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Plant and Equipment Wellness, Part 1: Observing Variability
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Published January 8, 2008
Turbocompressors, either centrifugal or axial,
are the heart of many industrial processes. Often, these compressors are critical to
the operation of the plant, yet they are seldom installed with a spare unit. Surging
Many believe that surging is analogous to cavitation in a centrifugal pump, but this is not the case. Surging is defined as a self oscillation of the discharge pressure and flow rate, including a flow reversal. Every centrifugal or axial compressor has a characteristic combination of maximum head and minimum flow. Beyond this point, surging will occur. During surging, a flow reversal is often accompanied by a pressure drop.
Surging is best illustrated by observing the movement of the compressor operating point along its characteric curve as shown in Figure 1.
Development of the Surge Cycle
Consider a compressor system as shown in Figure 2. The discharge pressure is marked Pd and the downstream vessel pressure is Pv.
Now, referencing Figure 3, assume that the system is operating at steady state at Point D. If the demand for gas is reduced, the operating point will move toward Point A, the surge point. If the load is reduced enough, the compressor operating point will cross Point A. Beyond Point A, the compressor loses the ability to increase the discharge pressure such that Pd will become less than Pv. This is the flow reversal observed during surging. The operating point will then jump to Point B.
Point B is not a stable operating point. When the flow reversal occurs, the discharge pressure drops. This forces the operating point to move from Point B to Point C. At Point C, the flow rate is insufficient to build the necessary pressure to return to Point A. Thus, the operating point moves to Point D where the flow rate is in excess the load demanded and the pressure builds until Point A is finally reached. This completes a single surge cycle. The next cycle begins again with another flow reversal and the process repeats until an external force breaks the surge cycle.
Consequences of Surging
Consequences of surging can include:
1. Rapid flow and pressure oscillations
cause process instabilities
Mechanical damage can include:
By: Prabhat Yadav, Indian Oil Corporation Limited, Guest Author
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