Over the years, well-defined standards have been established to address the requirements of process sealing between an electrical system and a process fluid, where a seal failure could allow the migration of the fluid into the electrical system. Defined by ANSI/ISA, NFPA/NEC and CEC, these standards historically have varied, allowing either single seal or dual seal devices or allowing only a dual seal system device, for process fluid barrier protection.
However, with the update to National Electrical Code, 2011 Edition, the NEC standards for process sealing now align with CEC and ISA, and all standardization bodies currently require the use of either a single seal or a dual seal to prevent process fluid coming into contact with the external electrical system.
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 its designed containment into the external electrical system.
Single Seal Versus Dual Seal
There are a number of reasons why both single seal and dual seal devices are now accepted under industry standards codes – and why a simpler system for meeting required specifications may be preferred.
Qualifications for single seal marking are more stringent: To meet process sealing requirements set forth by NEC, ISA and CEC, the qualifications for the primary and secondary seals within a dual seal system are not as robust as those applied to a single seal device.
Single seal is a robust solution: The popularity of single seal devices in process industries indicates that owners, operators and installers are more concerned with meeting the required specifications, rather than counting the number of seals used.
Need more information about the process fluid containment standards and the qualification requirements of single seal and dual seal devices? Magnetol® has produced a white paper on the subject. “An Overview of Recent Changes to the Single/Dual Process Seal Standards” provides detailed information about current ANSI/ISA, NFPA/NEC and CEC process sealing standards, as well as guidelines for selecting process sealing based on application and performance requirements.