What happens during a surge event
Figure 2 is a representation of a surge event. Only a single-speed line is shown for clarity. Assume that initially the compressor is operating at point (1) in the diagram.
The compressor is operating at its maximum flow capabilities at point (1). As the discharge pressure increases, the work the compressor must accomplish also increases. This pushes the compressor operating point along the speed line to point (2). If the discharge pressure continues to increase, the compressor operating point will move to point (3). If the control system cannot reduce the discharge pressure, the operating point will cross the surge line and flow will reverse through the compressor. When the flow reverses through the compressor, the compressor operating point will rapidly move to point (4). The surge event reduces the discharge pressure and increases the suction pressure of the compressor. The compressor will then re-establish forward flow and the operating point will move from point (4) back to point (2). Total time for a surge cycle is one to three seconds, but the flow reverses through the compressor in under a millisecond. This cycle will repeat until the compressor controls can intervene to change the operating conditions and stabilize compressor operation.
Common compressor control algorithms
Minimum flow recycle
There are several control approaches to prevent surge in compressors. The oldest and least efficient method is minimum flow recycling. This approach simply picks a flow rate that guarantees the compressor will not surge. If the flow drops below the set flow rate, the recycle or blow-off valve opens and maintains a redefined minimum flow through the compressor.
While this approach can be effective, it is not efficient. When the compressor operates at lower speeds, it requires a large flow to protect the equipment. This approach also does not take into account changes in gas properties, which may alter the compressor surge line.
However, while inefficient, this approach can be useful as a fallback algorithm in an advanced surge-control application. Fallback algorithms are used when field instrumentation faults prevent an accurate calculation of the compressor operating point. In fact, with degraded field instrumentation, minimum-flow fallback may be the only practicable control algorithm.
Maximum discharge pressure
This approach to surge control relies on the relationship between the maximum achievable discharge pressure a compressor produces at various temperatures. Discharge pressure control is commonly used on constant-speed, packaged-air compressors (typically integrally geared machines) where suction pressure does not vary. The advantage of this approach is that it is extremely inexpensive (read: cheap) to implement because minimal instrumentation is required: just a discharge pressure transmitter and an ambient temperature. There is not even a need to measure flow through the compressor.
There are several compressor maps supplied by the manufacturer to relate the maximum pressure the compressor can produce in both summer and winter conditions. During the winter, when air is colder and therefore air density is higher, the compressor can produce a higher discharge pressure before a surge occurs. In the summer, when the air density is lower, the compressor cannot produce the same high discharge pressure. As a result, control is very simple; with a discharge pressure proportional integral derivative (PID) featuring an adjustable variable pressure setpoint for ambient temperature.
Because variations in manufacturing require a conservative approach, many companies use discharge pressure control for packaged air compressors where the surge data is generic, rather than specific to a particular machine.
Unfortunately, this approach often wastes energy and does not provide adequate equipment protection. In addition, as the compressor impellers wear out or intercoolers become fouled, the maximum discharge pressure the compressor can achieve decreases. This requires a lower pressure setpoint to protect the compressor, which if ignored will cause compressor damage.
Similarly, for older packaged compressors using pneumatic controls, maintenance personnel often ignore outdated pneumatic temperature measurement because it is difficult to calibrate. This results in a pressure setpoint that does not change with temperature, which also can cause damage to the compressor.
More modern controls, even if ignored by maintenance personnel, allow for automatic adjustment of the surge line. Once a surge occurs, the margin can be adjusted to limit the number of surge cycles a compressor would experience. Multiple surge cycles also can be set to trip the compressor to help prevent damage.
Delta P vs. h
The Delta P vs. h algorithm, also known as Pressure Rise, was originally developed in the 1970s. It was developed based on observations that the pressure ratio across the compressor closely followed the measured differential pressure across a flow measurement device. Delta P vs. h is still widely in use today, due to its relative operational simplicity and low cost. In fact, this control method requires only a flow and a differential pressure measurement across the compressor to function successfully.
Unfortunately, while Delta P vs. h is a major improvement on minimum flow recycle, it still has significant problems. The method does not account for changes in gas properties and requires a suction pressure that does not significantly change during operation.
As control systems have become more advanced, it has become possible to implement more elaborate thermodynamic models for compressor control. As a result, some form of a compressor head model has largely replaced Delta P vs. h.
Compressor head vs. flow
The compressor head vs. flow algorithm calculates the head generated by the compressor and plots it versus the temperature- and pressure-compensated flow produced. Regardless of whether the algorithm is based on polytropic or adiabatic head, this approach can accurately predict the compressor operating point at various temperatures and pressures. In addition, it is not affected by changes in the molecular weight of the gas. The basic equations for this algorithm are shown below.
The basic equation for polytropic head is defined as:
The difficulty with using this equation for surge control is that not all the variables can be measured directly. Gas compressibility and molecular weight cannot be determined except by offline analysis.
To eliminate these variables from the equation, it is necessary to utilize the flow relationships of differential pressure-flow measurement devices. Orifice, venturi, annubar, and other head-type measurement devices have flow equations that include terms for molecular weight and compressibility.