Compressor Issues Get New Airing

Plants increasingly re-evaluate economic and technical factors

By Seán Ottewell, Editor at Large

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Rental compressors still may make sense for backup, Beals notes. As an example, he cites another customer that was paying $180,000 annually for rental backup compressors. A new compressor would cost $1.5 million installed; the simple payback was 8.33 years, hardly a strong inducement. On the other hand, for plants regularly operating diesel-engine-driven rental compressors, it's really quite easy to justify the new compressor, he says.

"What I don't understand is the thought process that seems to exist in the petrochemical industry that concludes it's better to spend maintenance money on the rental compressors and fuel year after year rather than spending capital on new compressors, which would reduce annual operating costs," he says.

OTHER OPPORTUNITIES
Beals also points to other areas now attracting industy's focus: on the supply side, centrifugal air compressor and desiccant dryer efficiency, plus system reliability, and on the demand side, leak surveys. Exemplifying this, he currently is conducting a compressed air system review at a major U.S. refinery that will keep him on site for 60 days — his longest audit ever. Beals and his team will conduct a supply- and demand-side review, a leak survey, and flow measurements to each unit.

"Years ago leak surveys weren't considered an essential part of an audit, but companies now appreciate that you can conduct a more in-depth demand-side audit if you carry out a leak survey at the same time. Most clients also supply a person to work with us during the leak survey so they can attempt to repair the leaks as we find them. For example, during a recent refinery audit, the company mechanic working with us repaired nearly 1,000 of the 2,700 identified leaks, which reduced demand by approximately 1,000 cfm."

Such leaks typically are found at filters, regulators, lubricators, air drops and control valves. Isolation valves with single packing nuts also are common culprits.

Another demand-side opportunity for optimization involves air-operated cabinet coolers. Although cheap and intrinsically safe, they often are installed without thermostatic controls. "Almost any plant can install the thermostatic controls and significantly reduce demand — and this is something that is sustainable that they can actually get implemented."

On the supply side, one of the biggest opportunities in most plants is to replace heatless dryers with more efficient dryers — doing so allows users to save energy and recapture air capacity. "While vacuum purge dryers, which are popular in Europe, may be the most efficient dryers, they haven't been able to break into the U.S. market; so, we typically recommend internally heated or blower purge dryers. We don't recommend heat-of-compression dryers because they are typically less reliable and we are often hired to provide justification to replace them," notes Beals.

There also is an argument to be made about the use of trim compressors, he says. In many other industries trim compressors are allowed to load and unload — but not in the chemicals sector, where the philosophy is to keep a constant pressure and maintain online backup capacity.

"The defense for this philosophy is that if the plant is shut down due to a compressed air issue it eats up all the energy savings you could have made and more in just a few hours. Plants are very worried about jeopardizing production, so your recommendations must be based upon sustainable demand reductions, such as installation of internally heated, blower purge or vacuum purge dryers rather than repairing air leaks."

Beals notes that in the last 20 years oil-free centrifugal compressors have become more popular because of their longer intervals between overhauls. To further extend the intervals, some plants now install 125-psi centrifugal compressors but operate them at a lower pressure — accepting reduced efficiency to increase the periods between overhauls.

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