Vertical steel tanks, known to everyone under the abbreviation “VST”, are used to store completely different liquids. In this document, we will consider one of the requests to the ZANDZ Technical Center with a question about the lightning protection of a water tank designed to store water for fire fighting at the facility.
ZANDZ Technical Center experts have proposed the following solution.
Object: vertical fire tank.
Height: 10.7 m
Diameter: 9 m
Ground: concrete foundation
Region: Samara region
pecific resistance of the soil used for calculations – 100 Ohm * m.
calculate external lightning protection system;
calculate grounding device.
Vertical fire tanks are conventional from the point of view of lightning protection in accordance with IS 153-34.21.122-2003 and refer to the 3rd category according to AD 34.21.122-87. Required reliability — 0.9.
Fire tanks are protected against lightning by lightning rods. A lightning rod is a device rising above the facility that diverts lightning current into the ground without entering the facility. It comprises a lightning rod that directly receives the lightning discharge, a down conductor and a ground electrode.
The set of measures to provide the required lightning protection is offered by the following solutions:
- installation of 4 lightning rods-masts with the height of 4 m by welding (see.figure 1);
- arrangement of down conductors with the use of copper-bonded wire D = 8 mm. Lightning rods are interconnected to organize at least two current outlets. Arrangement of equipment in Figure 1. All metal communications must be connected to down conductors;
- installation of a grounding device, consisting of four vertical electrodes 3 m long, united by a horizontal electrode (copper-plated wire with a diameter of 10 mm) along the contour around the object. The distance from the horizontal electrode to the facility shall not be less than 1 m, embedded 0.5 - 0.7 meters deep.
Grounding device resistance calculation:
The design resistance of the grounding device is 5,2 Ohm.
Results of the calculation carried out with the software developed by the OJSC "Energy Institute to the name of G.M. Krzhizhanovsky (JSC "ENIN"):
The density of lightning discharges into the ground – 4 strikes / sq. km per year
Full number of strikes into the system – 0.028 (once in 36 years);
The total number of breakthroughs is 0.00017 (every 5882 years)
System reliability - 0,9939
Probability of a breakthrough into all the system objects - 0.0061.
Figure 1 shows the layout of equipment and protection zone.
The list of required material is given in Table 1.
Figure 1 - Location of lightning protection equipment and grounding for vertical fire tanks. Protection zone.
Молниеприемник мачта высотой 4 м – Air terminal mast 4 m high
Проволока омедненная стальная диаметров 8 мм – copper-bonded steel wire 8 mm in diameter
Полоса омедненная стальная сечением – Copper-bonded steel tape with the cross section of 30*4 mm
Электрод - electrode
Table 1 - List of material needs.
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