Below are excerpts from a case study presented at the Middle Eastern Turbomachinery Symposium 2015.
Authors Muhammad Zahid (Sr. Engineer – Rotating), Devrajan Venkatesh (Rotating Equipt. Analyst), N. K. Raju (Lead Process Engineer) and Christopher G Holt (Proj & Facilities Engg Div Manager) presented this case study.
At the Qatargas condensate refinery, the Hydrogen Recycle Compressor’s compressor train is equipped with BN 3500 system, and the compressors are of barrel type construction. The problem is that about six months after commissioning in September 2009, LP compressor performance degradation was observed. The degradation was slow initially but accelerated with time, and the compressor efficiency dropped from 65% (new) to 35% (failure).
LP Compressor Failure
In July 2011, after about 20 months in service, there was a sudden increase of LP compressor vibration from 10 mic to 75 mic at DE. BN ADRE was installed on machine vibration rack to analyze the data. However, meanwhile vibration quickly increased to danger levels (beyond 1000 mic) at DE, and the machine was manually shutdown because vibration trip did not function.
(A damaged journal bearing)
On internal inspection, the following were observed:
• The first stage impeller was found broken
• Heavy deposits / fouling of impellers and diffusers
• Damaged Journal bearings, DE was severe
Note: HP Compressor was not dismantled based on vibration, bearings temperatures and thermodynamic performance data analysis. Internal inspection also revealed damaged journal bearings. The journal bearings were subjected to very high cyclic load due to unbalance rotor after impeller breakage. The max bearing temperature recorded was 130oC.
Deposits were analyzed and they were mainly Ammonium Chloride per laboratory analysis. Ammonium Chloride is produced in traces in the reactor due to the presence of N2, H2 and chlorides in feed. EDS Analysis of foulant on 1st Stage impeller showed high chlorides and iron.
(Characteristics of fatigue failure)
Metallurgical analysis revealed that “the fracture surface is smooth, relatively featureless, and contains some evidence of beachmarks and thumbnails which are all characteristics of a fatigue failure.”
Impeller Modal Analysis
Modal analysis was carried out with the OEM to know if the impellers’ natural frequencies are close to the operating range and could be responsible for broken impeller. A sufficient margin was found.
Root causes of failure
The root causes of failure were:
• Under deposits pitting corrosion of 17-4 PH SS impeller by chlorides
• Impeller failure was due to stress corrosion cracking and possibly combined with high cycle fatigue resulting from flow irregularities caused by excessive fouling of the gas flow passages and pitting.
(Corrosion of impellers by chloride)
Other contributory causes of failure were:
• Lack of understanding of fouling service and its harmful nature
• Insufficient process controls to prevent deposits formation
• Incomplete execution of vibration trip function by project team. BN3500 was not connected to compressor ESD for trip on high vibration or journal bearings temperature.
The following resolutions were taken:
• Compressor reassembled with spare rotor
• Vibration and bearing temp trip function commissioned per OEM guidelines
• Water injection upstream of compressor started on alternate days
• Compressor efficiency was closely monitored to see the effectiveness, and it remained steady at 65%
• After 3 years of service, max 2% efficiency dropped, indicating water injection was working very good.
The compressor was opened for inspection in a planned outage and the internals were in excellent condition which was in line with the efficiency trends, and the Rotor NDT revealed no defects.
Therefore, this shows that fouling can result in serious damage to vital components and cost heavy production loss, in addition to reducing throughput of centrifugal compressors. A thorough understanding of process stream constituents and impact on equipment is very important for reliable plant operation. Equipment surveillance for performance and mechanical health parameters can help in early detection of problem and preventing failures. The collaboration of process and rotating engineers is important. There should be gaps in project QA/QC.