Gas fire detectors – early fire detection.

Gas fire detectors — early fire detection.

Gas fire detectors — early fire detection.

Fire detectors, the most common at present, detect the following fire factors: changes in the optical density of air as a result of smoke, an increase in temperature or the rate of temperature increase, the appearance of an open flame.

At the same time, even without delving into the materials of combustion theory, it can be noted that any fire is accompanied by the spread of gaseous products, some of which any person senses in the form of odors. This is confirmed by studies conducted in 1998 by the VNIIPO MVD RF in a standard chamber used to simulate a fire with a volume of 60 m3. The composition of gases released at various stages of combustion was determined by precise methods using chromatography. («Application of semiconductor gas sensors in fire control systems», V. Antonenko, A. Vasiliev, I. Olikhov).

At the initial stage of a fire, during smoldering, the concentration of hydrogen increases to 10-20 ppm (molecules per million). Subsequently, the content of aromatic hydrocarbons and carbon monoxide CO increases to 20-80 ppm. And when a flame appears, the concentration of carbon dioxide CO2 increases to 1000 ppm.
These studies formed the basis of NPB 71-98 «Fire gas detectors. General technical requirements. Test methods.» The results obtained during the tests are confirmed by similar studies conducted in the UK, Australia, and the USA.

Carbon monoxide (carbon monoxide) is a tasteless, colorless, odorless gas that is released by all carbon-containing materials when ignited. Carbon monoxide is extremely toxic. Even at relatively low levels, it can cause brain damage or death in 1-2 minutes. When initially exposed, carbon monoxide causes disorientation, which prevents people from acting consciously during a fire.

Fire detectors that react to CO levels have been used as early fire detection devices since 1999. The action of a CO gas fire detector is especially indicative when detecting smoldering, slowly developing fires. The fundamental difference between CO gas fire detectors and CO gas alarms is their speed of response.

Since carbon monoxide is a gas that is more mobile than smoke, the positioning of the detector relative to the location of the fire or fire is less critical, which increases the likelihood of early detection. Smoke movement is determined by convection currents, the strength of which depends on the temperature of the fire source. Gas spreads not so much by convection, but rather by diffusion, so various partitions, beams, racks, stacks and other physical obstacles at the facility significantly less affect the detection of a fire by a gas fire detector.

Diffusion is a mechanism of gas distribution in which a gas molecule moves in volume, changing places with a molecule of another gas. This applies to any gases whose «task» is to uniformly fill the entire volume. If we operate with odors, in any case, they can be considered as a test perceived by a person, then with any source of odor (let's take a pleasant one — a cup of freshly brewed coffee), this odor will gradually spread throughout the room and go beyond the room (let's note this point, we will return to it later). At the same time, a bookcase or a beam will not interfere with the spread of odor. The penetration of smoke upward is prevented by the stratification effect: an air gap 15-20 mm thick appears near the ceiling, where smoke does not penetrate (which limits the installation location of smoke PI). There is no such layer for odor (respectively — for gas), no matter how hard we try to find it. Let's try to turn on the fan and stop the spread of the odor. Will it weaken it? Absolutely. Will it stop? No.

If the PI is installed on the ceiling of a metal hangar, warehouse, or on the ceiling of an atrium, then when heated by the sun, a fairly thick layer of warm air appears near the ceiling of the room, which repels ascending convection currents with smoke particles, which prevents smoke PIs from triggering. Diffusion allows CO molecules to penetrate through this layer, carbon monoxide reaches the ceiling, causing the GPI to trigger.

This is, of course, a simplified picture of gas propagation, but it allows you to feel the physics of the process.
Next, why do most gas fire detectors use CO as the target gas? Of course, hydrogen appears before carbon monoxide, and the level of carbon dioxide CO2 increases as the fire grows. However, as the fire grows, the level of hydrogen drops, and CO appears in the registered levels before CO2. That is, carbon monoxide is a kind of golden mean. In addition, what is very important: carbon monoxide is distributed more evenly in volume than H2 or CO2.

Another task that CO gas fire alarms simultaneously solve is protection against carbon monoxide poisoning in a fire. A disproportionately large number of fire deaths are caused by fires that start slowly. Of all fires, 80% of deaths are due to poisoning by deadly carbon monoxide. Studies of these types of fires have shown that smoke can be released for many minutes, or even hours, to a level that causes the smoke alarm to operate. At this time, carbon monoxide levels become such that a sleeping person often does not wake up, and if he does wake up, he is disoriented and can no longer escape. No other type of alarm can perform the task of detecting dangerous levels of carbon monoxide.

Now let's add a few flies in the ointment to this beautiful picture. Everything seems beautiful, the fire is registered at the smoldering stage, physical barriers do not interfere, sleeping people are rescued in a timely manner… But the energy of the fire stages, starting with smoldering, is of great importance. If the power of convection flows is stronger than the diffusion energy, then it is possible that the smoke PI will detect the fire before the gas one. I will note, however, that the possibility of detection is also affected by the state of the environment in which the fire detectors are used.

