Problem
The 1500 hp of compressed air that was running continuously couldn’t keep adequate pressure to the diverter valves creating occasional dumps of the silo contents. This productivity problem created a cleanup mess requiring significant operator attention and required product reprocessing.
Solution
Compressed Air Consultants was brought in to determine if there was an opportunity to turn this discretionary project into a ROI project. The audit showed that the opportunity was big enough, that the productivity issues which plagued the plant could be eliminated as part of the process without taking financial credit for those improvements.
The Traditional Solution
Burying a truck in cement is less than desirable, but that’s what was occurring to one cement mill. The 1500 hp of compressed air that was running continuously couldn’t keep adequate pressure to the diverter valves creating occasional dumps of the silo contents. This productivity problem created a cleanup mess requiring significant operator attention and required product reprocessing. In addition, moisture problems created small boulders in a finish mill nuisance dust collector reduced the uptime for the roller mill.
The customer called their local supplier of compressors and asked them to help them solve the problem. The solution developed by the sales engineer required an additional 300hp compressor with dryer and clean up equipment and would replace a 100 hp pack house com-pressor and leave it as a backup. The pressure to the silos would be increased to 100 psig solving the load out issue and it was estimated to increase the operating cost between $50,000 and $100,000 per year. $150,000 was set aside to purchase and install the equipment required to complete the job.
The new system included a drying system and split the plant into two distinct parts thereby solving a major moisture issue for one third of the plant.
A Different Approach with a “No Risk” Start
Compressed Air Consultants was brought in to determine if there was an opportunity to turn this discretionary project into a ROI project. A no cost feasibility study was conducted, and it became clear that the compressed air system created problems throughout the plant and the client had learned to live with the pain because “that’s just the way air is”. The feasibility study showed that there was a high probability of a six-figure savings with a payback that met the plant’s hurdle rate of three years and probably beat a two-year payback. It also showed that the opportunity was big enough, that the productivity issues which plagued the plant could be eliminated as part of the process without taking financial credit for those improvements.
CAC was hired for less than half the cost of the last air dryer the client had purchased and subsequently mothballed when the water problems didn’t go away. Using multiple logging devices, the various problems were analyzed simultaneously resulting in the ability to analyze the entire system in a short period of time.
The data first showed that the pressure problem was a distribution problem and not a capacity is-sue. In the graph above, pressure would fall on the roof of the load out (red line) while pressure would remain stable in the compressor room (black line). The compressors had no idea that there was a low air pressure problem anywhere in the plant and continued to load and unload. CAC discovered through concurrent pressure logging that there were multiple issues creating the situation which included:
Undersized pipe
Unnecessary separation equipment
A broken gate valve
An abandoned-in-place non-functional refrigerated air dryer
A failed drain which filled a receiver creating a head pressure the compressed air had to over-come.
The good news was that fixing the real root cause would cost $3000 to re-pipe that area of the plant. Not only did this eliminate the $150,000 project, but it would also not increase the operating cost of the facility.
The audit then determined that the there was a significant opportunity to reduce operating costs. It was originally thought that all the compressors were operating at a point where they were making all they could put out. Rather than showing every compressor was in drawdown, the data showed that there were events happening in the system that were happening too fast and too far away for supply to see resulting in a localized air pressure problem. By the time the event was seen in the compressor room, the event had gone away, and the system just caught up.
The controls were set up in such a way that if the plant reduced the air demand through improved operation of applications such as vibrators, baghouses, or FK pumps, the compressors energy re-duction would not be proportional. All the demand side improvements that were being contemplated wouldn’t have the return on investment because the compressors weren’t behaving the way the plant thought they were.
With the improvements in the control system, the changes in operation of the air system could reduce the operating costs by $182,000 (29%) Payback for the project was less than 18 months and showed additional opportunities or roughly 20%.
An additional benefit of the improvements was that fewer compressors would be required to operate the system. Turning off these compressors allowed the client to permanently turn off a two-stage rotary vane machine which was consuming significant maintenance dollars. It also created back up machines out of the existing compressors which meant that any expansion or failure could be met with their existing machinery rather than a capital expenditure.
The third problem the audit addressed was the moisture problem. Even though the plant was using desiccant dryers, moisture would from even on an 80°F day. There were multiple root causes for these problems, and they included failed drains, excessive compressor discharge temperature, and poor maintenance.
The moisture problems though were particularly bad at a nuisance dust collector in the finish mill. This unit had to be shut down regularly as the bags would blind from the moisture combining with the cement dust.
What made this particularly strange was that the moisture in this part of the plant was far worse than anywhere else in the plant.
During the study, a dedicated application was also analyzed. The process it served did not need drying, so no dryer was installed.
The concurrent drops in pressure showed us that contrary to the plant’s personnel’s beliefs, the systems were tied together and that this untreated air was the reason for the downtime on the finish mill nuisance dust collector which ultimately reduced the plant’s throughput.
The new design for the system included a new drying system which reduced the energy consumption by over one hundred horsepower while improving the overall air quality of the system.
The fourth purpose of the audit was to develop better specific process information. The client wanted to go from having intuitive knowledge of the system to hard facts. The audit also answered the following questions:
What are the demands of the various subsystems of the plant?
What is the impact of the air cannons and air lances on demand?
Does it make better financial sense to use high pressure air for silo load out aeration or should the felt pads be replaced so that low pressure air can be used?
In the customer’s opinion, this audit had significantly more data collection which was then tied back to the process, and there was far greater interface with operators creating a more precise and more “in depth” audit which in turn created a new level of understanding of the air system for operators and management.
In the words of the production manager, “This was the best money we ever spent on compressed air.”