The Importance of Purging

How to choose the correct purge system for your application.

Purging is an important part of a good calibration procedure and should always be practised when handling dangerous gases. Purging ensures that you have control over which gases are in your gas delivery system, and therefore which gases are being exposed to internal components, sensors, or other equipment. It also helps to prevent unwanted reactions from taking place, which can greatly increase the service life of related components.

There are several main reasons for purging:

1. Removing resident gases from a system before introduction of a new gas or gas mixture which may react.
2. Removing impurities from a system delivering a pure gas or sensitive gas mixture.
3. Removing dangerous or damaging gas from a system after use.

Failure of regulators, valves, connections, or any other components of a gas control or delivery system can pose serious risks to health and even cause death. Many gases are highly dangerous even in very small amounts. The utmost care should be taken in maintaining the integrity of your gas control system when dealing with dangerous gases. Part of an effective strategy to mitigate such risks it to perform regular purging.

Some methods exist for purging with a standard regulator. However, purging can cause an interruption to processes, waste valuable gas, or not be sufficiently effective unless an appropriate purge valve system is integrated. CAC Gas supplies a range of purge valve systems designed to maximise effectiveness, safety, and efficiency, while minimising wastage.

Removing resident gases from a system before introduction of a new gas or gas mixture which may react.

Gases can become more dangerous when they react with other compounds. A common result is the formation of corrosive liquids when some pure gases react with moisture in ambient air. These liquids can’t be totally removed the way gases can by purging. This, in turn, damages internal seals, valve seats, and threaded connections leading to leaks or even catastrophic component failure.

For example: A laboratory is conducting experiments using pure anhydrous ammonia. A technician chooses an appropriate regulator and connects the regulator to the cylinder. The technician is aware that the space inside the regulator is filled with the ambient air from the laboratory, which includes oxygen, nitrogen, and moisture. They are also aware that ammonia is hygroscopic (readily absorbs water), and that ammonia can react with oxygen. Therefore, the technician purges the regulator with high purity, dry nitrogen to displace all the oxygen and moisture before opening the cylinder of ammonia.

Removing dangerous or damaging gas from a system after use.

Many pure gases and gas mixtures are toxic, corrosive or otherwise reactive and should not be left sitting in gas delivery systems. Even the most resistant wetted materials will eventually degrade in the presence of strongly reactive gases. This can present danger to people using the gases and reduce the service life of components.  To reduce the risk of safety incidents, and extend the life of system components, such gases should be displaced with an appropriate purge gas after every use.

In our example: the technician has now finished the experiment for the day. The valve on the cylinder of ammonia has been closed, but there is still ammonia left in the cylinder connector, the regulator, and various valves and lines between the regulator and the experiment equipment. Even though the regulator has a special resistant coating, and is made from high quality stainless steel, the technician knows that the ammonia is very corrosive and can still cause damage. Therefore, the technician uses pure, dry nitrogen to purge the cylinder connector, regulator, and the whole system downstream. The system is plumbed into the lab’s fume hood, and the purged gases are disposed of safely though their scrubbing system. After performing the purge, the technician now removes the regulator from the cylinder, confident that all dangerous gases have been displaced, leaving the regulator and system ready for the next use.

Removing impurities from a system delivering a pure gas or sensitive gas mixture.

Purging is also an important part of a good calibration or analytical procedure, regardless of whether the gases are dangerous to health or not. The person performing the calibration or analysis should be sure that the only gases reaching the sensing elements are those which are to be measured or used for calibration. All other gases should be displaced before the analysis or calibration takes place. In such cases, the purge gas is the pure gas or gas mixture itself.

In our example: The technician now performs a calibration on the equipment. The gas mixture used is 500ppm ammonia in nitrogen. The technician knows that since the last purge was performed the gas delivery system contains pure nitrogen and the regulator again contains ambient air, since it was removed from the previous cylinder. They also know that the amount of nitrogen in the delivery system and air in the regulator can dilute or react with the mixture of 500ppm ammonia in nitrogen, skewing the results of a calibration. Therefore, these gases must be completely displaced before the calibration is performed. The technician connects the regulator to the cylinder of 500ppm ammonia in nitrogen, purging the system for an appropriate length of time to displace all the unwanted gases.

Types of purge systems:

Note: purge systems do not include regulator