Typical errors in the design of modular fire extinguishing systems and their consequences.
Typical errors in the design of modular fire extinguishing systems and their consequences.
INTRODUCTION
Unfortunately, the process of designing modular fire extinguishing systems still raises more questions than answers among specialists of design organizations. And each individual entrepreneur solves these questions at his own discretion, but not always correctly.
The problem is that the regulatory documents in force in this area, namely their terminology, content, style of presentation, imply the presence of a significant layer of basic knowledge among the specialists using them. If this knowledge is insufficient, then the process of making a technical decision becomes a kind of lottery — will the system work in a fire or not, will it be lucky or unlucky…
In 2003, VNIIPO issued, in my opinion, a very useful document. It is called “Fire-fighting automation systems. Scope. Type selection.” and has the status of recommendations. This is the very case when the status of the document fully corresponds to its purpose. These “Recommendations…” directly illustrate the process of designing fire-fighting automation systems, and do so in a coherent manner with references to the current (at that time) regulatory documents. The recommendations do not provide anything new, but consistently demonstrate and explain the adoption of certain technical solutions, i.e. they are simply a kind of “pilot” in the sea of SNiPs, GOSTs and Norms…
Interested specialists will easily find this document and will be able to familiarize themselves with it in detail, and in this article I will try to simply share my experience in solving a number of issues that periodically arise in the process of designing modular fire extinguishing systems, and draw the reader's attention to some points that are not obvious when considering, but are important when operating automatic fire extinguishing systems.
1. SELECTING A FIRE CLASS. SELECTING A FIRE EXTINGUISHING MODULE
When collecting initial data on the protected facility, in addition to space-planning solutions, it is necessary to determine the list of flammable substances (materials) in the room and the corresponding fire class or subclass (determined by Appendix to the Code of Practice 9.13130.2009 «Fire-fighting equipment. Fire extinguishers. Operating requirements.»).
If combined fire sources are possible, then it is necessary to choose a fire extinguishing module that is more universal in terms of the area of application. In this case, its fire extinguishing capacity is determined according to the manufacturer's data (passport) for the selected fire classes according to the minimum values.
A typical design error is an attempt to use the maximum values of fire extinguishing efficiency of the selected module. For example, it is unacceptable to use the characteristics of a module for extinguishing a class «A» fire at a facility with combustible (CL) and highly flammable (HL) liquids, since in the vast majority of cases these values do not match. Consequences — a lack of fire extinguishing agents leads to ineffective operation of the modular unit during a fire, i.e. no extinguishing occurs.
2. ANALYSIS OF THE FEP. DETERMINING THE TYPE OF FIRE DETECTORS (FD) FOR STARTING FIRE EXTINGUISHING MODULES
In accordance with GOST 12.3.04691, the AFPT must be triggered before the end of the initial stage of the fire.
The minimum duration of the initial stage of a fire tнсп in a room is determined in accordance with GOST 2.1.004.
If there is dust or smoke in the protected room, it is necessary to analyze the possibility of false triggering of a smoke PI with specified response thresholds. It should be taken into account that most fire extinguishing modules, when triggered, emit finely dispersed fractions into the extinguishing zone, which are perceived by the absolute majority of smoke PIs as smoke.
The calculation of the critical fire time required to ensure the timely evacuation of people is carried out according to the methodology set out in GOST 12.1.004. The task is to select a fire scheme that leads to the fastest development of one of the hazardous fire factors (HFF).
The development of HFF depends on the type of combustible substances and materials and the combustion area, which, in turn, is determined by the properties of the materials themselves, as well as the method of their laying and placement.
A typical design erroris an attempt to use smoke PI both to send an «Alarm» signal to the warning and evacuation control system (WECS) and to generate a command to launch fire extinguishing equipment. This is not always possible, since, on the one hand, the location of smoke detection is quite ambiguous in localizing the location of the fire, and on the other hand, the activation of a modular installation by such a signal is quite capable of causing an «avalanche effect» as the fire extinguishing agent spreads to neighboring control zones. Needless to say, for modular systems, which are systems with a limited supply of fire extinguishing agents, the timeliness of action is decisive in most cases… Probably, when working out the above-mentioned requirements in this section, it will be necessary to work out two fire alarm systems that respond to different fire extinguishing agents (for example, smoke + heat). This will be more expensive, but it will be correct.
