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 represents a major threat to compressors and these processes.  Surge prevention is an important process control problem in these environments as surging can result in costly downtime and mechanical damage to the compressors.  An effective anti-surge control system is critical for every turbocompressor.

Understanding Surge

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.

Figure 1: Examining the Characteristic Curve

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. 

Figure 2: Example Compressor System

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.

Figure 3: Graph of Operating Points

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
2.  Rising temperatures inside the compressor
3.  Tripping of the compressor
4.  Mechanical damage

Mechanical damage can include:

• Radial bearing load during the initial phase of surging.  A side load is placed on the rotor which acts perpendicular to the axis.

• Thrust bearing load due to loading and unloading.

• Seal rubbing

• Stationary and rotating part contact if thrust bearing is overloaded.

Anti-Surge Control >

By: Prabhat Yadav, Indian Oil Corporation Limited, Guest Author

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