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Uninterrupted flow from primary to reserve side increases convenience, cost savings, and safety in high consumption applications

Switchover or changeover systems use a primary gas source with a reserve side for use when the primary is depleted. The NFPA 99 Standard for Health Care Facilities stipulates that a third gas source be held in reserve for most patient medical applications. Medical switchovers and gas piping manifold systems are governed by NFPA 99 codes, while NFPA 51 details design and construction of industrial systems through the point of use.

There are manual switchover systems available in which the gas supply may be interrupted while the valves for the reserve cylinder or bank are opened. However, this article will concentrate exclusively on automatic switching systems that permit uninterrupted flow of gases from the primary to the reserve sides.

Automatic switching may occur either by mechanical or electronic means. A mechanical switchover is accomplished using two regulators set at different pressures with a common outlet. Each regulator is fed from its own cylinder or bank of cylinders. The regulator set with the higher outlet pressure shuts off flow from the second regulator during normal operation when both banks are full. When the primary bank begins to empty, the outlet pressure of its switching regulator will drop. When the outlet pressure from that regulator falls below that set for the reserve regulator, gas will flow from the reserve side. (See Figure 1) An electronic switchover accomplishes the same switching function. It utilizes electronic components such as transducers to monitor the pressures from both banks of cylinders, and opens or shuts flows from either side when it senses pressures from these sources falling below predetermined levels.

A line regulator is positioned downstream of the switching regulators. The switching regulators are normally preset to ensure an effective and seamless switchover. However, the line regulator usually permits adjustable outlet pressures, so the operator can set his working pressure. This line regulator also helps to maintain a steady outlet pressure, smoothing out the pressure variations from the switching regulators. The line regulator may be located at the switchover, at the point of use, or anywhere in between.

The gas sources for switchovers can be from either high pressure sources (e.g. cylinders, cradles or tube trailers) or liquid cylinders (e.g. dewars or microbulk). High-pressure sources generally do not present operational problems. The most commonly used gases are pumped at high pressures, which work well for pressure differential systems. With most non-liquefied gases, gas pressures are a good gauge of the cylinder’s contents as the pressure drops with gas consumption.

Liquid cylinders are a more economical gas source for high-consumption installations, but they do present unique operational problems. Most of the cylinders’ contents are stored in low-temperature liquid form and vaporized into gaseous state through a series of coils. The gas is stored in the cylinder’s headspace at lower pressures than those typical with high-pressure cylinders. This gas pressure will be below 230, 350 or 500 PSI, depending on the type of cylinder. If the head pressure exceeds these pressure limits, the relief valve will open and vent gas to the atmosphere until this pressure drops. However, when the gas is withdrawn for use, these types of cylinders have difficulty maintaining a steady head pressure during high-flow conditions, under continual withdrawal, or when the liquid contents dip below half full.

Another problem with a dewar is that its maximum gaseous withdraw is around 350 cubic feet per hour. Manifolding additional liquid cylinders will only increase the flow capacity moderately (approximately 20% of the original cylinder’s capacity), since multiple cylinders will retain varying gas pressures and the ones retaining higher pressures will shut off supply from those with lower head pressures.

The low gas vaporization pressures, fluctuating head pressures, and low gaseous withdrawal rates make liquid cylinders a difficult supply source. Since both pressure differential or electronic switchovers operate off pressure readings, problems of premature switchover or flip-flopping from bank to bank, depleting both banks can occur, if a steady inlet pressure cannot be maintained. Other than the IntelliSwitch™ switchover, which will compensate for any supply source, switchovers are designed for specific types of cylinders. Before installing any switchover, make sure it is designed for the correct type of high-pressure or liquid cylinders you intend to use and that it is suitable for the pressure ratings of those cylinders.

There are several options and accessories that often are offered with switchovers. Naturally, outlet pressure ranges, preset switchover pressures, flow capacities, and materials of construction need to be taken into consideration for compatibility with your particular application and the gas in use. If the application is one for specialty gases, particularly high-purity or corrosive gases, the switchover and all ancillary equipment must provide high integrity against contamination and also corrosion resistance. The switchovers can be either open style with the regulators exposed or boxed with the regulators protected inside an enclosure. (See Figures 2, 3 and 4) Less common is a semi-enclosed style with a panel covering the front, and the tops and bottoms exposed.

