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Aug-2011

An overview of recent changes to the single/dual process seal standards

Two is better than one, right? Applying this old adage is often good guidance, but sometimes it’s just not appropriate. Case in point comparing single and dual process seals.

Bob Botwinski
Magnetrol International
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Article Summary
Overview
• Overview of the process sealing requirements as defined by the ANSI/ISA1, NFPA/NEC2, and the CEC3
• History and explanation of recent changes to the relevant codes and standards
• Guidelines for selecting process sealing based on the application and performance requirements

Background

Over the years, well-defined standards have been established to address the requirements for process sealing between an electrical system and process fluids, where a failure could allow the migration of the fluids into the electrical system. The primary audiences for these standards are the owner/operators of process facilities and installers of electrical equipment and instrumentation.

Definitions: Process Fluid and Seals
Generically, a process fluid is any liquid or vapor used in, or is a byproduct of, an industrial process. As defined in the standards, a process seal is a device that prevents the migration of a fluid from a designed containment into an external electrical system.

Process seals are often grouped into two categories of devices: single seal and dual seal4:
• Single Seal: A device that incorporates a single sealing structure that is considered to have a negligible probability of failure when used in accordance with the manufacturer’s specification.

• Dual Seal: A device that incorporates, along any single potential leakage path, a primary process seal and one or more secondary process seals such that the failure of two or more independent seals is required to allow migration of process fluid from their designed containment into the external electrical system.

Brief History of Process Sealing Standards
Although they all have the same goal in mind, over the years ANSI ISA, NFPA/NEC, and the CEC have issued slightly different standards and code requirements for process sealing.

More than 20 years ago, NEC required the use of primary and secondary seals, and specified that the secondary seals must be able to withstand conditions equal to or greater than the conditions that caused the primary seal to fail.

Independently, the CEC required that secondary seals must be provided between devices containing a primary seal and conduit/cable seal, where failure of a single component in the device containing the primary seal could allow passage of process fluids.

In 2003, ISA required that dual seal devices incorporate a method that indicated or annunciated a primary seal failure (e.g., visible leakage, an audible whistle, or other means of monitoring5).

In 2005 and again in 2008, the NEC6 modified its requirements for process sealing to include the concept of dual seals consistent with the standards specified in ANSI ISA-12.27.01. The NEC still did not allow single seal devices. The NEC modifications required that either a device listed and marked as a dual seal was installed, or that the installer must add an external secondary seal (or other mitigation technique) to the system.

Reviewing All Seals
Some process facility owners/operators are replacing any equipment that has a non-compliant seal with a device that has a process seal that meets the ANSI ISA 12.27.01 standard.

These changeovers may be directed by local legislation or inspired by more stringent safety practices. On a case-by-case basis, owners/operators should review their equipment with single-seal that are not covered by the 
new standards. Some of these applications 
will be acceptable as is; some should be 
changed in order to be compliant with ANSI ISA 12.27.01.

Here’s an example. Consider a stainless steel thermowell isolating a temperature sensor from a process. A thermowell is considered a single seal device and covered by the standards. Since the thermowell is pressure-tight, solid container, it has negligible chance of a failure. It is a common and good engineering practice to use thermowells for safely separating electrical systems from a combustible media and does not need to be changed.

Alternatively, consider the same temperature sensor inserted directly into the process with a single o-ring forming a seal. Assume this process connection initially passes all standard agency testing. However, over time, the seal could age and weaken, thus allowing combustible gases to pass. This type of process seal has a significant chance of failure over time and should either be protected by a secondary seal or replaced with an approved single-seal device.

Recent Revisions to the Standards

The new NEC edition7 has expanded its process sealing standards and added detail as to what will meet the requirements for installation of process connected equipment and the prevention of process material from getting into the conduit of equipment.

The 2011 update to the NFPA 70 process sealing methods have brought the current version of the NEC into alignment with CEC and ISA. A new clause (501.17) replaces clause 501.15(F)(3) relative to process seals. The key statement in this clause is in the last paragraph: Process-connected electrical equipment that does not rely on a single process seal or is listed and marked “single seal” or “dual seal” shall not be required to be provided with an additional means of sealing.8

In summary, the NEC now allows devices to be marked as either single seal or dual seal per ANSI ISA-12.27.01. The NEC and CEC reference the testing standards as specified in the standard. A Nationally Recognized Testing Lab (NTRL) such as FM or CSA typically provides the certification for such devices.

Meeting Process Sealing Requirements
Single Seal Qualifications

To be certified and marked as “single seal,” a device must pass the following controlled tests:
• leakage and burst – must not show visible signs of leakage when subjected to overpressure. The maximum pressure requirements and duration are different for different devices and are dependent on the maximum working pressure.
• temperature cycling – must not fail when subjected to repeated changes in temperature.
• fatigue cycling – must not fail when subjected to changes in pressure.

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