Q3 2009 / Automatic Switchover Systems – Install gas delivery equipment that boosts profits and productivity

Specialty gas switchover systems use both a primary source of supply and a reserve for when the primary is depleted. Although switching from the primary to the reserve can be accomplished manually, automatic switchover systems offer the greatest benefits.

Automatic switchover systems come in two types – mechanical and electronic.

Mechanical systems use differential outlet pressure settings for the regulators tied to each gas supply side. Electronic ones use pressure transducers with the gas supply regulators.

Several switchover options are commonly available including various outlet pressure ranges, switchover pressures, flow capacities, materials of construction, open style or boxed, as well as several other features.

In addition, users can install remote alarms that signal the need to replace an empty primary source along with remote communication devices that monitor or manipulate operating conditions through auto-dialers, serial ports, and Ethernet connections.

Although automatic switchover systems offer convenience, safety, reliability, and cost savings, these benefits are often viewed differently by users and specialty gas distributors.

Continuous Gas Supply

Because an automatic switchover system ensures a seamless, continuous flow of gases from the primary to the reserve supply, there is no shut-down of a manufacturing process while cylinders are replaced. For the user, this translates to higher production rates and often higher quality products if the gas pressure is maintained at a fairly constant pressure level.

CO2 lasers, for example, have protection circuitry that automatically shuts down if minimum gas pressures are not maintained. Although laser gases are not specialty gases per se, high-purity gases are normally used to minimize the introduction of moisture, hydrocarbons and particulates that sap beam efficiencies and reduce parts lives. The restart cycle when lasing gas pressures drop below predetermined set-points can average 45 minutes, and critical parts can be damaged if a laser cut is inadvertently interrupted. Since one of the principal advantages of cutting with lasers is high production rates, continual downtime due to gas pressure fluctuations will seriously impact production.

In other high-purity applications an automatic switchover system reduces the risk of contaminants invading the gas stream by out-gassing through a cylinder’s wall when internal pressure falls below 200 psig.

For the specialty gas distributor, maintaining a continuous gas supply keeps the customer satisfied. Minimizing pressure fluctuations and contaminants from the gas source also helps reduce operational problems that divert the distributor’s sales and service staff to solve operational problems rather than focusing on new sales and installations opportunities.

Using an automatic switchover system allows users to install the supply system in a secure place. Switchover units can be bolted to a wall or a mount with cylinders chained, secured, and enclosed in a padlocked environment with gases piped to the point-of-use.

For the customer, a primary benefit of this installation is safety. High-pressure cylinders, flammable gases, and other potentially harmful gases and storage vessels kept from the work area help comply with National Fire Association Codes and Uniform Building Codes. Here, users should remember that local regulations can vary.

In addition, OSHA classifies compressed gases as hazardous materials under Standards 29 CFR- 1910.101 (www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS8p_id=9747). These standards also vary according to the type of gas and installation.

For the gas supplier as well as the user, keeping cylinders stored in a secure area helps alleviate liability concerns. Simply the fact that the cylinders are not carted around the work area and continually adjusted through every work cycle reduces accidents and improves productivity as well as prolongs their service life and appearance. 

Switchovers are especially beneficial in specialty gas applications. They prevent the introduction of humidity and other contaminants into the cylinders that could cause corrosion or require purging by retaining a minimum pressure in the cylinders, i.e. the pressure at which the switchover takes place. That pressure is normally factory set by the supplier of the manifold and not left to the operational staff at the customer.

Gas Mode Changes

Switchovers facilitate the change from high pressure to liquid cylinder gas. Liquid cylinders are often used in high or critical flow applications. However, care should be taken to ensure that the switchover system used is suitable for the specific gas supply. Some switchover systems are only suitable for high-pressure cylinders, some for liquid, and others for liquid primary cylinders with high-pressure cylinder reserve supplies. Only a few models are truly universal and can adjust to any type of gas supply.

The biggest benefit to the user in converting to a liquid mode is cost savings. Although costs can vary by market, negotiated arrangements can score large gas users substantial cost savings where space is at a premium since liquid gases require less space than comparable volumes of high pressure cylinders.

For the specialty gas supplier, converting to a liquid cylinder gas mode means a long term contract with a large gas consumer. In addition, liquid is an effective method of supplying large volumes. The volume of gaseous product that can be vaporized from a liquid gas depends on its specific gravity. With oxygen, for example, liquid yields 12.076 cubic feet for every pound. Helium vaporizes to 96.71 cubic feet of gas for every pound. Obviously, the transportation and servicing of a gas account becomes more cost effective if larger volumes of product are stored on-site.

Inventory Management

Because a single switchover with a cylinder or bank of cylinders on each side can feed multiple work stations, switchover systems can help reduce cylinder inventory and related costs compared to using individual cylinders.

