Sometimes plants need more gas flow than they can get from single cylinders. One solution is a bulk compressed gases supply. With this arrangement, gas is piped from a storage tank in back of the plant. Another common method is to use manifold devices that allow two or more cylinders to be ganged to increase gas flow rates. When acetylene is used, manifolding is the only way to increase the bulk flow of fuel beyond that available from a single cylinder. Manifolding increases the safety problems, because it increases the amount of gas being handled and used at one time. Certain rules should be followed by manifolding gas cylinders:

• Use fuel-gas manifolds and high-pressure (more than 250 psig) oxygen manifolds that are approved by Factory Mutual or Underwriters Laboratories for the gas used. The manifolds must be able to withstand pressures and must be leak free. When manifolds are tested, an oil-free noncombustible gas such as dry nitrogen, should be used.
• Use manifolds and parts only with the gases they are approved to handle.
• Label all low pressure (less than 250 psig) oxygen manifolds clearly.
• Install an approved flash arrester between cylinder and coupler block whenever acetylene cylinders are coupled to a manifold. If no more than three cylinders are being used outdoors, one flash arrester installed between the coupler block and the regulator is acceptable.
• Have at least one valve key or wrench available at the manifold for emergency closing of valves not equipped with a handwheel. Leave more than one key or wrench on the cylinders.
• Have approximately equal pressure in gas cylinders connected to discharge simultaneously through a common manifold. If a full cylinder at 2400 psig and a three-quarters empty cylinder at about 600 psig are connected to feed into the same line, the gas in the full cylinder will tend to back up into the nearly empty cylinder instead of feeding into the pipeline or hoses.

Procedures for placing manifolds help reduce the risk of fire damage:

• Do not use a fuel gas manifold with a capacity larger than 3000 cu ft if acetylene or nonliquid fuel gases are used indoors unless the manifolds are placed in a room built to local fire-resistant regulations and standards. Above this capacity, the building must meet regulations requirements or the gas must be stored in a separate outdoor location.
• Keep fuel gas manifolds at least 50 ft (15.2 m) apart. You can use two or more in one room if they are separated.
• Locate oxygen manifolds for pressures above 250 psig and capacities above 6000 cu ft, or below 250 psig and capacities to 12,000 cu ft, outdoors or in a building constructed to regulations requirements. No combustible materials may be kept within 20 ft (6.09 m) of the manifold.
• Be sure that manifolds connected to bulk (more than 20,000 cu ft) oxygen supplies meet local regulations and standards for bulk oxygen systems.
• Be sure that high pressure (above 250 psig) oxygen manifolds are at least 20 ft (6.09 m) from combustible materials or fuel-gas manifolds, or are separated by a barrier at least 5 ft (1.5 m) high that meets local regulations, codes and  standards has had a fireresistance rating of at least half an hour.
• Never put oxygen manifolds inside any acetylene generator room.