Gas compressors and blowers should, wherever practicable, be installed in the open air or in an open sided structure beneath a roof canopy.
Where gas compressors or blowers are installed in enclosed spaces (e.g. acoustic enclosures), ventilation requirements should be confirmed by the compressor supplier. These may need to be increased if considered necessary for area classification requirements as described below. Minimum requirements recommended by the Institution of Gas Engineers Publication IGE/UP/2 - Gas Installation Pipework, Boosters and Compressors on Industrial and Commercial Premises (38), comprise the following for natural ventilation of enclosed spaces:
• ventilation openings to the atmosphere should be provided at high and low levels;
• all high level, ventilation should be located at roof level, or as near to the roof as practicable;
• the total ventilation area should be equally disposed about the enclosed area with all openings at least 3 metres from any external source of ignition hazard; • the total effective ventilation area should be not less than 2 percent of the floor
area of the enclosed area;
• mechanical ventilation systems shall be fully interlocked with machine operation; and,
• when noise levels necessitate attenuation of the ventilators, such attenuation shall not reduce the effectiveness of the ventilators. The acoustic attenuation of all ventilation items must, also, be effective.
Gas compressor rooms or enclosures located within a larger building, should be fire resistant, with at least 30 minutes fire resistance or in accordance with Building Regulations.
7.16.1 Gas Compressors and Blowers Installed in the Open Air
This section applies to compressors and blowers in service on sludge gas (digester gas). For compressors on vaporised LPG see Section 7.19.
Compressors and blowers should be installed in the open air or in an open sided structure with only overhead weather protection provided by a roof with which is ventilated at its high point. (Note: the roof is not required for health and safety purposes and could be omitted if other factors allow). Guidance on compressors installed within acoustic enclosures is given in the next section.
Gas compressors and blowers are classified as Zone 1 internally. Secondary releases may occur due to leaks in compressor glands, seals and joints. A leak on the shaft seal on the compressor/blower would represent the worst case secondary release. Therefore for both rotary and reciprocating compressors the installation should be classified externally as shown in Figures 7.15 and 7.16. The Zone 2 distance ‘x’ around the periphery of the centrifugal/reciprocating compressor is dependant on shaft diameter and discharge pressure of the compressor (see Table 7.10 for Zone 2 distances on open air).
Table 7.10 Zone 2 Distances for Centrifugal/Reciprocating Compressors in Open Air
Shaft Diameter SD (mm) 10 15 25 40 50 80 100 Leak Area (mm2) = 16.5 24.3 40.1 63.6 79.3 126.4 157.9 Radius (m) at 100mbarg 0.6 0.7 0.9 1.1 1.3 1.6 1.8 Radius (m) at 200mbarg 0.6 0.7 0.9 1.2 1.3 1.6 1.8 Radius (m) at 500mbarg 0.7 0.8 1.0 1.3 1.5 1.8 2.1 Radius (m) at 1barg 0.8 0.9 1.2 1.5 1.7 2.1 2.4 Radius (m) at 2barg 0.9 1.1 1.5 1.8 2.1 2.6 2.9 Radius (m) at 4barg 1.2 1.5 1.9 2.4 2.7 3.3 3.7 Radius (m) at 5barg 1.3 1.6 2.1 2.6 2.9 3.7 4.1
Leak area calculation -
([
]
)
2(
)
22 / 2 / SD d SD π
π + − (from Area Classification for Landfill Gas Extraction,
Utilisation and Combustion; ESA ICoP 2, edition 1:Nov. 2005). d = leak diameter (assumed to be 1mm)
Zone 2 distances in open air are based on dispersion of a pressurised jet of gas in air to below the LEL.
Diaphragm compressors shall be classified as Zone 2 above with distance ‘x’ 3.0m around the periphery of the compressor. Note: Figures 7.15 and 7.16 do not include the zones associated with the gas pipework - reference should be made to Section 7.14.2
Blower installations should, also, be classified as shown in Figures 7.15 and 7.16. Any relief valves or drain vents on gas duty, including vents from any crankcase seals and lubricating oil system must discharge to atmosphere at a safe location. Such vents will have a zoned area at their discharge point to atmosphere. The classification will depend on the continuity of discharge as given in the zone definitions in Section 5.3. The zone will be a sphere centred on the discharge point having a radius as follows:
Table 7.11 Discharge Radius from Compressor Vents
Vent Rate at Ambient Conditions (m3/hour)
Radius (m)
Up to 10 1.5
Drains should be classified by reference to Section 7.14.2.
Figure 7.15. Zoning for Compressors and Blowers Beneath a Canopy in the Open Air.
Figure 7.16. Zoning for Compressors and Blowers in the Open Air.
7.16.2 Enclosed Gas Compressors and Blowers
Although it is preferred that compressors or blowers are installed in the open it is often necessary to enclose a compressor or blower installation, for instance to provide noise attenuation. When it is necessary to install a gas compressor or blower in an enclosed space, such as an acoustic enclosure or inside a room or building, secure proven ventilation and gas detection must be provided. Secure ventilation must be powered by an electrical source independent of a CHP engine, whilst proven ventilation requires a device to directly monitor the air flow. Where explosion relief is also provided, the sizing of such explosion relief must be supported by appropriate calculations provided by competent specialists.
