Converting NIS Instrument to IS Instrument 1

[Berrio]

I have a client who wants me to do something that sounds a bit dodgy to me and I just thought I'd see what advice or information anyone has.

We are changing out the control and safety system on an old offshore production platform and as part of this changeout a number of pressure switches connected to the safety system are being changed out for pressure transmitters. However pressure switches connected to the control system are not being changed out. Due to the age of the platform and the instruments many of these Exd rated pressure switches have enclosures that are not longer capable of meeting their certification due to holes, corrosion, addition of tape, etc (this in itself does not sound good). Therefore the client has informed us that these Exd rated NIS pressure switches are to be "converted" to IS pressure switches and connected to barriers in the new control system. They have stated that the Exd labels should be removed from the encolsures which will effectively make the enclosures non rated boxes with holes, etc.

The question is...is this acceptable? Their argument is that the pressure switches are simple apparatus and therefore can be connected to an IS circuit with no issues. To me this all sounds a bit dodgy. The Exd pressure switch will have no entity parameters, so I am unsure how I would be able to assess that the barrier was the right one for the job and that the circuit was indeed intrinsically safe.

Any assistance and advice you could give would be really appreciated.

 

[danw2]

A switch is a Simple apparatus, which does not need to be approved. It is a nonenergy storing device, as are thermocouples, RTDs, LEDs and resistors. A simple apparatus is still used with an approved barrier or isolator.

Here's what MTL (a barrier/isolator manufacturer) says about Simple apparatus:
2.5 Simple apparatus
In general, intrinsically safe apparatus is certified; usually by an independent body such as an Accredited Certification Body (ACB) under the IEC Ex scheme. Self-certification by the manufacturer of ‘ic’ equipment is also quite commonly accepted. The exception to the rule is ‘simple apparatus’, which is considered not to appreciably affect the intrinsic safety of the system. This apparatus is exempted from the requirement for certification. The simple requirements are clearly specified in the apparatus standard.
‘Simple apparatus’ should always be readily demonstrable to be adequately safe. The usual examples are switches, thermocouples, RTD’s and junction boxes.

 

[danw2]

I've seen pressure switches installed where the switch carries a fair level of current as a series interlock, as opposed to a signal to control system.

My assumption for an I/S 'switch' is that it is a logic switch, providing a logic signal to a control system, not carrying high currents that arc and spark when contacts are made or broken.

 

[Texagere]

Dear Berrio,

According to last 60079-11 IEC:2011 (Explosive atmospheres –Part 11: Equipment protection by intrinsic safety "i")
chapter 5.7 Simple apparatus
You can read:
"... Simple apparatus shall conform to all relevant requirements of this standard (IEC 60079-11)with the exception
of Clause 12. The manufacturer or intrinsically safe system designer shall demonstrate
compliance with this clause, including material data sheets and test reports, if applicable..."

and after you have the following comment:
"...The following aspects shall always be considered:
• simple apparatus shall not achieve safety by the inclusion of voltage and/or current-limiting
and/or suppression devices; ..."

So I suggest first to confirm with your client his idea and may be to read this part of international standard to clarify the situation.
For example, as you know, we find on the market a lot of mechanical switch with IS certification: they don't need only a barrier, they are part of the IS circuit (electrical, mechanical, enclosure, isolation from earth etc. ).

regards

Bruno