Guidelines for design of fixed water mist firefighting systems – Part 1

Authors:
Gniewosz Siemiatkowski, VID FIREKILL
Diana Georgieva, TechInfo

The design of water mist fire extinguishing systems is no more complicated than, for example, the well-known sprinkler extinguishing systems. However, the lack of wider knowledge about this technology, which belongs to the group of fixed water firefighting systems, is a factor blocking its development. The article examines the water mist extinguishing installations described in the European standards as well as the guidelines of the American and German standards. Examples of applications, projects and protocols based on the European standard EN 14972 in its parts 2 to 17 are also discussed.

In May 2021, the Bulgarian Institute for Standardization published on its website the new Bulgarian standard BDS EN 14972-1 [1], issued on the basis of the European standard approved by CEN at the end of 2020. Until then, the resource of design engineers lacked a normative document on which to base the design of fixed water mist firefighting equipment. Alternatives were American guidelines such as FM technical data sheets or NFPA standard 750. Several applications were also covered by the German VdS standard.

That is why it can be said that a new stage in the development of water mist systems in Bulgaria is beginning . Access to reliable knowledge has always been a catalyst for dynamic development. With this article, we start a series of publications devoted to the design of water mist systems, using as an example the low pressure water mist systems of the company VID FIREKILL.

Introduction to the series

EN 14972 is a series of standards whose first part EN 14972-1 [1] as a design standard is similar in scope to the well-known standard for sprinkler installations EN 12845 [2]. Many of the provisions are even borrowed from the content of the sprinkler standard. Completely new, unknown in the field of sprinklers, are Parts 2 to 17 of the standard EN 14972-1 [1], containing full-scale fire test protocols . A fire test report is required to demonstrate that a water mist system is properly chosen for its application . Annex A to the standard is also very innovative, allowing the development of new test protocols that are not included in the scope of 2-17.

Until 14 March 2023 the following documents from the series of seventeen EN 14972 standards have been accepted into the catalogue of the Bulgarian Institute for Standardization

BDS EN 14972-1:2021 Fixed firefighting systems. Water mist systems. Part 1: Design, installation, inspection and maintenance.

BDS EN 14972-3:2022 Fixed firefighting systems. Water mist systems. Part 3: Test protocol for office, school class room and hotels for automatic nozzle systems.

BDS EN 14972-8:2020 Fixed firefighting systems. Water mist systems. Part 8: Test protocol for machinery in enclosures exceeding 260 m³ for open nozzles systems.

BDS EN 14972-9:2020 Fixed firefighting systems. Water mist systems. Part 9: Test protocol for machinery in enclosures not exceeding 260 m³ for open nozzles systems.

BDS EN 14972-10:2022 Fixed firefighting systems. Water mist systems. Part 10: Test protocol for atrium protection with sidewall nozzles for open nozzles systems.

BDS EN 14972-14:2022 Fixed firefighting systems. Water mist systems. Part 14: Test protocol for combustion turbines in enclosures exceeding 260 m³ for open nozzles systems.

BDS EN 14972-15:2022 Fixed firefighting systems. Water mist systems. Part 15: Test protocol for combustion turbines in enclosures not exceeding 260 m³ for open nozzles systems.

BDS EN 14972-16:2019 Fixed firefighting systems. Water mist systems. Part 16: Test protocol for industrial oil cookers for open nozzles systems.

Source: Bulgarian Institute for Standardization

Fire tests also lead to development of so-called DIOM (Design, Installation, Operation and Maintenance) document, which is a guide for the design, installation, operation and maintenance of the fire extinguishing system with water mist. This document contains guidelines for nozzle spacing, maximum height to ceiling, design area size, and other important data. It is therefore not surprising that the word DIOM occurs about 100 times in the design standard [1]. An important difference in the design process is that with sprinkler systems, the choice of component manufacturer is not important to design parameters. Whereas with water mist systems it is crucial which system the designer uses.

