- Cables and Junction Boxes
- Location of Sensors
- Typical Sensor Mounting Options
- Typical System Configurations
- Installation Methods
The most common method employed to continuously monitor for leakage of hazardous gases is to place a number of sensors at the places where any leaks are most likely to occur. These are often then connected electrically to a multi-channel controller located some distance away in a safe, gas free area with display and alarm facilities, event recording devices etc. This is often referred to as a fixed point system. As its name implies, it is permanently located in the area (e.g. an offshore platform, oil refinery, laboratory cold storage etc).
The complexity of any gas detection system depends on the use to which the data will be put. Data recording allows the information to be used to identify problem areas and assist in the implementation of safety measures. If the system is to be used for warnings only, then the outputs from the system can be simple and no data storage is necessary. In choosing a system, therefore, it is important to know how the information will be used so that the proper system components can be chosen. In toxic gas monitoring, the use of multi-point systems has rapidly demonstrated their potential for solving a wide variety of workplace exposure problems and is invaluable for both identifying problems and for keeping workers and management aware of pollutant concentrations in the workplace.
In the design of multi-point systems, considerable thought should be given to the various components and to their interconnection. When using catalytic detection sensors, for instance, the electrical cable connections to the sensors would have three cores, each of 1mm squared, carrying not only the output signal, but also power to the electrical bridge circuit, which is located at the sensor to reduce signal voltage drop along the cables.
In the case of toxic (and some flammable) gas monitoring systems, the atmosphere is often sampled at locations remote from the unit and the gases are drawn by pumps to the sensors through a number of synthetic material, narrow-bore tubes. Care in design of such systems will include selection of suitable sized pumps and tubes, a sequential sampling unit for sampling each tube in turn and filters to stop particulates or water cutting off the flow of gas. The bore size of tubing can be critical, since it needs to be both large enough to allow rapid response times with standard size pumps, but at the same time should not be so large as to allow excessive dilution of the sample by air. Each sampling point must be connected to a separate tube and if a number of points are connected to a single, central sensor, it will be necessary to purge the sensor with clean air between samples.
The controllers used in fixed systems can be centrally located or distributed at various locations in a facility according to the application requirements. They come in a control panel and come in either single channel (i.e. one control card per sensor) or multi-channel configurations, the latter being useful where power, space or cost limitations are important.
The control units include a front panel meter or LCD to indicate the gas concentration at each sensor and will also normally have internal relays to control functions such as alarm, fault and shutdown. The number of alarm levels available varies between controllers but typically up to three levels can be set, depending on statutory requirements or working practices within the industry. Other useful features would include alarm inhibit and reset, over-range indication and analogue 4-20mA outputs. Often digital outputs are also available for interfacing the controller to a DCS/BMS. It is important to remember that the main purpose of a gas detection system is to detect the build up of a gas concentration before it reaches a hazardous level and to initiate a mitigation process to prevent a hazard occurring. If the gas concentration continues towards a hazardous level then executive shut down and hazard warning alarms are initiated. It is not enough to just log the event or measure the gas levels to which personnel have been exposed.
Cables and Junction Boxes
In a typical industrial gas detection system such as that just described, sensors are located at a number of strategic points around the plant and at varying distances from the controller. When installing electrical connections to the controller, it is important to remember that each sensor cable will have a different electrical loop resistance depending upon its length. With constant voltage type detectors the calibration process will require a person at both the sensor in the field and at the controller. With constant current detectors or those with a local transmitter, calibration of the field device can be carried out separately to that of the controller.
The sensor cables are protected from external damage either by passing them through metal ducting, or by using a suitable mechanically protected cable. Protective glands have to be fitted at each end of the cable and the sensor is mounted on a junction box to help in making simple, low-resistance, ‘clean’ terminations. It is also very important to ensure that all the gland sizes and screw threads are compatible with the junction box and the external diameter of the cables being used. The correct sealing washer should be used to ensure a weatherproof between the detector and junction box. A further point to remember is that sensor manufacturers normally indicate the maximum loop resistance (not line resistance) of their sensor connections when providing the information to calculate cable core diameters for installation.
