Fire alarm systems for explosive objects.

Fire alarm systems for explosive objects.

Fire alarm systems for explosive objects.

Objectives of explosion protection

    Currently, against the background of actively developing oil and gas industries, there is a significant increase in facilities using explosive and flammable substances in the technological production cycle. This leads to the widespread use of explosion-proof electrical equipment for various purposes, including fire alarm equipment (hereinafter referred to as FAS).

However, among some representatives of design organizations, and even among some employees of supervisory services, there is a misunderstanding of the goals and principles of explosion protection. Considering the main goal of explosion protection to be the protection of the electrical equipment filling from the energy of an explosion, a stereotype has developed that explosion-proof equipment must be massive, with a strong «explosion-proof» casing. Such a judgment is fundamentally wrong.

   Explosion protection has completely opposite goals, and explosion-proof equipment should not itself be a source of explosion in the first place. From this point of view, the definition of explosion-proof equipment is more correct, although it is not generally accepted.

At explosive sites, even minor sources of heating or sparking may cause an explosion and fire. Along with other electrical equipment, a working fire alarm system should also be considered as a possible ignition source. After all, most detectors, alarms and control devices contain circuits and elements that are dangerous in terms of sparking.

   Therefore, explosion-proof alarm systems are used to implement fire and security protection of explosive objects. Explosion-proof fire alarm systems must not only properly perform their functions in explosive environments, but also, first of all, guarantee that they themselves will not cause a fire or explosion, both during normal operation and in the event of malfunctions.

Types of explosion protection and principles of their implementation

   The design of explosion-proof electrical equipment provides for a number of special measures. There are strictly defined types of explosion protection, the methods for implementing which are standardized.

Let's consider the basic principles of implementing explosion protection used in fire alarm systems.

Containment

– is a principle of implementing explosion protection, in which parts (elements) of electrical equipment capable of causing ignition of an explosive gas environment are placed in a shell that can withstand the pressure of an explosion of an explosive mixture inside this shell and prevent its spread to an external explosive gas environment. Based on this principle, type «d» protection is implemented — an explosion-proof shell.

It should be noted that this type of explosion protection does not, in principle, exclude the occurrence of an explosion, it only prevents the propagation of a blast wave outside the electrical equipment. It is the «d» protection that many perceive as «real» explosion protection, however, as will be shown below, it is not always appropriate and not applicable everywhere. In addition, equipment with «d» protection has a number of disadvantages (large weight, dimensions and, as a result, cost), as well as strict restrictions (it is prohibited to turn on equipment with an open casing)

Insulation

is a principle of implementing explosion protection, in which parts of electrical equipment capable of igniting an explosive mixture due to sparking or heating are enclosed in a special insulating material in such a way that the explosive mixture does not have access to the elements of electrical equipment. Based on this principle, explosion protection of type «m» used in OPS is implemented — sealing with a compound. Among the disadvantages of explosion protection «m» can be distinguished low maintainability of equipment and switching limitations (lack of terminals for connection).

Inadmissibility

is a principle of implementing explosion protection, which completely excludes the possibility of releasing (in the form of a spark or thermal heating) in an explosive zone the energy stored in an electric circuit, capable of igniting the environment. This principle underlies the type «i» protection widely used in fire alarm systems — intrinsically safe circuit (hereinafter ISC). ISC is a special type of explosion protection, which is based on limiting the parameters of an electric circuit — current, voltage, dissipated power — to intrinsically safe values. Intrinsically safe values ​​are the maximum permissible electrical parameters at which the energy of the resulting discharges is insufficient to ignite an explosive mixture. Thus, type «i» protection basically excludes the possibility of an explosion. Therefore, many specialists consider equipment with type ISC protection to be explosion-proof equipment by definition.

   IBCs are divided into three levels according to their reliability: level ia – especially explosion-proof, level ib – explosion-proof, level ic – increased reliability against explosion. IBCs of level ia maintain intrinsic safety in case of any possible types of damage, therefore they are the most reliable. Despite all the advantages of intrinsically safe circuits, their use is limited to low-current and low-voltage circuits, i.e. signal and control circuits, such as alarm loops in fire alarm systems.

   Along with the above-mentioned types of explosion protection, in some cases other types can be used in fire alarm systems, in particular type «s» protection — a special type of explosion protection, which is based on a combination of the above-described principles and some additional measures.

Type of explosion protection and class of explosion hazardous area.

   Fire alarm system equipment, like any electrical equipment installed at explosive facilities, must fully comply with the zone in which it will be used. According to GOST R 51330.9, explosive zones are divided into 3 classes:
Class 0 — a zone in which an explosive environment is present constantly or over a long period.
Class 1 — a zone in which an explosive environment can be created during normal operation.
Class 2 — a zone in which an explosive environment is absent under normal operating conditions, and if it occurs, it is rare and does not last long.

The type of explosion protection applied in electrical equipment is indicated in its explosion protection marking after the symbols «Ex». Depending on the type of explosion protection used, the equipment may be installed strictly in a specific zone, as indicated in Table 1. For types «d» and «m» protection, a number (1 or 2) indicating the zone class should be indicated in the explosion protection marking before the symbols «Ex». For type «i» protection, this is not necessary, since the zone class is unambiguously determined by the level of intrinsic safety.

Table 1.

«+» — is allowed; «–» — is not allowed.

   Note that of the considered types of protection in zone class 0, only special explosion-proof equipment with the use of IBC level «ia» is permissible. Special explosion-proof equipment can also be made with a special type of protection «s».

Advantages of intrinsically safe OPS.

According to European statistics, the main type of explosion protection currently used at explosive facilities in Western countries is explosion protection of the intrinsically safe circuit type — «i». Explosion protection of this type is used in 90% of cases of implementing OPS at these facilities. Such popularity of IBC is associated with a number of advantages, compared with other types of protection, affecting the main criteria for selecting equipment, such as reliability, cost, ease of installation and maintenance.

As noted earlier, the level of electrical parameters in these circuits is limited to intrinsically safe values, so there is no need to use special wiring methods (in pipes or in armor) and explosion-proof junction boxes inside the explosion-hazardous area. According to GOST R 51330.13, the installation of the IBC can be carried out by any means, including open wiring of ordinary insulated wires with insulation tested at 500 V. IBCs allow any type of work related to equipment replacement, repair, maintenance, expansion of the fire alarm system, as well as commissioning and testing, without disconnecting the power supply, maintaining the operability of the entire circuit.

   According to GOST R 51330.13, intrinsically safe circuits and electrical equipment in these circuits do not require protection from overload, negative consequences of short circuits and ground faults. The reliability of the IBC is also confirmed by the fact that, according to the same GOST R, this type of explosion protection with level «ia» is especially explosion-proof and is used to build an FPS in class 0 zones, where increased requirements are imposed on the explosion protection of equipment and the circuit as a whole.

   Thus, the use of the IBC is often the optimal, and sometimes the only possible way to build an FPS system for an explosive facility.

Conclusion.

   Implementation of fire alarm systems at explosive facilities is a complex and responsible task that requires a detailed study of the regulatory framework, as well as taking into account all factors at the stages of design, installation and subsequent operation. A competent and rational approach to choosing the type of explosion protection will significantly reduce the installation time and costs of implementing and operating the OPS, and most importantly, guarantee the safety of human lives and property.