Industry News

    Dave Adams
Post by On 15 January 2013 In CPCA Corner

Process Seals, Secondary Seals, and “Single Seal” and “Dual Seal” Devices

There is a great deal of confusion surrounding the requirements for electrical equipment which is in direct contact with the process, specifically; is a Secondary Seal required? Does my device require the “Single Seal” or “Dual Seal” markings? Are there other options? etc. I’ll try to answer some of these questions. A Process Seal is a device to prevent the migration of process fluids from the designed containment into the external electrical system. Electrical equipment with an interface in direct contact with process fluids under pressure must incorporate a “process seal” to prevent migration of the flammable fluid into the wiring system. The Canadian Electrical Code (2012), Rule 18-072 and the National Electrical Code (2011), Rules 501.17 and 505.26, both contain requirements, which to paraphrase, state, “Process-connected electrical equipment that incorporates a single process seal such as; a single compression seal, diaphragm, or tube to prevent flammable or combustible fluids from entering a conduit or cable system capable of transmitting fluids, shall be provided with additional means to mitigate a single process seal failure”.

These additional means include, but are not limited to:

  • Use of a suitable barrier between the equipment’s process seal and the wiring system, such as:
    • A Certified Secondary Seal and indication of primary seal failure
    • A short length of MI cable
    • A drain or vent between the process-connected electrical equipment and the cable or conduit seal
  • Engineering Considerations – Leading to the conclusion that the probability of leakage or failure of the primary process seal will be negligible
  • Verification that the equipment does not rely on a single process seal

Process-connected electrical equipment that does not rely on a single process seal and/or is Certified and Marked “Single Seal” or “Dual Seal” does not require additional means to mitigate a primary seal failure.

To summarize, if electrical equipment has a single interface (process seal) in direct contact with process fluids under pressure and the equipment is not Certified and Marked either “Single Seal” or “Dual Seal”, additional means are required to minimize the possibility of fluids entering the wiring system. As a Certification Body, one of the questions we get asked most often is, “Does my electrical device require “Single Seal” or “Dual Seal” Marking? This is one of the reasons I wrote this article.

The answer is NO!! It is never REQUIRED to Certify and Mark electrical devices as “Single Seal” or “Dual Seal”.

The benefit to Certifying and Marking the equipment “Single Seal” or “Dual Seal” is that it eliminates the possibility of additional means being deemed necessary by the authorities having jurisdiction (AHJ).

If a manufacturer chooses to Certify a device as “Single Seal” or “Dual Seal”, it must comply with; ANSI/ISA-12.27.01-2003 (or 2011), Requirements for Process Sealing between Electrical Systems and Flammable or Combustible Process Fluids.
Note: The CEC and NEC both reference the 2003 edition; however the 2011 edition is now published and would be an acceptable alternative.

In general, the requirements of this Standard are as follows (based on 2003 edition):
General Requirements:

  • The manufacturer must specify the following;
    • The operational Pressure and Temperature Range
    • The materials used in construction
    And must make available “the information necessary to determine if the materials critical to the process sealing of the device are compatible with the process fluids”. This means this information must be supplied to customers with the device.

For “Single Seal” Devices:

  • Cannot rely on Bourdon tubes or seals with moving parts

For “Dual Seal” Devices:

  • Must incorporate a method on annunciation of a primary seal failure

Test Requirements:
For “Single Seal” Devices:

  • Temperature Cycling
  • Fatigue Cycling
  • Leakage test (up to 2 x MWP for 1 minute)
  • Burst Pressure test (up to 3 x MWP for 1 minute)

For “Dual Seal” Devices:

  • Primary Seal Leakage test
  • Primary Seal Burst Pressure test
  • Venting Pressure Determination
  • Verification of Annunciation
  • Secondary Seal Leakage test

Marking and Documentation:
Marking

  • “Single Seal” or “Dual Seal” as applicable
  • Process Temperature Range
  • Working Pressure Range

Documentation – To be provided with each device

  • Chemical compatibility of process wetted materials that form part of the Primary Seal
  • Instructions for Primary Seal Annunciation
  • Maintenance Instructions

I hope this helps with the general understanding of process sealing requirement contained in both the CEC and NEC, and soon to come internationally with the publication of IEC 60079-40, Explosive atmospheres – Requirements for Process Sealing Between Flammable or Combustible Process Fluids and Electrical Systems, coming soon.

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Read 20187 times Last modified on Monday, 16 June 2014 14:22
  Dave Adams

QPS Evaluation Services Inc.
Manager, Hazardous Locations Department

www.qps.ca

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