Q: What are the factors driving the increased demand for purge and pressurization systems?
A: The increased demand for purge and pressurization systems is driven by a number of factors. Following are some the key factors:
- The continuing focus on safety in hazardous working locations.
- The high-cost of designing, certifying, and installing electrical equipment as either explosion proof and/or intrinsically safe.
- The increased use of automation requiring electrical/electronic controls in hazardous work environments. For example: Robotics, programmable controllers, computers, printers, calibrators, analyzers, and video displays.
- The recent boom in the petro-chemical industry brought on by the lower cost of petroleum-based raw material.
Q: What is “Purging” in the context of purge and pressurization systems for hazardous environments?
A: In the context of purge and pressurization systems for hazardous environments, the term “purging” refers to the process of supplying pressurized clean instrument air or inert gas into an electrical enclosure in order to reduce or eliminate the hazardous material in the enclosure to a safe (non-explosive) level.
Q: What is “Pressurization” in the context of purge and pressurization systems for hazardous environments?
A: In the context of purge and pressurization systems for hazardous environments, the term “pressurization” refers to the process of bringing the same (as the purging air) clean instrument air or inert gas under pressure into the electrical enclosure. The pressure is enough to create a small positive differential pressure between the inside of the enclosure and the ambient pressure outside the enclosure. The positive pressure ensures there is no ingress of hazardous material into the enclosure.
Q: How much pressure is enough to ensure an electrical enclosure is “safe” within a hazardous environment?
A: The AB-CO PURGE pressurization system require a minimum of 0.15 inches of water (1 psi = 27.7 inches of water). Our system’s visual indicator shows a safe pressure range of 0.15 inches of water to 0.35 inches of water. Often, it is best to operate at the higher side of the safe pressure reading to ensure a proper safety factor.
Q: What is MIC?
A: MIC is the Minimum Igniting Current. This is the ratio of the minimum current required from an inductive spark discharge to ignite the most easily ignitable mixture of a gas or vapor, divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions.
In the “Class/Division/Group” system of hazardous area classifications, MIC ratios are used for Groups B, C,and D to define the types of hazardous gases in the surrounding atmosphere.
Q: What is MESG?
A: MESG is the Maximum Experimental Safe Gap. This is the maximum clearance between two parallel metal surfaces that has been found , under special test conditions, to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration. In the “Class/Division/Group system of hazardous area classifications, MESG distance values (in mm) are used for Groups B, C, and D, to define the types of hazardous gases in the surrounding atmosphere.
Q: What is SCFH?
A: SCFH is the Standard Cubic Feet per Hour is the volume flow rate of a vapor at standard conditions of pressure and temperature and humidity –14.7 PSIA, 68°F, and 0% relative humidity.
Q: What is SCFM?
A: SCFM is the Standard cubic feet per minute. It is the volumetric flow rate of a gas corrected to “standardized” conditions of temperature and pressure. It is related to the mass flow rate of the gas by a multiplicative constant which depends only on the molecular weight of the gas.
Q: What is a Hall Effect Sensor?
A: A Hall Effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications. In its simplest form, the sensor operates as an analog transducer, directly returning a voltage. With a known voltage field, its distance from the Hall plate can be determined. Often, this type of sensor is combined with threshold detection and as such acts as a switch. A Hall Effect sensor is used in the AB-CO Purge pressure switches DP-1B and DP-1C.
Q: What kind of “air” (protective gas) is required for a purge/pressurization system located in a hazardous environment?
A: Air of normal instrument quality, nitrogen, or other non-flammable gas is permitted as a protective gas. All protective gas is to be free of contaminants or foreign matter and shall contain no more that trace amounts of flammable vapor or gas. (This is per the NFPA 496 specification.)
Q: If using compressed air as a protective gas, where should the compressor intake be located?
A: The compressor intake should be located in an unclassified location.
Q: Is there a specific NEMA (National Electrical Manufacturer’s Association) enclosure specified by NFPA 496 for use with a purge/pressurization system used in a hazardous environment?
A: NFPA 496 does not specify a particular NEMA rating for an electrical enclosure that uses a purge/pressurization system in a hazardous environment. Most manufacturer’s of purge/pressurization systems recommend electrical enclosures that minimally meet the rating of NEMA 4 or NEMA 12 enclosures respectively. If the enclosure has windows, it’s important that the windows be shatterproof. It’s best to make sure there is no tape over cutouts and/or openings in the enclosure.
Q: What factors determine which purge/pressurization system is most appropriate for our application?
A:The following factors help determine which purge/pressurization system are appropriate for your applications:
- Hazardous area classification for the area in which the electrical enclosure is located.
- The hazardous area ratings of the equipment within the enclosure.
- The size and configuration (windows, doors, penetrations) of the electrical enclosure.
The power requirement to the electrical enclosure (applies to X-Type systems).
Q: What are the advantages of purge & pressurization systems as a method of protecting electrical equipment in hazardous locations?
A: Following are advantages of purge & pressurization systems:
- Overall safety
- Most cost effective (low cost of protection/volume of enclosure
- Low maintenance
- Not limited by quantity, volume, shape, power requirements, or physical location of electrical equipment
- Capable of reducing moisture, heat, dust, and corrosion within protected equipment
- Can improve performance of equipment within protected enclosure
- Allows for quick and easy access to protected equipment
- Adds to the life expectancy of instrumentation within electrical enclosure
No immediate danger created if system fails.