Retrofits to save energy in centrifugal compressors

Centrifugal compressors offer several opportunities for energy savings. Some of the typical saving opportunities lie in: Anti-fouling coating resulting in improved performance over longer period of time 2) Using non-metallic labyrinth seals which can produce lower clearance and also deform slightly 3) Changing of seals to dry gas seals from wet seals. 4) Performance enhancement using water or liquid washing systems.

Below are some case studies outlined in the paper, “Carbon Foot Print Reduction Techniques With Rotating Machinery,” by Vasanth Bhat and Quek Ser King Aaron of Singapore Refining Company presented at the 2016 Asia Turbomachinery & Pump Symposium.

The wet Gas compressor in FCC unit was proving to be a limiting as at the end of a four year run the compressor internal would be fouled and the extent of fouling would be so high that the steam turbine driver was not able to attain maximum speed conditions. So when a capacity creep was initiated it was considered if it was necessary to upgrade the compressor to handle higher flow with an incremental capacity increase in the steam turbine capability. The compressor OEM suggested that a more cost effective approach could be to coat the compressor rotor with anti-foulant coating

With a minor upgrade to the turbine the machine was put into to operation and this time around the compressor managed to operate at max operating speed at the end of the run without limiting plant through put. The coating managed to withstand the four and half years of continuous operation.( refer photo 2)

Lesson learnt: a) Initially it was assumed there would be no change required to the rotor or other parts but on assembly it was observed that the labyrinth seals rubbed against the rotor when the horizontal split casing was tightened. It was realized the coating thickness ate up the clearance resulting in jamming of the compressor rotor. b) The fouling in the compressor rotor reduced but the foulant continued to foul the as flow passages of the stationary diaphragms. We are considering providing a coating to the stationary parts as well so that the performance of the machine could be maintained. Option 2: Change to non-metallic labyrinth seal has been attempted but has not been very successful due to poor gas conditions resulting in rapid wear. The refrigeration compressor (refer photo 3) have these type of seals and there is no direct comparison data to show the difference in performance with a normal seal. Generally it has been accepted in the industry that it would reduce the internal leakage loss which could be in most cases be around 2%, so depending on the size of the machine the benefits could outweigh the costs. In terms of costs there could be a fivefold increase in costs between conventional labyrinth seals and use of non-metallic material.

Change from wet seals to dry gas seals

The wet gas compressor in the FCC (Fluid Catalyst Cracking ) unit had wet seals which had its own wet seal system. This seal system requires seal oil pumps, exchangers and the wet seal consumes some power loss. Due to reliability issues with the wet seal the compressor was retrofitted with dry gas seals and thus eliminated the need for a seal oil system and its associated losses. In many installation similar to this seal oil reclamation units are attached which also need power for pumps, heaters etc. The annual operation costs for seal oil system just based on energy costs was in the tune of US$150,000 per annum in this case. It could be higher or lower depending on the size of the prime mover for the seal oil systems. A typical loss in wet contact seals compared to dry gas seals would be in the range of 10 to 15 KW per casing, again a small number but when seen as a total system can prove to be significant.

1) As with any typical retrofit project keep some contingency in terms of schedule and costs to take care of surprise findings. 2) Keep an option for full reversal as in some rare cases Dry Gas Seals may not end up as reliable solution. 3) Depending on familiarity of the site personnel to dry gas systems we need to have a robust training program to ensure operators understand the dry gas seal panel which is more complicated in terms of looks compared to seal oil systems.

Online washing systems

Running a fouled compressor always results in higher specific power consumption. On line wash offers a quick remedy to performance degradation but has its own challenges. Water or naphtha has been used to wash but the effectiveness of the wash would depend on the location of the nozzles with respect to the fouling. With washing there are 2 major risks, namely machine could end up going to high vibration if the distribution of the fouling and the cleaning effectiveness varies with location on the rotor, secondly with dry gas seals it gets further complicated with possible risk of contamination reaching the dry gas seals and potentially resulting in its failure. Due to these risks industry has used it more effectively for gas turbines, main air blower and to much lesser extent in wet gas compressor and recycle gas compressors. Washing of steam turbine rotors has also been done but maintaining a good steam quality would be the area of focus rather than depending on water washing as a solution.