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14 February 2022
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What is Purge Testing and when should I be doing it?
Georgia Pratt
Marketing Executive

Purge testing is vital when installing, replacing or maintaining a natural gas pipeline or storage tank, or filling new pipework with flammable gas. This process uses an inert gas to clear the enclosed environment of flammable gases prior to air being introduced thereby preventing air and flammable gas mixing. Such mixtures could of course lead to explosive combustion.

What is Purge Testing? 

Purge testing is a key part of the process of making a working environment safe prior to entering it to carry out work. Analysis of the atmosphere in the pipe or enclosure shows the starting point – usually 100% flammable gas. Purge testing is the measurement and reporting of the atmosphere as an inert gas is introduced. As the flammable gas declines to a safe level well below concentrations that would be dangerous in air, the atmosphere is continually analysed, and the flammable gas concentration reported. Once a low concentration has been achieved, air may be introduced. During this phase the flammable gas concentration is analysed to check it remains low, and oxygen concentration is measured to indicate when the atmosphere becomes breathable. Work may then commence – all the while protected by the measurement of flammable gas and oxygen concentration. If, as is likely, the purge testing is being carried out via suction of atmosphere through a sample tube, then this sample tube must at all times and all along its length be held above the flash point of the flammable gas in the tank. This is vital to both your safety and the safety of those working with you.  

Purging removes or displaces hazardous gases from the tank or pipework to prevent them from mixing with the air you need to introduce into the tank to carry out the inspection or maintenance task. The most used and preferred purge gas is Nitrogen, due to its inert properties. After conducting the inspection or maintenance task the reverse process is carried out, reintroducing the inert gas and reducing the oxygen level to near zero prior to allowing natural gas to re-enter. Often a service valve on the line with a standpipe or diffuser attached is cracked to release the venting gas or nitrogen. Purging systems are generally designed to redirect additional gases away from the work area preventing them from remixing with the gas within the tank or pipework. 

Why Conventional Gas Detection isn’t enough 

Traditional gas detection systems are not designed to work in oxygen-deprived environments. This is because they are primarily designed as safety equipment with the specific purpose to detect small traces of target gases in otherwise normal breathable environments. Gas detection equipment designed for use in purge testing activities must be able to function in low oxygen environments and with all contaminants likely to be found in tanks and pipes being purge tested. If sensors can be poisoned by the contaminants present or if there isn’t enough oxygen in the air to enable the selected sensor technology to be used, it may lead to the sensors on the device producing inaccurate results, posing a threat to those working within that environment. An additional point to note to note is that certain gas combinations, concentrations and corrosive liquids may damage the gas detection equipment, rendering it useless. For these reasons, Infrared technology or thermal conductivity is usually chosen as the measurement technology of choice for purge tests. Crowcon uses infrared technology in these applications. A fortunate by-product of that design decision is better accuracy than required over the full sensing range. 

More about Purge testing 

Purge Testing is essential for workers as some may be breathing in toxic gases without even realising it if the sensors on their detection equipment have become defective, don’t measure the required gas type or don’t measure over the required gas range, or environmental range present. Toxic or asphyxiant gas exposure can lead to respiratory issues, significant injury, even death. 

Workers cannot merely rely on a standard confined space gas detection instrument to adequately test for safe conditions during this process, as the high gas level may overwhelm or damage an LEL (Lower Explosive Limit) sensor depending upon type. Or the sensor may not function in an oxygen-depleted atmosphere leading to an unreported dangerous condition. 

What products do we offer? 

Our Gas-Pro TK is a specialised tank monitor that is perfect for customers who want to purge, free, or maintain storage and transportation tanks due to its integrated auto switching dual range IR sensor technology. Other sensors in the product, for example the H2S (Hydrogen Sulphide) sensor option cover other potential risks if gases vent during purging. 

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