Purge and pressurization systems are protective methods that serve to safeguard electrical enclosures and control panels in hazardous or potentially explosive environments. These systems work by creating and maintaining a safe environment inside the enclosure. Hence, preventing the ignition of flammable gases, vapors, or dust present in the surrounding atmosphere. This article discusses why purge and pressurization systems are important, common methods, Z purge system and requirements vs X purge.
Why use Purge and Pressurization Systems?
Purge and pressurization systems offer an alternative to explosion-proof or intrinsically safe concepts when locating equipment in hazardous locations. Moreover, they have several benefits over these other methods, including:
- Purge and pressurization systems enable the use of general-purpose enclosures and electrical equipment. As a result, the system design, assembly, and commissioning are quicker and cheaper than other methods.
- By using standard enclosures, it is possible to deploy hazardous-rated vortex coolers or air conditioners. Also, standard enclosures make electrical panels easily accessible for maintenance and corrosion-resistant materials for their construction.
- In addition, purge and pressurization enable greater design flexibility, lower costs, and longevity of equipment.
Purge and pressurization systems generally split into three categories: type X, Y, and Z. Each reduces the hazardous content to which the electrical equipment is exposed via purging the enclosure.
The most common variant, the Z purge, makes a Div 2 or Zone 2 location become non-hazardous.
Purge and Pressurization Methods
There are two primary methods of deploying purge and pressurization systems to protect equipment in hazardous environment: continuous flow and leakage compensation.
Continuous Flow
In the continuous flow method, a constant, uninterrupted airflow is introduced into the pressurized enclosure, even after the purging phase is complete. During the purging process, the airflow helps remove any potentially flammable gases from inside the enclosure. Once purging is finished, the continuous airflow ensures that the pressure inside the enclosure remains higher than the surrounding atmosphere.
Leakage Compensation
The leakage compensation method begins with a high-flow purge to expel flammable gases from the enclosure. Afterward, a controller regulates a lower flow of air to maintain a higher internal pressure, thus, compensating for any leaks in the system.
Z Purge System
The major components of a Z purge system include a control unit, outlet orifice, and relief valve.
- Control Unit/Panel: This component ensures that the enclosure is purged until arrival at safe conditions before allowing power to the equipment. It measures and monitors both flow and pressure. Also, it provides the signal for monitoring the enclosed pressure to ensure it is at a proper level. It offers both automation and configurable functions that support purge and pressurization, as well as other functions such as alarms. For a z purge system, the control unit may have a manual or automatic purge controller and disconnect for electrical power.
- System Supply Tubing/Valve/Fittings: The system must supply gas via tubing, fittings, and valves. The gas supplies directly to the enclosure and is calibrated via the control unit to ensure appropriate flow and consistent pressure. Also, the gas supply must be a clean, non-flammable gas. This could be compressed air or some other inert gases such as argon, nitrogen, or even a mix of inert gases.
- Relief Valve or Enclosure Protection Vent: Generally, a relief valve or vent fits into the enclosure and limits the maximum pressure during operation. When operating a leakage compensation purge system, the relief valve serves as the outlet for measuring flow to ensure proper purge timing. Moreover, all relief valves require a spark arrestor to contain sparks within the enclosure, so it does not escape via the aperture.
Z Purge Requirements
Although every manufacturer has unique specifications for their z purge system, most requirements border around the NFPA 70, NFPA 496, and ISA 12.4 standards. The following table highlights some examples of these requirements.
| Attribute | Requirements |
| Examples of Certifications | IECEX, ATEX, UL, CEC, NFPA/NEC, EU DofC, and Korea Testing Laboratory Certificate. |
| Hazardous environment | Capable of dealing with gas and dust. |
| Protective gas supply | The IEC recommends a minimum enclosure rating of IP40. Foam-in-place gaskets and gaskets for typical UL type 4 and 12 sealing are adequate. Enclosures may have multiple doors, but overlapping doors should be avoided due to high leakage potential. Must be capable of resisting any corrosive element in use. |
| Enclosures | The IEC recommends a minimum enclosure rating of IP40. Foam-in-place gaskets, as well as gaskets for typical UL type 4 and 12 sealing are adequate. Enclosures may have multiple doors, but overlapping doors should be avoided due to high leakage potential. Must be capable of resisting any corrosive element in use. |
| Control unit | The control unit should have a built-in spark arrestor, flow meter, and pressure indicator. It provides an automatic alarm system, but purging and power could be manual. Leakage compensation. |
Z Purge vs X Purge
A common alternative to z purge is the x purge. Both z purge and x purge eject flammable and hazardous elements from enclosures. However, there are several differences between both systems as the following table highlights.
| Z Purge | X Purge |
| Reduces the classification in a pressurized enclosure from Div 2 or Zone 2 to non-hazardous. | Also reduces the classification inside the pressurized enclosure, but from Div 1 or Zone1, to non-hazardous. |
| Less stringent as it is used in less severe areas (Div 2 or Zone 2), where hazards are intermittent. | More stringent as it serves in more severe areas (Div 1 or Zone 1) where hazards are more probable or constant. |
| If the pressure drops after purging and pressurizing, an alarm must be activated, unless the power is turned off. | In the event of a pressure drop after the system has been purged and pressurized, the power supply to the enclosure must be disconnected. |
| The operator has the flexibility to manually initiate the purging process. In this case, the flow rate is measured at the enclosure’s inlet. | A timer is necessary for the purging process and the flow rate is measured at the enclosure’s outlet using a flow sensor. |