Another aspect. In the example above, the smell of coffee goes OUTSIDE the room. The same thing happens with carbon monoxide. Therefore, the GPI may be triggered due to a fire in an adjacent room. This must be taken into account when operating systems that include a GPI. Sometimes this is a plus, sometimes a minus. For example, if the GPI is installed in the corridor, it will also be triggered by a fire in the room whose door opens into the corridor.

All of the above features of the use of the GPI are well described in the «BFPSA application guidelines for carbon monoxide (CO) fire detectors».

So, which detector will provide reliable fire detection?
As noted in the recommendations of the All-Russian Research Institute of Fire Protection of the Ministry of Emergency Situations of the Russian Federation “Fire-fighting automation equipment. Scope. Type selection. Recommendations” — if it is established that the prevailing factor of the fire will be gaseous products, then it is advisable to use gas fire detectors. We will add that in this case it makes sense to use a GPI as the main fire detector. However, a combination of detectors of different types is most effective. However, this combination should not always be combined in one housing, for example, if a gas fire detector can detect a fire, and some conditions will obviously prevent a smoke detector from performing its task, then there is no point in using a multi-criteria fire detector.

Strictly speaking, gaseous combustion products are released in all types of fire, except for TP-6 ​​(Combustion of flammable liquid). But under normal conditions, GPIs are more effective in fires with a long smoldering time (TP-2 and TP-3), in this case the temperature of the source is low, the material undergoes pyrolysis with a large release of gaseous substances. That is, if smoldering of fabric, paper, wooden or plastic sheathing, smoldering of a cable is expected, then the GPI will detect the fire quite effectively, starting from the smoldering stage.

What do we consider normal conditions? A fairly clean room, without any special drafts, with a small presence of smoke and dust, moderate humidity.
Now let's consider other applications — industrial ones. Dust — a must, smoke — very often, difficulty of routine maintenance of smoke fire detectors — very often. In this case, the smoke chamber of smoke fire detectors becomes dirty, false alarms occur. GFIs do not respond to smoke and dust, they do not have a chamber (there is nowhere for dirt to accumulate), therefore, in such conditions, GFIs work more stably than other fire detectors and, as a result, more reliably detect fires of types TP-1 — TP-5. Gas fire detectors work stably even in conditions of heavy dust, up to 2.5 kg of dust per 1 m3. As for smoke («white» or «black» smoke is not important), a gas fire detector cannot respond to smoke, up to the appearance of carbon monoxide at levels indicating the occurrence of a fire.
The primary task of fire alarm systems is to detect fires at the earliest stage, when the fire can still be easily localized and thus save lives and property. This is especially true for fire- and explosion-hazardous facilities, where the spread of fire can be spontaneous.

It should be noted that the consequence of fires at fire- and explosion-hazardous facilities is also, in most cases, slow smoldering processes, for example, of the same notorious electrical wiring.

Of course, such phenomena as lightning strikes, exceeding the maximum concentration of explosive gases, exceeding the maximum autoignition temperature lead directly to an explosion. However, practice shows that the cause of most fires, even at oil storage and processing facilities, oil products, natural gas, and other flammable liquids or gases, is either sparking or ignition of secondary (not related to the main production) materials. In the latter case, there are already recorded concentrations of carbon monoxide CO.
In general, it should be said that objects related to oil, gas and their derivatives (hydrocarbons) in the total number of explosive objects occupy only 25…30%. And of course, at such objects, the means of first choice are, of course, flame detectors. But a fire at such an object may not start with an open flame. If a flame detector at such an object has already been triggered, and this is not a false alarm, then there is very little time left for the fire not to develop into a large-scale disaster.

But what about the remaining 70% of explosive industries and facilities not related to oil, gas, flammable liquids, alcohols? What type of detectors should be used to protect, for example, a woodworking shop, grain storage, flour mill, confectionery production, cement and cement products production, many chemical plants, mines and underground workings? A possible explosion at such facilities, in most cases, will not be associated with an open flame, and therefore the inefficiency of using flame detectors at such enterprises is obvious.

In addition, all of the listed industries are industries with a high dust content, which means that it will be ineffective to use not only flame detectors, but also smoke detectors with an optical channel. Thermal fire detectors, due to their high inertia, will not work at the very beginning of a fire.

Thus, the relevance of using gas fire detectors or combined fire detectors with a gas channel at fire and explosion hazardous facilities is obvious.

The detection zone of a gas fire detector is regulated by SP5.13130.2009 «Fire protection systems. Automatic fire alarm and fire extinguishing installations. Norms and rules for design» and the operating instructions for a gas fire detector. The detection zone for gas is the same as for smoke detectors, but when determining it, it is not necessary to take into account ceiling beams and floors, which is due to the different mechanism of gas and smoke propagation.
Gas fire detectors have three sensitivity options. The unit of measurement of sensitivity is ppm – the number of molecules per million (part per million). According to NPB 71-98, there are 2 classes: Class 1 triggering in the range of 21-40 ppm, Class 2 – in the range of 41-80 ppm. Some detectors provide high sensitivity and triggering in the range of 10-20 ppm. Foreign experience shows that even in Class 2, GPIs provide timely triggering with a small number of false alarms.

