Water treatment and many other technical processes frequently require the use of oxidising gases such as chlorine or chlorine dioxide. These gases are highly toxic and corrosive. Therefore special precautions have to be taken in areas where such gases are used. The Air is contaminated with oxidising gases will endanger plant personnel and equipment. If employees are revealed to low concentrations of these gases for extended periods, the safety limit will be reached. Higher concentrations gases will affect the respiratory system of the personnel and corrode machines resulting in costly repairs.
The gas leak detector monitors the contents of oxidising gases in the air, indicates leaks and warns the personnel in plants where chlorine or chlorine dioxide are generated, stored or metered. The unit will initiate a visual and audible alarm if the MAK value is exceeded. The MAK value is the maximum permissible gas concentration in the air of an area where people are working. MAK values are toxicological limit values.
Description of Operation
The gas leak detector consists of two crucial items; the detector cell assembly which is connected by means of a coaxial cable to a control box. The detector cell includes two platinum electrodes wrapped by a porous wick. The electrode is partly immersed into an electrolyte reservoir attached and thus kept constantly moist. One stuffing of the electrolyte reservoir is sufficient for approximately one year of continuous operation. The chlorine dioxide gas or chlorine gas be should present in the surrounding air, an electrochemical reaction by depolarisation will take place on the electrode; the electric current produced is proportionate to the gas concentration in the air.
An increase in the chlorine gas concentration is indicated on the electronic control box which is equipped with an indicator of 0...5 ppm Cl2. The desired alarm level is set by means of a potentiometer having a range between 0...5 ppm Cl2 If two alarm levels are required an amplifier with two operational set points will be used.
When the gas concentration in the air exceeds the preset value, the leak detector will release a visual and audible alarm and operate an alarm relay. This alarm relay provides a voltage-free single pole double throw contact for use by the customer to actuate safety devices such as a solenoid valve in the gas line or a neutralisation system.
When the gas concentration drops below the preset limit, the alarm contact will move back to its former position. If the preset value is exceeded for more than 10 seconds, the light emitting diode alarm horn will light and a further relay will be energised. The alarm horn is fixed to the potential free output of this relay. The relay will remain energised until it is reset either by means of the push button on the control box or by an external push button in the customer's control centre. After having stopped the alarm and after the gas concentration has dropped below the preset limit, the contact will automatically be in operating condition again.
The real functioning of the leak detector may be checked at any time by operating a test button. The test button is provided as a push button with a built-in light emitting diode.
The combined test button and cell failure alarm light will flash at the following conditions: Electrolyte reservoir empty
- Detector cell dry
- The cable between cell and control box interrupted
An internal circuit board provides for the continuous output signal of 4... 20 mA, 300 Ohms, corresponding to a gas concentration of 0...5 ppm C12. Industrial Devices manufactures also a chlorine gas generator capable of generating adjustable chlorine gas concentrations in the air. This unit is a very practical accessory item for the physicochemical calibration of the leak detector. The detector cell is placed close to the place where the air is to be monitored. As chlorine level is heavier than air, the cell is mounted approximately 350 mm from the floor.
The ambient temperature should be in the range between -10°C and +50°C. The control box containing the electronic amplifier, the alarm horn and other alarm devices connected to the alarm output should preferably be installed in a chlorine-free room. The distance between the control box and the detector cell may be adapted to the local requirements. While using a screened cable this distance may be up to 100 m. If the distances are more than 2 m between cell and control box, a connection box must be used.
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