Getting the Most Out of Compressed Gases: The Role of the Gas Pressure Regulator

by Mike Marone and Ron GeibTechnical Article

Suppliers of compressed gases go out of their way to deliver gas products that meet customer requirements for purity, mixture accuracy, and

other specifications. Most gases are deliv-ered in conventional high-pressure cylin-ders, while other gas products are delivered in bulk or small bulk containers and dew-ars. Whatever the means of delivery and storage, these gases are handled, regulated, and distributed by the user. In the process, the gases are exposed to different equip-ment and systems that can either maintain or degrade gas purity.

End-user gas handling systems can be as simple as a regulator, valve, and tubing; or as complex as a site-wide gas management system. They can also be a combination of the two. Whatever the configuration, the impact of the delivery system on gas quality is often underestimated, or worse, sometimes not even considered.

In addition, the use of hazardous gases such as acetylene, hydrogen, and chlorine can cre-ate an unsafe condition if the proper equip-ment is not used. Further, failure to consider the consequences of crossover usage of hard-ware with flammables, oxidizers, and corro-sives can lead to serious problems.

This article focuses on gas pressure regula-tors. Materials of construction, selection criteria, general precautions, and other issues are discussed in the context of safety and gas purity. Other articles by the authors deal with additional system elements.

RegulatorsRegulators are used in gas delivery sys-tems to reduce the pressure from a high-pressure source to a working pressure for use. The right regulator will maintain gas purity and deliver the gas safely to downstream gas delivery components, and ultimately to the point of use. Many different criteria need to be taken into account when selecting the right regula-tor for an application.

PurityThe first step in choosing a regulator is to consider the purity level that is required for the application. Typically, requirements fall into three categories: 1) general pur-pose (4.0 or 99.99% pure or lower), 2) high purity (5.0 or 99.999% pure), and 3) ultra-high purity (6.0 or 99.9999% or higher).

If, for example, an application requires high-purity gas, the use of general-purpose regu-lators should be avoided; only high-purity (or better) regulators should be used. As a minimum, regulators used in delivery sys-tems should be cleaned (by the supplier) for oxygen service to CGA G4.1/ASTM G93 guidelines. This will ensure both a minimum level of cleanliness and safe usage with high-pressure oxidizers such as oxygen and nitrous oxide. (Note that economy-grade regulators and gases are not suitable for laboratory use, and are not considered in this article.)

Body typeRegulator bodies may be forged or machined from barstock. Forging is used only with brass, whereas barstock is used for brass, stainless steel, and other mate-rials. Barstock bodies are preferred for higher-purity applications for the follow-ing reasons:

• Smaller internal volume: easier to purge, facilitating removal of moisture and impurities

• Smoother internal surfaces: do not retain as many contaminants, such as oxygen and moisture

• Tight grain structure of metal: The cold drawing process produces a very tight grain structure that is more resistant to adsorption of moisture and impurities.

Materials of constructionBrass is the most common material used in pressure regulators. Other materials, notably stainless steel, are used when gas compatibility with brass is an issue, or in higher-purity applications.

Inert gases and flammables are generally compatible with brass and most materials. Gases that have corrosive properties may require corrosion-resistant materials for all wetted parts. The most common corrosion-resistant material used for these applica-tions is 316 stainless steel, although Monel (Special Metals Corp., Huntington, WV), Elgiloy (Elgiloy Specialty Metals, Elgin, IL), and Hastelloy (Haynes International, Inc., Kokomo, IN) are also sometimes used. Most gas companies have material compatibility charts on their Web sites (see Figure 1).

Forged brass bodies are typically used in the lowest-cost regulators, and are suitable for use with the lowest-purity gas, 4.0–4.5, depending on the type of diaphragm (neo-prene or metal).

Brass barstock bodies have smaller internal volumes, and smoother internal surface finishes, in the 25-Ra range. Brass barstock bodies with stainless steel diaphragms can maintain 5.0 purity of noncorrosive gases.

Stainless steel should be chosen for semicorro-sive or corrosive gases or mixtures, or for higher-purity requirements. Stainless steel regulators are made from barstock material, and can be machined from a standard of about 25 Ra down

Figure 1 Example of a material compatibility chart (Figures 1, 2, and 3 courtesy of MATHESON, Montgomeryville, PA; source: MATHESON Gas Catalog.) Kel-F®: 3M Co. (Maplewood, MN); Teflon®, Tetzel®, and Viton®: DuPont (Wilmington, DE); Kynar®: Arkema, Inc. (Philadelphia, PA).

