HOW TO SELECT NBC / CBRN FILTRATION SYSTEMS FOR A SHELTER
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WHAT IS NBC / CBRN FILTRATION SYSTEM?
NBC filtration systems are part of life-support systems which are usually installed in the shelters, bunkers or protected facilities. The systems designed to filter the incoming fresh air that may be contaminated with nuclear, biological and chemical particles and agents. Such systems are usually designed for a specific range of threats and depending on the filters installed and filtration system characteristics can provide efficient protection from threats it has been designed for. Such threats can be air contamination as a result of NBC attack in wartime or terror act, natural disasters, industrial accidents with release of toxic industrial chemicals (TIC) or radioactive materials, dust and more.
From my personal standpoint, NBC filtration systems can be divided by the following categories:
Systems for shelters and protective structures
Systems for mobile and deployable applications – vehicles and mobile platforms
Systems for deployable stationary applications – tents and field camps
Each category have subcategories and the systems in each category significantly differ one from another in specification like airflow, size, AC/DC power supply, shock resistance, resistance to vibrations, operational environment, pushing or pulling configurations and many more.
NBC / CBRN / CBR / ABC- common abbreviations of similar meaning: Nuclear/Atomic/Radiological, Chemical warfare, Biological threats). In this article we will use NBC for all the above.
TFA – Toxic Free Area
Overpressure - Positive differential pressure inside a protective space compared with the outside
WHY IS NBC FILTRATION SYSTEM NEEDED?
NBC filtration systems are required for supplying fresh and filtered air for people and equipment which allows creating inside the shelter a toxic free area (TFA) and protect them from contaminated environment outside the shelter. The constant supply of fresh air allows to create replacement of air in closed space of the shelter and prevents creation of dangerous concentration of carbon dioxide (CO2) which can be harmful to people. In case the shelter is airtight the NBC filtration system by introducing fresh air into the protected space creates overpressure inside the TFA area that is regulated by overpressure valves that usually is part of the complete air management system in NBC shelters. The overpressure insures that contaminated air will not enter inside through small openings in shelter perimeter and only filtered by NBC system air will enter the shelter.
THERE ARE SEVERAL TYPES OF SHELTERS:
Simple / escape shelters designed for very short stay - usually not equipped with forced ventilation or air supply system
Shelters for longer stay (not NBC) – usually equipped with ventilation / fresh air supply system
NBC shelters – airtight and specially designed shelters equipped with NBC filtration system
Shelters isolated from outside environment – airtight and specially designed shelters equipped with air regeneration systems (CO2 scrubbers, Oxygen supply equipment and overpressure maintenance systems and more).
In many cases, NBC shelters additionally to NBC filtration mode can be operated in isolation mode for some periods of time if equipped with isolating systems or with air regeneration systems.
HOW FILTRATION PROCESS IS WORKING IN DIFFERENT SYSTEM CONFIGURATIONS
As mentioned above there are two main systems configurations depending on the application it is used in:
Pulling filtration system: such systems are commonly used in shelters and protective structures and the blower is “pulling” the air through the NBC filter.
Typical pulling system airflow configuration scheme: prefilter > NBC filter > blower > Release to TFA
Pulling configuration considered as a safer when the NBC filtration system is placed inside the TFA area. The blower pulling the air through the system creating under pressure inside the air duct of system and reduces the possibility of unfiltered contaminated air leakage before the filter. The main issue that need special attention is the possible radioactive contamination of the filter therefore it is recommended to place the filter in separate room (technical zone) separated by wall to reduce the radiation exposure.
Pushing system: commonly used in applications where the NBC filtration system is placed outside the TFA area like vehicles, containers, tents and some types of shelters.
Typical pushing system airflow configuration scheme: blower > prefilter > NBC filter > Release to TFA
Pushing configuration considered safer for vehicles or mobile applications where the NBC system influenced by vibration and other external forces and air leakage in the system will not cause contamination of the TFA area. An additional advantage in such configuration is that the filters placed outside of TFA and their replacement in most cases can be made from outside as well as system decontamination.
MAIN COMPONENTS OF NBC FILTRATION SYSTEM
We will focus on the pulling systems that are mainly used in shelters and bunkers.
The main system components include:
Blast valve – installed on the air inlets or integrated in ventilation duct and used to stop the blast wave entering trough the ventilation openings to the protected area and preventing blast wave destroying the filters and other system components.
