TURBO TIPS: CASE STUDIES

This article discusses how process and operation conditions can affect turbocompressors and their operation, performance, and reliability through three real-world case studies that address how downstream, upstream, the process itself, and all surrounding facilities and items impact turbocompressors.

TEMPERATURE SPIKES IN BEARINGS

Radial bearings of tilting-pad type in a critical centrifugal compressor showed spikes in bearing pad temperatures every 1 - 4 hours. Measured temperatures went from 96°C (baseline) to 115-119°C (spike). The recommendation by the operations team was to reduce the inlet lubrication oil temperature to limit the peak of the spikes below high temperature limits. The initial proposal was to reduce the temperature of the delive red lubrication oil to the compressor from 50°C to 45°C. It was implemented. However, the lubrication oil temperature reduction caused a higher temperature at the bearing pads. This might be because of more viscosity of lubrication oil and consequently more friction. In this case, a 5°C reduction in the lubrication oil supply temperature caused around 2-3°C increase in bearing (resistance temperature detector (RTD) temperatures. The reduction of supply lubrication oil temperature could not reduce temperature spikes at RTDs.

This is an example of how random actions cannot solve the problem. The root-cause of the problem should be identified and eliminated. After thorough studies, the root cause of temperature spikes was identified as varnish in the lubrication oil. Every time varnish settled on a bearing pad RTD, a spike in the bearing RTD temperature was recorded.

One indication that helped to correctly identify the root cause was a simple test of switching the lubrication oil filter to see if spike patterns changed. The switch of filter reduced varnish deposits on bearings temporarily and this reduced spikes in RTD bearing temperature and changed its patterns.

Investigation showed one of additives in the lubrication oil interacted with compressor process gas and produced varnish. It was decided to change the lubrication oil type for this compressor. The operation of the compressor has been smooth and satisfactory since this adjustment.

NEW PRESSURE RATIO

The pressure ratio of a process centrifugal compressor was adjusted due to a change in gas composition and operating conditions. The pressure ratio (discharge pressure to suction pressure) could move theoretically from 2.25 to 2.51 because of these changes (gas composition change, higher required discharge pressure, etc). The compressor was initially rated for a pressure ratio of around 2.39.

Process and operation conditions can affect turbocompressors and their operation, performance, and reliability

The compressor data including compressor map, design conditions, compressor details, and thrust bearing data were carefully reviewed. Rises in differential pressure and load on thrust bearings were accurately determined and verified. Based on these studies, it was confirmed that there was no issue for the thrust bearing regarding this differential pressure rise. A thrust bearing should often be sized to absorb this kind of change. In this case, these changes were within thrust bearing limits. In addition, there were temperature measurements and vibration monitoring for the thrust bearings. These could highlight possible malfunctions. The compressor has been operated safely and reliably with the modified operating conditions.

LIQUID CARRYOVER

The potential of liquid carryover to a refrigeration centrifugal compressor was raised by the operation team of a plant. Recorded data showed that prior to start-up of the compressor, the suction line was at 6.5 Barg and 5°C. Under these conditions, the refrigeration gas was theoretically liquid and could damage the compressor, seals, and inlet strainer when it started with liquid flowing through the centrifugal compressor. It was recommended by the operations team to depressurize the refrigeration gas to below 4.5 Barg prior to the start-up. In addition, they asked to investigate possible solutions to detect or automatically drain liquid refrigeration from the inlet piping at pre-start.

Investigations showed that liquid carryover was a false alarm. The temperature transmitter range was up to 5°C. It showed 5°C for any temperature above 5°C. The actual temperature in the suction line before the start-up was always above 20°C. Theoretically, there was a little chance for refrigeration gas conditions reaching the saturation point during a shutdown.

It was decided to change the temperature transmitter with a new one with a measurement range up to 40°C. There might be a remote chance that the gas condition during a shutdown could reach saturation point. In that case, draining of the collected liquid would be necessary. The operation procedure was corrected and a necessary step for the drain of liquid at the suction drain prior to the compressor start-up was included. ■

Amin Almasi is a Chartered Professional Engineer in Australia and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.