Compressed Air System Audit - Case StudyGreif Bros. Corporation
Western Division / Steel Drums
2400 Cooper Avenue
Merced, Ca 95344
Executive SummaryThe compressed air supply system at the Merced California location is well managed and is operating relatively efficiently. The header pressure fluctuates proportionally to the compressor dead band except in the test area where the pressure significantly falls off. These pressure fluctuations may have an impact on the consistency of production processes, however they definitely increase the costs of operating the system. The supply side of the system must operate at a higher-pressure range to allow for peak demand when production pressure drops. This increases plant air demand along with the supply energy in the system. The compressed air supply system consists of three 200 hp compressors with one in operation during off peak and two compressors operating during peak or when the eurothayne top line and fiber line are operating. The pressure fluctuates from 79 to 106 psig in the header as the compressors attempt to match the rapid changes in air demand created by the open blowing, dust collector and the test area events.
Contaminants were found in several drip legs throughout the facility. The dryer capacity is marginal for the compressor flow and ambient conditions that occur in the hot summer months. There are a number of air using applications, which are wasting compressed air and can be better served with a different approach. We are recommending point of use retrofits on the dust collector and the test area that will stabilize the pressure and reduce the air demand as illustrated on page 6 & 7 of the report. An additional supply receiver and flow controller in the compressor area will allow the peaks in demand to be supported with stored air, this will allow the pressure to be stabilized and will reduce the operating costs of the system. Combined with correcting the wasteful applications and supporting large demand events with storage, production pressure can be lowered and stabilized to an acceptable level. It will be advisable to install a smaller trim compressor to support any increase in demand without having to run one of the large compressors. This is for the peak months, March - September. We will need to re-log the system to size a smaller trim compressor. Completing all actions will generate a reduction in annual operating costs of $72,543.00 and a simple payback of 10.4 months based on an equipment cost of $ 60,750.00.
Production and Demand Issues
- During the audit, we were on site for two weeks recording and sampling the air system. We noticed there was a consistent presence of contamination in the compressed air at the final filters and isolated drip legs. There are three steps, which must be taken. First, additional controlled storage must be added to control the rate of flow across the filters and dryer. Second, a conservatively designed final filter must be added to the system to filter all compressed air just prior to exiting the compressor station. Third, a cycling refrigerated air dryer should be added to automatically adjust to high and low demands. The dryer and filter should to be sized for highest flow, highest temperature, lowest pressure to handle all possible conditions.
- Compressed air is being used in open blowing applications in the waste area, galvanized grinding table, roll weld line and a phew other small areas. A number of these blowing applications in the plant are not practical to convert, however the application in the waste area can be designed with blowers or pulsing controlled storage with metered recovery. The galvanized grinding table can utilize a small vacuum pump to pick up and feed the sheets. The roll weld line uses open blowing for clean up along with copper tubing for sheet separation. This application can work well with Zepher or Silvent type high efficient nozzles. These nozzles work off venturi, which creates more mass utilizing less pressure.
- The Fiber Line area operates with continuous blowing, a solenoid shut off device should be installed in this area to stop the blowing when the line in not in use.
- The Gasket Line utilizes air to spray eurothayne gasket, air driven pump and air agitation. This line runs from February through summer but during the off months the air continues to blow. A solenoid shut off device should be installed in this area to stop the blowing when the line in not in use.
- The Disc Line for punch utilizes a 6" bore with 2-1/2" stroke. See technical attachments for proper installation of supplemental storage for high speed cylinders.
- The Paint area utilizes -40 deg. air, however bulk water is present in isolated drip legs. There is a phenomenon called molecular diffusion, which has several definitions, one will draw contaminants through the air system due to differences in partial vapor pressures. When production facilities have separate pressure dew points within the same distribution system, vapor will seek lowest vapor pressure. Water was found in other areas of the plant and liquid is drawn against the direction of air flow in a vapor state and will later condense in the header or in the area of lowest vapor pressure. In this case the paint area.
- Hour Glass Weld Line utilizes a ½" feed line with approximately 35, 5/16 holes to blow debris off the sheets before their rolled and seamed. This application requires point of use low pressure high flow nozzles to reduce the demand while increasing the clean surface area.
- Drum Purge utilizes the same -40 deg. air as the paint booth. The air is used for food drums only using a ½" line open 4 seconds per drum. High efficiency nozzles are recommended.
- Waste Area operates open blowing to bubble and agitate water. There area a total of 4 pits approximately 14 feet deep. Maintenance department stated that Trico blowers were installed for this application but were removed for unknown reasons. Blowers or pulsating point of use storage are good alternatives for this large air user.
- The Test Area pressure is very unstable. This area pressurizes the drums to 7 psig and checks for leaks in the seam. The test will last between 3 and 9 seconds with very high flow rates. This area operates with a desiccant air dryer on a five minute purge cycle. When the operator activates the air to pressurize the drum, air is pulled from the receiver and overhead piping. With the dryer out of service it adds to the pressure decay and with the dryer in service it adds to the demand, which will increase the pressure drop.
The air must be stored and checked from the system so each test comes from the dedicated receiver allowing pressure to drop only 5 psig. This will stabilize the test area pressure with no affect to the surrounding areas. The air can be metered back into the receiver over a five second time frame to reduce the rate of flow. Once the supply side of system is retrofitted the desiccant air dyer will not be required. The entire system will be clean, dry and free of contaminants. The drums are pressurized from 0 to 7 psig during the five second test. The dedicated receiver will be checked from the system at 85 psig with 50 psig article pressure and 77 cf of usable differential per test.
