Purge my gas supply system after every cylinder change? It’s a “No-Brainer”

To purge or not to purge, that is the all-important question when you are preparing a high purity gas system for use. Seriously, purging is the most often overlooked step in the day-to-day operations in the laboratory. When a system is initially put into operation, the pipe or tubing is filled with air, moisture, and other contaminants. While virtually everyone understands the importance of purging, not all recognize the necessity to purge every time you change cylinders. For some gas services, it is critically important. For others, it is just very important!

Reasons for Purging

There are three basic reasons to purge a system.

Reason 1: To prevent corrosion. Most of the gases that are referred to as corrosive (chlorine, hydrogen chloride, sulfur dioxide, and other halogen gases) are non-corrosive in their anhydrous state; i.e., they are free from moisture. In the presence of moisture, these gases revert to the acid phase and will attack the materials within your system. We live and work in a sea of moisture; air is literally filled with it.

While a variety of materials are used to prevent corrosion—stainless steel, Monel, Hastelloy, etc., they cannot prevent system failure; they simply delay failure.

When you purchase high purity corrosive gases (pure or in mixtures), you are confident that the specialty gas supplier has purified them to remove as much moisture as possible. However, if you allow these gases to be exposed to the atmosphere you immediately negate this purification.

That is exactly what happens each time you change a cylinder. The inlet cavity of the pressure regulator becomes filled with air and its contained moisture. When you open the valve on your anhydrous HCl cylinder, the air (containing moisture) and HCl mix, forming hydrochloric acid. That very aggressive acid is injected directly into your regulator, and then downstream into your system. Purging can easily prevent this. I have seen stainless steel regulators that have failed after a few cylinder changes and I have seen stainless steel regulators that operate perfectly after 10 years of service. The difference is purging. (See Case History of a Failure below.)

Reason 2: To prevent the release of toxic or poisonous gases. When using toxic gases, every precaution must be taken to avoid the uncontrolled release of these gases into the environment. Even in small amounts, these gases can have long-term health consequences. When changing cylinders, a small amount of residual gas remains in the CGA connection and the high-pressure cavity of the regulator. Purging this gas to a safe location is an important safety consideration.

Reason 3: To prevent cavities from filling with air. Again, when changing gas cylinders, the inlet cavity or the regulator fills with air. Air contains approximately 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, and lesser percentages of carbon dioxide, rare gases, and a variety of air pollutants.

Also present in varying degrees is moisture. While many of the pollutants are present only in ppm levels, they can play havoc with a gas chromatograph or other sensitive analytical equipment.

The Choice is Yours

You have a choice when changing cylinders: allow your high purity or ultra high purity gas to push this air (contaminants and all) through your instrument, or you can remove it by purging.

Many lab operators are under the mistaken impression that after a gas cylinder is changed, you have to wait one to two hours for your chromatograph’s baseline to re-establish. This is clearly wasted time that can be avoided by simply purging the air from the system without sending it downstream.

Types of Purge Devices

Now that we have established the reasons for regulator purging, let’s take a look at the types of purge systems available. Again there are basically four types of purge devices, the straight purge, the tee purge, the cross purge, and the deep purge.

A straight purge is used for pressure cycle purging on a regulator that is equipped with an auxiliary high-pressure port. The tee purge is used on regulators that do not have an auxiliary high-pressure port, and the cross purge may be used on all regulators. The cross purge combines the purge inlet valve with a block valve to allow purging of just the inlet connection, as well as a bleed valve to exhaust the unwanted contaminants and/or purge gas from the inlet to the regulator. A deep purge includes a small siphon tube that carries gas back to the most stagnant location, the cylinder valve, and sweeps the contaminants out with the purge gas.

Purging Methods

Pressure Cycle Purging. The pressure cycle, or dilution purge, is performed by pressurizing the system to dilute the contaminants and exhaust them from the system. Repeated pressurization/exhaust cycles will gradually reduce the concentration to an acceptable level. The dilution occurs in a direct ratio between the applied pressure (psia) and the exhaust pressure, also in psia. In the following formula, the pressure must be expressed as absolute pressure, not gauge pressure.

Dilution per Cycle = Exhaust Pressure/ Applied Purge Gas Pressure. Let’s take an example using a pure gas in the regulator and an applied purge gas pressure of 100 psig, exhausting to atmospheric pressure (in a safe location).

Dilution per Cycle = 14.7 psia / 100 psig + 14.7 = 0.128. Therefore, the concentration of the pure gas would be diluted from 1,000,000 ppm to 128,000 ppm. A second cycle would reduce the concentration to 16,400 ppm. After the sixth cycle, the concentration would be approximately 4 ppm and the balance (99.996 percent) would be purge gas.

The efficiency of the pressure cycle purge can be greatly improved by the addition of a vacuum cycle to the exhaust side. Let’s use the same example as above, except pulling a vacuum down to 0.25 atmospheres on the purge outlet.

Dilution per Cycle = 0.25 atm / 100 psig + 14.7 = .03. Therefore, after the first cycle, the concentration would be down to 30,000 ppm, and after four cycles, the concentration would be down to less than 1 ppm.

Between cycles, you must allow time for all components of the gases to mix. This is particularly important where dead-end passages exist. In the case of purging a regulator inlet, a period of 30 seconds between cycles is appropriate. For pressure cycle purging of complex systems, a longer period may be necessary.

Displacement Purging: Displacement purging is used for purging complete systems where there are no dead-end passages and gas can flow from the beginning to the end, such as in a pipeline. Displacement purging is best performed with a low pressure and high flow of gas to avoid stratification and mixing between the purge gas and gases to be removed. This will allow the purge gas to physically push the gas from the system. Normally, four to five pipe volumes are adequate to achieve a near zero concentration of contaminants in the system.

While system purging is an often-overlooked step, it is a very important operation in high purity, corrosive or toxic gas systems. It can save time, increase the life of equipment and improve workplace safety.

Value, Not Price

Many customers look at price rather than value – and ultimately regret it. That is no more evident than with corrosive resistant purge assemblies. These assemblies may cost almost as much as the regulator itself. But the value they bring is in the regulator life expectancy, which routinely is extended many times over.

Perhaps the best sales approach in this case is to make an economic justification rather than a technical one. Purge assemblies simply save you more money than they cost!

Dave Durkin is a Senior Product Manager for Victor Equipment. He can be reached at Dave_Durkin@thermadyne.com.

Specialty Gas Report THIRD QUARTER 2007 //