However, if gas fire detectors have been used abroad for a long time, then in Russia this direction is poorly developed. Such detectors have been manufactured since 2006 by Etra-Spetsavtomatika (Novosibirsk) IP101/435-1-A1/2 «Expert», NPP Delta (Moscow) produces the «Sensis» detector, ZAO PO Spetsavtomatika (Biysk) — GPI IP435-1. NPP Ural-kompleks (Ekaterinburg) produces ASPS, intended for ore mines, which uses GPI. Recently, foreign detectors have appeared on the Russian market: TYCO (801CH, 801PC, 801CH Ex), Apollo presented a fire detector of the Discovery line.

«The ERVIST Company (Moscow), together with Etra-Spetsavtomatika, began production of the explosion-proof and mine gas fire detector IP 435-4-Ex «Segment» in 2010.

However, today, the number of gas fire detectors used in Russia is several orders of magnitude less than in Europe, not to mention the USA.
It is worth hoping that gradually in Russia the gas fire detector will finally take its rightful place in fire detection.

And in conclusion, we will list once again the areas of application of fire detectors with a gas channel.

Application areas of a gas fire detector for CO
CO gas fire detectors are not a general replacement for other types of fire detectors. Certain characteristics distinguish them favorably in a number of cases when a CO fire detector is advantageous for detecting a fire hazard. But, as with other types of detectors, there are applications where CO detectors are good to use and others — where their use is limited or not recommended (clause 13.1.7 SP 5.13130.2009).

If the fire is slow and smoldering, which is typical for fires of types TP2 — smoldering wood, TP3 — glowing cotton smoldering (GOST R 53325-2009), TFX — hidden smoldering cotton (according to LPS 1274), then fairly high levels of carbon monoxide concentration are recorded. In addition, at the initial stage of fires TP1, TP4, there is a certain amount of CO. The CO level drops when an open flame appears.

With clean combustion or rapid combustion, such as a liquid fuel fire (TP5 — heptane combustion, TP6 — alcohol combustion), low levels of CO gas are obtained, since complete combustion occurs.

As the fire develops, the air supply may not be sufficient to continue the fire, and oxygen depletion begins to occur. Under these circumstances, the CO level will increase.

Like smoke detectors, a CO gas fire alarm will benefit from convection currents created by the heat at the fire source. These currents help the CO reach the detector's sensing element. However, as a gas, carbon monoxide diffuses throughout the protected volume in a way that allows a CO gas fire alarm to operate effectively in areas where the presence of physical barriers may limit the spread of smoke. Examples of such barriers include heavily intersected ceilings, suspended ceilings, gas migration into adjacent rooms, and hot air currents.

It is possible that the use of CO fire detectors can sometimes lead to early detection of a fire in adjacent rooms and may be mistaken for a false alarm and ignored, since the location of the fire will not be accurately determined. Installation and operating organizations must be informed of this.

Cases in which a CO gas fire detector provides the earliest detection of a fire
If there is a high probability of a slowly developing fire, smoldering. In this case, a CO gas fire detector gives an earlier reaction, since carbon monoxide is likely to be received before smoke particles (TP2, TP3, TFX).

If it is not possible to use smoke detectors due to the potential for false alarms, which may occur in cases where steam, dust or process fumes are present (TP1-TP4, TFX).

If a smoke detector does not provide complete protection, for example in a sleeping area. In these cases, only a CO gas fire detector can provide protection for sleepers from fire and/or carbon monoxide poisoning (TP1-TP4, TFX).

If the oxygen supply is low, it causes incomplete combustion. Examples include closed storage rooms, laundry rooms, cabinets, where a CO smoke detector located behind the door can detect a fire starting before smoke begins to spread outside the room (TP2, TP3, TFX).
When smoke movement from a fire source may be restricted by hot air layers (stratification effect). In these circumstances, the diffusion process of CO gas propagation assists the fire detection by a CO gas fire detector. In addition to diffusion through thermal barriers within the room, gas diffusion will also result in penetration into other locations such as roof areas and voids. (TP1-TP4, TFX).

Cases in which a CO gas fire detector is not recommended as a primary means of fire detection
If the fire starts with rapid ignition (TP5, TP6) and the heat released ensures rapid and complete combustion. In this case, the CO detector will only operate when the oxygen supply decreases as the fire progresses.

Limitation of fire detection due to cable overheating (TP4). It is very important to know the cable insulation material. Carbon monoxide cannot be produced in detectable quantities where pyrolysis (decomposition and other transformations of chemical compounds when heated) of the material occurs rather than self-established combustion. Please note: pyrolysis is defined as the decomposition of the plastic insulation material by the heat of the overheated cable, i.e. the insulation burns without a flame.

For the detection of liquid fuel fires (TP5, TP6), such as heptane and other flammable liquids, since CO levels will be low.
If the main objective is preventing the spread of smoke within the building.

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E.G. Saidulin
Director of the company «Etra-spetsavtomatika»
M.V. Rukin
General Director of the company «ERVIST»