Stewart Kidd outlines the applications and advantages of direct and indirect modular fire protection systems

THE USE of bursting tube systems is not perhaps as new as some people believe. Burst tubes combined with low pressure cylinders containing a suitable firefighting agent provide low cost, easily installed automatic fire suppression systems (AFSSs) that are now a reality – and they have been around for 25 years. A small bore (typically 4mm) plastic tube can act as both a fire detection and suppression device. 
Such systems are often called direct low pressure (DLP) applications systems and indirect low pressure (ILP) application systems.
While the use of AFSSs employing water (sprinklers and watermist) are well understood, these are mainly used for area protection – often a whole building or at least a very large building compartment. Other AFSSs employing inert or chemical gases are typically used for protecting equipment or objects or, in some cases, smaller compartments in buildings. Typical uses include transformer or switchgear chambers, IT and data equipment, and the engine compartments of mobile plant. 
In many such cases, DLP and ILP systems may do a satisfactory job more cost effectively. It’s true that the loss of Halon 1211 (BCF) left a void in the market for such ‘local protection’ and large, cylinder based gas systems (and increasingly, modular cylinder based water mist systems) are now being specified where a sprinkler system might be inappropriate or where there is a restricted water supply availability. In some locations, an entire building may be sprinkler protected, but a gas system may be provided for areas where there are concerns about the impact of water on the equipment or materials found there – for example, in an archive store with mobile racking. 
Modular systems
In a modular or DLP or ILP system, the key element is the burst tube – which acts as a detection device. The tubes are specifically manufactured thermoplastic sealed units which are designed to rupture at a specified temperature, usually 70ºC (for comparison, the most commonly used sprinkler heads operate at 68ºC). The benefits of direct and indirect modular systems are:
  • flexibility afforded to the specifier in respect of the choice of the most appropriate agent for the risk
  • ease of installation
  • low probabilities of spurious actuation
  • lower recharge quantities and cost
  • cost effective transport 
  • low costs both of installation and maintenance
  • cylinders can be located inside the protected space – even inside a cabinet or engine bay 
Direct and indirect modular systems are essentially agent delivery systems and a wide choice of firefighting agents1 is available, typically including:
  • dry powder (ABC)
  • chemical gases (eg Novec 1230)
  • water and watermist
  • foam/water solutions
  • wet chemical (Class F)
  • inert gases (eg nitrogen)
Of the commonly used agents, only carbon dioxide would not be appropriate in the modular systems presently available – this is due to the pressure under which the CO2 gas is stored.
Structure and layout 
DLP or ILP systems using burst tubes rely on the stored pressure that is contained in the detection tubing and the agent storage cylinder. The most frequently used pressurisation gas for powder and liquids
is nitrogen. 
The detection tubing is extremely flexible and can be run extensively through the protected area. Once the tube bursts, it releases the pressure of the detection tubing, which then creates a differential pressure in the valves located in the top of the cylinder(s). 
As the tubing is normally charged to 10-20 bar/140-280psi, the moment the tube ‘sees’ a flame it will burst due to the differential pressure between the atmosphere and the pressure in the tube. 
System types
Burst tube systems using low pressure (maximum 20 bar) can be divided into two types: direct and indirect. In the direct system, the burst tube acts as a detection device and also a discharge mechanism. Some manufacturers now offer direct systems which are totally self contained, with no requirement for an external power supply – a major advantage in systems such as those fitted in mobile plant or vehicles.
Indirect systems use the burst tube solely as a detection device; when it loses pressure due to the effect of external heat, this actuates the sophisticated differential pressure valve. This then discharges the agent down a separate system of stainless steel pipework or a discharge hose, which will be fitted with nozzles to direct the agent to the desired locations. 
Other components
Cylinder valves are invariably supplied with a pressure gauge so that a quick visual check can be made by the user to confirm that the system is still charged. Additional, optional equipment can be added to the basic modular system, including:
  • manual actuation devices (for indirect systems)
  • input for operation by smoke or heat sensors 
  • remote reading pressure gauges
  • system isolation switches 
  • outputs for disconnecting power supplies – probably essential for modular systems protecting electrical or electronic equipment – disconnecting the protected racks from mains power will usually prevent reignition.
Range of applications
Originally designed for use in racing cars, DLP and ILP systems are now found in a wide range of other applications, one of the most common being in heavy mobile plant such as earthmoving equipment. Some vehicle manufacturers now offer the fire suppression systems as a factory fitted option.
Recent fires in recycling plant sites have also demonstrated the benefits of fitting fire suppression in the grinding and shredding machinery (which is often also mobile) used to process waste, including plastics.
Other proven applications:
  • public service vehicles (coaches and buses)
  • electrical control cubicles
  • data network controls
  • IT/server cupboards
  • small alternators – especially in unmanned hydro stations
  • wind turbines – gearboxes, yaw ring hydraulics, PF capacitors
Standards and certification
As all DLP and ILP systems are based on pressurised cylinders, it is a given that all manufacturers will supply only equipment which meets the requirement of the Pressure Equipment Directive (2014/68/EU) and be CE marked accordingly.
There is, however, at present no British or European standard for DLP, ILP or modular fire suppression systems. However, there is a 2017 BRE Loss Prevention Standard – LPS 1666: Requirements and test procedures for the LPCB approval of direct low pressure (DLP) application fixed fire suppression systems. There are at present three companies holding product certification to this at the time of writing. 
The LPS scheme so far only accredits single container systems (which some will view as a retrograde step, as it excludes twin agent systems, which have a proven track record in large mobile plant). It’s worth noting that while LPS 1666 does not specifically exclude mobile plant in its scope, in Section 1 it mentions only ‘enclosures such as electrical switchgear cabinets, server racking and similar installations’. There is also a restriction on the size of systems at protected areas of a maximum of 2 cubic metres and a maximum detection tube length of 10m. These limitations implicit in the scope of 
LPS 1666 have prompted some UK interest in an alternative US standard, FM 5970. 
Outside the UK, a design standard for fire systems on mobile plant was issued by US insurance group FM Global, through its approvals service, as FM 5320: 2000. This standard only related to dry powder systems, but in the course of time, it was, de facto, adopted by some organisations as a generic standard for all direct/indirect low pressure (DLP/ILP) systems, even though the standard may not have 
been wholly suitable for systems using water, foam and chemical and inert firefighting gases.
In respect of DLP/ILP systems in heavy mobile plant, FM 5320 has been superseded by the issue of ANSI FM 5970: 2017: Heavy Duty Mobile Equipment Protection Schemes. To date, no UK manufacturer has yet achieved a listing to this standard.
The question of certification and approvals is perhaps most important in connection with the protection of public service vehicles, as there is now an EU requirement (based on a UN mandate) for all buses and coaches used for the transport of more than 22 people. The Regulations, contained in UNECE R107 Revision 32, require that all fire suppression systems so used must comply with a specified test method, SP 49123. So far, only a limited number of UK suppliers have systems that comply with this test standard.
There can be no doubt that LP modular systems have substantial benefits for small area or object protection, and in areas where there are space limitations they may well become the system of choice. In particular, alternative energy production, including wind and solar power, may well provide a whole new market for these flexible systems.
Stewart Kidd is a fire and security consultant and chartered security professional. For more information, view page 5
  1. For a wholly independent and impartial view of the agents available, refer to BS 5306-0: 2011: Fire protection installations and equipment on premises. Guide for selection of installed systems and other fire equipment – note that this version of the standard does not consider modular systems. A review of the standard will take place in 2018 and the author will be pressing for reference to be made to modular systems.

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