STEL: (Short Term exposure Limit - )
TWA: (Time Weighted Average - )
Rising Latching Alarms For Line Safety Applications
|First Alarm:||Second Alarm:||Third Alarm:|
|10-20% LEL||20-40% LEL||Optional|
Flammable Gases and the Lower Explosive Limit
Many gases and vapours when mixed in the correct proportion with an oxidant, normally air, will ignite. If the mixture is too weak or lean when there is too much oxygen and not enough fuel gas or vapour then the mixture cannot ignite. Similarly if there is too much gas or vapour the mixture will be too ‘rich’ and so also not capable of ignition. So for a gas or vapour to ignite there must be three things present. Fuel and Oxygen in the right proportions and something (heat or energy) to ignite the mixture. This is usually represented by the fire triangle.
Remove any part of the fire triangle and there is no possibility for ignition. In practice removing Oxygen from everyday life is impractical and ignition sources are everywhere so in the world of gas detection focus is on detecting the fuel hazard to limit its impact.
For the gas to be capable of ignition the fuel air mixture has to lie within a flammable range. At the point where there is just enough fuel mixed with the air (Oxygen) to be capable of ignition is the point we call the Lower Explosive Limit. However each gas will form this flammable mixture at a different concentration. The following table illustrates this.
Flammable Gases and Vapours
Each listed percentage volume represents 100% of the Lower Explosive Limit for that gas or vapour, that is the point at which the gas is just capable of ignition. So flammable gas detectors must be calibrated to be best response for the flammable gas hazard in question and must measure in the region from no gas (zero) to the point the gas is just flammable, or 100% LEL. All flammable gas detectors are scaled to read from 0 to 100% LEL. In this way anyone looking at the flammable gas hazard level does not need to know the particular percentage volume at which the gas forms its flammable mixture. Instead the display shows the % LEL level. In this way anyone looking at the display can quickly quantify the risk level. For example 10% LEL and the gas is only 10% of the way to being flammable, 50% LEL and its half way to being flammable, which gas at that stage does not particularly matter in assessing the risk.
Normally the gas detection system will have alarm levels programmed, typically 10-20% LEL for a first alarm (warning) and 20-40% LEL for a second stage alarm to evacuate or take further action.
Some gases or vapours that are flammable may actually pose a toxic hazard that is more relevant than the flammable hazard. For instance Carbon Monoxide is listed on the table above and is flammable at 12.5% by volume in air. However its STEL (Short Term Exposure Level) is only 300ppm (0.03% by volume) for 15 minutes. In this case a toxic gas detector with ppm level arms is more relevant as it is toxic before it is flammable.
There is also an Upper Explosive Level (UEL) for each flammable gas or vapour. This is the point where there is so much flammable gas or vapour that there is insufficient air to combust with it. For gas detection we are not normally interested in the UEL as we are detecting the leak well before the LEL, Lower Explosive Limit.
Typically there are two main accepted methods for flammable gas detection:
1. Catalytic or Flammable Gas Detectors
2. Infra-Red Flammable Gas Detectors
Which or these two technologies is best? Well this is not a fashion choice, each has its own merits and problems and so the application is usually what determines the choice. For example.
Pellistors have the following advantages:
- Capable of detecting almost all flammable gases and vapours
- Capable of performing more reliably in dusty or dirty environments that would soil the optics of an infra-red detector
- Not significantly affected by pressure changes
- Can detect Hydrogen which is ‘invisible’ to Infra-Red
- Less sensitive to humidity
- Capable of performing more reliably in high temperature applications
- Generally cheaper than Infra-Red
- Well characterised meaning one calibration gas can be used to cross calibrate for others
Infra-Red Detectors have the Following Advantages
- Immune to poisons such as lead and silicone compounds that can affect pellistors
- Performs better in sampling or flow applications
- Is more fail safe than a pellistor as its electronics can self check and reveal most failure conditions
- Continuous built in monitoring and correction can mean longer calibration periods depending on the application
- The only really accepted method for monitoring CO2
So the Pellster is an excellent first responder due to its ability to detect almost all flammable gases or vapours, its then a question if problems with the application mean that an Infra-Red detector may offer advantages.
IGD have a wealth of experience in detecting flammable gases and vapours across a broad spread of applications. Call to access our knowledge so we can help at an early stage with your application.