Position is the key to effective gas detection

Dr Wolfgang Jessel, of Draeger Safety UK Ltd, in this review of factors affecting the installation of a monitoring system, says that position is everything when designing a gas detection system.

When planning a new gas detection installation or considering making changes to an existing system, there are a number of questions that need to be answered. Only then can the correct system be specified. For example, some systems are designed to eliminate the risks to health in areas where toxic substances may be found. Others are used to detect combustible gas leaks and prevent fire and explosions. According to Draeger Safety, the area to be monitored, the type of sensor, the control methods and ongoing maintenance requirements will all have a bearing on which is the right system for the particular application.

Answers needed

Typical questions that need to be asked might include: What is the main purpose of the monitoring? Where, how often, and in what concentrations and quantities are the hazards likely to be released? Which sensor is the most appropriate, and how many sensors are needed? Where and how should sensors be positioned and calibrated? What alarm thresholds are appropriate and, of course, how will the alarm information be processed?

When working with potentially explosive gases, for instance, a gas detection system can be designed to trigger a countermeasure as soon as a predetermined gas concentration level has been reached. If this measure proves to be inadequate and the gas concentration continues to rise, then a second alarm threshold will be crossed and all potential ignition sources can be automatically switched off.

Sensor know-how

Different sensors can be used in different applications. Broadly speaking, electrochemical sensors are used to detect lower concentrations of toxic gases, whereas catalytic or infrared (IR) sensors are used to monitor combustible substances in concentrations below the LEL.

Spot, area or perimeter monitoring?

It makes sense that sensors and sampling points should be positioned so that gas accumulations are detected before they create a significant hazard. To ensure maximum performance, different sensor positioning strategies can be implemented to suit different workplace environments. Whilst these can, of course, be combined or modified, they are generally used to provide spot, area or perimeter monitoring. Spot monitoring is used where the potential source of the leak is known and the sensors can be positioned to ensure that leaks are detected quickly.

Where the source of the leak is not known an increased number of sensors will be needed to cover an entire area. This type of application is known as Area Monitoring. In applications where combustible liquids are stored, for instance, sensors are often distributed in a grid like fashion, each measuring an area of between 50 and 100m2. In solvent storage areas they are sometimes placed in a circular pattern with a diameter of about 10 metres, to measure around 75m2. Perimeter monitoring is, as the name suggests, used in applications where the outer limits of the installation need to be checked and where it is important that potential hazardous gases do not reach neighbouring areas.

European Standard BSEN 50073:2000 also states that fixed sensors and sampling points should be installed in locations where they will not be vulnerable to mechanical or water damage as a result of normal operations in the area. In addition, sensors should be readily accessible to allow regular calibration, maintenance and electrical safety inspection. Under item 6.3 of the Standard, the regulations also state that if it is impossible to achieve regular direct access to the sensor then, as a minimum requirement, some form of automatic remote gas calibration facility should be provided.

Guidelines for positioning

Draeger Safety offers these straightforward guidelines for positioning sensors

  • The vapours emanating from com- bustible liquids are heavier than air and will be located close to the ground. For this reason, position sensors close to the ground.
  • Methane, ammonia and hydrogen are also combustible but these three gases are significantly lighter than air. Unless they are extremely cold, these gases rise and accumulate near ceilings.
  • As the distribution or dispersion of toxic gases that are heavier than air, but which are present in low concen- trations, is mainly dependent on con- vection and thermal currents, they should be monitored at head height (ie the breathing zone).
  • If there is a preferred, defined airflow or airflow guided by appropriate baffle plates, the sensor should be located on the intake side. In ducts, factors such as dilution and alarm delay should be considered.
  • When detecting combustible gases or vapours, position the sensor between the leak and any potential ignition points. The design should also take into account both the reaction time and the time needed for the intended countermeasure to come into effect.
  • Although it is often not possible to take into account obstructions to air flow, the potential flammable volume can be estimated by determining the maximum expected source-strengths and the air change speed.
  • If a sensor cannot be positioned close to a potential leakage source, it is pos- sible to sample air continuously by moving the gas past the sensor. However, this method is expensive because not only does the airflow need to be monitored but also, in addition, the adsorption in the pipeline, or con- densation due to a temperature gradi- ent, must be considered. Another potential pitfall is that air sampling increases the reaction time of the gas detection system.
  • In summary, BSEN 50073:2000 lists a number of factors that should be taken into account when determining suitable locations. These include the location, ie indoor or outdoor site, potential sources such as the location and nature of the potential vapour/gas sources (pressure volume and/or mass, source temperature, density and distances), as well as the chemical and physical data of the potential gases/vapours present. Other factors include leak control, the nature and concentrations of possible gas releases, the presence of cavities and jets and the general topography of the site. Air movements should also be taken into consideration as well as temperature effects, the local environment of the plant, the location and number of personnel in the plant, and the location of potential sources of ignition. Any structural arrangements such as walls, troughs or partitions, which could allow gas to accumulate, should also be considered.

    Expert advice

    Perhaps most importantly, the Standards also state that the placement of the sensors and sampling points should be determined following the advice of experts having specialist knowledge of gas dispersion, experts with a knowledge of the process plant system and equipment involved, and safety and engineering personnel. It also advises that the agre

    ement reached on the locations of sensors and sampling points should be recorded.

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