ARMAC Fire Defence System - Technical Notes Part B
Final
Tank and pump shed.
The lower DN50 copper pipe is the inlet from the tank to the pump. The upper DN20 pipe recirculates water to the tank when the system is in test or during the off phase of the intermittent cycle (when battery voltage < threshold). The tank water level sensor cable in a protective silicone sleeve is attached to the recirculation pipe.
Layout of plumbing in Pump Shed
Planned
Final
Outline of the water control system
The water system is controlled by an electronic system based on multiple layers of communication and control to increase resilience in the event of failure of one sub-system under the pressure of a fire emergency.
The pump itself has its own commercially constructed pump control module, and that can be operated directly by mobile network text messages, but also by external signals coming through one of its terminals.
The pump shed, being all metal and thus a near 'Faraday Cage', is not a good place to receive wifi, even though a magnesium oxide window (transparent to wifi and mobile network signals) has been set into the side of the shed where the pump control module is situated. In order to ensure excellent wifi connectivity and insulate the more sensitive electronics from heat, the bulk of the electronics are established in the house, with an underground multicore cable connecting the house control system with the shed controls.
Various devices (pumps, sprays, and sensors) are controlled or monitored remotely via a dashboard which can be accessed online via a mobile phone (iOS/Android), or tablet, or computer. A backup wifi router is established and powered from a battery charger/inverter, and is connected through an LTE SIM card, to enable automatic fallback wireless connections if the normal network/wifi becomes unavailable.
Pipe Supports
The copper pipes to the sprinklers are supported at regular intervals by poly irrigation pipe straps which are screwed to treated pine battens.
Feeder pipe
Since under the soil conditions at this site, and the distance from the house to the tank is only approx 8 metres, a trench dug by hand implements is adequate and was used. The trench depth varies considerably but since it is normally > 250 mm, the backfill of approx 100mm places the 63 mm external diameter polypipe (i.e. DN50 or 2" internal diameter) at a depth which has been shown to be adequate.1 The feeder pipe runs around the house passing close to each sprinkler. Normally the feeder pipe is about 300mm from the house itself (measured to the nearest point of the pipe). The exception to the feeder pipe being placed as close as possible to the house is where the ground around the house is bedrock. In this case the pipe is run around the bedrock. This means that the feeder pipe runs about 1 metre further from the house in places, even though the bedrock itself is only a centimetre or so under the soil. This is necessary since a rock saw would be required to cut the bedrock, and even allowing for that, there is still the problem of adequately placing the copper risers to the sprinklers in a vertical position. Given that most of the (impact) sprinklers have a 12-24 m range, any disadvantages from being a little further from the house are trivial.
Pump
The pump chosen was a Garpen diesel pump. The requirement was for a reliable pump with a high flow rate and pressure, and a diesel motor driving the pump itself so that it would not require electrical power and could run after electricity blackouts at times of fire.
The pump performance is shown below. At 180 kPa (1.8 Bar / 26 PSI) the pump delivers 800 l/min (48 m³/hr), and at 420 kPa (4.2 Bar / 61 PSI) the flow rate is approx 650 l/min (39 m³/hr).
Pump Shed
The pump shed was custom built from colorbond steel, and has two compartments. The main compartment houses the pump and the associated electronics and plumbing. A smaller compartment stores the Barricade gel supply (should gel be added to the water as a final stage, or for other valuable equipment).
The shed is mounted on a 1200 x 2400 x 100 mm concrete pad, reinforced with steel mesh. The concrete was poured and the mesh placed in the wet mix.
Pump shed protection
The pump shed itself is protected by 5 fan spray sprinklers. These are fed from a DN12 (12mm internal diameter copper pipe) run from the main sprinkler low volume line, upstream of the low/high motorised valve, and run through a NC (normally closed) solenoid valve. This allows the shed to be cooled at any time, independently of the sprinkler system proper, even when the pump is off.
The sprinklers are angled to spray the walls and roof of the pump shed.
Pump shed ventilation
Adequate shed ventilation is critical since the pump produces considerable heat (and fumes). A large louvred vent has been installed in the back wall of the shed, and smaller vents in the side walls. An extractor fan on a thermostat was initially considered but was not installed since (a) it would require electrical power which might not be available in a fire emergency, and (b) the natural convection through the louvres has proved adequate.
Consideration of further insulation
Some consideration was given to lining the pump shed with insulation board. However, this was not done for several reasons:
- The shed cooling sprays keep the external temperature of the shed down
- The insulation would need to be fire-rated
- The diesel pump can operate at quite high ambient temperatures (practical experience suggests well above the manufacturer's stated maximum)
- Insulation would reduce the internal volume and make access more difficult
Water system components
The water system is composed of:
- 30.3 kL water tank (3.5m diameter x 3.1m high)
- Diesel pump (Garpen)
- Pump control module (commercial unit with LTE connectivity)
- Remote control system (Blynk dashboard via mobile/tablet/computer)
- Autonomous control system (Arduino-based)
- Sprinkler system (impact and fan spray heads)
- Valve control system (motorised 3-way and solenoid valves)
- Sensor system (temperature, flow, water level, ember detection)
- Communication system (WiFi with LTE failover)
Tank
The tank specifications are:
- Capacity: 30,300 litres (30.3 kL)
- Dimensions: 3.5m diameter x 3.1m height
- Material: Colorbond/Aquaplate steel
- Outlet: DN50 (2" internal diameter)
- Inlet: 3/4" with float valve
- Water level sensor: Ultrasonic (mounted at top)
The tank is fed from the mains water supply via a 3/4" stainless steel float valve which automatically refills the tank after use. The valve is rated to 1 MPa which exceeds normal residential water pressure (0.3-0.5 MPa).