If the company either pays deposits and rental fees or else owns its own cylinders, a smaller inventory represents cost savings. If all the cylinders are maintained in a central location, switchover installations also allow for a more efficient layout of work space.

For the specialty gas supplier, cylinder inventories often represent their greatest capital expenditure. Switchover systems increase a cylinder’s fill rotation, reduce total cylinder stock levels, and maintain gas consumption rates. Storing the cylinders in a secure location also minimizes cylinder loss, prolongs their useful wear life by reducing abuse, and improves delivery efficiencies.

Cylinder changeout time

An individual cylinder’s replacement time can vary depending on the facility’s layout and the user’s changeover procedures. However, half an hour to 45 minutes is typical. During changeover production is idled. The operator, the warehouse, and the maintenance staff can experience disruptions while cylinders are replaced.

On the other hand, switchover systems can be sized for less frequent replenishing. Optional alarms and notification systems can even be added to alert operators when the primary side approaches depletion. Obviously, the elimination of cylinder replacements at the point-of-use means higher production rates and more efficient use of workers’ time since there is no need to shut down during changeouts. 

For the gas supplier, maintaining the cylinders at a central location improves gas delivery efficiency since gas deliveries are made to a single location. Also, because switchover systems allow for a reserve supply of gases, delivery schedules may not be required as frequently and the number of “emergency” deliveries can be reduced. Finally, systems with optional automatic e-mail or phone notification alerts and/or remote monitoring can help suppliers monitor cylinder contents and boost delivery logistics.

Purging

Purging is not unique to switchover systems, but they are a type of installation where purges are frequently offered as an option.

In high-purity applications a purge removes most gas stream contaminants. In many applications humidity levels must be kept low, particularly with corrosive gases. Different types of purges are available to remove contaminants. The most effective is the cross-purge, since it can purge contaminants throughout the supply stream from the supply to the point-of-use.

Minimizing the amount of contamination in specialty gas applications can be accomplished by ensuring both the switchover system and piping installation feature high leak integrity. The wetted components in contact with the gas stream must be compatible with the gas or gas mixture used. Pigtails with internal check valves on a switchover system will limit the amount of air and humidity that will fill the pigtails during cylinder replacement. Using stainless steel pipe and compression fittings is also helpful in preventing leaks and contamination in the piping.

Purging benefits the user by maintaining gas purity, ensuring process reliability, and reducing the number of erroneous measurements or “ghosts” in its quality checks.

Prevention of corrosion will prolong the life of all the components in the pipeline. For the gas supplier it helps prevent false claims that its gas is contaminated and enhances its image as a quality supplier.

Gas Loss

Compared to individual cylinders, gas supplied continually from a switchover system avoids losing gas at start-up and during equipment calibration. The amount of gas saved will vary, depending on the type of application and installation. Gas savings, for example, with a chromatograph will typically be a few cubic feet, while in a laser assist gas application it can be a few hundred cubic feet.

Compared to individual cylinders, gas supplied continually from a switchover system avoids losing gas at start-up and during equipment calibration. The amount of gas saved will vary, depending on the type of application and installation. Gas savings, for example, with a chromatograph will typically be a few cubic feet, while in a laser assist gas application it can be a few hundred cubic feet.

Mitigating these start-up savings in switchover use is the fact that switchovers retain some of the cylinder contents in the depleted cylinder or bank. The amount of gas retention is determined by the control regulator’s preset switching pressures. Although the gas retained is not used by the customer unless the switchover pressures are high, the amounts are not great. This is special benefit in high purity applications that must retain at least 200 psi in the cylinder in order to avoid out-gassing contaminants from the cylinder walls. 

If a liquid cylinder is kept in reserve over a prolonged period of time a switchover system ideally will incorporate an economizer to prevent overpressure venting. Dewars include relief valves normally set at 230, 350, or 500 psi that vent gas to atmosphere to protect against excessive internal gas pressures. The excessive gas pressures can build, if these liquid cylinders are held in reserve over a prolonged period of time. The economizer feature in a switchover bleeds off the excess pressure before it vents until the pressure drops to safe levels. Thereafter the system reverts to the primary cylinder or bank of cylinders.

In all, switchover systems provide an array of benefits in terms of cost, convenience, and safety. Users benefit because gas is not lost in the start-up and during calibration processes and enjoy a reduction in time and effort spent on start-up procedures. Gas suppliers benefit because although their customer’s gas consumption may remain steady, switchover systems help prevent the introduction of contaminants into cylinders and prolongs their service lives because of the reduction in cylinder handling and contamination.

About the Author Dan Cruz

Email: dan.cruz@concoa.com Web: www.concoa.com