The design standard BDS EN 14972-1 [1] consists of 10 chapters and is similar in layout to the standard for sprinklers, which is intended to facilitate the work of designers with the new document:

1. Scope
2. Normative references
3. Terms, definitions and abbreviations
4. Design
5. Installation
6. Water mist system components
7. Main pumps for water mist systems
8. Testing, acceptance and commissioning
9. Inspection and maintenance
10. Documentation

Scope

The EN 14972-1 [1] standard covers all water mist applications using automatic or open nozzles that are described in test reports in parts 2-17 . It is important to note that with Annex A providing access to new protocols, this scope is almost unlimited.

The developers of the standard adopt a procedure for future development of the list of protocols. If, based on a given protocol, at least two manufacturers have successfully tested their solution and in absence of contraindications for using this test from any country, the CEN working group agrees to add this protocol to the list of EN series standards 14972. This means that extinguishing performance assessment methods developed by FM, VdS, BRE or other internationally recognized institutions can easily be introduced into the EN 14972 series.

The protection of potentially explosive atmospheres and fire protection systems intended for the protection of wagons and trains are excluded from the scope of this standard [1].

Definitions

In the definitions chapter, there are several unique terms that apply only to water mist systems. Among the most interesting are:

• Water mist – water spray for which the Dv 0.9 * is less than 1 mm measured in a plane 1 m from the nozzle at its minimum operating pressure.
  *Dv 0.9  is the drop diameter such that the cumulative volume, from zero diameter to the respective diameter, is 90% of the corresponding sum of the total distribution.

The above definition guides the discussion of water mist. It is not necessary to distinguish between high, low and medium pressure water mist or between droplet size classes. The standard covers all systems. The burden of evaluating the quality of a given system is to meet the fire test criteria, which are the same for all types of water mist. Of course, the difference in pressure is important for design reasons.

• Firefighting pump unit – a pump unit consisting of one or more pumps, one or more motors or engines and one or more control panels (controllers) governing the operation.

This is a completely different approach to pumping units than that presented in the sprinkler standard [2]. This is because high pressure water mist systems do not work with one pump and one controller due to the limitations of the pumps hydraulics. Therefore, a multi-pump-in-series standard has been developed that schematically resembles conventional multi-pump booster sets. The scheme of such a kit unit has become a standard for water mist. However, sprinkler pumps can power systems with low pressure water mist, and it is enough that their parameters optimally provide the pressure and flow requirements of the system.

• DIOM – Water Mist System Design, Installation , Operation and Maintenance Guide . Definition of a new and very important document that enables proper design. The requirements for this document are discussed in Chapter 10.

Project

The most important part of the standard from the designer’s point of view is chapter four – design. She is the focus of this publication. In the Sprinkler Standard [2] this scope was extended to Chapters 4-7. In the water mist standard [1] some aspects were taken out and left for DIOM and special protocols .

In the introduction we learn that water mist systems are designed to protect against specific hazards or are intended for premises that are covered by fire test protocols in BDS EN 14972, part and 2-17. For situations outside the existing/planned parts of the standard, the procedure in Annex A should be followed. The design should be developed taking into account the guidelines and application limitations contained in the DIOM. The second requirement for the designer, as well as for the installer and service technician of water mist systems, is the obligation to train corresponding to the activities performed. Such clauses are also found in the standard for sprinklers. The difference is that in BDS EN14972 [1] the training must be carried out by the system manufacturer, which is not applicable to sprinkler systems. There, competences can be acquired in the free market of education.

Section 4.2.2 describes the minimum requirements for DIOM. This is especially important for designers working with various water mist systems. Each manual should contain at least the following:

a) System type and manufacturer identification
b) Classification of protected premises with restrictions on use
c) Description of protected risks
d) Restrictions on interaction with ventilation or external conditions
e) Limitations of the protected area/premises
f) Requirements for fire barriers between compartments
g) Design guidelines (e.g. nozzle type with pressure and discharge parameters, maximum heights, nozzle spacing, obstruction treatment, design area size or design number of nozzles operating simultaneously).
h) List of critical components that must interact in the system
i) Minimum requirements for the quality of water or gas used in the system
j) Rules for calculating installations
k) Additive requirements, if applicable (e.g. foaming agents).
l) Important instructions for the correct operation of the system ( e.g. for nozzle covers or mounting accessories)
m) List of relevant test reports for each application and references to reports

