Good afternoon! What do you mean by hidden opportunities in compressed air systems?
Good afternoon. Hidden opportunities is a term I use to describe less-than-obvious losses in performance and energy efficiency in compressed air systems. I’ve been conducting compressed air system assessments, at chemical plants located in the southeast and southwest regions of the U.S., for many, many years. The reliability, of the compressed air system, has always been the major issue and focus for my clients, but in the past assessments were undertaken to reduce energy consumption. Over the past three years, however, as the cost of energy continued to drop, the assessments are undertaken only to improve system reliability. They don’t mind if they also save energy, but that’s not the focus of the assessment.
How would you describe the challenges to reliability at a chemical plant?
Chemical plants, due to their size and complexity, pose many challenges to the efficient and reliable operation of a compressed air system. There are so many places for hidden opportunities to be found in these large industrial complexes. We are normally dealing with several large centrifugal and rotary screw air compressors scattered across the complex. We encounter sites with well over thirty (30) desiccant air dryers of different types. Compressed air leaks can be found almost at will across the vast lengths of compressed air piping. Add to this the fact they are outdoor installations exposing all compressed air system components to the extremes of summer and winter. As you can imagine, it is a big task to simply understand the system.
You’ve said that installing stainless steel inlet piping to the compressed air dryers would make a huge difference. Please review.
The first mistake the industry makes is not installing schedule 10 stainless steel pipe upstream of the compressed air dryers. Carbon steel pipe, in an oil-free compressor system creating aggressive condensate, creates a lot of rust particulates that plug the pre-filters on the compressed air dryers and plug up condensate drain lines. I think the engineering contractors, who design these systems, either don’t think about the need for stainless steel piping upstream of dryers in an oil-free system or they falsely perceive there to be a big price difference.
Installing schedule 10 stainless steel piping upstream of the dryers would improve system reliability by increasing the life of pre-filter elements and auto drains. For example, one plant I visited had a 15 psi pressure drop on the dryer pre-filter and the filter element had been changed only one week earlier! Another chemical plant, while retrofitting their system , took a 16 inch diameter header down and found sheets of rust inside that you could peel off in 3 foot sections.
Another hidden opportunity is excessive pressure loss across the inlet filter and inlet piping to centrifugal compressors. We have found inlet piping with “witches hats” restricting the inlet flow to compressors.. In one case, after removing the witches hats from the inlet piping to two compressors, the power was reduced by 200 kW.
“Hidden opportunities is a term I use to describe less-than-obvious
losses in performance and energy efficiency in compressed air systems.” — Chris Beals, President, Air System Management
What are the most common hidden opportunities associated with centrifugal air compressors?
One of the most common hidden opportunities on the supply side of a system is centrifugal compressor blow-off, often called “venting” in the chemical industry. While venting is an obvious air loss, eliminating blow-off in the chemical industry is complicated by the fact that they often don’t want to unload or shut off a compressor. The reason they provide for not wanting to unload or shut off a compressor is they aren’t confident it will load or start back up when needed. During one system assessment, I showed a refinery how to shut down all their compressors and dryers. Their response was they just wanted them to remain unloaded (and consuming energy) because they weren’t sure if they would start back up. Lastly, it’s hard to get chemical plants and refineries to build trim stations due to capital being too tight.
The compressor industry has addressed this issue by developing compressor automation that has the ability to load-share across centrifugal compressors. One can manually load-share across centrifugals if at least one control panel allows the pressure to be adjusted by 1/10th of a psi; however, manual load-sharing often ends when the pressure differentials change Load-sharing is better addressed by installing new control panels on each compressor that have the ability to load-share or by installing a master controller. Many of the new control panels and master controllers have the capability for remote access, which reduces maintance costs by allowing trouble-shooting to occur over the phone.
How effective are compressed air leak surveys at chemical plants?
Compressed air leaks are a hidden opportunity in plain site for everyone to see. It is not difficult to amass a long list of compressed air leak locations with the appropriate estimation of compressed air lost. I have done many leak surveys identifying costly compressed air leaks that the customer never got around to fixing. Now I only do a survey if the customer is willing to assign an operator that can make a first attempt to fix them while I’m doing the survey. That action alone pays for the leak survey.
Chemical plants are outdoors in the sun, snow and rain. You can’t install a lubricator with a plastic dome - it has to be metal or it will leak. Most compressed air filters have internally mounted automatic condensate drains inside the housings. Over time almost all of these condensate drains will leak compressed air as they get stuck in the open position. We recommend replacing them with a filter that has a manual drain that has a thumb screw - these don’t leak. With a well maintained -40°F dryer no moisture will collect in the filters so there’s no need for an automatic drain. Sight glasses on the filter housings are regularly found to be leaking. Don’t purchase a filter with a sight glass if you can avoid it. Copper tubing is also a big source of compressed air leaks. Control valves should be installed with stainless steel (not copper) tubing. In a chemical plant this January, I did a leak survey finding a significant number of compressed air leaks on control valves installed with copper tubing.
