What the SIL are you talking about?
Have you had a chance to catch Bill Goble’s Hydrocarbon Processing (October 2007 issue) editorial titled “Still not using a safety PLC?”
Essentially Bill addresses the question of whether you can use conventional equipment (PLC or DCS) in compliance to ISA 84.00.01-2004 (IEC 61511 Mod.)? I think he did a nice job summing it up as follows:
SIL 3, must be certified to IEC 61508
SIL 2, maybe, if it’s accompanied with a formal assessment (not very practical) SIL 1, okay with justifying documentation
Bottom line, a safety PLC is required for all but SIL 1, and even with that comes baggage, makes one wonder why anyone would elect to go outside the slew of certified offerings.