Prefilter – used for filtering coarse dust and larger particles to prevent them from entering the shelter and to prevent clogging of the HEPA filter inside the NBC filter.
Air ducts – there are two definite air duct lines. The dirty line, which is the duct before the filter, which has contaminated air inside and must be built stronger and gastight. The clean line, which is the duct after the filter, which holds only clean air.
Gastight Shutoff Valves (GSV) and airflow control valves
NBC filters – the filter can be radial, or flatbed type and typically built for two stages filtration:
Mechanical filtration of biologic particles and aerosols is performed by high efficiency particulate filter (HEPA).
Adsorption of gases and chemical components is performed by the activated carbon layer that is specially impregnated to comply with specific requirements and threat range.
Electric blowers – the system has to include high pressure blower to be able to overcome the pressure drop of the complete system (NBC filter, valves, duct and other components)
Overpressure valves – normally closed valves designed to release the pressure in the shelter over a specific value. The pressure created by combination of airtightness of the shelter and the air pushed by the NBC filtration system.
HOW TO CHOOSE NBC FILTRATION SYSTEM FOR THE SPECIFIC SHELTER
These are the main questions we ask the customers prior offering the NBC filtration system:
Number of people?
Size of the TFA area?
Specific threats or standards the system needs to comply with?
Is manual backup required or generator is installed in the shelter?
There is no united civil defense standard accepted by all countries and the system is chosen based on accepted standards or local threat analysis, environment conditions (temperature range, humidity and more) as well as specific local demands.
The volume of air required per one person: usually determined by different standards and presented in Cubic Meter Per Hour (CMH or m3/h) the values can be from 2 CMH per person and higher than 20 CMH. The most common is between 3-6 CMH for Civil Defense and and 17+ CMH for military/government facilities. Such a wide range can be explained based on the specific factors described above as well as shelter purpose and the activity of people sheltering inside. As an example we can compare the activity of people in command and control center which is much more intensive than the activity of people in civil defense shelter. As well command and control center may be equipped with various systems that may require large amount of air for proper function.
Air changes by the size of the shelter / TFA can be small or extremely large relative to the number of people sheltering inside. Besides the number of people in the shelter the size factor must be taken into consideration. When choosing the airflow of the NBC filtration system and based on the calculation of number of people vs size of the shelter we always take the higher between them. Typically we will consider 1-2 Air Changes per Hour (ACH) as a norm but will have to consider other factors that can influence this values.
National standards and specific regional threats - There are only a few countries around the world that has local civil defense standards that determines and regulates the shelters specifications and population protection requirements. Among such countries Finland, Switzerland, Israel, Singapore, Ukraine and more countries. Although most of the standards have a lot of similarities they were adapted to the unique local conditions and regional threats.
Of course, there is a long list of considerations that can be added, and each shelter project require a detailed professional evaluation of the existing and future unique technical, structural and general details.
MAINTENANCE FOR NBC FILTRATION SYSTEMS
Usually, life support systems are designed for long standby time and have to be completely operational in emergencies, even after years of storage. The parts in NBC filtration systems that are most vulnerable to long periods of storage are the NBC filters, EPDM / rubber seals and flexible duct.
It is important to follow the manufacturers maintenance schedule recommendations as well as local standards requirements that usually include visual and physical checks of the moving parts of all system components and weighting of the NBC filter for checking if they in allowed weight range. Filters that gained additional weight may have not been correctly stored, damaged or not hermetically sealed and the carbon absorbed too much moisture from the air. In some cases the filter can be dried or need to be replaced if gained moisture over allowed values.
GENERAL LIST OF THE MAIN EQUIPMENT USED IN SHELTER PROJECTS
Blast valves for every air opening (if not included in the filtration system set)
Gastight Shutoff Valves (GSV) for ventilation and sanitary ducts
Multi Cable Transmitters (MCT) for the safe passage of cables and pipes
Blast doors and windows (if applicable)
Airlock system (if required) for entry and exit under NBC conditions
Differential Pressure Meters (manometers) for measuring the overpressure in the shelter
CO2 removal system, if an extended lock-up mode is considered
Decontamination showers, if entry and exit of the shelter are permitted during CBRN conditions
Wall sleeves for the blast valves, GSV, MCT, and manometers
Shelter accessories, like toilets, beds, water reservoirs, radio, communication, batteries, etc.
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