All centrifugal compressors experience the phenomena known as surge. Surge is defined as a sudden reversal of air flow at a point when the compressor can no longer increase its pressure or flow. Surge can make a very dramatic sound that shakes the intake piping but at times is much more subtle and may not be heard but rather observed as sudden pressure swings easily seen on analog pressure gauges. Multiple surging can be very damaging to the compressor, causing severe axial thrust and severe heat loads on the rotor assembly. Surges can cause immediate and/or long term accumulative damage. Surge occurs in two different states.
Natural surge is the maximum pressure that can be achieved under ambient conditions with the inlet valve fully opened. (Note: Undersized motors may prevent maximum flow.) With full flow, the bypass valve can be slowly closed off to achieve a natural surge. This is how units are typically tested to compare their current condition, or health, in reference to the design point of the compressor. Ambient conditions can significantly affect surge, so if you are performing surge as a comparison to design conditions, always factor the differences between design and ambient for a fair comparison. Inlet air temp and inlet cooling water temp significantly influence the surge value.
The other type of surge is a throttle surge. A throttle surge occurs when the air flow of the compressor is somewhat below the actual flow capacity; in other words when the inlet valve is partially closed or some other flow impedance is present, such as plugged air filters, etc. Unlike the health comparison that can be made by performing a natural surge, the throttle surge allows one to determine the amount of flow reduction that can be achieved to produce a surge while maintaining a predetermined pressure set point at the discharge of the compressor.
Why is this important? A throttle surge test determines how low the flow can be reduced (while retaining a desired discharge pressure) before the unit surges. With this information we can now set up the controller of the compressor to operate the compressor in the most efficient manor possible. The point slightly above the throttle surge line and up to the full flow of the compressor or maximum amperage of the drive motor is known as the throttle range. This is the useful range of flow in which the compressor can operate without causing either a throttle or natural surge.