In addition, the DIOM must include information about the use of the system:

n) Working principle
o) Instructions for installation and commissioning
p) Instructions for working with the system
q) Guidelines for inspection and maintenance and their frequency

The availability of the above data for all parties involved in the investment process is crucial for the success of the design of the installation and operation of the stationary water mist extinguishing system. Therefore, these materials should not be held as trade secrets of the manufacturer.

Picture 4: Automatic nozzles for low pressure (above) and high pressure (below)

Chapter 4.9 sets out design guidelines on a systems using automatic nozzles. Water mist protection should cover the entire building or space, taking into account all possible risks. Exceptions include spaces:

1. Toilets and bathrooms constructed of non-combustible materials and without a storage function for combustible materials.
2. Stairways and service shafts, provided they are constructed of non-combustible materials and are separated from adjacent rooms by fire barriers.
3. Premises protected by other fire protection systems .
4. Wet process zones (e.g. the wet part of the paper making process) .
5. Storage areas less than 2 m2 .

The water extinguishing system should not be use it in rooms where water can create additional hazards. Examples of this are liquefied gases at low temperatures or the long list of substances that react dangerously with water.

Unless specific requirements are stated in the DIOM, the values for the minimum actuation area or the minimum number of simultaneously operating nozzles (whichever is higher) shall be observed according to the table below:

The sprinkler guidelines in the different standards specify the need for control and alarm valves in different ways. The water mist standard specifies a threshold for the maximum size of a dry or wet valve fed area . This area must not exceed 10,000 m2 . This is a non-restrictive requirement that allows the system to be designed with a small number of zone valves. At the same time, it allows the use of simpler site-specific trigger indicators in the form of flow switches. Fittings are recommended to be fitted together with non-return valves and a drain hole to test the sensor’s performance at a flow rate equivalent to that of a single nozzle.

Chapter 4.13 considers the water supply and describes the requirements for the supply of gases if this is necessary for the operation of the system. Water mist systems must have at least one automatic water source. The following water sources are acceptable:

1. Fire water tank and pump station
2. Water supply from the water network with a possible pumping station to increase the pressure
3. Pressure Vessels (hydrophore tanks)
4. System with water cylinders, activated by pressurized gas
5. Gravity tanks.

Of particular note is the paragraph in the Water Mist Standard [1] that describes the water supply requirements for systems used to protect escape routes. This shows an additional advantage of water mist , which has not been touched on so far in the sprinkler standard [2]. The working group in the CEN committee pays special attention to the protection of life. The water source for the protection of escape routes must meet the following requirements:

1. Water supply from two separate sides of the installation with the possibility of saving:

a. full design capacity and pressure of both connections
b. independence of connections
c. with one pump to increase the pressure in case of loss of pressure in one connection
d. with the use of two or more pumps to increase the pressure in case of lack of pressure in both connections.

2. Supply from a gravity tank or an above-ground tank with a back-up pump, where the tank must:

a. to be in the full design volume
b. be protected from light and pollution
c. be made of non-corrosive materials and designed so that draining is not required more often than once every 10 years.
d. is filled with clean water (according to the manufacturer ‘s DIOM specifications) .

In the case of property protection systems, partial capacity tanks may be used, provided that at least two tank filling devices are provided which operate automatically without the need for an electrical supply. Systems designed to control and limit the size of a fire can be fed from tanks with a minimum size of 30% of the design quantity , ensuring a filling capacity that allows water supply throughout the design time of operation. Fire extinguishing systems (primarily Class B fires) may be tank fed with 5 minutes of pump operation for systems designed to operate for 30 minutes, and 10 minutes of pump operation for systems operating longer than 30 minutes. The water supply must be calculated in such a way as to provide the necessary flow throughout the entire design time of operation.

