SURGE - WHAT IS IT?

Historically, the author has worked with technicians from Danfoss Turbocor, Multistack and Smardt, Canada, Smardt Australia and Smardt, Germany. The author is aware that there is a higher than normal percentage of compressor failures at certain sites and has discussed these failures with technicians and engineers from various organizations.

The conclusion drawn from this set of information is that allowing Turbocor to routinely enter Surge condition on a continuous basis will eventually lead to a bearing failure. Occasional Compressor surges can be expected during chilled water pump exchange or unusual operating conditions. However, the routine occurrence of surge is dangerous to the long term health of the compressor.

What is Surge? 
This is the condition in which the compressor enters an operating condition where the differential pressure generated by the centrifugal action of the compressor impeller is not sufficient to overcome the difference between the suction pressure and the compressor discharge pressure. When this occurs, there is an instantaneous reversal of refrigerant flow through the compressor. In the case of Turbocor application, there is a discharge check valve. When this instantaneous inversion occurs, the check valve closes abruptly. When this occurs, the pressure differential between suction and compressor discharge disappears. Flow through the compressor returns to normal, the check valve opens and the original pressure differential returns, causing the cycle to repeat. All of this happens very quickly. This phenomenon is accompanied by a ¨clacking¨ from the check valve in the compressor discharge and vibrations in the discharge line of the compressor. Normally, this Surge condition will last 5 to 10 seconds as the compressor controls adjust compressor speed to get the compressor out of the Surge condition.
Why is this bad for the compressor? When the pulse occurs, there are rapid pressure changes within the compression area on the compressor. Typically, the compressor shaft rotates between 20,000 and 30,000 revolutions per minute. The compressor bearing control acts to keep the shaft precisely located radially and axially. Movement of the shaft at a distance less than the thickness of a sheet of paper during operation may result in the shaft entering contact with the contact touchdown bearings at a high rotational speed. Touchdown bearings are designed to support the shaft once it is completely stopped. The shaft that contacts the landing bearings at normal operating speed has a high chance to destroy the touchdown bearings. The rapid changes in pressure that occur during surge act to shift the axis from its normal operating position. Magnetic bearing control measures shaft position 10,000 times per second and adjusts for any shaft movement. However, under surge conditions, with rapidly changing pressures and related forces on the shaft, there is an internal safety limit to the ―orbital displacement‖ of the shaft. Under Surge conditions, when these internal forces cause shaft displacement from its normal position, if the displacement exceeds the ―orbital displacement‖ limit, the compressor will shut down automatically and normally this will be accompanied by a message ¨FRONT RADIAL BEARING DISPLACEMENT FAULT¨. In certain instances, the internal forces are such that the shaft enters in contact with the touchdown bearing at a high rotation speed, possibly causing and damage the touchdown bearing.

The touchdown bearing is a standard type roller bearing which is part of the magnetic bearing assembly, specifically designed to support the shaft when the shaft is de-levitated. The touchdown bearing is not designed to support contact with the shaft at operating speeds.

Under these circumstances (damaged touchdown bearing), the compressor must be returned to the compressor manufacturer (Danfoss Turbocor) for repair. Repair of the touchdown bearing locally is expressly prohibited by the compressor manufacturer.