Fire cause analysis and fire prevention design of cables in thermal power plants

I.
Overview

For many years, there has not been a set of feasible and effective fire prevention methods for cable fire prevention in power plants: in the past projects, the measures for cable fire prevention were relatively simple, and most of them only used fire doors in
tunnels.
Once the cable in the tunnel catches fire, the fire door is isolated to prevent the spread of the fire and reduce the loss
.
Because the contradiction between fire door and ventilation is not easy to solve, so although the design considers the fire door, and in order to ventilate in operation, it is generally difficult to close the door often, and the fire door is often in an open position during operation, once a certain section of the tunnel is on fire, and it is too late to close the fire door, in fact, the fire door does not play the role of fire retardant, and some engineering fire doors are not installed
at all 。 Especially the cable laid overhead in the main plant, the fire rate is relatively high, the number of cables is large, and there is no good fire prevention facilities, so the cable fire prevention is more not a good countermeasure, once the overhead cable fire, due to the difficulty of fire extinguishing, often serious losses, repair difficulty, so cable fire prevention for the power plant can not wait to close the eye
.

Second, cable fire cause analysis

Over the years, there have been many fire accidents and ignition of cables in the power plant, resulting in serious
losses.
According to the survey data, about 25%
of the fire accidents in power plants are ignited due to the failure of the cable itself.

The fire accident caused by the failure of the cable itself is mainly the accumulation of water and ash in the cable structure, so that the cable is immersed in water for a long time, resulting in the decline and aging of the cable insulation and the rotting of the sheath; Breakdown short circuit
due to severe moisture in the insulation.
In addition, fire accidents caused by explosions of cable heads, etc
.
No matter what cause the fire, it has the characteristics
of flame resistance, difficulty in fire extinguishing and large loss.

Secondly, the cable is caught on fire
caused by external fire sources.
According to the analysis of external factors, there are the following main aspects of fire:

1.
Due to the oil leakage of the oil system of the steam turbine or the rupture of the oil pipe, the pressure oil is sprayed onto the high-temperature pipeline with incomplete insulation to cause a fire, and the nearby cable is ignited and spread, causing a fire
.

2.
The explosion-proof door of the boiler's milling system is blasted, and the flame is sprayed and rushes to the nearby cable, causing the cable to catch fire; Or due to the spontaneous combustion of pulverized coal accumulated in the boiler's high-temperature equipment, causing a fire in nearby cables and spreading into a disaster
.

3.
Explosion of oil-immersed electrical equipment and fire with oil injection, flame flowing into the cable trench with oil causing cable fire, etc
.

Third, to prevent cable fire and ignition design measures

Cable and electrical equipment fire problem, foreign in the power plant design is quite important, first of all, in the cable selection, usually the use of flame retardant cable, electrical equipment generally with closed type
.
In this way, once a fire starts, it can stop the spread
of the fire.
Electrical equipment such as high and low voltage power distribution devices and relay rooms, commonly used 1301 fire extinguishing materials, using flue gas or temperature sensitive components to alarm, automatic spray fire
extinguishing.
The cable density in the plant is generally equipped with smoke or temperature sensitive components, once the cable fire, there is a fire alarm device in the control room and automatically start the fire fighting facilities
.
Water spray is used to extinguish the fire
.
It is more reliable to lay sub-sensitive cables
at the same time according to the cable routing path.
Obviously, the cost of these measures is relatively expensive, and some countries also use cable threading and laying to ensure that the cable is laid in the pipe accordingly, but it is necessary to consume a lot of steel
.

In order to make the power plant operate safely and reliably, do a good job in the cable fire design of the power plant, in the spirit of prevention, learn good experience, and actively create conditions to reduce the cable accident rate to a minimum
。 When formulating specific measures to prevent cable fire and ignition, we should draw on the experience and measures summarized in the investigation and induction of cable fire accidents in power plants in recent years, and make full use of newly developed domestic fireproof materials for the cause of the fire, such as modified amino acid expansion fireproof coatings produced by Sichuan Fire Research Institute, non-combustible inorganic composite panels and fireproof plugging materials produced by Guangzhou Guoneng Electric Power Engineering Equipment Co.
, Ltd.
, and fireproof wrapping tapes produced by Sichuan Cable Factory, etc.
, to constitute the fireproof and flame-retardant section
of cables.
The selection of structures also varies
depending on the installation environment.

