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CASE STUDY: Advanced analysis of reciprocating engines, compressors and gas pipelines

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This paper refers to a detailed study conducted in March 2017 by SUMICO Technologies to evaluate the mechanical performance and condition of critical reciprocating engines, compressors and associated pipelines using state-of-the-art technologies at a natural gas booster station in Mari Gas Field (Sindh, Pakistan).

Results of the study revealed various hidden areas requiring attention to increase efficiency, reduce operational costs and increase reliability of the plant. This paper describes the technologies used, performance parameters analyzed,some interesting findings and the benefits gained through the study.

Prepared by Abdul Basit Bashir (SUMICO Technologies)
https://www.linkedin.com/in/abdulbasitb/

Published in: Technology

CASE STUDY: Advanced analysis of reciprocating engines, compressors and gas pipelines

  1. 1. Background The gas compressor station is run by seven identical compressor sets (GE Ajax model DPC-2804) connected in parallel to boost natural gas for fertilizer production. As the station is critical for manufacturing process’ reliability, it is essential to regularly monitor compressor’s health to get early warning of any developing faults and avoid catastrophic failures. It is not commercially feasible to repeatedly shutdown and disassemble compressor sets and their auxiliaries to inspect their internal conditions. Condition Monitoring activities were therefore in place, however they only involved vibration analysis and infrared thermography techniques. As the mechanical behaviour of reciprocating machinery is more complex than rotating machinery, additional analysis techniques were recommended for their better health evaluation. Advanced performance and condition analysis of reciprocating engines, compressors and gas pipelines March 2017 Executive Summary This paper refers to a detailed study conducted in March 2017 by SUMICO Technologies to evaluate the mechanical performance and condition of critical reciprocating engines, compressors and associated pipelines using state-of-the-art technologies at a natural gas booster station in Mari Gas Field (Sindh, Pakistan). Results of the study revealed various hidden areas requiring attention to increase efficiency, reduce operational costs and increase reliability of the plant. This paper describes the technologies used, performance parameters analysed, some interesting findings and the benefits gained through the study. Advanced Machinery Analysis Solutions by SUMICO Technologies
  2. 2. Study overview SUMICO’s engineers collected data from each of the seven compressor sets and adjacent pipelines while they were operational using Windrock™PA6320and EmersonCSI2140 analyzers. The data types collected included dynamic pressures, vibration, ultrasound, spark voltage and temperatures, each measured simultaneously with the dynamic crankshaft position data. The technologies provided by SUMICO, allowed recording faster than 50,000 data points per second from each sensor point. In that way, every performance parameter’s dynamic behaviour could be thoroughly studied with reference to the compressor’s dynamic crankshaft position. 3D computer models were also designed by SUMICO’s engineers for all compressor sets and the measured vibration data was input to analyse ODS (Operational Deflection Shape) of the discharge pipelines which had been subject to high vibrations. ODS (Operational Deflection Shape) computer model SUMICO’s engineers then analysed the recorded data using specialized computer software through various types of charts; PV (Pressure-Volume) curves, ultrasonic waveforms, spark traces, vibration FFT spectra and vibration waveforms, to name a few. Analysis examples 1. Identified piston ring leakage Image 1A shows ultrasound data measured initially from the four cylinders of engine no. 1. The pattern revealed that in cylinder no. 2 (red) there were momentary high levels of ultrasound (highlighted by black circle), occurring every time the crankshaft had rotated 45° after passing cylinder’s TDC (Top Dead Centre) which are symptoms of gas leakage through piston rings. Furthermore, the P-V (Pressure vs. Volume) curve (image2A)ofcylinder no.2 showed ahighrate ofpressure drop after achieving the peak firing pressure. After analysis of the data, SUMICO’s engineers recommended to inspect the piston rings. Image 1A – Ultrasonic patterns of engine cylinders (initial) When the engine cylinder was later opened by plant maintenance team, piston rings of cylinder no. 2 were found to be broken (image 1B). Image 1B – Broken piston ring from engine #1,cylinder #2 After the piston rings were repaired, data retaken showed significant improvements (image 1C). Image 1C– Ultrasonic patterns of engine cylinders (after repair) Leaving such an abnormality undetected could have continued to waste fuel through the leakage. Moreover, the fault could have further worsened over time and if any debris from broken ring interacted with internal moving parts, it could have ultimately caused catastrophic failure of the engine and unscheduled downtime which would be very costly.
