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.
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:
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.
Figure 4: Compressor Operating Map
The compressor surge limit is not fixed with respect to any one measured variable such as compression ratio or pressure drop across the flow meter. Instead, it is a complex function that is dependent on the gas composition, RPM, suction temperature, and pressure. A closed loop PI controller would be unable to prevent surge during large or fast disturbances. Therefore, such a controller would be unable to stop surge. Rather, the controller would simply cycle the recycle valve open and closed in response to successive surge cycles. For a PI controller to act quickly, the "b" value would need to be high. This would result in a decreased operating region for the compressor when the recycle valve is closed.
Thus, an open loop control is used in conjunction with the closed loop in an anti-surge controller. The overall configuration is shown in Figure 5. A Recycle Trip Line (RTL) is used between the SLL and the SCL. Small or slow distrubances are managed by the closed loop controller which keeps the compressor operating point to the right of the RTL. For large or fast disturbances, the compressor operating point will reach the RTL. At this point, the open loop control will be initiated. This will add a step change which is a function of the compressor operating point at the moment it reaches the RTL. In this manner, the fast opening valve will be sufficient to stop surging.
Figure 5: Compressor Anti-Surge Control Scheme
Anti-Surge Valve Requirements
1. The valve must be large enough to prevent surging under all possible operating conditions. However, a valve which is too oversized will result in poor control.
2. Stroke speed - A fast stroke speed is very important.
3. Ensure adequate air supply to properly operate the valve.
4. Tubing run should be minimized to reduce lag time.
5. One or more volume boosters are required to ensure fast response and equal opening and closing time.
6. Fail position should be open
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