A compressed air system's piping serves many purposes. The most obvious is to transport the air from the supply side of the system to the downstream equipment. It also provides limited storage capacity and controls velocity to and from various process parts of the system. A variety of  piping materials that can be used, as well as many approaches to assembling them. Many types and styles of valves  can be used within the air system. Different piping configurations can also be used to suit a broad range of needs. One thing is for sure: there are rarely straightforward answers regarding piping and piping systems.

Types of piping and assembly methods

Many piping materials are available for compressed air systems. The most common are:
• black iron,
• carbon steel,
• galvanized steel,
• copper,
• plastic (PVC and CPVC),
• ABS, and
• stainless steel.

Any of several different connecting or assembly approaches can also be used. (Naturally, some of them will only work with specific types of pipe materials.) The most common are:
• butt weld,
• slip-on weld,
• threaded,
• socket weld,
• sweat fit,
• chemical weld,
• flared and threaded, and
• grooved and clamped.

The goal of the compressed air system's piping designer should be to construct a safe and efficient system that is easy to work with and flexible for future needs. When selecting material, remember that each has a rating for pressure and temperature. Determine the highest operating or working temperature and pressure for the location in the system that the material will be installed at. Codes may dictate the test requirements for your area or application. Most burst tests are 152% of rated working pressure.

From the compressor to the aftercooler, the temperature could range from 175° to 425° F, and the air could be thoroughly saturated with water vapor. If Schedule 40 black iron pipe is used for a discharge line between a compressor and the aftercooler, a substantial amount of iron oxide will build up in the pipe due to corrosion. It has to come out somewhere — probably the separator drain line. This means that the drain-line size must be generous, a Y-strainer installed with a trash-out valve, and the trap will have to be frequently serviced. Choosing carbon steel, stainless, or galvanized for this application will eliminate significant pipe contamination, and will require no extraordinary approach toward piping or maintenance.

Plastic and ABS-based materials are light and easy to assemble, making them especially convenient overhead piping arrangements. However, they must be approved for compressed air service. They resist corrosion and are cost effective. This material is particularly good for inlet piping where the inlet is located outside, remote from the compressor. It also works well in many underground installations. Some local codes will not allow this type of piping in the overhead systems because of potential byproducts in the event of a fire. Expansion joints must be provided in these types of pipe, as they will expand and contract due to both interior and ambient temperature changes. With some materials, as much as ¼-in. of expansion must be provided per 10-ft length per 10° total temperature change from rated temperature. It is important not to clamp down on this piping with hangers or make 90° turns without provision for expansion and contraction.

The compressor room

In many smaller systems, copper Schedule L pipe with K fittings may be the best choice. Valves can be wafer- or butterfly-style installed between flanges in the copper pipe. ACR copper can also be an excellent choice of materials over stainless steel where cleanliness and low porosity are desirable, such as system dew points below –40° F. With line sizes over 3 in., copper is no longer economical.

In larger systems, galvanized and carbon steel offer relative cleanliness, but are harder to work with than several other choices. One particular assembly method grooves carbon steel or black iron, which offers the ability to prefabricate large piping systems.

Inlet piping differs considerably from pressurized piping. It is most important that the piping material be clean and not capable of developing contaminants such as rust or oxides. It is also important to assemble piping in such a way that there are no parts that can dislodge and go into the compressor. It is a good practice to install a cone strainer in the final flange before the inlet with the cone facing against the inlet flow.

Another common problem is remote mounting of the inlet to the outside. Although there are many reasons to install the inlet outside, it can be difficult to service. When this is done, it is particularly important to properly instrument the inlet so it will alert maintenance personnel or operators to the need for maintenance. But a better approach would be an inlet filter, set up for on-compressor mounting with remote connections. When this is done, a pre-cleaner should be installed on the remote inlet to capture the larger inlet contaminants. Figure 1 shows examples of good approaches to remote mounting of the inlet.