3. SELECTING THE SIZE OF FIRE EXTINGUISHING ZONES, INTERACTION OF FIRE EXTINGUISHING ZONES WHEN FIRE IS TRIGGERED
When analyzing the fire hazard of materials stored at a protected facility, among other parameters, there is one such as the linear speed of flame propagation along the surface of the combustible material. It should be determined using reference data at the stage of forming the technical specifications, but in practice it is almost never encountered in the design of modular installations. But it is precisely this that is decisive in determining the size of the extinguishing zones and their response time.
In the ideal case, the fire starts in the center of the detection zone and in some time, not exceeding tнсп (!), the PI is detected. By the time the command pulse passes to the module, the source has not had time to spread beyond the extinguishing zone. Extinguishing occurs «in normal mode» and, as a rule, is successful.
But it is almost impossible to predict the location of a fire in reality! The larger the room and the more extinguishing zones it has, the higher the probability of a fire breaking out at the boundary of such zones. Regulatory documents allow for some technological delay in starting adjacent fire extinguishing zones, but is this delay small enough? If there is a combustion of solid combustible substances (SCS, fire class «A»), then during the time between the fire detection and the moment of supplying the extinguishing agent, due to the relatively low linear propagation speeds, the fire source, as a rule, «does not go far» and ends up in the zone of direct impact of the extinguishing agent. And the nature of SCS combustion allows for a gradual effect on the fire source. But in the case of extinguishing a spill of flammable and combustible liquids (fire class «B») that occurred at the boundary of zones, such a delay can be fatal, since the fire can return through vapors to the already «extinguished» zone and continue to develop.
A typical design mistake is that designers do not want to analyze the process of fire development and extinguishing, but blindly follow regulatory documents, not paying attention to all possible features of this process.
4. SUPPLYING A COMMAND IMPULSE, CALCULATING THE CROSS-SECTION OF SUPPLY WIRES, CALCULATING STARTING CURRENTS
Fire extinguishing modules are activated by supplying them with a command current pulse of certain parameters from the fire control device (FCD). The usual value of the starting current is 0.10.7 A per module. It is generally accepted that the greater the current the FCD can produce, the more modules can be “hung” on it and … the better, because it is cheaper. This is not true at all.
The output pulse of the fire extinguishing module itself “does not know” that it must correspond to the values at the input of the extinguishing module that are sufficient for starting. With a sufficiently long start line (SL), it can either lose most of its energy on the wires or “arise” in the form of induced EMF. If we compare the requirements for alarm loops, controlled during certification tests of the fire extinguishing module, with the requirements for the SL, then the fair rigidity of the former and the practical absence of the latter are obvious. The task of ensuring the passage of the command pulse to the most remote “consumers” lies entirely with the design engineer.
A typical design erroris the lack of knowledge and/or non-application of Ohm's, Kirchhoff's and Joule-Lenz's laws by specialists. As a result, such a situation, when a functional and fully diagnostic (in standby mode) modular fire extinguishing system does not operate during a fire or operates without a fire, becomes traditional. A separate mention in this section is the need to monitor the capacity of the battery — the backup power source (RPS) during operation. All motorists know that this parameter decreases over time. At some point, the accumulated energy will simply not be enough exactly when it is needed most, but this is rather a question that requires design specialists to at least mention in the explanatory note to the project, since it relates to the scope of operation of the AUPT.