The specific practices of fire protection facilities are:

1.
The overhead laying of the cable in the steam engine room, in the path planning, and the heat engine professional close cooperation, so that the overhead cable and the layout of the equipment, pipelines unified consideration, as far away as possible from steam pipelines and oil pipelines, to avoid high temperature and oil leakage caused by accidents, if the distance between the cable and the oil pipeline can not be increased locally, the cable local wrapping fireproof package, etc.
, the wrapping method of the wrapping tape is usually to use the wrapping tape on the cable to 1/2 cover the wrapping round trip to two layers, or apply fireproof paint, constitute a fireproof section
.

2.
The cables laid overhead in the boiler room are not allowed to directly cross the above the explosion-proof door of the boiler, nor are they allowed to directly face the explosion-proof door to avoid causing fire accidents; The cable laid overhead in the coal transmission channel, due to poor environmental conditions, there is an unsafe hidden danger of pulverized coal accumulation on the cable, so in the coal transport plank, the cable laid overhead is laid in sections by light closed refractory tank box to prevent fire ignition, and the setting of the fire section in the plank road can be set
in a place where the plank road is long and the cables are grouped.
In addition, in order not to improve the operating conditions, reduce the occurrence of accidents, and improve the reliability of operation, the cable laying in the chemical equipment room adopts overhead laying, which overcomes the phenomenon of serious corrosion and insulation aging of the cable due to the ground flushing water flowing into the cable trench
.

3.
In the cable mezzanine under the power distribution device room of the factory, according to the bus bar segment of the furnace factory, set up the fireproof section, the implementation of this fireproof section, can be composed of fireproof coating and fireproof partition, that is, on the cable at the selected fireproof section with modified amino acid expansion fireproof coating along the cable twice, the thickness and time of the brush should meet the specified requirements, and in the cable section coated with fireproof coating, each two layers are isolated with fireproof partitions to constitute a fireproof section; Considering the large construction workload and high technical requirements of this fire retardant section, the light closed refractory tank box is also considered to constitute a fireproof section, and one of
them can be selected according to the construction force and order situation.

4.
The cable tunnel is set up with a fireproof wall according to the furnace, and the specific practice of the fireproof wall is: when the cable is laid, select the place
where the fireproof wall is set.
The wall is made of ordinary red bricks with a thickness of 240mm, the width of the wall is equal to the width of the cable bracket plus 100mm, the height is the same as the tunnel, and the lower side of the wall is left with drainage holes
above the drainage ditch.
After the brick wall is built, a fireproof paint is applied to each side of the wall along a 3m long section to form a complete fireproof wall
.

Cable channels are fire-retardant walls with slag cotton at the entry into the building, or at the intersection of cable
trenches.

5.
The cable shaft, the hole of the protective cover, and the hole of the cable through the floor slab are mostly rectangular, and the shape of the hole is relatively regular
.
Such holes can be closed with asbestos plates, according to the size of the holes, processed into the required rectangle, installed on a pre-reserved support frame, after installation, with asbestos mud yellow sand and lime mixture as a sealing filler, it is sealed
.

6.
In the cable mezzanine, cable tunnel, etc.
of the whole plant, with the furnace as the unit, the use of the characteristics of material combustion must produce smoke, set up a fire smoke alarm device, in each furnace centralized control room and network control room, each installed a set of alarm device, once a fire occurs, smoke through the alarm, the alarm can automatically send out alarm signals in time, so as to take fire extinguishing measures in time, prevent the spread of fire, make the fire extinguish in the initial stage, avoid or reduce fire losses
.

IV.
Concluding Remarks:

Plastic cables are easy to fire and spread, although the development and application of cable flame retardant tape and fireproof coatings have been carried out, but these materials can only be used in a local range, and the wrapping or painting construction is troublesome, the process requirements are high, and the service life has yet to be further summarized in practice, so it is necessary
to develop fire-resistant cables as soon as possible.

Cable fire retardant is a comprehensive treatment problem, in addition to taking practical and effective measures in the design, we should also pay attention to construction requirements and quality, and at the same time strengthen operation management
.
We believe that with sufficient fire retardant measures and strict management, the probability of fire can be greatly reduced
.