Explosion Protection

HOW IT WORKS

The spark enters the material conveying system via normal industrial process or by foreign objects in the system striking metal surfaces. Detectors are placed on the duct work to create a zone where no sparks can pass without detection. As the spark propagates and passes a detector, a signal is sent to the microprocessor in the Control Unit. Here the information is processed and Control Unit triggers the appropriate extinguisher, eliminating the spark and preventing a potential fire or explosion. At this point, the Control Unit can activate any user programmed instructions to open abort dampers, shutdown machinery, etc. The Control Unit executes its commands and eliminates a spark in less than 0.25 seconds.

As illustrated above, the Control Unit is the heart of the Spark Detection System. Henderson can supply you with two different types of Control Units depending on your system requirements. The most commonly used is the HIS1. This Control Unit is dedicated for only a single zone of protection, such as a single piece of equipment like a dust collector. This unit cannot be expanded for use with multiple zones and has to be replaced for system expansion to multiple zones. For multiple zone protection there is the HIS64 Control Unit. This Control Unit has a maximum of 64 zones of protection. You can start with a single zone installation similar to the HIS1, however the HIS64 can be expanded to handle up to the 64 zones without changing the Control Unit. This allows for more flexibility for future expansion of systems. Please see below for a more detailed description of the individual Control Units.

FEATURES AND ADVANTAGES

• LCD Display and keypad Command allows easy access to the software program. Here you can perform custom system programming and information retrieval. Alarm conditions are automatically displayed.
• Sensitivity Checking compares detector sensitivity with historical and/or initialized readings.
• Diagnostics provides positive identification of any problems on system hardware on a per unit basis (detectors, pressure transducers, extinguishments assemblies, programmable output modules).
• Programmable Output Modules connect Control Panel with individual plant process equipment. Spark event reaction can be customized to suit plant process needs.
• Output Isolation by a touch of the keypad gives you control of individual outputs, such as water sprays and abort dampers, for ease of commissioning, testing and servicing.
• Security Access Code ensures access to Control Panel is by authorized personnel only.

A spark detection system is composed of three basic components. First, the detectors, which are highly sensitive to optical radiation emitted by hot or burning particles. The detectors receives an optical signal and converts it into an electronic signal. The electronic signal is received and processed by the central control unit, which in turn triggers the extinguishing system. The three components: 1. Central Control, 2. Detectors and 3. Extinguisher, are all integral parts of a successful spark detection and extinguishing system.

A spark detection system is typically used in pneumatic particle transport systems where there is potential for combustion or explosions. These include wood chips, cellulosic fibers, sawdust, chemical dusts and any other combustible material. Other applications can be in mechanical transport systems such as conveyors or in the drop chutes. The detector reacts to rapid changes in the optical signal so it is especially suited for viewing a cross section of a duct where the particles are in the field of view for only a short period of time. The purpose of the system is to provide a zone in the path of transport where no potentially hazardous spark can pass without being seen by the detector or detectors. Usually, two detectors must be used in order to ensure that the entire cross section of the transport zone is within the field of view of at least one detector.

The extinguishing system used is a solenoid operated water spray located downstream from the detection zone. The water spray is set up to create a curtain through which the hazardous particle must pass. The Spark Detection controller provides the essential timing and the impulse to open the solenoid valve for a fixed period of time during which we are ensured that the spark has passed through the curtain of water. The timing is important, as the activation of the spray must not be too late as it will miss the spark. Also, to avoid unnecessary shut down and cleanup, the spray should not be on too long.

The system can also control a supplementary deluge water flooding; shut the dust collection system down or activate an abort gate.

The basic Spark Detection System includes two detectors, a control panel and an extinguishing system. This is the basic requirement for a successful spark detection system.

Described below are the basic components for the Spark Detection System.

HIS1 Control Unit

The HIS1 is a single zone, low cost, full featured Spark Detection and Extinguishment control panel. It is ideally suited to single dust collector facilities. This Unit cannot be incorporated into multiple zone systems.

The Main Control Unit is housed in an industrial cabinet 12” wide X 14” high X 5.5” deep. It consists of the Control Electronics, Backup Batteries and the Keypad/Display. The Control Electronics are mounted on a single printed circuit board. There are a total of 12 program switches on the printed circuit board which are utilized to configure the system as required. All external connections are conveniently made on terminal blocks at the top of the printed circuit board. The following equipment may be connected:

• Two Spark Detectors
• Supervisory Switches (optional)
• Water Flow Switch (optional)
• Heat Detectors (optional)
• Two Water Spray Assemblies (one optional)
• Alarm Systems or PLC's (via Alarm and Trouble Dry Contacts)
• Abort Damper Control (via Heavy Duty Relay Contacts)
• Conveyance System Shutdown (via Heavy Duty relay Contacts)

The system may be set to test the complete operation of the detectors, including sensitivity, every 6 hours by the setting of switch 10. Detector sensitivity testing can also be accomplished using the Keypad/Display.

HIS64 Control Unit

The HIS64 is a true Distributed Processing Spark Detection and Suppression System supporting up to 64 Intelligent Zone Controllers. The Zone Controller can detect, spray and shut down the dust collector even if communications to the main control unit is interrupted. The HIS64 is the most powerful and versatile system available with each zone configurable to your unique requirements. The HIS64 maintains a time stamped history for all activities which may be printed on demand or transmitted to a plant PLC or Computer - it even stores a Spark Count for every detector in the system. Automatic sensitivity testing, heat detector monitoring, waterflow monitoring and a variety of other sophisticated features are all standard in the HIS64.

