Explosion protection and Purging and Pressurizing systems are synonymous with each other as they are the go to options in this particular territory. The Zone 1 or 2, Division 1 or 2 principles form the base for these systems. An initial purge process is carried out which removes the gas mixture from the housing which has the possibility of igniting. This process is followed by sufficient supply of inert gas in the enclosure. This entire process is to achieve a state of overpressure using inert gas or compressed air which prevents the explosion in the enclosure.
The internal pressure of 3.5 to 12 mbar is achieved during the rapid exchange of pruge phase. The internal pressure is between 0.6 to 3 mbar during the operation. Certain specific cases like Zone 22/Class II (dust) the purge process is cannot be carried out as it will result in increase of explosive dust. As an alternative, the housing is manually cleaned.
Advantages of Purge/pressurization over other processes
Purge/Pressurization systems bring in a lot of advantages when compared to other methods. The most important of all is that the method makes use of general purpose equipments which are easily available. The time consumed is much lesser in terms of design and assembly. They are not confined within boundaries when it comes to applications. They can be used for both passive and active cooling elements. Certain simple customizations like white paint or sun shields will help in reducing the heat absorptions.
For these enclosures corrosion free material are easily available. The purge and pressurization also aids in improving the access and indication status. Compared to most other methods the flexibility aspect of it is very high. They are of low cost and light weight when compared to explosion proof counter parts.
There are four parameters that come into play while zeroing down to the appropriate purge system.
- Area classification
- Equipment rating inside the enclosure
- Door position, enclosure size, enclosure type, and windows
- The requirement of power of the equipment
The Parameters In Detail
The requirement of the purge system is determined by the area classification. For example for zone 1/ division 1 the requirement is either Ex px/ Type X system (general purpose equipment) or Ex py/ Type Y system (division 2/zone 2 rated) .
The equipment rating is an integral parameter that has to be taken into consideration while deciding on the purge systems to use. If zone 1/ division 1 area contains a minimum of one general purpose equipment, it requires an Ex px/type x system. If the rating of the devices are for zone 2/division 2 then an ideal choice would be Ex py/ Type Y system. There are a few special conditions which involves flammable gases in enclosures like chromatographers and gas analyzers.
For analyzers that are taking hazardous gases or liquids an extra consideration needs to go into it. Taking into account of a potential leak of hazardous material you can opt for two options. The first is dilution and the other is use of inert gases. There are certain conditions that might demand zone 2/division 2 area to opt for Ex px/Type X system majorly due to the leakage.
The size of the enclosure cannot be taken on a lighter note as it holds a paramount importance when it comes to deciding on the purge system. The free volume within the enclosure is the major factor that determines the size of the enclosure. In most cases the equipment mounted inside the closure cannot be included to the free volume. The purge time that is based on 4 volume changes for N.A standards requires the volume of enclosure to determine it. If you take a motor as an example, the purge requirement for N. A. , IEC and EN standards is 10.
The control unit operates the power for Ex px/Type X systems. In case of the power requirement exceeding beyond the contact rating addition of control relay will do the trick. Make sure that it is rated for hazardous area if the relay is located in hazardous area. The enclosure power can be disconnected if there is a drop of purging below the minimum value. A connection can be established if the enclosure is pressurized.
For Ex pz, py/Type Z, Y systems there is no requirement for the power to get disconnected provided an alarm is activating the pressure loss.
Methods of Purge and Pressurization
There are two methods when it comes to purge and pressurization process. They are
- Continues Flow
- Leakage Compensation
Continues Flow systems are predominantly used in small enclosures. To be more precise the area should be less than 0.4 cubic meters or 17 cubic feet. These continues flow systems pose a considerable number of advantages. The most important being the operation simplicity. Based on the convenience and requirement we can also choose between automatic or manual. The initial cost that includes purchase and installation cost less as it has a very simplified operation. But the flip side of it is that since there is constant consumption of compressed air. This constant functioning results higher operation cost. The flow rate is low at a very high purge time on large enclosures.
The majority of the users consider 30 minutes or less than that is an appropriate purge time. This is one of the important aspects that make leakage consumption apt for all sizes of enclosures. Unlike continues flow systems they consume very less compressed air which results in less operating cost. Within a reasonable time large enclosures can be purged, thanks to initial high purge flow rate. The downside to it is that, the operation is a bit complex and the initial cost is very high when compared to the continues flow.
Be is leakage compensation or continues flow, there are two major components that are involved in the purge and pressurization process. They are
- Control Unit
- Relief Valve
To put in simple words it is the entity that keeps track of the flow and pressure. The A dry contact signal is supplied by the control unit. This signal shows whether the enclosure pressure is at appropriate levels. Specifically for type X systems a fully automatic purging controller and automatic electrical power disconnect.
A relief valve is the one that is fitted to the enclosure that limits the pressure exceeding the prescribed levels in the enclosure. All of the relief valves have a built in spark arrester which prevents the sparks being ejected.
The Continues Flow purge and pressurization systems include one more component called the outlet orifice.
The outlet orifice is pre calibrated in such a way that the pressure drop is known at a particular flow rate. This component helps in maintaining the pressure in the enclosure with help of sensors by providing adequate flow. It also has a spark arrestor attached to it.