Dust Collector purges every 16 seconds for .50 seconds. While the total volume of air is approximately 4 cubic feet, the rate of flow created is 480 scfm (4 scf x 60 seconds / .50 seconds). This high rate of flow will cause the header pressure to drop with each pulse and could cause more critical users in the vicinity to require a higher pressure than otherwise necessary. The pulse occurs every 16 seconds so the recovery of the receiver pressure can be metered over 10 seconds which will reduce the rate of flow to 24 scfm (4 scf x 60 sec. / 10 sec.).
- The pressure in the system is unstable due to the size of the events and the amount of open blowing, compared to the total storage available. The pressure fluctuates 31 psi in the test area and 27 psi in the header, please refer to Attachments A-E for overall system performance during the audit. This unstable pressure forces the header pressure to be maintained at higher levels than actually required to allow for the increases in air demand. Elevating the system pressure will significantly increase the cost to operate the system but will have little to no effect creating a higher quality product.
- Stabilizing the air pressure will be key to achieving the savings projected in this report but it also offers the potential to eliminate existing process and production problems, which may not have been identified as related to compressed air. In order to stabilize the pressure, we are recommending that a pressure/flow controller be installed in conjunction with additional storage.
- The leak load was calculated at 202 scfm base on the power required to support the plant during non-production periods. This is 22% of the base load in the system. Repairing leaks in an uncontrolled system only causes the pressure to rise, which forces all other leaks and application to consume more air. After pressure is controlled, the larger leaks should be repaired as necessary to keep the next compressor off. Please review an article in the Technical Attachments,"Controlling Leaks In Compressed Air Systems"
Piping Distribution And Storage Issues
- We recommend completing the header loop in several areas. The piping is currently looped in the lid forming, spray booth, cooling tunnel area but not in the die rack, through the horn press area. These recommendations are relatively straight runs and should not be too expensive to accomplish. Completing these items will allow the system to economically support any future requirements for compressed air.
- The storage capacitance in a compressed air system controls the rate of pressure change and the size of fluctuations in the system as the demand for air changes. Greater storage and faster compressor controls will manage smaller pressure fluctuations. In this system the pressure fluctuates in the header as much as 27 psi as demand events are added and removed from the system. On average, the system is maintaining 20 -23 psi more pressure than necessary. This higher pressure increases the volumetric demand of any unregulated air in the system, such as leaks, open blowing and any equipment with the regulators turned wide open. This increase in demand is called artificial demand and is not contributing in any way to the manufacture of acceptable product. A larger, additional air receiver is required to help stabilize the system pressure and support the peak demands in the system. A larger receiver allows the compressors to replenish storage rather than responding directly to changes in air demand. This approach is much more stable and creates a reliable system. This receiver should be located down stream of all clean up equipment and spooled with existing receiver. The air will be clean, dry and free of contaminants before it enters the plant. (see Attachment F for proposed layout). We are recommending the addition of a 10,000 gallon receiver to bring our supply capacitance from 19.36 cf/psi to 111.56 cf/psi. The 10,000 gallon receiver is to protect the peak load in the system as depicted in Attachment C.
- In addition to the larger storage, the key to stabilizing the header pressure and creating useful storage in the system will be the installation of a pressure/flow controller as discussed above. This allows the supply side of the system to operate independently from the demand side of the system. The pressure/flow controller will eliminate artificial demand while creating useable control storage to average the demand in the system. At the same time, the compressors can be operated at the pressure, which maximizes the value of storage and minimizes the operating cost of the system. This controller is different from a regulator where it does not introduce a fixed pressure drop in the system but rather controls the flow across the valve to match the production requirements. The addition of a 10,000 gallon receiver increases the usable control storage to allow the system to cope with coincidental events peaks using storage rather than compressor horse power. The system pressure can be reduced to 85 psig after the action plan items are completed.
Supply Issues
- As mentioned earlier, there is evidence of contamination in the compressed air system due to oil and water carryover from the main compressor station. The recommendations to add the proper clean up equipment and controlled storage as discussed above will help to prevent contamination by controlling the dynamics of the system with clean, dry air.
- We are recommending a specific technology for condensate drains. Failed or poorly installed condensate drains are the most common reason for system contamination. The demand style no-loss drain is the best technology available for compressed air service. This technology of drain does not dump unless condensate is present, does not waste air and will not create surges of air demand in the system.
- During the audit, power and pressures were monitored in the system continuously. The results of these recordings were utilized to calculate actual power and air consumed during the audit period. The tables in Attachment H depict the compressor power and air delivered for both existing and proposed arrangements. The power and volume relationships may vary on individual compressors but will be consistent for the system on an annual basis.
Compressed Air System Audit ReportOur Compressed Air System Audit report for each client contains a number of items - including detailed analysis, worksheets, and action plans. The following partial list examines some of the components of our report structure for Grief Bros. Corporation.
A. Test Area Event Graph
B. Normal Production System Pressure Graphs
C. Peak Event System Pressure Graphs
D. Demand Graphs
E. Existing Process Flow Diagram
F. Proposed Process Flow Diagram
G. Constituents of Demand
H. Supply Power and Volume Tables
I. Energy and Financial Calculations
J. Compressor Worksheet
K. Prioritized Action Plan
L. Technical Reference Articles
- Using Storage to Reduce Costs in Compressed Air Systems
- Long Term Leak Management
- Maximizing Compressed Air System Efficiency
