Effective suppression
Published: 01 January, 2008
Wallop Defence Systems, owned by Esterline Aerospace and Defence, is a producer of traditional point source Magnesium Teflon Viton (MTV) Flares (anti-missile) for the UK Ministry of Defence and other defence ministries around the globe. To meet an increased demand it decided to build a new manufacturing facility. However, choosing the best suitable protection for this new building, in which the highly volatile flares would be produced, presented a challenge.
Anti-missile flares are handmade blocks containing large quantities of magnesium designed to burn rapidly without oxygen and are designed to generate a high heat output.
At first, Wallop Defence Systems considered Inergen as a choice of fire protection to replace the CO2 systems that were used traditionally. CO2 didn’t cut it for this high-risk application because the extinguishing capabilities of these type of systems are limited as well as unsafe for humans.
Wallop Defence Systems consulted Tyco Fire and Integrated Solutions about the most suitable application for its specific risks. Bob Whitely, Engineering and Standards Manager, was put on the case. He advised against the use of Inergen because anti-missile flares do not need oxygen to burn and Inergen works by reducing oxygen in an enclosed space. He decided to carry out trials with an ultra-high speed deluge system, called Pilotex, which is was developed for all types of [n1]rapid combustion applications, including those in oil, gas and chemical industries.
“Pilotex is suitable for high-risk applications such as in the munitions and explosive industries. Systems such as Pilotex are usually installed at points in the production chain where there is an ignition risk to the fuel or where there is a potential of rapid fire spread,” Bob explains.
It works as follows; high velocity projectors, which are mounted in a shuttle valve, are held closed by the sheer pressure of the water. Adjacent to each valve is a high-speed, solenoid-operated, water dump valve connected to a special high capacitance panel – a device designed to provide the quickest response time from fire detection to water discharge. Solenoids are electrically operated valves and are installed very close to each valve on the pilot line. A small (approx. 0.5 HP) pump increases the standing pressure to 170 psi (11,7 bar) to boost the velocity of the initial water discharge.
Bob continues, “The valves are kept closed by differential pressure. In situations where the water pressure has been taken off the pilot line, the decreased water pressure drives the piston upwards and the valve is opened. When pressure is restored the piston is driven back down and the valve is resealed.”
In flare production installation each workstation is monitored by ultra sensitive UV flame detectors, which send off an alarm as soon as they pick-up the tiniest spark of UV radiation. The detectors have 90° cone of vision and are carefully positioned to view only the hazard/work station. These areas do not have extraneous UV light sources. In less than 100 milliseconds, the system delivers the water onto the source of combustion and extinguishes the flare.
As all workstations are independently protected it means that when one valve releases water the remaining inactivated valves in the system will still work by simply re-setting the control panel.
The tests were carried out at the Wallop facilities in a quality control test shed. The team assembled a specially formulated rig, comprising a wooden frame supporting two Pilotex nozzle/valve arrangements. A flame detector was integrated with the test system as well as a 200-litre pressure water tank. The test started off with the smallest flare set on a metal carrier and in order to test system viability it was fitted with an electronic igniter. When this flare was successfully extinguished, the tests continued with a 117 and a 217-sized flare placed side by side. The results were surprising; the 217 was burnt partly, the 117 however – although ignited – did not burn at all.
In order to probe operator safety, a mannequin hand fitted with a latex glove was placed next to every test flare and in all tests the rubber glove on the mannequin hand was unharmed.
The test were very successful, however it was felt that the two nozzle array would not be sufficient for the larger flares so a second set of tests was carried out – but with four nozzles protecting the test platform. The array was again supplied by a 200-litre pressure vessel but with a fire pump for ongoing system operation.
A HS4-type flare was burnt and fully suppressed although this time with only a residual amount of flare material left. The testing continued with the very large 55mm flare with similarly spectacular suppression results, without any damage at all.
The final trial entailed burning a HS4-type flare without activation of Pilotex, which resulted in a huge fireball engulfing the test area.
Wallop Defence Systems was extremely satisfied with the results and will look to Pilotex going forward.
