|
|
This review examines the four essential steps in setting up a gas detection system and important considerations to bear in mind as you take each one. Joe Corsi, Wherever hazardous gases or vapor-producing liquids are used, transported, or stored, there exists the possibility that they could accidentally leak into the surrounding area. Continuous monitoring of such hazards is therefore an essential part of any safety program. The important steps in installing a hazardous gas detection system are: 1. Selecting the proper sensor. 2. Understanding what happens during a gas leak. 3. Installing the sensor properly. 4. Connecting the sensors to an alarm system. Step 1. Selecting the Proper Sensor The sensors used in area monitoring applications are typically diffusion in design. This means that the sensor is not part of an active sampling system that draws the sample to the sensor, but instead relies on diffusion and convection to obtain the sample. That is, the gas will mix with ambient air and diffuse through the sensor's flame arrestor without requiring a pump or aspirator.
Selecting the appropriate sensor for a given application is dictated by the gas or gases to be measured, the background gases present, and the conditions around the sensor location. Flammable hazards are measured in the 0–100% lower flammable limit (LFL) or lower explosive limit (LEL) range. Toxic hazards are measured in the low parts per million (ppm) range. Several sensor technologies are available in diffusion designs: catalytic and IR sensors for LFL range monitoring of flammable gases, and electrochemical and solid-state sensors for ppm monitoring. Catalytic Sensors. Catalytic sensors are appropriate for detecting flammable gases and vapors in the LFL range. When a flammable gas enters the sensor, it reacts with a catalyst-coated electrical coil. The resulting resistance change offsets the balance of a Wheatstone bridge circuit. The output signal is proportional to the concentration of flammable gas. Catalytic sensors have numerous strengths, including low cost, long life, and simplicity of design. But they can be affected by "catalytic poisons" such as silicones, plasticizers, and sulfur compounds that coat or corrode the sensor's catalyst. Infrared Sensors. The IR (point IR) has proven useful in monitoring methane in the LFL range. This sensor's chief advantage over the catalytic type is that it is not subject to catalytic poisons. Because it is an optical device, however, care must be taken to prevent fouling of the optics. Its usefulness in LFL monitoring of gases other than methane is limited to applications where gas mixtures and background interference are not issues. Electrochemical Sensors. Electrochemical sensors are excellent for detecting low parts-per-million concentrations of a select gas. They contain an electrolyte that reacts with a specific gas, producing an output signal that is proportional to the amount of gas present. Electrochemical sensors exist for gases such as chlorine, carbon monoxide, hydrogen sulfide, and hydrogen, but cannot be used to measure hydrocarbons. The number of gases that can be detected using this technology is relatively small, but is increasing from year to year. Solid-State Sensors. Solid-state sensors, typically based on a tin oxide semiconductor, respond to gases by changing resistance. They are used to measure numerous gases in the parts per million range. The devices are relatively inexpensive and have a long operating life. They have a low selectivity, however, and background gases can create inaccurate readings. In addition, the sensor's nonlinear output signal makes calibration more complicated. Step 2. Understanding What Happens During a Gas Leak Dispersion Characteristics of Gases. When a gas leak occurs, the gas tends to disperse into the atmosphere in a pattern based on its physical characteristics--its vapor density in particular. The diffusion rate of a gas into air is proportional to its density relative to that of air. Hydrogen, for example, with a density much lighter than air's, will diffuse very rapidly. Because the resulting hydrogen-air mixture has a density lighter than the surrounding air, convection currents lift the mixture in a plume similar to smoke rising from a cigarette in an ashtray (see Figure 1). For gases denser than air, the inverse is true. Most of the gases heavier than air are generated by liquids and are referred to as vapors. Gases with a density greater than air (see Figure 2) tend to settle along the ground or into a pit. Gases with densities very close to air do not diffuse much and tend to follow local air currents. Understanding Air Movement. In many cases, the movement of air is the greatest force in the dispersion of gas. When positioning sensors, careful thought must be given not just to the density of the gas but also to prevailing air flow. In some cases, it may be necessary to locate sensors counterintuitively. In Figure 3, a sensor placed on the ceiling at A will detect a lighter-than-air gas leak. But if the air flow in the room is as shown in Figure 4, the sensor at A will not detect the leak. Once the air flow in the room is understood, correct detector placement at B can be made.