Cylinder manifolds are available in a variety of configurations and materials, most commonly brass, chrome-plated brass, or stainless steel. They can be designed as wall mounted or free standing. They also can be of a modular threaded or flared design that permits extensions for additional cylinders, or of a rigid hard-piped design. Pigtails are also available in different lengths, materials and pressure ratings. These pigtails may be rigid or flexible. Some pigtails may incorporate a check valve in the CGA gland that helps prevent contamination from getting into the gas stream, protects against possible trans-fill of empty cylinders from the full bank, and provides safety against whipping if the pigtail breaks loose from the cylinder valve. If the gas in use is acetylene, the pigtail should also incorporate a flashback arrestor.

A common option for switchovers is an audio and/or visual alarm to signal that a bank is depleting and the empty cylinders should be replaced. These alarms can be placed at the switchover or any remote location (e.g. a maintenance office) within the distance limitations determined by the specific alarm. Other possible options are telephone dialers that will automatically dial anyone programmed to receive a prerecorded message, typically the gas supplier or maintenance office, advising of the need to replace the cylinders. If the alarm or switchover incorporates a serial port, it can be connected to a computer to program an automatic e-mail sending a similar message.

If the gas in use is flammable and the switchover incorporates alarms or any other electrical component, an accessory that protects against ignition needs to be used. These may be explosion-proof enclosures or intrinsic safety barriers. Explosion-proof enclosures will contain any ignition within its chamber, so that no switchover components will be expelled. However, it provides no protection against the conditions that can set off the ignition. Intrinsic safety barriers will limit the amount of thermal and electrical energy that can reach the switchover, so it eliminates one of the legs of the combustion triangle (i.e. fuel, oxygen and energy source). When combustible or flammable gases are in use, the switchover should be stored in a safe area, and any electrical or spark-producing components should be kept out of that area.

For liquid-liquid switchover systems, there are several options to help increase gas withdrawal rates and reduce product loss. These options include the following:

Economizers: To prevent reserve liquid cylinders from venting with the subsequent loss of that gas, an economizer will temporarily shut off flow from the primary side and consume the gas from the reserve side until the excessive gas pressure drops. When the pressure drops to a predetermined level, the switchover will switch back to the original primary.

Look-Back Feature: Since liquid cylinders are prone to pressure fluctuations when the head pressure drops below the switchover pressure, the switchover will switch to the reserve side even if the primary maintains a residual amount of product. A look-back feature monitors the head pressure in that original primary side. If the liquid cylinder can rebuild and maintain its head pressure, the look-back feature allows the system to continue drawing from that original side. It is not uncommon to enjoy 15 to 30% additional liquid cylinder consumption rates with the look-back feature.

Cryogenic Vent Manifolds: Since liquid cylinders maintain gas pressures at varying pressures, liquid cylinder manifolds are quite ineffective. As described above, each additional dewar added to a manifold only increases the flow capacity to approximately 20% of the first one’s capacity. By manifolding the vents of all the dewars, it will equalize the head pressures in all of them, so that you get uniform withdrawals from each of them. This option increases the additional flow capacity of each incremental dewar to approximately 80% of the first one. (See Figure 5)

Pushers: To increase flow capacity from a liquid cylinder, you can withdraw the product in liquid form and have it gasified in an external vaporizer. It is possible to substantially increase the liquid withdrawal from a dewar by using a second pusher dewar. The gas from this pusher is fed through a regulator to the vent of the primary liquid cylinder. This gas pushes down on the liquid contents at the bottom of the cylinder, facilitating a greater withdrawal of the liquid.

Summary
Switchovers are a convenient and cost-effective method of supplying gases in high- consumption applications. It assures continuous supply of the gas by maintaining a reserve. It can substantially lower costs for those using large volumes of gases by allowing for more economical gas sources and eliminating the inefficiencies inherent in individual cylinder usage. It also ensures greater safety by removing cylinders from the work area.

DAN CRUZ IS DIRECTOR OF SALES, CONCOA, MANUFACTURERS OF GAS FLOW CONTROL SYSTEMS AND EQUIPMENT, HEADQUARTERED IN VIRGINIA BEACH, VA, 800-225-0473, DAN.CRUZ@CONCOA.COM, WWW.CONCOA.COM. A GRADUATE OF OHIO STATE UNIVERSITY AND A GAS INDUSTRY VETERAN, HE IS RESPONSIBLE FOR CONCOA SALES IN NORTH AND SOUTH AMERICA.

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