Select The Right Gas Delivery System

Pay particular attention to pipe size and materials compatibility.

By Larry Gallagher, CONCOA

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Check that the bulk supply for these types of gases is appropriate — either by the number of cylinders or the size of both the bulk tank and any external vaporizing device. Table 2 indicates the maximum withdrawal rate in ft.3/hr. for propane based on cylinder temperature and liquid remaining in a 100-lb. cylinder.

Samples for process stream composition or purity usually involve tiny volumes. Keep the pipe diameter as small as possible — 1/8-in. outside diameter or less is common. Also, limit the length from the sample valve to the analyzer and use pressure or flow control devices with minimal internal volumes. This ensures as real-time a sample as possible.

In drawing gas samples consider source pressure and necessary inlet pressure of the analyzer, along with required sample size or rate. For analytical processes the sample must be delivered as a gas or at a certain temperature. Samples in liquid phase or having condensable components normally need a vaporizing pressure regulator (Figure 3).

Material Considerations
Today’s designers have more materials options than ever. New alloys have greatly extended the life of lines in corrosive service. Improvements in welding processes, joining techniques and fittings have reduced the labor burden and maintenance on many types of installations while improving leak integrity and purity of the resulting systems.

To select piping system material, start by considering properties of the gas and the process or instrumentation. For process and instrumentation gases, maintaining purity is a key requirement. It does little good to purchase high-purity grades of instrumentation gases if the delivery pipeline contaminates them.

All materials chosen should be cleaned to very high levels, delivered to the site capped, and installed by persons using processes capable of maintaining that level of purity. Copper tubing or pipe should be at least “cleaned for oxygen service,” a standard that guarantees it’s as free of hydrocarbon contamination as possible and safe for use in oxygen piping systems. Type 316L and other stainless steels or more exotic alloys benefit from electropolishing although this isn’t required.

 computer controlled
Figure 2. More sophisticated option:
A computer-controlled system with a
primary and backup supply source
can adjust pressure to the piping inlet.
For inert or non-reactive gases, it’s now just as expensive to use copper as 316L stainless. So, choose stainless because of its many advantages: it’s generally suitable for even corrosive or reactive gases, the leak integrity of compression-fitting or orbitally-welded joints is far superior, and installation costs are less. For extremely corrosive or reactive sample or process lines, consider high-nickel alloys. System durability and improved safety often outweigh the higher cost.

Piping materials and installation for oxygen deserve special care. At concentrations above 23%, oxygen can be extremely hazardous; even a small amount of a contaminant that won’t ignite in air will burn or virtually explode when exposed to a pure oxygen atmosphere. Therefore, the piping system and components must be certified as cleaned and suitably installed for “oxygen service.” Many facilities require piping systems carrying high-pressure (above 300 psig.) oxygen to be made of Monel, which strongly resists promoted ignition in pure oxygen. Lower-pressure systems that don’t need high-purity components typically use copper pipe that’s silver brazed.

Never use copper for an acetylene piping system because the acetylene will react with the copper to form hazardous and explosive compounds.

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