The technique is simple if done properly, but users often implement it incorrectly if they don’t know the standards or system’s characteristic gas flow.
By Chris Romano, product portfolio manager for Purge and Pressurization, Pepperl+Fuchs, Inc.
Editor's Note: This article is adapted from a white paper, "Avoid These Eight Common Mistakes When Purging and Pressurizing Enclosures." Download the free, full white paper with additional in-depth information about mistakes and how to avoid them.
Purge and pressurization is a method of protection that allows nonhazardous enclosures and equipment to be located in hazardous areas. It uses air or an inert gas to keep a positive pressure within the enclosure and prevent hazardous gases or combustible dust from entering.
For hazardous gas environments, air or inert gas is used to purge the inside of the enclosure before it can be energized. For combustible dust environments, dust is physically removed, and the enclosure pressurized before it can be energized. As long as the enclosure is pressurized, the equipment within the enclosure can operate normally in those environments.
The purge and pressurization technique is a simple concept if done properly, but too often this method is implemented without knowing the required standards or the system’s characteristic gas flow. Even if the protection components are certified, users might implement them incorrectly. Here are five common mistakes:
1. Failure to Include a Pressure-Relief Vent
Many times, the designer will neglect to include a pressure relief vent in an enclosure. In hazardous gas atmospheres, purging is required, and a pressure relief vent is necessary. The cabinet’s atmosphere must be purged of any hazardous gases and pressurized before the equipment inside can be energized. Without a vent, the enclosure’s internal atmosphere has nowhere to escape; the increasing pressure within the enclosure will prevent proper purging.
In hazardous dust environments, purging isn’t required, but the enclosure must be cleaned before pressurizing. Most users do not use a pressure relief device, which is acceptable under normal use. However, if the regulator being used to pressurize the enclosure fails, a large increase of flow into the enclosure could damage the enclosure if a pressure-relief vent isn’t present.
2. Forgetting to Purge Before Pressurizing
Often in a hazardous gas atmosphere, purging is neglected, and the enclosure is just pressurized. Users neglect the fact that the atmosphere inside the enclosure, before pressurizing, may contain hazardous gas. Industrial enclosures rated Type 4x or IP66 are not gas tight, so hazardous gases can still leak into the enclosure.
3. Not Knowing the Required Volume Exchange
To help ensure all hazardous gas is removed when purging an enclosure, standards exist that specify the number of volume changes required before the enclosure is considered safe.
For example, in an accident at a pharmaceutical plant that manufactured pill casings, the material for the casings was not combustible. However, the process of making them produced a very fine dust that collected over the support beams and machinery. When a maintenance worker using a torch accidentally ignited the dust, a small explosion suspended the dust into the air. The fire followed the dust path, which then caused a larger explosion.
5. Not Knowing the Standard
Putting a certified purge system onto an enclosure and populating that enclosure with equipment doesn’t necessarily make the complete enclosure a certified system. The applicable standards for purging/pressurization might require further system testing to make sure it complies with the area classification.
Uninformed users won’t know how much safe gas is entering the enclosure, which means they won’t know the proper volume exchange. A flow measurement is required to determine the necessary time that must pass for a successful purging. Some purging systems will measure the flow for determining purge time.
4. Failure to Address Combustible Dust
Another common and dangerous mistake is not recognizing that dust being produced in the plant is combustible. A raw material used during the manufacturing process might not be listed as combustible, but dust formed during the process itself could be combustible.
Additional testing is normally required for full and complete certification, including heat measurements from the equipment for T-codes, overpressure testing of the enclosure to help ensure it doesn’t break or permanently deform, and flow checks to confirm all points within the enclosure are purged and pressurized properly.
Other standards and requirements might apply to other parts of the system, including wiring, terminations, gas handling elements and alarm management. By using certified purge and pressurization components, the implementation and final certification of the system will be easier, faster, and hopefully less expensive.
Although the area classification is important in identifying the type of purge system required for the application, the above issues apply to both NFPA 496 Type Y, Z, X (North American requirements), and EN/IEC 60079-2, Type pyb, pxb, pzc (European and International requirements) and local codes.
An understanding of the applicable standards and the hazardous area classification is important in applying any hazardous area protection method. Safety is the ultimate goal and should always be the first consideration in operating equipment in hazardous areas.
Pepperl+Fuchs, Inc., based in Twinsburg, Ohio, is participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™ program. The company develops and manufactures hazardous location protection products and components for electronics used in the global process automation market.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.