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Understanding the Benefits of Fixed Gas Detection

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Courtesy of a1-cbiss

'Fixed gas detection is an essential method of ensuring maximum protection for personnel and plant. There are a variety of technologies available for a multitude of applications. Understanding fixed gas detection is the first step in providing safety.'

What is fixed gas detection?

Fixed gas detection is installed with the aim of providing continuous, area and personnel protection. Fixed systems typically have a long-life and require minimal maintenance and are able to connect to building management systems as well as other safe systems.

Why do we need fixed gas detection?

Fixed gas detection systems are installed to offer continuous monitor of potentially dangerous environments. A risk assessment is required to identify potential hazards in both manned and unmanned areas of the site.

A gas hazard can be found in three forms; oxygen depletion, toxicity, and explosive gases.

Oxygen depletion can occur where a there is potential for high concentrations of gas other than ambient air. Even non explosive or non toxic gas can displace enough ambient air to reduce the oxygen level to a level where is it unsafe for an operator to enter. Particular risk is in areas where there are gas storage tanks housed in a confined space where access is rare but required, i.e. boiler room.

Toxic gases can come in many forms, a risk assessment will identify all toxic gases that may occur, every toxic gas has an exposure limit regulated by the HSE with the EH40 document. In an area in which there is potential for toxic gases to be released into atmosphere there should be a specific sensor to monitor that gas and should alarm before the concentration exceeds the exposure limit.

COMMON MISUNDERSTANDING is to use an oxygen sensor to monitor toxic gases on the assumption that it will alarm when oxygen is displaced. A typical oxygen sensor will offer resolution to 0.1% volume. This will alarm when there is a concentration of 0.1% volume or 1,000ppm. This is not a suitable technique for toxic gas detection as many any gases are toxic in concentrations below 1,000ppm.

Explosive gases are monitored using either IR (infrared) or Catalytic sensors. Many gases at the right concentration can be explosive, therefore both technologies can detect a number of explosive gases, however, they offer different sensitivities for different gases. A risk assessment must identify all potentially explosive gases then the gas with the lowest LEL (lower explosive limit) must be selected as your target gas; this is often Methane or Pentane. Typically the fixed system should alarm at 50% of the LEL. Methane is explosive at concentrations of 5% vol. therefore the LEL should be set to 2.5% vol. which equates to 50% of its LEL.

Benefits of fixed gas detection

Protection

A fixed gas detection system protects you and your assets. In addition it also protects your workforce against asphyxiation, poisoning and explosions/fire.

Continuous monitoring

Fixed gas detectors provide monitoring 24 hours a day. All areas even when the majority of the time it is unmanned must be monitored to warn personnel of dangerous environments before they enter. Having fixed detection will reduce safety risks onsite creating a safer environment, often satisfying insurance policy criteria.

Looking after the costs

Although it is not a replacement for personal gas detection, a fixed gas detection system can reduce the time is which it takes to asses, manually, a potentially explosive area often reducing man hours and ultimately cost.

Reduced Maintenance

Fixed gas detection systems are often easier and cheaper to maintain than portable gas detection as they are less susceptible to loss and damage. The recommended service and maintenance schedule of a fixed gas detection system is calibration every 6 months with low sensor replacement frequency and cost.

Little training is required to operate a fixed gas detection system and its use is less likely to be affected by misuse or ignorance.

Customised fixed gas detection to suit your application

Every fixed gas detection system is different - each

installation should follow a site survey from the supplier as there must be consideration of the following;

Gas Hazard

All gases have different molecular weights. The gas hazard must be assessed and system specified appropriately. when detecting for CO2 the sensor must be positioned below the potential CO2 leak source at the molecular weight is 44

compared to 29 of air and will sink in atmosphere.

Location

The location in which you are monitoring should be assessed, for example the air flow. Fixed point of detection sensors reply on the gas passing across the sensor, to be detected. Therefore if there is a known, consistent direction of the air flow the gas detector must be positioned 'down wind' of the potential leak source.

Alarms

What do you want the fixed gas detection to tell you? In cases where you are monitoring for explosive gases or oxygen depletion it is to tell you that there has been a gas leak. As a result a sound or beacon alarm will providing you with a 'SAFE – NOT SAFE' indication controlled by the relay output of the controller.

In cases where you are monitoring toxic gases with average exposure limits STEL (Short term exposure limit over 15 mins) or TWA (Time weighted average exposure limit over an 8 hour period) you will need data acquisition software to calculate these average over these two periods and ignore safe peaks on concentrations.

Closed Loop

You are able to used fixed gas detection systems to monitor and control an environment. The outputs of the control panel can active emergency extraction for example to ventilate a confined space to reduce the unwanted gases. In some cases fixed gas detection systems activate dry contacts to shut down processes as a safety precaution to allow for corrective maintenance.

Choosing the right sensor

It is important to know which sensor is best for which gas and application.

Catalytic Sensors

  • Detects: Explosive gases in the LEL range only
  • Cost: Low cost £300-500
  • Service: Must be calibrated every 6 months
  • Life Span: 2-3 Years
  • Things to consider: If exposed to high concentrations it can burn out the sensor and will need to be replaced. If exposed to a silicone based product it can coat the sensor desensitising the to the target gas and will false zero

Infrared Sensor

  • Detects: Typically CO2 and Explosive gases in the ppm, LEL and %Vol range.
  • Cost: Low cost £500-800
  • Service: Must be calibrated every 6 months
  • Life Span: 10+ Years
  • Things to consider: Very stable sensor with a long life span but is more expensive, they can detect multiple gases but only tuneable to one gas at a time.

Electrochemical Sensor

  • Detects: Typically O2, CO, H2S, NO, NO2, SO2, NH3.
  • Cost: Low cost £300-500
  • Service: Must be calibrated every 6 months
  • Life Span: 2 Years
  • Things to consider: Over exposure to gas can reduce the life span of the sensor. Ideally it needs to be calibrated more frequently as some gases can be cross sensitive.

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