The same logic applies to a heavier-than-air gas leak in a similar situation (see Figure 5). Locating the detector at B (see Figure 6) will result in earlier detection and warning than will be provided by the same sensor placed at A. Understanding Temperature Effects. In addition to density and air flow, temperature can also affect the dispersion of leaking gas. Most important, it can change the way a gas might normally behave. If the temperature of the air at the ceiling is much hotter than the rest of the room air, the ceiling air will have a lighter density because hot air rises. This "thermal barrier" may slow down the diffusion of the leaking gas enough to delay or prevent detection at the sensor (see Figure 7). Also, many lighter-than-air gases are stored as compressed liquids. When these gases escape into the atmosphere, their density may at first be heavier than air (see Figure 8) until they are warmed by the ambient temperature and become lighter than air (see Figure 9). Understanding Dilution Effects. Figure 10 illustrates the dilution effect that occurs when several rooms are monitored by a single sensor placed in the ventilation system. If the air volume moving through each of three rooms, for example, is roughly the same, a hazardous concentration in one room would be diluted to one-third of its true value because of the air movement from the other two rooms. Outdoor Monitoring Concerns. When installing sensors in outdoor applications, careful attention must be given to prevailing wind conditions. It may be necessary to monitor a single hazard (such as a storage tank) using several sensors (see Figure 11) so that an accidental leak can be detected regardless of wind direction at the time of the leak (see Figure 12). When monitoring gases and vapors outdoors, wind and weather become of particular concern. The equipment may be subjected to very hot and very cold temperatures in the course of the year, and may even experience large shifts in temperature from daytime to nighttime. Equipment will be exposed to rain, snow, ice, dust, and dirt. For outdoor applications, a rugged, robust instrument and sensor are essential. To prevent rain from entering the cell, the sensor should always point down, or at least never point above horizontal. When monitoring pump seals, pressure vessels, flanges, etc., hoods, tubing, or small ducts can be used to direct the escaping vapor toward the sensor. Step 3. Installing the Sensor Properly Installation is the most important aspect of the gas detection system. Calibration. To function properly, all of the sensors reviewed here require routine calibration. Installation is not complete until the system has been installed, allowed to warm up and stabilize, and calibrated. The sensor's zero and span response must be checked before putting it into operation. Typically, clean, bottled air (or, in some cases, room air) is used to set the zero; a known concentration of test gas must be used to set the span. Number of Sensors. There are no published guidelines or standards indicating the volume or area effectively protected by a diffusion gas leak sensor. In fire protection schemes where diffusion smoke detectors are used, the recommendation of Underwriters Laboratories is that each detector be assigned
Once the approximate sensor location is defined, final placement should take the concept of early warning into account. Early warning is accomplished by placing the sensor near the most likely point of a gas leak, while at the same time maintaining overall coverage of the entire area. Early warning means that a gas leak will reach the sensor and cause an alarm before the gas disperses into the entire protected volume. In many instances, more than one sensor may be needed to monitor a single hazard. Each SmartMaxII gas monitor can continuously monitor the readings from as many as four independent sensors. This capability lowers the cost of a gas detection system in three ways: there is less equipment to buy, less equipment to install, and less equipment to maintain. In many applications, costs can be cut as much as 50%. Step 4. Connecting the Sensors to an Alarm System Alarm Action. The sensors must be connected to a controller that is capable of producing alarms. All gas detection systems require three levels of alarm. The first level should provide early warning of a developing hazard and notify supervisory personnel to initiate corrective actions. The second must warn personnel and automatically stop the process or the flow of gas. If stopping the process is not feasible, then some action must be taken to control the hazard (for example, a water deluge or curtain to confine the flow of vapor). The third type of alarm will warn operators of malfunctions, loss of signal, loss of power to the system, or communication errors. Malfunction alarms should be connected to either the warning or danger alarms so that corrective actions are taken as soon as possible. Relays. Built-in relays provide maximum safety and ensure that critical alarms are initiated directly by the sensor. Direct action is more reliable than the use of a secondary device or an intermediary connection. The SmartMaxII includes three internal relays that can be programmed to activate external horns and lights and to indicate when the system is undergoing calibration. Output Signals. The SmartMaxII is equipped with both a 4–20 mA analog output and an RS-485 Modbus digital I/O port. This facilitates the transfer of readings to a PLC, plant-wide data acquisition system, or process control system. The digital port also allows access to and control of hundreds of sensors from any PC or laptop, either directly or through a modem.
Joe Corsi is Director of Sales, Control Instruments Corp., 25 Law Dr., Fairfield, NJ 07004-3295; 973-575-9114, fax 973-575-0013, sales@controlinstruments.com
|
|
|
900 square feet of ceiling space. While this guideline is helpful, it does not directly apply to gas detectors. Lacking set rules based on area or volume, the total number of sensors required must be determined by considering actual conditions, especially those highlighted in
