. one or two main contactors feeding all others, if one welds in it opens the closed loop feedback to the safety relay so as not allow the two main contactors to pull in.
In the event of one of the main contactors or motor contactors welding, the 3 phase power will not be energised by resetting the safety relay.

Monitored Safety Systems

In Australia here, we use Safety Relays, together with main or cascading safety rated contactors that afford monitoring feed back to the safety relay. Generally here, we design to a Category System (3, 4+).
I am actively involved in the design of these such systems, and usually afford appropriate PLC input information via these mentioned safety relays or Multi Safety Relays to give me I/O process stops, under safety operated conditions.

The below comment by "parky" is generally what I have said above. Hope it helps in your quest!

Jarrod

I don't know what a lime slaker is either, so I will refrain from making any E-Stop recommendations without fully understanding the machinery...

OkiePC said:

I don't know what a lime slaker is either, so I will refrain from making any E-Stop recommendations without fully understanding the machinery...

Click to expand...

Ditto...a safety assessment specific to this equipment would be a great place to start.

I wholeheartedly agree with OkiePC and jstolaruk.

The project manager is directing me to put a mushroom type switch on each slaker to interrupt the control voltage in the control panel.

Click to expand...

Is the project manager a safety expert? It is good that he recognizes that some sort of safety protection mechanism is in order, but slapping some E-stops on the machine is not the solution. I too, do not know what a lime slaker does. But if this were plopped onto my lap, I would conduct a process hazard analysis and be sure to invite those who know something about the upstream and downstream processes as well as OSHA and local safety requirements as they apply to your industry.

The unit has no "factory" e-stop provisions.

Click to expand...

I also find it curious that the unit OEM has not provided a standard means of interconnecting E-stops. Can this thing kill or injure someone? Is it very old? Seems odd.

CeCo3

For the people who like to know stuff:

This is 'Slaking Lime' (adding water to Lime) the reaction as you can see is very exothermic. And that was starting with large chunks image if you added powder.
http://www.youtube.com/watch?v=eSKX0gzESwI

Here is a Safety Sheet for Lime.
http://www.limebase.co.uk/safety.htm

And some pictures of a Lime Slaker
http://www.sodimate.com/en/LimeSlaker.html

And some Safety Precautions for Lime Slaking
http://www.chemcosystems.net/SAFETY_PRECAUTIONS_SLAKER.pdf

You had better get used to this stuff, if energy keeps going up in price we will all be using Limecrete instead of Concrete.

Bryan

Doesn't concrete already contain lime?