I am upgrading the system to incorporate a new PLC to control the main operations, but I do not know if all circuits can be safely operated by a PLC.

If you have any information regarding what circuits can be controlled by a PLC and what are required to be hard wired (safety circuits such as emergency stop buttons for instance), I would greatly appreciate the help. Any other links, contacts or regulations would also be appreciated.

Thank you.

 

The emergency shutdown system acts independently of all other systems and should be totally independent from other systems. So you'd need 2 PLC's (which is a cost factor depending on your I/O).

If you are using a PLC as an ESD system it has to be TUV approved and certified for Safety Integrity Level (SIL) 3. ISA S84.01 is a good guide.

Cheers & good Luck
Gray

 

Adam:

The first thing you need to know is the Safety Integrity Level (SIL) required for each Safety Instrumented Function (SIF) you want to implement in your PLC.

There are three (SIL 1 to SIL 3)Safety Integrity Levels as defined by the ISA standard S84.01 and four as defined by IEC-61508 (SIL 1 to SIL 4), the higher the SIL is, the lower the probability of failure under demand of the complete safety loop is.

There are several Safety PLC manufacturers that have products certified (normally by TÜV) to work up to SIL 3 applications, among others are:
HIMA, TRICONEX, TRIPLEX, SIEMENS, GE, ALLEN BRADLEY. It is important to mention, that the PLC is not the only participant of the Safety
Instrumented Function, you have to consider also the sensors (Transmitter, limit switches, etc) and the actuators.

For simple logic functions you can use relay panels instead of a PLC, however regardless you use a relay panel or a PLC you still have to
verify that the proposed solution meets your SIL requirements.

In addition to the ISA S84.01, IEC-61508 and IEC-61511 you also need to consider other standards like NFPA 85 if you are working with boilers.

Hope it help,

Juan Calderon
MCL Control S.A.
www.mclcontrol.com
Ph. +58(212)238.2996 Ext. 119
Fax. +58(212)232.1223

 

Hi,
You do no have to run a redundant or dual PLC setup if is too complicated.
What I would suggest is that if you are using a PLC for control that put a safe bus onto the existing bus.
What I mean is for example using a Siemens S7 300 series with or without profibus.
You can put all the safety control on an ASi bus, this is a safe bus which monitors the status of all E-stops etc.
This system is very easy to set up and is a very good safety controll in machine or process applications.
I would be happy to answer any questions that you have on Siemens ASi, other wise check out their web site.
Regards,
Paul Pierce.

 

Thanks for the reply, Gray!

I had not thought about using 2 PLC's for the system, and I don't think buying another would be worth the trouble.

What I was thinking of was using the PLC to control the operation of the boiler and only to monitor the status of the E-Stop and the Safety Line. I had designed a safety line to be hard wired in series to include the e-stop, low water limit switch, high pressure cut out, high temerature cut out, and flue gas damper, so that if any of these switches open, it will open a contactor on each power feed to the motors, while the PLC only monitors the status of the switches.

Do all these switches listed have to be hard wired in the safety line? Am I missing any key element that must be hard wired?

 

I do know that Allen Bradley has a safety plc by a name like Guard Plc . May be you could use this for emergencies shut down and then utilise an
ordinary plc for control.
Rgds,
Gabriel

 

Adam,

You will probably want two separate control systems. One for the automation portion. One for the safety portion.

KTI offers a redundant PLC set up with safety relay built in to the controller. For more information, visit their web site -
http://www.ktitechnologies.com

Hope this helps.

God Bless,

Stephen Luft
Entertron Industries
3857 Orangeport Rd.
Gasport, NY 14067
Tel: 716-772-7216
Fax: 716-772-2604
web: www.entertron.com
wwjd