Reprinted from American Laboratory March 2011

to 10 Ra or lower for ultrahigh-purity (UHP) requirements. Purity lev-els of 5.0–6.0, depending on the type of connections, can be expected.

For highly corrosive gases, Monel or Hastelloy may be used. These materials provide a higher degree of corrosion resistance, but at a higher cost. While these materials are more corrosion resistant than stainless steel, they cannot be supplied at the lowest Ra sur-faces finishes, and are not normally used for 6.0 purity or better.

Delivery of highly corrosive gases at ultrahigh purity is a chal-lenge to the materials. Downstream purification may be used. One’s gas supplier should be consulted.

Regulator diaphragmIn lower-cost regulators, the diaphragm is an elastomer, typically neoprene. A regulator with a neoprene diaphragm should not be used when it is important to maintain purity levels of 4.5 or better. Elasto-mers can adsorb and diffuse contaminants, and may not be compatible for use with some gases. Metal diaphragms (stainless steel) provide a more secure metal-to-metal seal, are more broadly applicable, and are less prone to adsorption and diffusion of contaminants.

Pressure serviceSource pressure (inlet) and delivery pressure (outlet) are impor-tant parameters for proper regulator selection. Typical regulators can be separated into high-inlet-pressure regulators for use on high-pressure sources (such as cylinders), and lower-inlet-pres-sure regulators for use with lower-pressure sources such as some liquefied gases, bulk, small bulk, and dewars. Low-inlet-pressure regulators may also be used downstream from a primary regulator at the point of use (including line regulators; see below).

Most standard high-pressure cylinders used in the laboratory have cylinder pressures up to 2500 psig. Cylinder regulators are usually rated for a maximum inlet pressure of 3000 psig. Occasionally, higher-pressure cylinders are used, and these require regulators rated for the higher pressure, sometimes up to 6000 psig.

Cylinder regulators reduce the source pressure down to a safer, practical delivery pressure. Outlet delivery pressure is regulated by the control knob, and various delivery pressure ranges are available, such as 30, 100, and 250 psig. “Oversizing” is not recommended; a delivery pressure of 15 psi, for instance, is more stable and settable using the lower-range regulators, such as 30 psig.

Required flowRegulators are designed for various flow rates, determined by the Cv (coefficient of velocity) and orifice size. The larger the Cv, the higher the flow capacity of the regulator. Regulators with a Cv from 0.02 to 1.0 are typically the standard range, although much higher coefficients of velocity are available.

The orifice not only determines the flow capacity, but can have an effect on the droop and cylinder pressure decay characteristic of the

regulator.* In order to have adequate flow capacity while limiting the droop and decay characteristics, the regulator should be sized correctly based on specifications for the application.

Compressed Gas Association connectionsThe Compressed Gas Association (CGA) has promulgated the specifications for a standard set of cylinder valve outlet fittings to be used on compressed gas cylinders (see Figure 2). CGA fitting configu-rations (thread type, direction, and size) are specific to various classes of compressed gas. Typical classes are inert gases, flammable gases, highly corrosive gases, and oxygen. The different CGA fittings are designed to prevent mismatches of equipment and gas (e.g., it is not possible to use an oxygen regulator on a hydrogen cylinder).

Importantly, users should never attempt to defeat this safeguard with the use of adapters. Equally important, PTFE tape, oil, or any other lubricant should not be used on the inlet connection of a regulator. PTFE tape can shred, and fluid lubricants will find their way into the flow stream; neither should be inside the regulator.

Difficulty in attaching a regulator to a cylinder suggests the wrong CGA connection, or that replacement may be necessary. Safety note: The regulator fitting and the regulator itself should be

* Note: 1) Droop is a change in the delivery pressure as flow increases. 2) As the cylinder is depleted, the cylinder pressure declines. This reduces the pressure on the inlet side of the regulator, which causes an increase in the delivery pressure (a much smaller issue with dual-stage regulators; see below); this is known as cylinder pressure decay.

Figure 2 Table of CGA valve outlet connection numbers.

Line regulatorsLine regulators are single stage, and have only a single pressure gauge (output). Line regulators are installed at point of use and are intended for use downstream from a primary pressure regula-tor, as would be the case with manifold-supplied or house-sup-plied gas distribution systems.

Typical line regulators have an inlet pressure rating of about 400 psig maximum; in normal use the actual inlet pressure is much lower (and controlled by the upstream device). As a result, the output pressure creep typical of

single-stage regulators is not a problem with a properly deployed line regulator.

