RTD's intrinsically safe 4

[Lc85]

Can RTDs be used as intrinsically safe items without the need for a barrier?

We have an RTD in an EExd enclosure and we wish to reuse the rtd with a tank gauge. My question is would the tank gauge need to have barriers fitted to allow the circuit to be intrinsically safe?

Thanks

 

[waross]

My understanding of the codes is that the devices must not only be intrinsically safe but must also be tested, certified and labelled as intrinsically safe.
If the RTDs are rated for the location they are probably OK.
Are you using transmitters to transition from the resistance measurement to a 4-20 mA loop? Are the transmitters field mounted or remote mounted? Are the transmitters intrinsically safe?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

The Ex'd' RTD is, in principle, OK to install within the hazardous area without a barrier. Based upon what you've disclosed isn't dual certifed as an IS piece of equipment as well as Ex'd'. It will not be an intrinsically safe circuit, and nor does it need to be, because it is a flameproof device. If the gauge is located outside the hazardous area you do not need a barrier.

 

[itsmoked]

Scotty; How can you say an RTD is not a flameproof device? May be I'm missing the precise definition of flameproof?

If the power comes into the hazardous area from outside it it must come thru barrier protection unless it's powered thru an I.S. approved device, which is essentially providing its own barrier protection.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[moltenmetal]

An RTD is a simple circuit device in the parlance of NFPA 70 (USNEC) and the CEC. It is incapable of storing energy. As long as it is supplied and monitored by an IS circuit, it does not need a specific approval- it needs no inspection, identification or certification.

The same is true of a thermocouple.

 

[waross]

Would it be correct to say that an RTD may be mounted in an explosive area provided that the circuit powering it is rated as intrinsically safe, even if the monitoring circuit is located outside that hazardous area?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

That is what I understand it to be, but I also understand you don't need an "approved" RTD, as molten is describing, because an RTD is a "simple" non-energy storing device.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Hi Keith,

I'm saying that the RTD is certified using the Ex'd' protection concept: our 'flameproof' is broadly equivalent to your 'explosion proof'. As such it already has all the protection it needs for use in a hazardous area, without use of a barrier. If some hypothetical fault caused the RTD to reach a temperature where it ignited flammable gas inside the enclosure then the flameproof enclosure would contain the explosion and vent it through a controlled flamepath at a temperature where it couldn't cause an external ignition. Flameproof protection isn't especially common for instruments because it makes them bulky and expensive, but it's certainly possible.

I'm not saying that an intrinsically safe solution isn't possible for an RTD, but having spent a considerable sum on a flameproof RTD why would you want all the additional requirements of using it in an IS application when there's no need to do so?

 

[itsmoked]

Greetings Scotty!

Yes, yes, I'm with you now! Your flameproof~>ExP cleared that up handily. I totally agree with you.

Never seen an ExP RTD before. Seems likely to lag the poor thing horridly.

I could see them wanting to change out ExP for two reasons. One would be having to run the requisite thick-wall conduit and bulkhead plugging hassles and the other would be maybe needing a more responsive sensor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

We have two classes.
Explosion proof is what Scotty described.

Explosion-proof — enclosed in a case that is capable of withstanding without damage any explosion that may
occur within it of a specified gas or vapour and capable of preventing the ignition of a specified gas or vapour
surrounding the enclosure from sparks, flashes, or explosion of the specified gas or vapour within the enclosure.

Another option is intrinsically safe.

Intrinsically safe — that any spark or thermal effect that may occur in normal use, or under any conditions
of fault likely to occur in practice, is incapable of causing an ignition of the prescribed flammable gas, vapour,
or dust.

In most all of the Petro-chemical plants that I see, heavy wall conduit has been replaced by hazardous location rated cable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[moltenmetal]

waross: just to clarify a bit what you've said. Most locations are not Div 1 (normally hazardous) but rather Div 2 (hazardous only during an upset/leak). In Div 1, only explosion proof and IS instruments are permitted, whereas devices certified as nonincendive are also permitted in Div 2. If the device itself is protected by means of explosion proofing only, then you need to pour a seal whether you're in a Div 1 or Div 2 environment in order for that protection to work. The location of that seal will determine whether you need a bit of XP conduit first or you can transition right away to area-rated cable. If the device is, like most but not all process instrumentation, certified as both XP for Div 1 and nonincendive for Div 2, then you don't need to pour the seal in a Div 2 area and you can cable right into the instrument. That tends to confuse people who don't understand that the devices are dual-certified.

 

[waross]

We are now using sealing connectors or glands on HL cables. In many instances the only difference between a div 1 installation and a div 2 installation is the connector or gland on the end of the cable. The conductors typically pass through an ex rated union into the instrument so that the instrument may be disconnected without disturbing the seal.


Bill
--------------------
"Why not the best?"
Jimmy Carter