Section 6 Ventilation

6.1.1 Mechanical ventilation systems are to be capable of continuous operation by the provision of adequate standby/redundancy capable of maintaining the required flow rates and pressure differentials. Machinery spaces are, where practicable, to be served by redundant air intake ducts.

6.1.2 Open or semi-enclosed spaces which are designed to be ventilated by natural means are to achieve a minimum of 12 air changes per hour for 95 per cent of the time. This natural ventilation may be augmented by mechanical means.

6.1.3 Non-hazardous enclosed spaces are to be maintained with an overpressure of at least 50 Pa in relation to any adjacent more hazardous areas or spaces. The non-hazardous area ventilation with positive pressurisation is to be designed to help mitigate against potential gas ingress to the non hazardous area so that where there is any doorway, hatch or other opening in the contiguous boundary, the ventilation helps to prevent the transmission of fluids from the more hazardous area or space to the less hazardous space.

6.1.4 Accommodation spaces are to be maintained at a positive pressure in relation to the outside atmosphere.

6.1.5 Ventilation services to areas for pipe-laying operations (e.g. pipe welding and coating, as defined in Pt 3, Ch 17, 1.1 Application 1.1.1) or drilling utilities areas and to wellhead areas should, where practicable, be separate from services to other hazardous areas.

6.1.6 Air supplied for combustion and/or cooling of engines or other fuel-burning equipment is to be supplied separately from general ventilation services. The ventilation system for engine or boiler rooms is to be independent of all other ventilation systems. Induced draught fans, or a closed system of forced draught may be employed for fired equipment, or the fired equipment may be enclosed in a pressurised air casing.

6.1.7 System design is to be arranged for individual isolation to enable continuity of operation and purging of spaces following contamination.

6.1.8 The system design is to take due regard to the possible weathervaning of the unit and periods when the current is the prevailing factor, such that the air intake, at low wind speeds, may be partially starved of air.

6.1.9 Ducting materials, including associated fittings, are to be of a non-combustible material, to be of all welded construction adequate to withstand likely damage and corrosion and to be suitable for a marine saline atmosphere. Ventilation fans are to have non-overloading, non-stall characteristics and are to be fitted with anti-sparking tracks.

6.1.10 For aspects of ventilation systems relating to fire integrity, see Pt 7, Ch 3 Fire Safety, and for gas detection requirements, see Pt 7, Ch 1, 5 Protection against gas ingress into safe areas.

6.2.1 Ventilation systems and ducting for spaces designated as hazardous areas are to be entirely separate from ventilation systems and ducting for spaces designated as non-hazardous areas.

6.2.2 All enclosed hazardous spaces are to be adequately ventilated by a mechanical ventilation system providing at least 12 air changes per hour. Air change calculations are to be based upon empty volume of space. The mechanical ventilation is to be such that hazardous enclosed spaces are maintained with an underpressure of at least 50 Pa in relation to any adjacent less hazardous areas or spaces.

6.2.3 To ensure that the required relative underpressure is maintained in any hazardous enclosed space, the supply and exhaust fans are to be interlocked so that the supply fan cannot be run unless the exhaust fan is running.

6.2.4 Ventilation arrangements should ensure that the entire space is adequately ventilated, giving an even air distribution, with special consideration to locations where there is equipment which may release gas, and to locations within the space where stagnant pockets of gas could accumulate.

6.2.5 Electric heating elements are to be fitted with automatic temperature control, a high temperature alarm and an independent sensor and cut-out with manual reset. The surface temperature is to be restricted to a maximum of 200°C, or below the ignition temperature of any flammable gas likely to be present in the area.

6.2.6 The presence of gas within the enclosed hazardous area and/or the ventilation system air extracts from this area is not to initiate the shut-down of the area’s ventilation system as this will result in the build-up of hazardous gas in this area. In these circumstances all ventilation equipment must be rated to operate in a Zone 1 hazardous area.

6.3.1 Ventilation systems and ducting for any space containing a source of release of a flammable substance, but not designated as a hazardous space in its entirety (e.g. by virtue of compliance with Pt 7, Ch 2, 1.2 Definitions and categories 1.2.5), are to be entirely segregated from ventilation systems and ducting for other non-hazardous areas or spaces.

6.3.2 The mechanical ventilation is to be such that the space and ducting serving it is maintained at an underpressure of at least 50 Pa in relation to adjacent non-hazardous areas or spaces.

6.3.3 Where the ventilation air flow rate within the space in relation to the maximum release rate of flammable substances reasonably to be expected under normal operating conditions is sufficient to prevent any concentration of flammable substances approaching their lower explosive limit, consideration will be given to regarding the entire space, including the zone around equipment contained within it, its ventilation systems and other openings into it, as non-hazardous. Ventilation airflow is to be monitored and appropriate measures taken in the event of failure. For requirements particular to gas turbine rooms and hoods, see Pt 7, Ch 2, 6.5 Gas turbine ventilation.