Where the minimum ventilation rates specified in Table 7.12 are provided within the enclosure for the removal of flammable gases, the enclosure shall be classified as Zone
2 inside. Ventilation rates may be greater than this if required for other duties such as cooling.
Table 7.12 Minimum Ventilation Rates (m3/s) for Enclosed Centrifugal/Reciprocating
Compressors Shaft Diameter SD (mm) 10 15 25 40 50 80 100 Leak Area (mm2) = 16.5 24.3 40.1 63.6 79.3 126.4 157.9 MVR (m3/s) at 100mbarg 0.05 0.08 0.13 0.20 0.25 0.40 0.50 MVR (m3/s) at 200mbarg 0.07 0.11 0.18 0.28 0.35 0.56 0.70 MVR (m3/s) at 500mbarg 0.12 0.17 0.28 0.45 0.56 0.89 1.11 MVR (m3/s) at 1barg 0.15 0.23 0.38 0.6 0.74 1.18 1.48 MVR (m3/s) at 2barg 0.26 0.38 0.63 1.00 1.25 2.00 2.49 MVR (m3/s) at 4barg 0.49 0.72 1.19 1.89 2.36 3.76 4.68 MVR (m3/s) at 5barg 0.61 0.90 1.49 2.36 2.94 4.68 5.85
If the minimum ventilation rate is provided by mechanical means, then the ventilation system should operate whenever the gas compressor is operating. Air flow monitoring shall be provided that alarms on ventilation failure.
Where the minimum ventilation rate is not achieved within the enclosure or for mechanical ventilation where air flow monitoring is not provided, the enclosure shall be classified as in Figure 7.17.
Figure 7.17 Zoning of Enclosed Compressors and Blowers with Inadequate Ventilation or No Air Flow Monitoring
The Zone 2 distance ‘x’ around any source of continuous vent from the enclosure is dependant on shaft diameter and discharge pressure of the compressor (see Table 7.13 for Zone 2 distances).
Table 7.13 Zone 2 Distances for Enclosed Centrifugal/Reciprocating Compressors
Shaft Diameter SD (mm) 10 15 25 40 50 80 100 Leak Area (mm2) = 16.5 24.3 40.1 63.6 79.3 126.4 157.9 Radius (m) at 100mbarg 1.2 1.4 1.9 2.4 2.8 3.6 4.0 Radius (m) at 200mbarg 1.4 1.7 2.3 3.0 3.3 4.3 4.9 Radius (m) at 500mbarg 1.8 2.2 2.9 3.8 4.3 5.5 6.3 Radius (m) at 1barg 2.2 2.7 3.6 4.6 5.2 6.7 7.6 Radius (m) at 2barg 2.7 3.3 4.3 5.6 6.3 8.1 9.2 Radius (m) at 4barg 3.8 4.7 6.2 8.0 9.0 11.7 13.2 Radius (m) at 5barg 4.0 5.0 6.5 8.4 9.5 12.3 13.9
Leak area calculation - π
([
SD+d]
/2)
2−π(
SD/2)
2 (from Area Classification for Landfill Gas Extraction, Utilisation and Combustion; ESA ICoP 2 (47)).d = leak diameter (assumed to be 1mm)
Zone 2 distances based on low velocity release from enclosure opening with dispersion occurring at minimum wind speed of 0.5m/s.
A higher ventilation rate may be employed in the enclosure to achieve dilution ventilation (refer to Section A2.5.2), for the purpose of reducing the enclosure classification to Non-hazardous. Where dilution ventilation is employed in the enclosure, the adequacy of the ventilation must be confirmed by air flow monitoring, that automatically isolates the suction and discharge valves on the gas supply in the event that inadequate air flow is detected. Gas detection should be fitted within the enclosure to detect gas leaks and isolate the gas supply as above. Any gas detection system being used to initiate a gas supply shut-off as a means of eliminating a zonal classification inside the enclosure is deemed to be a Safety Instrumented System and therefore must comply with the requirements set out in Section 10.2.3. The use of a Safety Instrumented System must be approved by the Project Manager.
Gas detector settings should be as low as practicable without causing spurious alarms. Alarm settings should be no higher than 10% of the lower explosive limit (LEL) and trip settings no higher than 25% of LEL.
For small enclosures (no access except by removing panels), located in the open or in a building where dilution ventilation is being provided in the enclosure to achieve a Non- hazardous classification, then explosion relief panels shall be fitted to the enclosure. The sizing of such explosion relief must be supported by appropriate calculations provided by competent specialists.
For large enclosures, such as a room within a building, where dilution ventilation is provided to achieve a Non-hazardous classification, then explosion relief is not required but local gas detection providing alarm and interlocked with isolation of the gas supply must be installed.