The question of galvanized piping comes up often in compressed air system piping instead of schedule 40 black iron for the nominal 100 psig air systems. To help evaluate this, let’s look at inlet and discharge piping separately.
General Guidelines for Inlet Piping
The proper inlet pipe brings the air from the filter to the compressor with no pressure loss, and should not create operational problems with any type of self-contamination on the inside. It is important to realize that the ambient inlet air condition may well dictate the selection of one type of pipe over another.
Galvanized inlet piping has the advantage of resisting corrosion better than standard iron pipe. However, overtime when the corrosion does set in, the galvanizing material peels off. The inlet pipe is now a producer of potentially very damaging, solid contaminants between the filter and the compressor. This would be particularly dangerous to the mechanical integrity of a centrifugal compressor. We do not recommend this.
During high-humidity weather it is quite conceivable that condensation will form in the inlet pipe (therefore, the OEM installation manual usually recommends a drain valve be installed on the pipe before the inlet). Condensation in the pipe will obviously accelerate the time frame before the coating breaks down. This time frame is dependent upon where the thinnest portion of the coating is applied.
Stainless steel inlet pipe is an excellent material for such large-diameter, low-pressure inlet air, as long as it is installed properly and the inside is properly cleaned.
There are also many grades of thermoplastic material suitable for inlet air piping.
Extruded Aluminum - aluminum tubing that can be easily assembled with normal hand tools can bring a great deal of flexibility to an operating air system or sub-system. These are particularly effective for specific work areas, which may have to change on a routine basis.
Summary: We recommend either stainless steel or proper thermoplastic-type material for inlet piping and do not recommend galvanized piping.
Discharge and Distribution Piping
Here we have more complex considerations:
The discharge air from the compressors can be at 250 to 350°F (for centrifugal, oil-free rotary screw and reciprocating types), or from 200 to 220°F (for lubricant-cooled rotary screw compressors), so the pipe must be able to withstand those temperatures.
Even if there is an aftercooler that drops the temperature to 100°F, consideration must be given to the consequences if the aftercooler were to fail.
Compressed air-generated condensate tends to be acidic. In oil-free compressors (such as centrifugals and oil-free rotary screws), it is usually very aggressive.
The basic objective of the interconnecting piping is to deliver the air to the filters and dryers and then to the production air system with little or no pressure loss, and certainly with little or no self-contamination.
Galvanized piping will have the same problems once it begins to peel, as we described on the inlet application. In all probability, due to the aggressive acid characteristics of the condensate, the galvanized coating life may be much shorter.
Regardless of the thermoplastic pipe manufactures claims; we never recommend any plastic type material for interconnecting piping and rarely for distribution header piping. Most of these materials carry cautions not to be exposed to temperatures over 200°F and to avoid any types of oil or lubricants.
Here again, stainless steel is our number one recommendation for the interconnecting piping from the compressor to the filter/dryers when the compressed air is oil-free. It will obviously resist corrosion much better than standard schedule 40 black iron. Some other considerations:
Most areas will allow schedule 10 stainless steel in lieu of schedule 40 black iron.
For the same diameter pipe, stainless steel will be much lighter and easier to handle, usually lowering the labor cost.
For welded connections, stainless steel usually just requires one bead, while black iron pipe usually requires three beads (Weld-fill-cover). This should also lower the labor cost.
Stainless steel does not usually seal well when threaded. It will do much better with Victaulic type connections when welding is not practical.