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Chemistry and Compressed Air Piping

By: Neil Mehltretter

While as engineers we typically get really excited about designing and building new facilities, in reality, most compressed air systems are a hybrid of new and old. In these cases, piping connections can be made simply by putting two joints together, flanged or threaded. However, when it comes to chemical composition, connecting aluminum directly to brass or copper can cause a chemical reaction called galvanic corrosion. Over time any water present (such as condensate) will serve as a conductor, shedding electrons from one material to another. This can cause pitting corrosion, even in your new aluminum piping.

Let’s take a look at a specific case where we found such pitting corrosion, and a whole lot more.

The customer in question noticed a pinhole leak in their aluminum piping after about one year of service. The pinhole leak was in the piping directly after the oil-flooded screw compressor, which was installed at the same time as the aluminum piping. Obviously the customer was concerned.

Upon inspection of the removed piping, the bottom of the pipe had a considerable amount of rust and scale (Picture 1). How did that happen?! Aluminum is supposed to be corrosion free. However, it was obvious that condensate had been pervasive in this pipeline. After cleaning the pipe, we also found pitting on the bottom of the pipe (Picture 2). In the end, to solve this problem we have to look at the system. We can’t solve the problem in a vacuum.

When looking at this particular piping run, the piping ran directly to the ground at the discharge of the compressor (Picture 3) and then along the floor (Picture 4) before running to another room for any clean up equipment (filters and dryers). There was no drip leg or centrifugal separator to remove any potential condensate. So how did the condensate get there?

Every system has condensate. Water vapor is present in the air, in the summer months considerably more than the winter months (just ask my hands in the winter!). This water vapor gets ingested into the compressor. Since the ambient air gets heated up to significantly higher temperatures inside the compressor, that air can hold a lot more water. As the air cools that water vapor and condensate mixture (with oil-flooded rotary screw compressors) gets dropped into the compressed air. A 20 F drop in air temperature will drop 50% of the evaporated condensate into the compressed air. So that’s how it gets there.

When reviewing the screw compressor operation, we observed a 40% duty cycle. Is a 40% duty cycle good or bad? The simple answer is: yes, it’s a bit low, and therefore would be defined as oversized.

Read our blog posts on oversizing for more information on this topic and register for our May 20 webinar with Plant Services magazine.

One additional point about the screw compressor: this unit did not have an internal centrifugal separator, and therefore no means to remove condensate within the unit. So our only chance was to remove that condensate outside the compressor.

What happens in systems with low duty cycles, especially ones that are very lightly loaded, is that the compressed air in the piping starts to cool. If piping cools enough, the air can reach its temperature dew point thus dropping out condensate. The best thing to do is remove the condensate.

In this particular case, like I said, we didn’t have any means to remove that condensate. There was no drip leg, so the condensate just sat in the piping. Should that have eaten through the aluminum piping creating the pinhole leak? Well, with direct contact of copper or brass and aluminum at the compressor discharge (in this case we had brass) we had galvanic corrosion. The electrons started to eat away at the aluminum, as we saw in the pitting. If there hadn’t been any standing water in the pipe (which clearly looks like the case) this may not have been an immediate problem. But since the original compressor discharge went straight to the ground with no drip leg, the condensate had nowhere to go, and the pitting eventually ate a hole in the pipe causing our leak.

The external environment could also have caused the pinhole. Pitting is a classic example of corrosion in saltwater and humid environments where salt is present. Chlorides or sulphates are the most common culprits. In the case of alkaline or acidic salts, the pitting rate is even higher. These salts can also be ingested into the compressor from the ambient air and then make their way out in the condensate as well. In this case pitting was only evident on the inside of the pipe, not the outside. However, salts in the air could have certainly affected the rate at which pitting in this case study was occuring.

For this system, we corrected the floor piping and installed a drip leg. We would also recommend replacing the brass fittings at the compressor discharge with steel as well as adding a no-loss demand drain to the drip leg.

Corrected piping with drip leg

So what’s the best way to avoid such galvanic corrosion in your new aluminum piping?

Aluminum compressed air piping systems have been on the market for almost 20 years now. There’s a lot to be excited about with aluminum despite the fact that material costs for both piping and fittings tend to be higher than steel or copper. However, since aluminum is light-weight and aluminum compressed air piping systems require little pipe and fitting preparation, there’s a rather large potential savings in labor when comparing an overall installation to steel or copper.

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