Instrument seals and Division I II boundary

[HiLevel]

Ref: thread242-135230

I rountinely encounter the situation in chemical plants, where we are installing an instrument on a vessel that will contain a Class I Div.1 environment and the surrounding environment is Class I Div.2. The assumption is that the vessel contains the Div 1 environment that will only be released if something goes wrong, thus Div. 2 outside the vessel. Folks sometimes miss this Div 1 area because they don't consider the inside of vessels when they do their electrical area classification.
Some instruments inherently must apply limited voltage or energy to the sensed media in order to work. I'm talking about Capacitance probes, and Guided Wave Radar to name a couple. These instruments typically are designed within their electonics to limit the energy that they can pass to the media, and this is certified by a 3rd party such as FM, or UL. They also have a separate compartment for the field wiring terminations to segregate them from the electronics. Therefore is is important to specify the proper instrument for these applications to be sure the instrument can't be an ignition source.
To the point- The instrument vendors tell me to specify a Class I Div 1 instrument to get the current limited sensor wiring. The NEC has a provision for "nonincendive field wiring" in 501.10 b3; however, it only applies to Div 2. In this configuration, the instrument and it's internal seals are the Div 1/Div 2 boundary. The instrument vendors won't tell me that I don't need a conduit seal. The NEC says that in Div 2 if the instrument needs to be "Explosionproof" it requires a seal. I'm prone to install the seal just to play it safe; however, I'm thinking that the seal doesn't really buy me anything. And, I encounter this scenario routinely. I'm wondering if there is anyone out there who has worked through this and established a rationale for either eliminating or installing this seal.
Thanks for your input!

 

[JLSeagull]

Your post could be easier to read if split into paragraphs that represent the various ideas. You really need to look at a few related issues. One issue pertains to the dual process seal. Check a few other locations within the codes and standards to assure isolation of the process from the electrical components.

Review ISA 12.27.01, Requirements for Process Sealing Between Electrical Systems and Flammable or Combustible Process Fluids.

Also review NFPA 70. I made note regarding NEC 501.15( F)(3) under Canned Pumps, Process, or Service Connections etc.
Process-connected equipment that is listed and marked
“Dual Seal” shall not require additional process sealing
when used within the manufacturer’s ratings.
FPN: For construction and testing requirements for dual
seal process connected equipment, refer to ISA 12.27.01,
Requirements for Process Sealing Between Electrical Systems
and Potentially Flammable or Combustible Process
Fluids.

Additional conduit seals are not prohibited, just costly.

 

[JLSeagull]

To reduce conduit seals, the instrumentation requires reliable dual process seals compliant with the NEC section 501.15 (F)(3) (or 505.16 (E)(3) for zones). For offshore applications review API RP 14F 6.8.2.2. This API standard applies to conduit seals and drains; and clarifies that bourdon tubes, thermowells, etc. are a single seals. The additional seal must meet the temperature and pressure requirements. Not stated and equally important is the material compatibility. This is an especially important consideration for elastomers exposed to process fluids. Further it should be obviously detectible if the primary or secondary seal were to fail.