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40 www.coalage.com January/February 2018 operating ideas continued imeter probes can measure gaps down to 1/100 of a millimeter or even less. A good practice to avoid too much trouble with high vibration inside of centrifugal com- pressors is to make sure that the lubrica- tion system is operating properly. The signal conditioning unit, which monitors and analyzes the proximeter probes, will generate a signal to a central machine monitoring system. Each individ- ual model can be set with a specific "high" alarm and a "high high" shutdown, just as described with the reciprocal compressors. Other than vibration, what is another type of critical control? For centrifugal compressors, controlling surge is a critical component. Surge oc- curs when there is no forward flow of gas through the compressor and a reversal of flow occurs. The gas that is being com- pressed gets pushed backward through the centrifugal compressor. When this surge (flow reversal) occurs, which can happen in milliseconds, the rotor is affected. It will shift to one end in its normal running po- sition. Then it will slam backward, and in many cases, the thrust bearing in that op- posite thrust situation cannot handle this because it is sized smaller than the main thrust bearing, often due to cost. Addi- tionally, the dry gas seals are slammed into and with the tolerances being merely a few ten thousandths of an inch, they can be severely damaged. Needless to say, surge should be avoided and thus, anti-surge controls were created. Prior to anti-surge controls, surge was managed by running the compressor very far out on the curve, in a zone that was rel- atively inefficient. Then, an algorithm was invented that allows the operator to run the compressor up closer to the surge line, a more efficient operating point. However, events can happen quickly and if an opera- tor is running up close to the surge line, the compressor can be damaged. Anti-surge controls measure pressure, temperature and flow, and feeds that information into a special purpose computer complete with algorithms that monitor the operat- ing point of the compressor relative to the anti-surge line. This means when a fast condition occurs, and the operating point moves quickly to the surge line, the special purpose anti-surge controller will take spe- cific actions to open the recycle valve, al- lowing the discharge gas to flow to a lower pressure or bypass any downstream block- age and recycle back into the compressor. The anti-surge system is dependent on this valve located on the discharge of the compressor. The sensors and the comput- er determine where the operating point of the compressor is relative to the surge line, and if necessary, open the anti-surge valve. Vibration controls and anti-surge con- trols are crucial protection elements to keep reciprocating and centrifugal com- pressors operating effectively. Through monitoring and quick corrective actions, operators can effectively increase output, avoid unnecessary costs, and prolong the lifespan of compressors. Jim Slama is senior systems engineer for Valin Corp., (www.valin.com), a technical solutions provider. Valin offers personal- ized order management, on-site field sup- port, comprehensive training and applied engineering services using automation, fluid management, precision measure- ment, process heating, filtration and fluid power products.