Specialty Gas Delivery Systems

Three important considerations

When specialty gases are used in significant volumes, a centralized gas delivery system is a practical necessity. A well-conceived delivery system will reduce operating costs, increase productivity and enhance safety, and may be accomplished with either a single regulator system or a more sophisticated switchover and manifold.

A centralized system will allow the consolidation of all cylinders into one storage location. With all the cylinders in one place, inventory control will be streamlined and cylinder handling will be simplified and improved. Maintaining gas purity also is simplified with a centralized system.

A properly designed specialty gas delivery system should fulfill the following objectives:
  • Materials of construction should be compatible with the selected gas or gases.
  • Maintenance of gas integrity and purity.
  • Automatic shutoff of gas in the event of catastrophic failure.
  • Effective control of residual gas during cylinder changeout.
Purity
One of the most important considerations in designing a gas delivery system is the level of gas purity and materials of construction required at the end-use point.

The first level is usually described as a multi-purpose or analytical application. Typical applications are AA, ICP and general gas chromatography. Manifolds for multi-purpose applications are designed for safety and convenience. Acceptable materials of construction include brass, copper, teflon, tefzel and viton. Packed valves such as needle valves and ball valves often are used for flow shut-off.

The next level, called high purity, requires a higher level of protection against contamination. Applications include gas chromatography, where capillary columns are used and where system integrity is important. Materials of construction are similar to multi-purpose manifolds, except flow shut-off valves are the diaphragm packless type to prevent diffusion of contaminants into the specialty gas.

The third level, referred to as ultra-high purity, needs the highest level of purity. Trace measurement in gas chromatography is an example of an ultra-high purity application. Wetted materials for manifolds at this level must be selected to minimize trace component adsorption. These materials include 316 series stainless steel, teflon, tefzel and viton. All tubing should be 316 cleaned and passivated. Flow shut-off valves must be of the diaphragm packless type.

It is particularly important to recognize that components that are suitable for multi-purpose applications may adversely affect results in high or ultra-high purity applications.

The frequency of cylinder changeouts required is reduced in a centralized system. This is achieved by connecting multiple cylinders to manifolds in banks in such a way that one bank can be vented, replenished and purged while a second bank provides continuous gas service.

Since cylinder switch-over is accomplished automatically by the manifold, cylinders in a bank will be uniformly exhausted. The gas manifolds used in these systems should be equipped with check valves to prevent gas backflow and purge assemblies to eliminate contaminants from the system during changeout.

Specialty Gas Delivery System

Specialty Gas Delivery System

Types of Gas Delivery Systems
Single Cylinder Manifold Systems: In some applications, specialty gas is used only to calibrate the instrumentation. For example, a continuous emissions monitoring system (commonly referred to as a protocol station) may only require calibration gases to flow for a few minutes each day. A single cylinder manifold with bracket is an ideal solution for this type of application.

Multiple Cylinder Manifold Systems: Many applications require a flowrate of gas beyond what can reasonably be supplied by a single station manifold, but are not of such a critical nature that they cannot tolerate occasional shutdown for cylinder changeout.

The single header manifold connects two or more cylinders to the same line for continuous gas supply. Each cylinder connection point, or station, is fitted with a valve to permit individual cylinders to be isolated for changeout. In order to preserve system purity, these valves should be the diaphragm packless type to eliminate oxygen, nitrogen, water vapor or other contaminants from entering the gas stream.

Continuous Uninterrupted Gas Supply: If the operation runs 24/7, an important consideration for many users is the requirement for a constant, uninterrupted supply of gas. Manifolds provide the capability to switch from a primary to a reserve bank without interrupting the gas supply, minimizing or eliminating costly downtime. The selection of the correct manifold for a given application depends on a number of factors.

Change-Over Method
There are a number of different methods used to effect cylinder bank change-over. These methods vary substantially in their level of sophistication. Selecting the correct manifold, then, depends on the application, since the additional features in the more sophisticated versions can justify their expense in critical applications.

Manual Differential Change-Over: The simplest manifolds are designed to change over based on a sensed drop in pressure of one cylinder bank relative to the other. The regulator pressure gauges must be visually monitored to determine when change-over has occurred.

Differential manifolds require regulator monitoring and resetting and generally are selected for applications where cylinder bank change-over is relatively infrequent, and where a drop in delivery pressure at change-over will not cause a problem.

Automatic Change-Over: A change drop in delivery pressure can, in some instances, result in an adverse effect on instrument performance. To avoid this problem, an automatic manifold may be selected. The operation of this type of manifold is also based on differential pressure, but delivery pressure is held virtually constant during cylinder bank change-over.

When outlet from the primary bank drops to the set pressure of the reserve bank, the reserve bank opens to provide uninterrupted flow of gas.

When used in conjunction with a pressure switch and audio alarm to provide remote indication of change-over, the automatic manifold need not be monitored. Automatic manifolds are used in applications where change-over is relatively frequent and variations in delivery pressure cannot be tolerated. Manual selection of the primary bank is still required after depletion of gas supply.

Electronic Logic-Controlled Manifolds: In some critical manufacturing and laboratory processes, an uninterrupted gas supply is an absolute necessity. Failure of the gas supply in these cases could result in loss of an entire lab’s in-process experiments or even shut-down of a production line. A logic-controlled manifold generally is selected for these applications.

By using pressure transducers, the logic-controlled manifold can provide the user with the ability to constantly monitor the pressure in their supply from a remote location. It also can indicate which side of the manifold is currently in use.

When the supply reaches a pre-set pressure, the manifold automatically switches to the reserve side, which continues to deliver gas without interruption. This change-over is accomplished by energizing a solenoid valve, which sends an air signal to open a pneumatic valve on the reserve cylinder bank. At the same time, another solenoid de-energizes to close a pneumatic valve on the supply side, shutting it off. The outlet of both pneumatic valves is connected to the inlet of an internal regulator, which maintains constant delivery pressure.

With the logic-controlled manifold, the danger of inadvertently draining the reserve bank is eliminated since there is no reliance on the operator to switch a change-over valve or reset regulators. Fail-safe operation is ensured by using normally closed pneumatic valves on each cylinder bank. In the event that either electrical power supply should fail, these valves automatically will default to a pre-selected bank to ensure supply of gas.

Gases and Welding Distributors Association
David Bell Meet the Author
David Bell is manager, Special Gas Products Group for Harris Calorific, a Lincoln Electric Company, in Gainesville, Georgia.