Flow, pressure and sprinkler requirements
The table below summarises the flow and pressure requirements for the different operating modes:
| Mode | Pressure | Flow Rate | Run Time |
|---|---|---|---|
| Shed Cooling Only | 100 kPa | 5 L/min | Unlimited (with refill) |
| High Volume | 420 kPa | 250 L/min | 30 minutes |
| Low Volume | 180 kPa | 150 L/min | 3+ hours |
| Intermittent Low | 180 kPa | 150 L/min (cycled) | 6+ hours |
| Low Volume + Gel | 180 kPa | 150 L/min | Limited by gel supply |
The high volume mode is designed to run when the fire front is approaching or has arrived (external temperature > 65 deg C). The low volume mode is for ember attack prevention. The intermittent mode extends the available run time by cycling between spray and recirculation.
Intermittent spray Mode
The intermittent spray mode works in one of two ways:
Method 1: Valve switching - A motorised 3-way valve switches between directing water to the sprinklers and recirculating it back to the tank. This conserves water while maintaining pump operation.
Method 2: Pump cycling - When battery voltage is adequate (> 12.25V) and cycle length is >= 5 minutes, the pump is started and stopped to achieve intermittent spray. This is more water-efficient but places more stress on the pump starter.
The intermittent cycle is controlled by the autonomous module based on:
- External temperature
- Rate of temperature rise
- Ember trap activation
- Battery voltage
- Time since last ember detection
- Manual override settings
Extending run-time in intermittent spray Mode
Run time in intermittent spray mode can be extended by:
- Increasing the off-time in the spray cycle (reduces average water consumption)
- Using valve switching instead of pump cycling (maintains pump readiness)
- Reducing the number of active sprinklers (requires manual valve adjustment)
- Adding gel at the final stage (increases surface protection per litre)
The table below shows the effect of different spray/pause ratios on total run time:
| Spray Time | Pause Time | Duty Cycle | Effective Run Time |
|---|---|---|---|
| 2 min | 2 min | 50% | 6 hours |
| 2 min | 4 min | 33% | 9 hours |
| 2 min | 8 min | 20% | 15 hours |
| 1 min | 4 min | 20% | 15 hours |
Note: These times assume a 30.3 kL tank starting full, low volume mode (150 L/min spray rate), and no tank refilling during operation.
Table: Summary of intermittent spray mode
The key parameters for intermittent spray operation:
| Parameter | Value | Notes |
|---|---|---|
| Tank Capacity | 30,300 L | Starting full |
| Spray Flow Rate | 150 L/min | Low volume mode |
| Continuous Run Time | 3 hours | = 30,300 / 150 / 60 |
| Intermittent 50% Duty | 6 hours | Spray 2 min, pause 2 min |
| Intermittent 33% Duty | 9 hours | Spray 2 min, pause 4 min |
| Intermittent 20% Duty | 15 hours | Spray 2 min, pause 8 min |
| Minimum Cycle Time | 5 minutes | For pump start/stop method |
| Battery Threshold | 12.25 V | Minimum for pump cycling |
The autonomous module automatically selects the appropriate duty cycle based on:
- Detected threat level (ember traps, external temperature)
- Rate of temperature increase
- Time elapsed since last threat detection
- Available battery power
- Manual override settings from the dashboard
Copper Pipe Risers to Sprinklers
Each sprinkler is fed from the feeder pipe via a copper riser. The risers are:
- Material: Type B copper pipe
- Sizes: 15mm or 20mm depending on sprinkler
- Fittings: Brass compression or flared fittings
- Valves: Brass ball valve at each sprinkler for individual control
- Connection to feeder: Saddle clamp with brass compression fitting
The copper risers run underground from the saddle clamps on the feeder pipe up to ground level where they connect to the adjustable sprinkler mounts.
Anti-Syphon Control
To prevent syphoning of water back into the tank when the pump stops, anti-syphon valves or air gaps are used:
- Recirculation line: Returns to tank above water level (air gap)
- Main spray lines: Check valves prevent backflow
- Vacuum breakers: Installed at high points in the system
The vacuum breaker on the clerestory window sprinkler is visible in the earlier photos.
Footnotes
1 See https://bushfireresilience.org.au/wp-content/uploads/2022/03/Video-07-Sprays-2.pdf Justin Leonard: "Minimum 100mm depth is adequate unless a vehicle or pile of wood or a fence is burning above the line, in which case needs to be deeper."
Continue to: Part C
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