Detector

The detector is housed in an outdoor NEMA 4 rated aluminum enclosure. It is equipped with a quick-release mounting bracket for mounting onto the surface of a duct. The quick-release mounting bracket allows for easy periodic cleaning of the lens surface. Each detector is equipped with an LED located behind the protective lens. This LED is used to test the opposing detector when more than one detector is used in a duct. In this way each detector can verify the sensitivity of the opposing detector.

The specifications of the detector are:

• Sensitivity = 1 .0 µW/cm²
• Response Time = 60 µsec
• Angular Acceptance = 140 degrees
• Half Power Cone of Vision = 120 degrees
• Spectral Response = 0.4 - 1.1 microns
• Response Peak = 0.9 microns
• Temperature Range = -40°F to 140°F
• Current = 15 mA

Spray

The water spray system is comprised of the Mounting Flange, Gasket, Strainer, Nozzle and Valve. This unit is always supplied as an assembly and is specifically designed to operate and be installed as an assembly.

The solenoid valve portion of the assembly partly relies upon differential water pressure across a diaphragm to open and close the valve. This flow of water takes place in small diameter orifices, which should be kept free from debris as much as possible. Therefore, the Nozzle and Spray Assembly includes a strainer immediately upstream of the valve assembly. It is recommended that all newly installed piping upstream of the assembly be thoroughly cleaned out prior to startup. This will help reduce the chances of foreign matter lodged in the pipes from entering the assembly.

An option available with the system is a water flow switch to ensure that the assembly actually sprayed water when a spark was detected. A pressure switch may also be located upstream of the Spray Assembly. This allows verification of the presence of adequate supply pressure of the water source with pressure within the operating parameters of the Spray Assembly. The minimum water pressure is 50 PSI and the maximum recommended water pressure is 100 PSI - achievement of which may require a pressure reducer.

Deluge Water Spray

The Dust Collector Deluge Water Spray system is comprised of a Strainer, Valve, and the appropriate number of nozzles, each fitted with the Mounting Flanges and Gaskets. The size of the valve and the number of nozzles are determined by the dust collector size and shape. The Solenoid Valve Assembly is normally installed in the plant facility where temperature remains above 5°C. Piping from the valve must naturally drain from the valve or must be fitted with a drain cock to ensure water lines to the dust collector do not freeze.

The Solenoid Valve relies upon differential water pressure across a diaphragm to open and close the valve. This flow of water takes place in small diameter orifices, which should be kept free from debris as much as possible. Therefore, a strainer is required immediately upstream of the valve assembly. It is recommended that all newly installed piping upstream of the assembly be thoroughly cleaned out prior to start-up. This will help reduce the chances of foreign matter lodged in the pipes from entering the assembly.

Sensors

The HIS1 system is compatible with most available sensors to enhance the power and reliability of your spark detection system. Types of sensors avialable on the market include:

• Water Flow switch
  • The water flow switch is generally employed before the duct nozzle and is used to verify that water actually sprayed on a detection of a spark. If water failed to flow on detection of a spark, a trouble condition will be initiated. A trouble condition will also be initiated if water flows when there has been no spark detection.
• Pressure Switch
  • The pressure switch can be employed to ensure that proper water pressure is available when it is needed. Optimum water pressure for the spray assembly is 50 – 100 PSI therefore we recommend that you set the pressure switch to generate a trouble condition if water pressure falls below 50 PSI. This switch would connect to the supervisory input and would generate a trouble condition when the pressure is too low. If trouble conditions are common because your water pressure is too low, you will need to install a booster pump.
• Plug Up Detector
  • The Plug Up detector provides early detection of plugging or bridging of material such as wood shavings, saw dust or wafers in cyclones, dust collectors, hoppers or ducts. It consists of a light source mounted on one side of the transport and a detector on the other side of the transport. A high density of material in the transport will block the light from the receiver and cause an alarm. This switch would connect to the supervisory input and would generate a trouble condition when a plug up condition exists.
• Broken Bag Detector
  • The broken bag detector can be installed after the baghouse of filters to ensure proper filtering is taking place. This sensor provides a closed contact when the particle count of the filtered air is too high. This normally means that a filter bag has broken and should be replaced. This switch would connect to the supervisory input and would generate a trouble condition on detection of a broken bag.
• Heat Detector
  • A heat detector is available to monitor for excessive heat in the dust collector. The heat detector provides a contact closure when the temperature exceeds the detectors rating. The system may be programmed to activate a deluge water spray into the dust collector. Activation of the heat detector generates an alarm condition.

An effective Spark detection system must monitor all potential paths of a spark or ember through all branches of ducts and once a spark or ember is detected, it must establish a curtain of water through which the spark must pass. Each main duct system is considered a protection zone. If your dust collector has a single main duct, then only one zone is required.

The air in the ducts is traveling at a very great rate of speed and therefore carries a spark or ember very rapidly. Air transport speeds of 6000 ft/min. are not uncommon which means a spark can be carried 100 feet in one second. Detecting the spark, turning on the water and establishing a spray curtain across the entire duct can take as much .288 seconds or .0048 minutes. If the spark is traveling a 100 feet per second, we will therefore require the detector to be placed at least 28.8 feet ahead of the water spray nozzle since that is how far the spark will travel in .288 seconds. If there is insufficient length available on the main duct, detectors can be placed on the branch lines or the main duct must be extended. Note that these are minimum distances.