Please discuss your findings with desiccant air dryers.
While they can provide a steady and reliable -40 F (and lower) pressure dew point, heatless and even heated desiccant air dryers can often offer significant opportunties on the supply side of the system. Dryer purge on heatless desiccant dryers can be the largest compressed air leak source in a chemical plant. Often the purge air may be turned up unnecessarily or partially loaded dryers are operating in a fixed cycle because the purge-saving controller hasn’t been maintained. The biggest purge rates (leaks) occur when a valve fails and you dump air from one side to the other and it goes out the purge exhaust valves. During one compressed air system assessment, I found one failed valve on a desiccant dryer causing a compressed air leak of 1200 cfm. In a huge chemical complex, these kinds of leaks can go unnoticed for years unless you have a flow meter on the inlet and outlet of the dryer
Replacing heatless and heated desiccant dryers with blower purge dryers can often recover a significant amount of compressor capacity. For example, one chemical plant was able to recover 5,000 scfm by replacing their heatless dryers with blower purge dryers, which saved energy while improving reliability by giving them a backup compressor.
One chemical plant had forty-two (42) point-of-use heatless desiccant air dryers scattered across the complex. It took me five days to analyze them all. Out of the forty-two, only twelve (12) were working.
The problem here was they had wet air piping-headers and they would slug all the dryers with moisture. It took them five years to find the capital to put in two 15,000 cfm internally heated desiccant dryers-one serving as back-up. This significantly reduced compressed air pressure drop and leaks in the system – while delivering quality compressed air. A lot of chemical plants and refineries prefer internally heated desiccant dryers over blower purge type dryers because they’re easier to install in in Class 1 Div II classified areas..
Heated desiccant dryers often use compressed air (purge air) to cool the desiccant in order to minimize the temperature and dewpoint spikes that occur at tower switchover. Depending on the manufacturer the cooling air flow is equal to between 7.5 and 14.5 percent of the dryers rated capacity and it can last for between 30 and 90 minutes. In the southern part of the United States we routinely shut off the cooling air, while in the Northern states we only turn it on during the winter months.
|“While venting is an obvious air loss, eliminating blow-off in the chemical industry is complicated by the fact that they often don’t want to unload or shut off a compressor.” — Chris Beals, President, Air System Management|
What impact does being outdoors have on these compressed air systems?
The larger chemical plants I visit, in the southeast and southwest, are all outdoor installations. I was just down in Houston this winter and we saw 30 F ambient temperatures. The air compressor’s aftercooler outlet temperature was 80 F and the compressed air dryers were 500 feet away. A lot of condensate was forming across this distance due to the temperature differential and the distance to the dryers. This is an example of why we recommend stainless steel inlet piping and a receiver upstream of the compressed air dryers, along with insulating the piping between receiver and the prefilters and the prefilter and the dryer inlet.
Another time I was in a metallurgical plant in Texas. They were using 1300 cfm in the plant and had significant issues with moisture in the compressed air lines. We suggested a 2600 cfm blower purge desiccant dryer, which they installed along with a Mist Eliminator. After installing the new dryer they could still only get a -20 F pressure dew point. Upon further review we found the air compressor discharge was 117 F on a day with a 95 F ambient temperature. We found that moisture was condensing in the pipe upsteam of the Mist Eliminator, between the Mist Eliminator and the prefilter, and between the prefilter and the dryer inlet, which resulted in condensate hitting the desiccant . We put a precooler in front of the Mist Eliminator to cool the air down to 100 F degrees, and the problem went away.
Any final thoughts?
Yes. It’s worth taking the time to thoroughly understand your compressed air system. Another hidden opportunity is what we call “system events”. System events puzzle many compressed air service providers who are only interested in the supply side of the system. I was asked by one compressed air service provider to figure out why the backup air compressors kept kicking on. After a thorough review, which included following the compressed air lines throughout the facility, I discovered a booster station (boosting plant air from 100 to 350 psig) that was causing the big demand event.
Thank you for your insights.
Chris Beals is president of Air System Management, Inc. Designated as an energy expert for compressed air by the U.S. Department of Energy, he is a founding member of the Compressed Air Challenge®.
To read more System Assessment articles, visit www.airbestpractices.com/system-assessments.