Select The Right Gas Delivery System

Pay particular attention to pipe size and materials compatibility.

By Larry Gallagher, CONCOA

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You can take several steps to ensure that your plant’s high-purity carrier, support, process or calibration gases are supplied safely and cost efficiently without compromising purity and sample integrity. Adherence to codes and specifications is vital (see “Properly Deliver Compressed Gases”), as is following a comprehensive series of procedures that also improve efficiency. You also should pick conditions optimal for the specific application and hardware components compatible with the gas.

This article focuses on the two latter considerations, namely:

• pipeline size relative to flow and pressure drop rates; and
• materials compatibility for gas process lines and sampling systems.

Pipe Sizing
Designers should size piping so that pressure drop at the furthest point in the system doesn’t exceed 10% of the inlet pressure under actual flow conditions. Table 1 lists the maximum flow rate of nitrogen in ft.3/hr. for pipe diameters from 1/8 in. to 3 in. and pressures from 5 psig. to 250 psig. — the data are for 10% pressure drop per 100 ft. for ½-in. and smaller sizes, and 5% pressure drop for larger diameters. To convert this table to the gas of interest, you must multiply the flows by a factor based on the difference between the specific gravity of nitrogen and that gas. While there’re more accurate equations for this conversion factor, you can get a reasonable value by dividing the square root of nitrogen’s specific gravity by the square root of the gas of interest’s specific gravity.

However, this doesn’t provide the entire picture. The source must be able to provide this flow rate at the required pressure. Some gases, such as nitrogen for large process applications, are delivered in pipelines direct from a source such as an air separation plant, while other gases are supplied in high-pressure cylinders or from portable or permanent cryogenic tanks. Source pressure must be reduced to piping inlet pressure using either a simple single-pressure control device (Figure 1) or a computer controlled system that has a primary and backup supply source (Figure 2).
 simple device
Figure 1. Pressure control:
A simple single-pressure device
can reduce source pressure to
that needed for the application.

Size the source and the pipeline itself with an adequate but affordable safety factor to allow for peak demand and future growth. It’s not unusual to apply a safety factor of two times the current anticipated flow and from 1.2 to two times the required pressures. This provides a margin so future additions or changes in process requirements don’t necessarily demand a totally new piping system.

For a pipeline with more than one use point, calculate flow assuming total flow occurs at the farthest use point from the inlet of the system. This ensures that each point won’t be starved for flow because one high-flow use point creates a huge pressure drop every time it’s initiated. Another way to compensate for disparate flow rates at different use points is to run the entire pipeline in a loop, beginning and ending at the same inlet point, so no point is any further from the supply than any other point. For large piping systems made with exotic materials this can be cost prohibitive but for smaller systems or critical use processes it can make the difference between a system that works and one that doesn’t.

The safety or supply limitations that apply to specific gases is another consideration. With acetylene, for safety reasons (to prevent spontaneous ignition in air) pipeline pressure can’t exceed 15 psig. Certain gases that come from liquefied or cryogenic sources have maximum-flow-capability restrictions. For carbon dioxide, ammonia, chlorine or liquid hydrocarbons like propane, the bulk of the product actually is in the source tank or cylinder in liquid form; the use valve withdraws gas from the vapor phase. This gas must be replaced by vaporizing liquid. Achievable withdrawal rate depends upon both volume and temperature of the liquid.
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