3.2.6.1 Road tanker unloading
11 As described in 3.2.5.6.
12 Hazard radii distances due to small losses have been derived using Aeroplume dispersion model X m (Note 1)
X m (Note 1)
Figure 3.12(b) With Stage 2 vapour recovery installed
Figure 3.12 Typical composite hazardous area classification around a dispenser11 during refuelling
Note 1: Size of hazardous area equal to length of dispenser hose
3.2.6 Autogas12
3.2.6.1 Road tanker unloading
Road tankers delivering autogas to retail facilities should be assessed for hazardous area classification together with the provisions of safe entry of vehicles covered in 3.2.2 (e.g. in the event of a spillage of flammable materials at the site).
Whilst road tankers delivering liquid fuels are discharged under gravity at ambient conditions, autogas will be at its vapour pressure at ambient temperature and will therefore have to be pumped into the storage vessel but without the need for vapour recovery and without creating vapour discharges to atmosphere.
11 As described in 3.2.5.6.
12 Hazard radii distances due to small losses have been derived using Aeroplume dispersion model.
Delivery of autogas is typically from a rigid tanker with an onboard discharge pump.
Providing the pump is of a high integrity type, this will give rise to a Zone 2 hazardous area around the pump. In cases where an external pump is used for delivery, a Zone 2 hazardous area will be present around any hose connection points around the tanker and the pump will require classification at its fixed location. Further details of pump classification can be found in EI 15. It should be noted that road tankers used for autogas are highly variable in design and the site operator should ensure they are advised of the type of tanker likely to deliver to their site and be provided with a hazardous area classification drawing of the tanker which may be used. Hazardous area classification of the site will be incomplete without this.
The hazardous areas around the delivery area will need to take account of the hazardous areas created during unloading of autogas. This includes (but is not limited to) hazardous areas around hose reels, gauges, pumps and relief valves on the tanker. The integrity level of these fittings and components will impact upon any associated hazardous area.
3.2.6.2 Autogas storage1314
For autogas storage in buried storage vessels, typical classification is given in Figures 3.13(a) and (b). For cases where storage is above ground, typical classification is given in Figure 3.13(c).
Buried storage vessels will have vessel access chambers which should be classified as Zone 1 hazardous areas. Where connections are made (e.g. fill point or ullage level indicator operation), this will create a transient Zone 2 hazardous area of 1,5 m above ground around the access chamber during the unloading operation. Where fill points and ullage level indicators are offset (above ground), these will give rise to Zone 2 and Zone 1 hazardous areas respectively of 1,5 m15 radius, see Figure 3.13(c).
Relief valves with a soft seat, which are regularly maintained and tested, are not considered under the design relief condition for area classification purposes; however, fixed electrical equipment should not be installed within the direct path of discharge. To allow for small, infrequent leakages they should be classified with a Zone 2 hazardous area of 0,5 m radius.
Where other types of relief valves are fitted, this hazard radius should be increased to 2,5 m.
Provided autogas storage facilities are purged with nitrogen prior to filling and emptying, within the vessel the ullage should never contain a flammable atmosphere due to air, and therefore the ullage space can be considered as non-hazardous. Where nitrogen purging is not used, the ullage space within in the vessel should be classified as Zone 0.
Where it is necessary to classify single flanges they should have a Zone 2 hazardous area of extent appropriate to the integrity of the flange and process conditions encountered in the pipework, the extent of which should be at least 1m.
3.2.6.3 Autogas dispensers14
The design of autogas dispensers incorporates such safety measures as back-check valves, hose breakaway couplings (safebreaks) and other isolating valves to limit releases during normal operation to those associated with the filling hose. Dispenser hoses with self-sealing valves in the filling nozzle limit the loss of autogas on disconnection to 10 cm3. However, this can occur at any position between the dispenser and the full extent of the hose and is therefore classified as a Zone 2 hazardous area equivalent to the length of the hose, to a height of 1,2 m above ground (i.e. typical height to which the hose will be handled during the refuelling operation).
Dispensers with hydrostatic relief valves should only give rise to infrequent releases of autogas into the dispenser casing due to the valve lifting. Whilst this results in the inside of the dispenser casing being classified as Zone 1, the resultant hazardous area around any casing vents will be limited to a Zone 2 of 1 m16 radius around the apertures.
13 In addition to hazardous areas there is also a 'separation distance' around the storage vessel and the main components. For further information see UKLPG CoP 1 and 20.
14 Area classification is typical only. Site-specific conditions may lead to changes in the size and extent of hazardous areas.
15 Valid for losses up to 100 ml.
16 This is equivalent to a leakage of 0,01 kg/s of autogas. A 1,5 m radius is suitable for losses within the casing of autogas at 10 bar(a) of up to 0,015 kg/s.
DESIGN, CONSTRUCTION, MODIFICATION, MAINTENANCE AND DECOMMISSIONING OF FILLING STATIONS
With adequate low-level venting of the dispenser casing, autogas should not be vented from the upper part of the dispenser. However, to ensure uncertified electrical equipment is not mounted on the hydraulic part of the dispenser casing, a nominal Zone 2 of 150 mm should be applied surrounding the casing.
Dispensers with hydrostatic relief valves should only give rise to infrequent releases of autogas into the dispenser casing due to the valve lifting. Whilst this results in the inside of the dispenser casing being classified as Zone 1, the resultant hazardous area around any casing vents will be limited to a Zone 2 of 1 m16 radius around the apertures.
With adequate low-level venting of the dispenser casing, autogas should not be vented from the upper part of the dispenser. However, to ensure uncertified electrical equipment is not mounted on the hydraulic part of the dispenser casing, a nominal Zone 2 of 150 mm should be applied surrounding the casing.
(a) Buried
16This is equivalent to a leakage of 0,01 kg/s of autogas. A 1,5 m radius is suitable for losses within the casing of autogas at 10 bar(a) of up to 0,015 kg/s.
1,5m 1,5 m
Fill point
Ullage level indicator
Figure 3.13(a) Buried
(b) Buried with offset fill point and ullage level indicator
(c) Above ground
1,5m
1,5m Fill point Ullage level indicator
1,5m 1,5m
Fill point Ullage level indicator
0,5m Figure 3.13(b) Buried with offset fill point and ullage level indicator (b) Buried with offset fill point and ullage level indicator
(c) Above ground
Figure 3.14 - Typical hazardous area classification for autogas storage 1,5m
1,5m Fill point Ullage level indicator
1,5m 1,5m
Fill point Ullage level indicator
0,5m
Figure 3.13(c) Above ground
Figure 3.13 Typical hazardous area classification for autogas storage
Notes: 1. Area within dispenser casing defined by manufacturer.
2. Suitable for dispensers using hose lengths of up to 4 m.
Figure 3.15 - Typical hazardous area classification around an autogas dispenser during refuelling