Compressed Air Best Practices - May 2008 - (Page 31)

Compressed Air Industry AUTOMOTIVE ASSEMBLY | 05/08 | REAL WORLD BEST PRACTICES GUIDELINES FOR STARTING AIR FOR CENTRIFUGAL COMPRESSORS the lowest flow generator. This will develop the most temperature drop, 70–80 °F and when equipped with a proper temperature control to shut off the air when not needed, they will only use the actual required air. In the real world, most of these are installed without this shut off and should be modified. This air has to be considered in the total instrument air. Recommendation: A cabinet cooler is not required to start the compressor, and if one is in place, it should, at a minimum, be connected to the compressor starter. When the unit starts, it should activate a timer to open the air to the cabinet cooler 15 minutes after start up so it won’t expand the control air start up flow demand. Investigation may reveal that cabinet cooling is not required particularly if the electrics have been updated. Other constant bleeds: There are other constant bleeds that may or may not be part of the main air supply which, if not removed at time of start up, will have to be considered. Many of these, like the cabinet coolers, will not be required for start up and can be bypassed during start up as described with the cabinet coolers. To be sure, always check with the OEM’s service and/or engineering group before taking any specific action. Some of these are: p Balancing control regulators with a small constant bleed p Seal air bleed off if used p Control valves with constant bleeds p Venturi vacuum generators to evacuate certain areas p p Some of the obvious leak locations are: p Small to medium size (2–10 cfm): Hose/pipe connec- tions; unions; cylinder/valve connections; push/pull connections; quick disconnects; regulators; filters, etc. p Large to extra large (15–30 cfm or larger): Leaking or misadjusted blow off valves; flange connections between stages; discharge air to aftercooler; worn control valve bodies/cylinders; etc. In the field, we have observed centrifugals with more than 100 cfm of leaks in the flanges and valving. V. Establishing general packaged system specifications for optimum centrifugal starting air by type and size of the compressors General specifications and action guidelines: p Main air supply should be from a non-lubricated or oil-free compressor or properly filtered lubricated unit Package to be aircooled for flexibility and portability Package dedicated only to centrifugal start up air • Run 30 minutes once a month to be sure of availability p p p Pre-selection work: Working with professional service support, identify any constant bleeds and isolate the demand from the start up as required Determine the need for (or no need for) seal air List all the control valve, leaks, open bleeds, etc., that can be identified. All distribution tubing/piping should be sized to the maximum potential flow and should be of non-corrosive material to avoid self generating contamination. The air delivered to the compressor should be clean and dry — defined as “instrument air” — in reality, we have three choices of types of dryers: refrigerated, desiccant or membrane. such as control cabinets, oil sumps, etc. This can range from a constant bleed of 10 cfm to 25 cfm with a probable average of 15–20 cfm. p Condensate drains kept open all the time by plant p personnel or a “vee notch” ball valve designed and applied not to completely close. There will be a minimum of one per intercooler and aftercooler, plus potentially any “wet air” risers. This demand is usually around 20–25 cfm; however, it can be much higher. And the most obvious — LEAKS. In real life there will always be leaks. The volume or magnitude of these in the compressed air supply package is very dependent on the quality and level of maintenance. p Refrigerated dryers have been generally satisfactory in the past, but the +40 ºF or more pressure dew point can be high enough to have some liquid water in the compressed air — particularly the control air. www.airbestpractices.com 31 http://www.airbestpractices.com

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Compressed Air Best Practices - May 2008 Contents From the Editor Utility-Air News The Compressed Air Audit of the Month Pneumatics Real World Best Practices Delta Industries: Doing What is Right for Customers Delta Industries: Six Truths You Need to Know about Really Putting the Customer First Power Supply Industries Resources for Energy Engineers Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - May 2008

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