There are several different types of purging equipment – the tee purge, the cross purge, and purge panels (panels will be covered in a future posting). The type of equipment selected depends on the gas service, purity requirements, and budget.
REDUCE SYSTEM CORROSION - All of these devices reduce moisture in corrosive gas service, and will extend the life of equipment in the process stream by preventing/reducing acid build up.
PRESERVE GAS PURITY - Purge devices help prevent/reduce inbound contamination of UHP streams during equipment changeovers.
REDUCE RELEASE OF TOXIC GASES - Cylinder connections, regulators, and process lines can be purged prior to cylinder change to prevent the escape of hazardous gas into the work area during cylinder change outs.
Tee Purge
The Tee Purge is the simplest piece of equipment used for purging gases for safety and purity, and is usually installed on the inlet of a regulator, and connected directly to the cylinder valve. Since there is no vent or isolation valve on a tee purge, it requires a line purge, through the regulator, down stream to a vent, to a safe area prior to loosening the CGA connection.
Recommended Gas Service: Flammable gases (not pyrophoric), low ppm concentrations of semi-corrosives (ex.: SO2, H2S), mild toxics with a TLV greater than 25 ppm.
Purity: Allows for a continuous supply of purge gas through the CGA while changing the cylinder, reducing the amount of moisture and Oxygen that enters the line during cylinder change Moisture and Oxygen can be reduced in the regulator and process lines by purging after cylinder change. A tee purge can help maintain 4.5 – 5.0 levels of purity.
Drawbacks: Requires more purge gas and longer purge times than cycle purging, while not reducing the impurity levels as low as cycle purging.
Cross Purge
The Cross Purge is an economical piece of equipment used for cycle purging gases for safety and higher purity levels, and is usually installed on the inlet of a regulator, and connected directly to the cylinder valve. The cross purge includes both a vent and isolation valve, and can be used with vacuum assist.
Gas service: Flammable gases, pyrophoric, corrosives, toxics.
Purity: Allows for cycle purging before and after gas cylinder change out. The isolation valve reduces the amount of volume that needs to be purged during cylinder changes by isolating the regulator and downstream system, saving time and gas.
Cycle purging can reduce the amount of contaminates to sub ppm levels. The vent valve can be connected to a vacuum to enhance the purge cycle, reducing contaminates further.
Allows for a continuous supply of purge gas through the CGA while changing the cylinder, reducing the amount of moisture and Oxygen that enters the line during cylinder change.
Allows for purging the process line if required. Moisture levels can be reduced dramatically with a complete cycle purge. Can maintain 6.0 levels of purity.
Drawbacks: When connected to the inlet of a regulator, the assembly must be supported in some way to prevent bending / kinking of the process tubing after the regulator.
This article covers some specific examples of regulators for corrosive service, but it is not the complete story. Contact MATHESON, or visit our website for more details.
Standard Corrosive Service. We offer the 3630A Series and the high-purity 3610A Series for corrosive gases like Ammonia, Hydrogen Sulfide, Nitric Oxide and Sulfur Dioxide. Models 3510A and 3810A can also be used for corrosive service.
High Purity Miniature Regulators. These regulators are recommended for use with corrosive gases where stainless steel systems are required, and the gases are in lecture bottles. The proven, low maintenance design features a Kel-F® seat and Teflon® seals for wide media compatibility. Model 3570 or 3870 Series is recommended.
Deluxe Monel® Corrosive Service. Designed for the pressure control of corrosive halogen gases, the deluxe corrosive service regulators Model 3210A Series have all-wetted parts of either PCTFE®, or Monel® materials.
Fluorine Corrosive Service. Specifically designed for use in pure fluorine gas service the Model 3225A is recommended. This is a Monel® construction with bronze filled Teflon® seat and Kel-F® seals. The Model 3610A is recommended when mixtures 10% or below of fluorine gas are being used. Cross-Purge strongly recommended.
Specialty Line Regulators. We offer Model 3450A Series high flow line regulator for use with dichlorosilane, ammonia and amines. And the Model 3494 for absolute pressure delivery applications. Cross-Purge strongly recommended.
Cross-Purge Assemblies. Cross-purge assemblies Model 4774 and 4775 provide effective purging during cylinder change out. Hazardous gas traces are eliminated before opening the cylinder connection, and atmospheric gases are removed after reconnection.
The quarter-turn diaphragm valves allow easy, fast purge cycles, while the color-coded lever handles indicate gas flow and mode of operation. These assemblies are also designed to prolong the service life of regulators and other gas system components.
Each cross-purge assembly incorporates the use of an integrated check valve to prevent backflow of process gas into the purge line.
We'll cover more about cross-purge assemblies in a future article.
Not Just Another Pretty Face.
Gas pressure regulators are beautiful pieces of hardware. Perfectly machined. Brilliantly finished.
But we often ask these handsome products to do the dirty work of handling some pretty nasty chemicals.
More Than Meets the Eye.
The corrosion you see could also be causing a physical problem that you can't see. Possibly leading to a failed connection or a regulator seat that won't seal.
A failed connection can of course lead to a leak (of a chemical you probably don't want to leak), which further compromises the connection.
Keep in mind that the corrosion you see and touch can itself be toxic.
Corrosive Gases
The list of commonly used corrosive gases includes ammonia, hydrogen chloride, chlorine, nitric oxide and others.
When using these gases the correct selection of equipment is critical.
Importantly, these chemicals do not need to be at high concentrations in order to cause problems.
These gases are capable of burning and destroying organic tissue (human tissue) on contact, representing a threat to people on the job.
Corrosive gases can also attack and corrode metals, interfering with your equipment's performance, and reducing its lifespan.
You can't neutralize the gas, and no choice of material will eliminate corrosion. But you can make equipment choices that resist corrosion, and that minimize the impact of corrosion (the health and safety impact, as well as the financial impact).
Read the details in the next entry. If it's not here yet, be patient, it's coming.
Single Stage Regulators.
MATHESON high purity single stage cylinder regulators are available in plated brass or stainless steel. Both have stainless steel diaphragms. Available ranges (outlet pressure) are from 0-15 psig up to 0-500 psig.
All single stage regulators exhibit a characteristic known as cylinder pressure decay. As the cylinder is depleted, the cylinder pressure decreases. This reduces the pressure on the inlet side of the regulator, which causes an increase in the delivery pressure.
Single stage regulators are the choice when delivery pressure is not critical, when the application calls for short term flow, or when periodic re-adjustment of the delivery pressure is not a problem.
Single stage regulators are also appropriate when using liquefied gases, such as CO2, which maintain their vapor pressure until depleted.
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Dual Stage Regulators.
MATHESON high purity dual stage cylinder regulators are available in plated brass or stainless steel. Both have stainless steel diaphragms. Available ranges are from 0-15 psig up to 0-500 psig pressure at the outlet.
Dual stage regulators are essentially two single stage regulators in one body, with the first regulator supplying the second regulator. As a result, since the inlet pressure at the second stage is stabilized by the first stage, dual stage regulators do not exhibit significant pressure decay.
Therefore, dual stage regulators are the choice when the delivery pressure of the regulator must be stable over a long period of time as the cylinder pressure decreases. A dual stage design will supply a delivery pressure that is essentially constant.
It is important to remember that dual stage regulators are not definitively "better than" single stage regulators. The distinction between the two types boils down to how a dual stage regulator deals with pressure decay. The other advantages of dual stage design are incrementally small.
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