Diagram 1 shows the arrangement of the partial volume tank where under numbers 1 and 12 are the two measuring devices necessary to regularly test the operation of the connection (1) and the fire pumps set (2). A flow meter is sufficient to test the connection. Regular tests should demonstrate sufficient link capacity for the calculated fill parameters.

Diagram 1 : Diagram of a partial volume tank with a pump 1. flow meter to measure the capacity of the connection, 2. automatic water inlet, 3. normal water level, 4. overflow, 5. effective capacity of the tank, 6. minimum level of the water (min. 10 cm above the vortex inhibitor), 7. bottom, 8. vortex inhibitor, 9. shut off valve, 10. pump set, 11. water mist system inlet, 12. pump flow test meter.

The premises with tanks, pumps or compressors must be a separate fire hazard area with fire resistance exceeding the operating time of the system, or be a separate building with non-combustible construction. Shut off and control valves must be installed in places that are accessible in case of fire. This means that there is no need to install control valves in the pump room. They can be installed in another section of the installation, ensuring that they are in a different fire zone than the zone protected by this valve. Rooms with water sources (tanks, pumps, compressors) must be protected with a fixed water mist extinguishing device. Pump rooms with electric motors can be protected with OH1/HC1 nozzles, while pump rooms with diesel drives must be protected with open nozzles in accordance with the protocols intended for machinery spaces.

Documentation

Chapter 10 specifies the minimum scope of documentation to be included in the design of a water mist installation.

• The system must be fully described, indicating the names of the technical solutions, a description of the risk areas in the facility and the distribution of the types of nozzles chosen for their protection.
• The design should include the results of hydraulic calculations and tank size calculations, as well as the selection of pumps or pump sets.
• A complete system diagram should be drawn up, including all connections.
• The drawing must show the location, arrangement and designation of the types of nozzles, pipes and valves used, indicate the straps and clamps, etc.
• Description of the principle of operation of the system, including the intended sequence of operation, delay time, etc.
• The division and size of the sections must be indicated in the plans and sections.
• The bill of materials for the entire system should be included in the text or in the appendices.
• The technical data sheets of the components used should be included.

The above guidelines for complete project documentation are particularly difficult to implement in public procurement projects, where it is often impossible to specify specific parameters . In this case, the specified requirements will apply to the work project prepared by the company that won the tender.

Appendix A

Annex A is a set of guidelines for the development of new test methods for water mist systems and is a tool that allows fire tests in accordance with EN14972-1 [1] to be carried out faster than the normative way . However, this method should not be abused. The testing methodology must be developed in close cooperation between the interested parties – the accredited testing laboratory and the contracting authority, the designer, the assessor and the insurer. Such a configuration of the working group ensures that a reliable and hard-to-challenge test protocol, adequate of the hazard under consideration. The standard describes the necessary steps in the procedure for creating a new protocol and the results of the specific steps to be included in the documentation.

Conclusion

Designing water mist systems may seem difficult, but it is usually no more difficult than designing a sprinkler system. A sense of insufficient knowledge of the subject is a factor that blocks the further development of this technology, while water mist, like foam systems, complements the portfolio of stationary water extinguishing systems. Therefore, water mist should not be missing from the toolkit of the professional design office for fire protection. This publication aims to explain the meaning of the DIOM design standard and guidelines. The following parts of a series of publications will be devoted to specific examples of applications, designs and protocols based on the current and forthcoming parts 2-17 of the standard BDS EN14972.

See also:
Guidelines for design of fixed water mist firefighting systems – Part 2
Guidelines for design of fixed water mist firefighting systems – Part 3

Референции:
[1] BDS EN 14972-1:2021 Fixed firefighting systems. Water mist systems. Part 1: Design, installation, inspection and maintenance.
[2] BDS EN 12845:2015+A1:2020 Fixed firefighting systems. Automatic sprinkler systems – Design, installation and maintenance.
[3] Regulation № Iз-1971, October 29, 2009 on building and technical rules and norms for ensuring fire safety.

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