A typical oil and gas process skid package will incorporate only the process equipment. This is primarily due to the requirement to classify the immediate area surrounding the process equipment for hazardous area classification purposes. Depending on the probability of a flammable release, the duration of a release and the level of ventilation available, a process skid may be classified Class I division 1 or Class I Division 2 using API RP 500 as a basis for design, or Zone 1 or Zone 2 using API RP 505 or IEC 60079-10 as a design reference.
Once the process module has been classified as a hazardous location,the electrical and instrumentation equipment must be certified for location. Hazardous location certified equipment is typically more costly and often of limited availability. For this reason, the majority of the electrical and control equipment is located off-skid in a non-hazardous location with only the end device sensors located on the process skid. This results in a large number of wiring interconnections that must be completed in the field.
Integrated Modular Process Package Concept
The integrated modular design package concept incorporates all process, instrument, electrical and control systems on a common skid. To install non-hazardous location certified utility, electrical and control equipment on a process skid handling flammable materials, a '' safe'' area must be created. The design of the safe area is part of the hazardous area classification design for the skid package. This can be accomplished in one of three ways.
1. Physically separate the non-hazardous location from the hazardous location on a common skid.
2. Create a positive pressure zone within the hazardous location to prevent the migration of flammable gas/vapours into the non-hazardous location.
3. Incorporate the use of a physical barrier to segregate the hazardous location from the non-hazardous location.
A Option 1- Physical Separation
Option 1 separates the hazardous location from the non-hazardous location by distance. A building incorporating the electrical and control equipment is located outside the classified area on a common skid. The distance between the buildings and the process equipment is determined by the extent of area classification design for process equipment. This is a function of the process parameters and vapor density of the flammable materials and the level of ventilation within the immediate area
The primary disadvantage of using distance as means to separate a hazardous from the non-hazardous location is the size of overall skid package is increased impacting the size and weight of the shipping envelope.
Option 2 - Pressurized Control Building within a Classified area.
Option 2 involves creating a non-hazardous location within a process unit by using a positive air purge. An enclosed building is pressurized to a value slightly beyond normal ambient pressure to prevent any flammable fugitive emissions from entering the designated non-hazardous location.
The basic requirements for a purged room or building as defined by IEC 60079-13 are as follows:
1. The positive pressure air system must provide 25pa (0.1 inch WC) of pressure with 50% of all outlets opened. The minimum air velocity through an open door is 0.3 m/s. This requirement may require the use of an air lock entry system.
2. The purge system must be monitored by a pressure or flow switch. If flow detection is not possible at the outlet, gas detectors shall be used to ensure that the air within the room is at or below 25% of the lower flammable limit.
3. All safety devices, the pressurization system and its controls and the electrical isolation means, system fan and motor shall be suitable for the area external to the room unless located in a non-hazardous area.
4. The purge volume for pressurization and for general artificial ventilation shall be minimum of 10 room volumes. The purging flow rate is required to be minimum of 5 air changes per hour.
5. In the event of an interruption to the purge system, a procedure is required to prevent equipment from becoming potential ignition sources.
6. The pressurization system is required to energized independently from the electrical power to the room.
7. The entry of cables, electrical conduits and other penetrations into the room is required to be sealed so that the necessary pressure differential or ventilation integrity can be maintained.
8. Personnel access doors are required to be automatically self-closing, close fitting and designed to close and latch against the normal pressure differential. Restricted access doors are not required to be self-closing.
The building envelope penetrations require careful consideration. They must facilitate the installation of piping and cables and must maintain the structural and fire integrity of the wall, the physical barrier or pressurized building requirements and allow for future expansion. A mechanical transit barrier design meets these requirements and incorporates innovations to reduce installation costs and facilitate future expansion.
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