5. INTERACTION…
The interaction of a modular (aerosol, powder, water (TRV) and gas) fire extinguishing system with door status monitoring sensors, information boards, local and remote manual start devices (buttons), building engineering systems, etc., is usually reflected in sufficient detail in regulatory documents. But this is done, unfortunately, at the level of the required «fact» … But the numerical values of various delays, pauses, moments and times are practically not specified anywhere. And there are no direct methods for their calculation. To correctly determine the interaction parameters of all technical devices that ensure fire safety, formalized requirements of the Code of Practice are not enough! Here again, it is necessary to use, for example, the methods of GOST 12.1.004 and build the entire interaction system! Only in this case will there be an understanding of the most likely (!) way the situation at the facility will develop during a fire.
Typical design errors are:
— complete blocking of automatic start-up of installations (gas, powder and aerosol) when smoke protection is operating. This is how the requirement of paragraph 14.6 of SP 5.13130.2009 is often interpreted, but this is only a ban on the simultaneous operation of such systems. Its purpose is obvious — to maintain the required concentration of the fire extinguishing agent at the time of extinguishing and to prevent its leakage through the operating smoke removal system. In other words, the smoke removal system should not remove the fire extinguishing agent at a time when this fire extinguishing agent is needed to extinguish the fire. And before the extinguishing process and after its completion, the smoke protection must work!
— the absence in the design documentation or in the attached tasks of the requirement to equip doors with door closers in those protected rooms where fire extinguishing installations are used, regulatory critical to door condition monitoring.
Such errors lead to the fact that a serviceable AUPT often physically cannot operate and safely burns out along with the protected object.
6. TAKING INTO ACCOUNT THE GEOMETRICAL CHARACTERISTICS OF FIRE-EXTINGUISHING WHEN PROTECTING VARIOUS OBJECTS
All modular fire-extinguishing systems consist of a certain set of individual extinguishing means – modules. During certification tests, each type of module is checked for fire-extinguishing efficiency. Although this is done using different methods (since there is still no single method for checking fire-extinguishing efficiency!), it is done without fail. The results of these tests are reflected in the technical documentation (passport).
A typical design error is inattention to the specified values of fire extinguishing characteristics and notes to these values, which usually describe the conditions under which the values were obtained. The result is incorrect placement of extinguishing agents and/or their insufficient quantity. The consequence is the inability to supply a sufficient amount of fire extinguishing agent to the right place, i.e. low efficiency of work during a fire.
7. CONTROL OF INTEGRITY OF STARTING CIRCUITS
Modular installations are characterized by the presence of a large number of starting circuits that need to be monitored. This requirement is fair and fully justified. It is clear that the methods for its implementation can have many options, but the difficulty is that in many cases, when designing modular extinguishing systems, design engineers perform “commercial optimization” of the requirements of design standards, which leads to a sharp decrease in the reliability of the extinguishing system.
A typical design erroris a parallel connection of fire extinguishing modules in a single starting circuit, based only on its load capacity. And to be sure that the modules will start at the right time, it is necessary to control each starting circuit. The difficulty is that for most fire extinguishing units the rule applies: «One starting channel — one module!» In this case, the requirement is met, but the number of fire extinguishing units, wires, the volume of installation work and, accordingly, the cost increases. Some companies produce special interface units installed next to each module and monitoring its status. But in the case of their use, the issue of providing power supply for such devices in standby mode from the RIP is added to the listed problems. Taking into account the usual current consumption of 20-30 mA for such «devices», it is enough to simply calculate the required capacity of the RIP batteries. There is a system solution from one of the companies that produces both fire extinguishing modules and control equipment for them, when, when powder fire extinguishing modules are connected in parallel to the starting line, each is monitored for an open circuit without any additional devices. It is possible to include up to 30 modules in each of the four launch lines. But this is rather an exception to the general rule… In other cases, the control method is forced to be invented by the chief engineer.
CONCLUSION
Fire extinguishing modules differ from other systems in that they allow extinguishing fires with minimal means and maximum efficiency! But a prerequisite for realizing this advantage is the competent construction of the entire fire protection system. Human life often depends on it!
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A. Matsuk
Deputy General Director for Science, ETERNIS Group of Companies
SAFETY ALGORITHM No. 3, 2011?