4.2　Location of Outdoor Units

4.2.1　For outdoor units in petrochemical plant，lightning protection shall be designed for any one of the following cases:

1　Large and tall process equipment installed on grade.

2　Process equipment installed on/in structure or support at height and pipes to flare.

3　Machinery equipment such as large size compressor and grouped pumps installed on grade.

4　Flare，chimney and vent stack located in open fields.

5　Lighting fixtures installed at height that are vulnerable to direct lightning flash.

4.2.2　For outdoor units in petrochemical plant，lightning protection may not be designed for any one of the following cases:

1　Non-grouped water treatment facilities（except for key equipment）constructed in open fields.

2　Non-grouped pumps and small size metal equipment installed on grade.

3　Pipes and pipe racks installed on grade.

4.2.3　Metal body of process equipment should be served as the air terminal against direct lightning flash；however，the following requirements shall be satisfied:

1　Process equipment served as air terminal shall be integrally enclosed and welded metal static equipment，and machinery equipment shall not be used as air terminal.

2　Process equipment served as air terminal shall have a metal enclosure，and its top and upper outer section vulnerable to direct lightning flash shall have sufficient thickness.Wall thickness of steel equipment shall be larger than or equal to 4mm，and wall thickness of other metal equipment shall comply with the thickness value，t，indicated in Table 6.1.5 of this Code.

4.2.4　Additional air terminal shall be provided for the following equipment that is vulnerable to direct lightning flash and located beyond the protection zone afforded by adjacent tall process equipment，structures and large-size pipe racks（already served as air terminal）:

1　Machinery equipment；

2　Metal static equipment that is not served as air terminal；

3　Static equipment with non-metal enclosure.

4.2.5　Protection zone afforded by air terminal shall be determined using one the following methods:

1　Protection zone shall be determined by rolling sphere method as specified in the current national standard GB 50057 Design Code for Protection of Structures against Lightning，the radius of rolling sphere is taken as 45m；

2　The elevation difference between the top of air terminal and the reference plane for protection and the protection angle shall comply with Table 4.2.5 of this Code or the current national standard GB/T 21714.3 Protection against Lightning-Part 3:Physical Damage to Structures and Life Hazard.

4.2.6　Down-conductor against direct lightning flash shall comply with the following requirements:

1　Metal enclosure of large and tall process equipment installed on grade shall be served as downconductor；

2　When process equipment is installed in/on structure or support，the metal framework should be serviced as down-conductor；

3　No fewer than two down-conductors shall be used for large and tall furnace，tower，silo，large-size equipment and structure，and the spacing of down-conductors shall not be larger than 18m；

4　Down-conductors shall be provided at both ends of long horizontal vessel and pipes（pipes to flare）installed at height，and if the spacing of down-conductors exceeds 18m，additional down-conductor（s）shall be provided；

5　Down-conductor shall be directly connected to earth electrode as straight and short as practical，and shall have sufficient cross-sectional area and thickness，mechanical protection shall be provided for the section of down-conductor above grade；

6　If the main longitudinal reinforcement in column is served as down-conductor，it shall be hooped or welded.

4.2.7　Measures against lightning induction shall comply with the following requirements:

1　For location of outdoor unit，all metal equipment，frameworks，pipes，cable protection shields（cable armor，steel conduit and cable tray）and vent nozzles shall be all connected to earth-termination system against lightning induction.If special down-conductors are provided，down-conductors shall be led from the reinforcing bars in reinforced concrete column at the highest top extremity and near to grade，and then connected to earth conductor（grid）.

2　The separation distance from the metal objects described in Item 1 of this Article to adjacent down-conductors shall be calculated according to the following formula:

where:

S—separated distance，m；

kc—partitioning coefficient，it is taken as 1 for single down-conductor，0.66 for two down-conductors and multiple down-conductors with air terminals that are not connected to a closed loop，and 0.44 for multiple down-conductors with air terminals that are connected to a closed loop；

lx—length from the calculated point of down-conductor to the earthing connection point，m.

3　If the separation distance indicated in Item 2 of this Article is not satisfied，additional earth conductor shall be provided at a location higher than the connection point.

4　For metal pipes，structures and metal cable shields arranged in parallel，if the clearance between them is less than 100mm，metal bonding shall be made in every 30m，and if the clearance at cross-run or adjacent location is less than 100mm，metal bonding shall be also made.

4.2.8　Earth-termination system for lightning protection shall comply with the following requirements:

1　For process equipment where its metal enclosure is used as air terminal，no fewer than two earthing points shall be provided at the base of metal enclosure for connecting to earth electrodes.

2　Additional air terminals（rods，catenary wires and meshed conductors）as required in Article 4.2.4 of this Code shall be all provided with down-conductors for directly connecting to earth electrode.

3　For the earth electrode to which each down-conductor against direct lightning flash is directly connected，it shall have an impulse earth resistance not larger than 10Ω，and shall comply with the following requirements:

1）For calculating ground resistance，the length of branch conductor at each earth electrode shall be less than or equal to the effective length of earth electrode，le；

2）le calculation and conversion of impulse earth resistance shall comply with the current national standard GB 50057 Design Code for Protection of Structures against Lightning.

4　The power frequency earth resistance of earth electrode against lightning induction shall not be larger than 30Ω.

5　Earth electrode against direct lightning flash should be connected with the earth electrode against lightning induction and the earth electrode used for electrical equipment to form a complete earth system.However，the earth resistance of earth electrode against direct lightning flash shall meet the requirements specified in Item 3 of this Article.