Specialty regulatorsSome gases or applications require specific operating parameters or limitations. For example, an acetylene regulator must be lim-ited to a maximum of 15 psig delivery pres-sure due to the instability of acetylene above this pressure. Acetylene regulators are sup-plied with a delivery gauge that has a red area marked on the gauge face above 15 psig.

Some regulators may have specialized interior surface treatments or platings. An example is a regulator designed for use with low ppm lev-els of hydrogen sulfide (H2S) mixtures. This prevents the small amount of H2S molecules from sticking to the interior surfaces of the regulator and leaning out the mixture.

Outlet valvesMany regulators are fitted with outlet valves. These are useful for turning flow on and off, but for proper flow control a rota-meter or mass flowmeter should be used. Often, at point of use, a line regulator will supply proper delivery pressure, and the instrument or laboratory device will pro-vide appropriate final flow controls.

RisksGas pressure regulators are necessary in any application that requires the use of compressed gas. As a result, all laboratories have a selection of “assets” that have seen previous use. It is up to the user to select and prepare regulators and other equip-ment properly for appropriate and safe use.

• Indiscriminate use of old components and regulators, even those that are in good condition, can lead to problems. A record of usage for every gas flow component should be kept.

• Regulators should be dedicated to a single gas service. If it is necessary to change ser-vice and redeploy a regulator, that regula-tor should be appropriately cleaned prior to reuse. Users should seek expert assistance if they are unsure of the appropriate measures.

• Along similar lines, it seems intuitive that with a gas delivery system com-prised of centralized or manifold-sup-plied sources, changeover of gas service on previously used tubing (and other components) should be done only when necessary, and only after system-atic purging and cleaning of the entire flowstream. Again, it is good practice to seek expert assistance.

• Regulators with obvious evidence of corrosion should not be used (anything that is visible on the outside is sure to be inside as well).

• Regulators with damaged threads or seal-ing surfaces of the cylinder connection should be carefully leak-tested before deployment. If a proper seal in the cyl-inder valve cannot be achieved, the regulator should be taken out of service. PTFE tape or pipe sealant should not be used in an attempt to remedy the leak.

• If one of the gauges on a regulator appears to be stuck or malfunctioning, it should be assumed that there is a damaged component in the regulator. Qualified repair (gauges and internal parts must be replaced with equivalent parts) must be enlisted or the regulator replaced. Oil or spray lubricant should not be used in an attempt to loosen a stuck gauge.

SummaryIn any laboratory or production setting, there are numerous circumstances that can lead to degradation or contamination of gas prod-ucts prior to their application at point of use, sometimes with serious performance or safety consequences. Mindful attention to applica-tions and equipment choices will help ensure that the gas used is of the same quality the supplier worked so hard to deliver.

Mr. Marone is Senior Project Manager, and Mr. Geib is Product and Technology Marketing Man-ager, MATHESON, 166 Keystone Dr., Mont-gomeryville, PA 18936, U.S.A.; tel.: 215-641-2700; e-mail: MMarone@MathesonGas.com.

checked thoroughly for leaks each time they are attached for use.

Single- and dual-stage designsCylinder regulators are available in single-stage and dual-stage designs. From the front, outward appearances are similar. Regulators use two gauges to indicate inlet and outlet pressures, regardless of whether the design is single or dual stage (see Figure 3 for com-parison of single- and dual-stage designs).

Many users prefer dual-stage regulators because they believe they provide better purity perfor-mance. In reality, the only advantage of a dual-stage regulator is pressure performance, and the difference can be an important one.

Dual-stage regulators are essentially two single-stage regulators in one body, with the first regulator supplying the second regulator. A dual-stage regulator is used when the delivery pressure of the regulator must be stable over a long period of time as the cylinder pressure declines.

Typically, with a single-stage regulator, the delivery pressure will rise about 0.5 psig to 2 psig for every 100 psig of decline in the cylin-der pressure, which translates (roughly) to a delivery pressure increase of 10 psig up to 40 psig over the life of a cylinder. Output pres-sure can of course be manually readjusted, but the effect is not continuous and operator intervention is required. When this is not practical or cannot be tolerated, a dual-stage regulator is used. Even though the first stage will still exhibit the delivery pressure rise as the cylinder pressure decays, the change is easily managed by the second stage, and out-put pressure is virtually constant.

Figure 3 Cutaway drawings of single-stage (left) and dual-stage (right) regulator.