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Location of Sensors
‘How many detectors do I need?’ and ‘where should I locate them?’ are two of the most often asked questions about gas detection systems, and probably two of the most difficult to answer. Unlike other types of safety related detectors, such as smoke detectors, the location and quantity of detectors required in different applications is not clearly defined.
Considerable guidance is available from standards such as EN50073 Guide for selection, installation, use and maintenance of apparatus for the detection and measurement of combustible gases or Oxygen. Similar international codes of practice e.g. National Electrical Code (NEC) or Canadian Electrical Code (CEC) may be used where applicable. In addition certain regulatory bodies publish specifications giving minimum gas detection requirements for specific applications. These references are useful, but tend to be either very generic and therefore too general in detail, or application specific and therefore irrelevant in most applications.
The placement of detectors should be determined following the advice of experts having specialist knowledge of gas dispersion, experts having knowledge of the process plant system and equipment involved, safety and engineering personnel. The agreement reached on the location of detectors should also be recorded.
Detectors should be mounted where the gas is most likely to be present. Locations requiring the most protection in an industrial plant would be around gas boilers, compressors, pressurised storage tanks, cylinders or pipelines. Areas where leaks are most likely to occur are valves, gauges, flanges, T-joints, filling or draining connections etc.
There are a number of simple and quite often obvious considerations that help to determine detector location:
To detect gases that are lighter than air (e.g. Methane and Ammonia), detectors should be mounted at high level and preferably use a collecting cone
To detect heavier than air gases (e.g. Butane and Sulphur Dioxide), detectors should be mounted at a low level
Consider how escaping gas may behave due to natural or forced air currents. Mount detectors in ventilation ducts if appropriate
When locating detectors consider the possible damage caused by natural events e.g. rain or flooding. For detectors mounted outdoors it is preferable to use the weather protection assembly
Use a detector sunshade if locating a detector in a hot climate and in direct sun
Consider the process conditions. Butane and Ammonia, for instance are normally heavier than air, but if released from a process line that is at an elevated temperature and/or under pressure, the gas may rise rather than fall
Detectors should be positioned a little way back form high pressure parts to allow gas clouds to form. Otherwise any leak of gas is likely to pass by in a high speed jet and not be detected
Consider ease of access for functional testing and servicing
Detectors should be installed at the designated location with the detector pointing downwards. This ensures that dust or water will not collect on the front of the sensor and stop the gas entering the detector
When siting open path infrared devices it is important to ensure that there is no permanent obscuration or blocking of the IR beam. Short-term blockage from vehicles, site personnel, birds etc can be accommodated
Ensure the structures that open path devices are mounted to are sturdy and not susceptible to vibration
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Typical Sensor Mounting Options (click for .pdf)
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Typical System Configurations (click for .pdf)
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Essentially three installation methods are used Worldwide for electrical equipment in hazardous locations:
1. Cable with indirect entry
2. Cable with direct entry
These are mainly used in Europe (although the US and Canadian Electrical Codes list Metal Clad and Mineral Insulated cables for use in Class 1 Div 1 or Zone 1). Explosion-proof standards state that cable systems with suitable mechanical protection must be used. The cable is often Steel Wire Armoured (SWA) if used in areas where mechanical damage may occur, or it may be laid in protective conduit which is open at both ends. Certified cable glands are used to safely connect the cable to the enclosure.
Indirect Cable Entry
Indirect entry is into an increased safety ‘Ex e’ terminal area. Line barriers are used on the wires between the terminal chamber and the main enclosure. The installer need only open access the terminal area, not the flameproof enclosure.
Direct Cable Entry
Direct Entry is made into the flameproof enclosure. Only specially certified glands may be used. The type and structure of the cable must be carefully matched to the correct type of gland. The integrity of the protection is reliant on the correct installation by the installer.
Conduit is the main method of installation in hazardous areas throughout the USA. The electrical wires are run as individual wires inside enclosed metal tubes. The tubes are connected to the housings by means of unions and must have a seal within 18 inches of each entrance point. The entire conduit system is flameproof.
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