A typical surge cycle is represented by the circuit between points B, C, D, and back to B (Figure 200-17). If events take place which alter the system curve to establish operation at point B, the pressure in the system will equal the output pressure of the compressor. Any transient can then cause reverse flow if the compressor discharge pressure falls below the downstream system pressure.
For reverse flow to occur, compressor throughput must be reduced to zero at point C which corresponds to a pressure called the “shut-off head”. When the system pressure has decreased to the compressor’s shut-off head at C, the machine will reestablish forward flow since the flow requirement of the compressor is satisfied by the backflow gas (compressor capability now greater than system requirements).
Now that the compressor has sufficient gas to compress, operation will immediately shift to the right in approximately a horizontal path to point D. With the compressor now delivering flow in the forward direction, pressure will build in the system, and operation will follow the characteristic speed curve back to points B and C. The cycle will rapidly repeat itself unless the cause of the surge is corrected, or other favorable action taken, such as increasing the speed.
Several internal factors combine to develop the surge condition. From the surge description, you can see that the domed shape of the head-capacity characteristic curve is fundamentally responsible for the location of the surge point at a given speed. On the right side of the performance map (Figure 200-17) the slope of the curve is negative. As inlet flow is reduced, the slope becomes less negative until it reaches zero at the surge point. As flow is reduced further to the left of the surge point, the slope becomes increasingly positive.
Section 210, “Engineering Principles” covers internal factors and their effect on location of the surge region.