Compressed air: don't forget it.
Rotary compressor and dryer equipment designed for 100 psig is not happy when operating at 115 to 120. The compressor runs hotter, and exit air temperature is higher. Exit air carries over a little oil vapor, and the filters that protect the dryer from hydrocarbons begin to foul. Overall dryer performance decreases and pressure differential increases.
The situation begged for an explanation, so we asked the production superintendent if this was normal or if something had recently changed. He explained that initially the plant operated two units. It began running the third unit in trim mode after some converting machines were upgraded. A new, larger converting machine was recently installed, and air pressure quickly became a production issue. Since capital funds were tight, the project engineering team determined that the third compressor had sufficient capacity.
Our preliminary investigation had revealed that all three compressor systems were running at 120 psig (with no crucial operational issues), yet system pressure on the production floor was marginally low and causing intermittent production quality issues. The compressor motors had high amps and high airtemperatures, and the dryers were overloaded because of the hot air.
This type of situation occurs occasionally when manufacturing systems engineers manage plant expansion projects. They occasionally have tight project budgets and do not always conduct a thorough assessment of the plant utility equipment. It is the responsibility of site plant engineering to be involved and identify how new loads could impact the utilities: electrical motor control centers, air compressors, process water, HVAC, the steam system, etc.
Our experience is that compressed air is the most often overlooked utility during expansion projects.
The project team apparently had not conducted a thorough due diligence of the entire system to determine if it could adequately supply a relatively higher system demand. Hey, three robust compressors and dryers with a 4-inch air header should work.
But when it comes to standard air dryers, there is a "Three-100s Rule": Operating conditions should not exceed 100 psig inlet pressure, 100[degrees]F inlet temperature, or 100 percent rated flow.
The compressors and dryers had been installed in parallel mode to provide system flexibility; however, piping restrictions (fittings and valves) at the compressors, the in-line filters, and dryer connections were causing downstream pressure problems with all three units in service. More review confirmed that the original design of the system was for only two compressors to be in normal operation. But, since the third unit was not fully loaded, the project team had determined (theoretically) that it should provide the additional capacity for the new converting machine. But, unforeseen piping restrictions were causing compressor performance issues.
These relatively new compressors (150 hp, 750 cfm) were capable of supplying over 2,000 cfm to the plant. However each air dryer line could handle only about 600 cfm before incurring excess pressure loss. In addition, 400 feet of 3-inch main air line to the production area had a nominal capacity of about 1,600 cfm. The new converting equipment (300 cfm air usage) had overloaded the plant piping distribution system. Moreover, additional artificial demand in converting, the use of air hoses to clean the machinery, was out of control.
We recommended changing the valves and fittings at the compressors and dryers from 2-inch to 3-inch size and running a new 2-inch air line from the 4-inch main header (after the flow meter station) to the new converting machine. Pressure storage options were not practical because of the 3-inch header in converting; but, we did suggest 10-gallon surge tanks at the two case-packer machines.
It took a few weeks to get funds approved and make the changes. Some preventive maintenance work also improved dryer performance. Later the compressors were back to operating at 95-100 psig (with 85-90 psig in converting), and production improved.
We recommended that a formal rental compressor delivery plan be established for major maintenance service and emergencies until a fourth compressor/ dryer line was installed.
The production issue was resolved. Moreover, every 2 psig drop in the compressor discharge pressure produces a 1 percent reduction of motor power. So 20 psig yielded 10 percent power savings on the 400 hp load. In addition, compressors and dryers operating at 95-100 psig will have a much longer life and fewer maintenance issues than those operating at 115 to 120 psig.
When conducting a problem resolution exercise, be sure to fly your helicopter high enough above the rooftop to get the big picture. It also helps for the "elephant in the room" to have good operational experience.
GARY WAMSLEY is an engineering consultant at JoGar Energy Services in south Florida with over 40 years of industrial utility systems experience. He can be reached at www.jogarenergy.com.
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|Title Annotation:||TECH FOCUS: FLUID HANDLING|
|Comment:||Compressed air: don't forget it.(TECH FOCUS: FLUID HANDLING)|
|Date:||Aug 1, 2015|
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