11. Features of a control system for enterprises with potentially explosive (1.14, с. 103)

11.1. Basics of explosion protection

Often, DCS, introduced in the mining, chemical, petrochemical and gas industries are established in areas of production that are characterized by potentially explosive consequence of any permanent presence of an explosive environment, or a high probability of occurrence of such an environment in case of accident or violation of the flow process. Failure to comply with the rules of explosion can cause as many casualties as well as irreversible damage to the environment. That is why the use of highly reliable and secure technical solutions for explosion protection is one of the biggest challenges facing designers APCS.

The emergence of the risk of explosion cause the following adverse conditions:

1. The presence of flammable vapors, liquids, gases or combustible dust.

2. The presence of oxidizer - air or oxygen.

3. Formation energy of ignition - electrical or thermal.

To provoke the explosion must have the above components in certain proportions. Thus, in order to blast air / gas mixture must contain an oxidizer in a certain range of concentration. In this explosive mixture should be in contact with the body, which could send him enough for ignition energy (for example, with strongly heated conductor or sparkling contact).

All known and practiced methods of protection are aimed at reducing the risk of explosion to an acceptable level. Thus, if the system is designed correctly, single fault in any of its components should not lead to an explosion.

11.1.1. Methods of explosion

In general, all methods of explosion protection can be divided into four main groups.

A. Reducing the likelihood of an electric spark.

By this method, the following types of protection are:

1. Explosion protection type "e" (increased safety) provides additional structural measures against the possibility of exceeding the permissible temperature and the occurrence of arc and spark discharges, which during normal operation does not occur.

2. Explosion protection type "n" provides additional structural measures against the possibility of exceeding the permissible temperature and the occurrence of arc and spark discharges in normal and some abnormal operating conditions.

3. Explosion protection type "s" (special) can be achieved through: the conclusion of the electrical circuits in a sealed envelope with the degree of protection IR67; electrical sealing material with insulating properties (compound, sealants); impact on the explosive mix of devices and substances to absorb or reduce the concentration of the latter .

B. Isolation of electrical circuits of explosive mixtures.

The method assumes the conclusion of electrical circuits in a special envelope, filled with gaseous, liquid or solid dielectric so that the explosive mixture was not in contact with electrical circuits.

By this method, the following types of explosion:

1. Explosion protection type "m" - fill by a special compound.

2. Explosion protection type "o" - an oil filling of the shell.

3. Explosion protection type "a" - filling the shell with quartz sand.

4. Explosion type p "- fill or blowing a shell explosion-proof gas under excess pressure.

C. Holding of the explosion.

Under this method is implemented explosion protection type "d" (blast shell).

It is understood that the electrical circuits are placed in a special cased with a small gap. This does not prevent contact of electrical circuits with an explosive mixture and the possibility of ignition, but it is guaranteed that the shell constrains resulting from the blast overpressure, ie flash does not go beyond the flameproof enclosure.

As the hot gases have different penetrating ability, it is taken into account the subgroup of gases.

D. Limiting the power of the spark.

Under this method is implemented protection type "i" (intrinsically safe circuit). This method assumes that in the event of a spark of its power will be insufficient to ignite an explosive mixture. However, this method does not preclude contact explosive mixture with electric circuits.

Due to its versatility, safety and ease of implementation, the type of protection "intrinsically safe circuit" (IS, intrinsically safe circuit) is most often applied in the construction of CAM, and therefore more speech will focus on it.