commercial gas installations - Gas Purge Testing

What Is The Purpose Of Gas Purge Testing And How Is It Done?

Safety and efficiency are crucial when working with commercial gas installations. A key element in maintaining the integrity of the systems is gas testing, purging, and decommissioning. Here’s a look at the process in more detail, to understand when and why these actions might be necessary.

What does gas testing involve?

Commercial gas testing is the process of ensuring that gas installations are fit for purpose and in safe working order. It not only mitigates against the risk of accidents, but also ensures that the gas installation is running efficiently and is cost effective. 

During the process, a gas engineer will carry out a thorough inspection of the system to identify any leaks, potential hazards, and for general compliance with safety standards. Pipework, appliances, and interlock systems will be examined to test if they are in a sound condition or if any improvements or repairs are required.

What is the purpose of purge testing?

Purge tests are designed to assess the structural integrity of natural gas pipework installations in compliance with safety standards. Purge testing is an essential part of installing, maintaining, or replacing a commercial natural gas pipeline or storage tank. It is also necessary to carry out purge testing when filling new pipework with flammable gas. 

Purging refers to the process of removing all hazardous gas from the pipeline to remove the risk of combustible mixtures and ensure that the working environment is safe for routine maintenance work or decommissioning. Without this crucial step, it is highly possible that a fire or explosion may occur when flammable gas mixes with air. 

Purge testing involves introducing an inert gas such as nitrogen at a certain pressure and flow rate into the system. This decreases the concentration of flammable gases in the system. The atmosphere is analysed with specialist equipment to measure when the flammable gas has decreased to safely acceptable levels for air to be introduced.

There are two main methods of purging: direct purging (sometimes called displacement purging), and indirect purging (sometimes called dilution purging). During direct purging, the inert gas is injected into an open apparatus to displace the hazardous gas.

During indirect purging, an inert gas is injected to lower the concentration of the dangerous gas. The diluted gas may then be safely discharged into the atmosphere. Sometimes, vacuum purging is carried out, which involves extracting the flammable or toxic gases with a vacuum pump. This method is usually used for very large spaces.

Purging is a safe and effective method of removing the risk of unsafe levels of hydrocarbon and air mixing and causing an explosion or fire. Direct and indirect purging does not involve using pumps, which is another safety advantage because it removes the risk of sparks igniting the flammable gas or of cross contamination occurring. 

Purging is sometimes also carried out as a control method to lower the risk of oxidation during welding or manufacturing processes. After the purging has been carried out, the inert gas is reintroduced to reduce the oxygen levels and allow natural gas to safely be returned to the system. 

It is important to note that it is not safe to rely on a regular gas detection system as an alternative to purge testing. This is because standard testing systems are not designed to function at very low oxygen levels; they are designed to test for the presence of alien gases in a normal working environment.

The detection equipment used during purge testing is specifically designed to operate at very low oxygen levels and to detect all the types of residual gas that may be present in a commercial gas system.  They will generally incorporate specialist infrared technology or thermal conductivity systems to produce accurate and reliable readings.

Relying on standard gas detection systems can lead to an unsafe working environment that may contain toxic gases. These can cause breathing difficulties, serious illnesses, and even be fatal. 

What does decommissioning involve?

Decommissioning is carried out when a commercial gas system is at the end of its lifespan. This may be due to redevelopment, upgrades to more modern equipment, the shutdown of a plant or factory. It is the final stage after purge testing to ensure that there is no remaining gas within the pipework.

When the engineers are satisfied that it is safe to do so, the supply lines will be disconnected and the remaining infrastructure will be dismantled and safely removed. 

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