6.3.4 The presence of gas within the enclosed hazardous area and/or the ventilation system air extracts from this area is not to initiate the shut-down of the area’s ventilation system as this will result in the build-up of hazardous gas in this area. In these circumstances all ventilation equipment must be rated to operate in a Zone 1 hazardous area.

6.4.1 Supply air intakes are to be located in external non-hazardous areas, at least 3 m from the boundary of any hazardous area.

6.4.2 The siting of supply air intakes should be such as to avoid the possibility of drawing in combustion products from equipment exhausts or hazardous/toxic gases from process equipment.

6.4.3 Ventilation intake and outlet ducts should not pass through spaces of different classification. Where this is unavoidable, ducts may pass through a more hazardous space than the ventilated space provided; such ducts have an overpressure in relation to the space through which they pass. Where necessary, ducts should be of welded, gastight construction. The internal space of such ducts is to have the same zone classification as the ventilated space.

6.4.4 Ventilation outlets are, as far as is practicable, to be located in external areas of the same or lesser zone classification as the ventilated space. Where this is not practicable, appropriate measures are to be taken to prevent backflow into the ventilated space, in the event of ventilation failure.

6.4.5 The separation between air intakes and outlets should be at least 4,5 m. The siting of inlets and outlets should be such as to avoid the possibility of cross-contamination.

6.4.6 Ventilation intakes and outlets are to be located and arranged to avoid ingress of rain, snow and sea-water, even under predicted worst storm conditions.

6.4.7 Gas turbine intakes and exhausts are to be positioned well clear of the unit’s structure. Turbine exhausts are to be safely located, so as not to endanger personnel or interfere with helicopter operations.

6.4.8 Where practicable, ventilation outlets from non-hazardous areas should not discharge into a hazardous area.

6.4.9 Air intakes for internal combustion engines (unless certified for use in a Zone 2 hazardous area), fired boilers and other fired units are to be located at least 3 m from hazardous areas.

6.5.1 The turbine room is to be designed as a non-hazardous space, mechanically ventilated with at least 12 air changes per hour and arranged so that an overpressure of at least 50 Pa is maintained in relation to the turbine hood.

6.5.2 The turbine hood is to be mechanically ventilated by means of one duty and one 100 per cent stand-by extraction fan with a ventilation rate to remove adequately heat from the turbine and equipment, and to dilute any flammable gas. Potential leakage from under the turbine hood is to be considered. The ventilation rate is to be at least 12 air changes per hour and arranged so that an underpressure of at least 50 Pa is maintained in relation to the turbine room. On failure of the duty fan, an alarm is to be given in the control room and the stand-by fan automatically activated.

6.5.3 Provided it can be shown that no exposed surface of the turbine or equipment inside the hood will have a surface temperature exceeding 200°C, or that the surface temperature will not exceed 80 per cent of the auto-ignition temperature of any flammable gas which may be present, the ventilation rate may be as per Pt 7, Ch 2, 6.5 Gas turbine ventilation 6.5.2 where the turbine is in operation. Under these conditions, the space inside the hood will be classified as a Zone 2 hazardous area.

6.5.4 Where the surface temperature of the turbine or equipment inside the hood could exceed 80 per cent of the auto-ignition temperature of any flammable gas which may be present, the space inside the hood is to be ventilated with at least 90 air changes per hour. Under these conditions, the turbine hood need not be classified as a hazardous area when the turbine is in operation.

6.5.5 The turbine hood ventilation fans referred to in Pt 7, Ch 2, 6.5 Gas turbine ventilation 6.5.2 are to be interlocked with the turbine starting sequence, to provide at least five air changes in the turbine hood before start up of the turbine or the energising of any associated electrical equipment, other than that suitable for installation in a Zone 1 location. On shut-down, the duty fan is to purge the turbine hood until the turbine has stopped. At least one of the fans is to be supplied from an emergency power source. See also Pt 6, Ch 2, 3.7 Alternative sources of emergency electrical power 3.7.9.

6.5.6 Equipment which is required to remain activated after shut-down or hood ventilation failure, is to be certified for use in a Zone 1 hazardous area.

6.5.7 Gas detectors are to be installed inside the turbine hood to shut down the turbine on detection of gas.

6.5.8 For gas turbines utilising gas fuel from the production and process facility, the arrangement and capacities of the ventilation system and fuel gas piping are to comply, where applicable, with the requirements of Pt 5, Ch 16 Gas and Crude Oil Burning Systems, taking into account any additional requirements which may be necessary during start-up or shut-down of the plant.