Lightning protection and grounding project of a gas-distributing plant

Lightning protection and grounding project of a closed stationary gas-distributing plant

Task:

Object: closed stationary gas-distribution plant 24х6х6,75 m (LxWxH)
Soil resisitivity: 100 Ohm*m

It is necessary to make calculations and create lightning protection and grounding project with the resistance of 10 Ohm.

Solution

In accordance with the AD 34.21.122-87, gas distribution plant belongs to category II of lightning protection and must be protected from direct lightning strikes, its secondary effects and import of high potential via on-ground (above-ground) metal communications. Protection zone B is chosen in accordance with p.2 of the chart 1 of the AD 34.21.122-87 and the expected number of lightning strikes per year determined below. In accordance with p.2.6 AD 34.21.122-87, if there are straight gas outlet pipes and respiratory pipes for a free disposal of gases, vapours and explosive suspensions into the atmosphere, there must be a space above the pipe cut limited by the hemisphere with the radius of 5 m to in the protection zone of lightning rods. For gas outlet and respiratory pipes, equipped with boots or air taps, there should be a space above the pipe cut in the protection zone of lightning rods. This space should be limited by the cylinder with H heigh and R radius for gases lighter that air at the excessive pressure inside the installation more than 25,25 KPa H=5 m, R=5 m.

Protection of secondary effects of lightning and import of high potential

Protection against secondary effects of lightning and import of high potential is carried out by connecting metal constructions of gas distribution plant columns to the grounding device.

Expected number of lightning strikes into the object per year is defined by the formula:

N=[(S+6h)(L+6h)-7,7h²]•n•10^-6,
where h=6,75 m - the highest point of the object;
L=24 m - object length;
S=6 m - object width;
n - density of lightning strikes per 1 square km² of ground serface a year
n=6,7•Т_гр/100,
where T_гр - average duration of a thunderstorm in hours.

For Moscow region, the average duration of thunderstorms is 20-40 hours. Consequently:
N=[(S+6h)(L+6h)-7,7h2]•n•10-6≈0,05296
The expected number of lightning strikes into the obect is 0,05296 lightning strikes per year.

Protection of the object against direct lightning strikes

Protection of the object against direct lightning strikes is carried out with the help of ZANDZ lightning rod 18 m long, having the following dimensions:

h_o = 0,92*h = 0,92*18 = 16,56 м — height of protection zone cone,
where h=18 m — lightning rod height;
r_o = 1,5*h = 1,5*18 = 27 m — cone radius on the ground level;
r_x = 1,5*(h-h_x/0,92) = 1,5*(18-6,75/0,92) = 16,0 м — высота конуса на уровне защищаемого объекта.
Boundaries of protection zones, see the layouts of the present project.

Grounding device of ga-distribution pant consists of 3 vertical electrodes (copper-bonded threaded rods D14, 1.5m ZZ-001-065 - 2 pieces per 1 electrode) and copper-bonded tape 30x4 GL-11075, connected with the help of a clamp for connecting conductor ZZ-005-064 in accordance with GOST 21130-75 (see the complete specification and links to the description of equipment below).

Carry out hydroinsulation for bolt connections and foresee the possibility of inspection of connections at any time. Connect the grounding device to the lightning rod with the help of welding. The grounding device is laid at the depth of 0,5 m from the ground surface at the distance of 1 m from the lightning rod.

According to paragraph 3.2.2.5 IS 153-34.21.122-2003 , the own structure of vertical lightning rod 18 m high (galvanized steel with embedded parts under the foundation) can be used as a down conductor.

Electric continuity between the different parts is provided for a long term. The grounding device is organized at the nearest territory outside the protective zone of underground communications, its precise location is defined when building it.

Arrangement of the external grounding device, see the layouts of the present project. Carry out installation of lightning protection and grounding according to EIC, ed. 7, AD 34.21.122-87 "Instructions for lightning protection of buildings and structures" and IS 153-34.21.122-2003 "Instructions for lightning protection of buildings, structures and industrial communications", GOST 21130-75 "Electrotechnical products. Grounding clamps and signs. Design and dimensions".

Calculation of grounding device resistance:

Vertical electrode resistance:

formula-1

where p - soil resistivity, ohm-m;
L - vertical electrode length, m;
d - vertical electrode diameter, m;
T - burial - the distance from the ground surface to the ground electrode, m;

Horizontal electrode resistance:

formula-2

where ρ- soil resistivity, Ohm* m;
b - horizontal electrode tape width, m;
L_гор - horizontal electrode length, m.

formula-3

where t - deepening of the electrode top, m

Impedance of the grounding device:

formula-4

where n - number of kits;
K_исп - utilization ratio.

formula-5

formula-6

formula-7

The design resistance of the grounding device is 5.88 ohms, which is less than the permissible resistance of 10 ohms.

The list of required materials:

№ п/п	Fig.	Product item	Name	Quantity	Weight, unit, kg
1.		ZZ-001-065	ZANDZ Copper-bonded threaded grounding rod (D14; 1,5 m)	6	1,9
2.		ZZ-002-061	ZANDZ Threaded coupler	4	0,08
3.		GL-11703A	ZANDZ Starting tip	3	0,07
4.		ZZ-004-060	ZANDZ Driving head for a breaker hammer	2	0,09
5.		ZZ-005-064	ZANDZ Clamp for connecting conductor (up to 40 mm)	5	0,31
6.		ZZ-006-000	ZANDZ Conductive grease	1	0,19
7.		ZZ-007-030	ZANDZ Water-proof tape	3	0,442
8.		ZZ-008-000	ZANDZ Driving head for a breaker hammer (SDS max)	1	0,48
9.		GL-11402	GALMAR Inspection pit	3	2,6
10.		GL-11075	GALMAR copper bonded tape 30x4 mm	30	0,98 (in meters)
11.		ZZ-201-018	ZANDZ Vertical lightning rod 18m (galvanized steel;. with embedded details under the foundation)	1	-