  3. 3. 2. Identified abnormal spark plug timings P-V curves (Cylinder Pressure vs. Volume) of engine no. 1 showed that cylinder no. 3 and 4 were experiencing improper ignition and low PFP (peak firing pressure) values. The 1St derivative of Pressure-Time curves of cylinder no. 3 showed inconsistent firing from cycle to cycle (image 2B). It was therefore recommended to replace spark plugs. Image 2A – P-Vcurves (Cylinder pressure vs. Volume) before repair Image 2B – P-T (Cylinder Pressure vs. Crankshaft angle) and dP/dT (1st derivative) curves of multiple cycles overlapped for comparison After spark plugs were changed, the behaviour of cylinder pressures improved significantly (image 2C) which resulted in great improvement in engine performance and efficiency. Image 2C– P-V curves (Cylinder pressure vs. Volume) after repair 3. Identified damaged crankshaft bearings Analysis of vibration data in crankshaft angle domain showed abnormal patterns of crankshaft bearing no. 2 on compressor set 1 with a transient appearing once per revolution. Image 3B – Main bearings vibration vs crankshaft angle When the bearing was later disassembled for inspection, it was found to be damaged at the inner race (Image 3C). The data retaken after repair showed significant improvement in the vibration waveforms and spectra (Image 3D). Image 3C – Eroded inner race of compressor set #1, crankshaft main bearing #2 Image 3D – Main bearings vibration vs crankshaft angle 4. Compression efficiency evaluation The collected data also allowed performance evaluation of everycompressorcylinder individually through P-V(Pressure vs. Volume) curves. As the example shows in image 4A, the measured PV curve was compared with an ideal performance curve(dashed)accordingtothe cylinder’sdimensions. Thearea of difference was highlighted by shaded area to easily visualize the energy losses within every cylinder individually. Image 4A – Pressure-Volume curve of compressor cylinder 5. Identified locations requiring additional piping supports Some pipe sections had been experiencing excessive vibration but the causes were unknown before the study was conducted. Taking compressor set 1 as an example, by collecting vibration at the discharge pipeline and analysis through frequency domain (spectrum) it was found that vibration was highest at 20 Hz (image 5A) which was 3X compressor running speed when operating at 400 rpm. Image 5A – Compressor #1 discharge piping vibration spectrum Inconsistent pressure rise from cycle to cycle during ignition period Ignition period Dominant vibration at 1200 rpm (20 Hz) = (3 X crankshaft speed) Measured PV curve Ideal PV curve
  4. 4. ` CORPORATE HEAD OFFICE 185/J-1, M.A. Johar Town, Lahore, Pakistan KARACHI E-24, Block-A, Railway Society, Gulshan-e-jamal, Karachi, Pakistan ISLAMABAD No. 307, Main Margala Road, F-11/3 Islamabad, Pakistan Tel: +92 42 35315541 Tel: +92 21 34682766 Tel: +92 51 2225472 Fax: +92 42 35315540 Fax: +92 21 34688265 Fax: +92 51 2225151 Email: info@sumico.com.pk Email: info@sumico.com.pk Email: info@sumico.com.pk www.sumico.com.pk The Advanced Machinery Analysis division of SUMICO Technologies is the leading provider in Pakistan of critical machine health analysis including gas turbines, steam turbines, reciprocating enginesand reciprocating compressors using in house state-of-the- art technologies and a strong team of internationally trained engineers. SUMICO has personnel certified in Reciprocating Compressors and Engines Analysis from Windrock Inc. Knoxville, TN, USA. In addition, there are ISO-18436-2 certified vibration analysts up to Category III certified by Mobius Institute, Australia who can be deployed anytime to your plant for analysing health of your critical machinery. Advanced Machinery Analysis Solutions Image 5B – Compressor #1 discharge piping pressure curve Vibration bump test conducted on the pipeline showed that no natural frequencies existed near 20 Hz which confirmed that the issue was not related to structural resonance. By further analysis through ODS (image 5C) and the compressor’s discharge pressure data plotted against compressor’s crankshaft angle (image 5B), it became clear that the 20 Hz vibration was mainly caused by compressor’s pressure pulsation and the back and forth gas flow occurring betweenNRV (Non-Return Valve) downstream of compressor set #1 and the main discharge header. Image 5C – ODS (Operational Deflection Shape) of existing piping system Image 5D –Piping modifications recommended Based on the analysis, it was recommended to add additional piping support and change location of the existing NRV as shown in image 5D so that it can prevent any reverse flow of gases from the main discharge header towards the compressor. The test results together with the recommendations acted as evidences to help plant teams justify the modification actions required. Findings summarized Along with the few examples described in detail, a number of beneficial findings were made through the study:  Enginecylinders withearlycombustion, latecombustion and peak firing pressure instabilities were identified  Cylinder valve leakages and lash issues were identified  Engine and compressor efficiency benchmarks were established for future comparison  Compressor skid and piping vibrations were compared against API 618 standards and benchmarks were established for future comparison  Engine cylinders with poor performance were identified and engine balancing requirements were determined accordingly  Requirements for modifying piping supports were determined to reduce vibration significantly  Engine spark plugs which were causing improper ignition were identified  Identified bearing faults of compressor crankshafts, cooler fans, lubrication oil pumps and hydraulic pumps  Identified gear mesh problems of lubrication oil pumps Conclusion The findings helped identify key areas to reduce operational costs and increase reliability of the plant. As shown through examples, the study not only helpedincrease plant efficiency and performance by identifying poor performance areas, but also helped identify faults which could have further progressed ifleftundetected,leadingtocatastrophic failures and unplanned downtimes. For equipment with normal performance, the result reports provided by SUMICO would be useful for future comparison in case of performance deviations and would also help in any future fault investigation for the plant engineers. Piping section with excessive vibration Existing NRV location New NRV location suggested New NRV location suggested Location suggested for additional pipe support To main discharge header Updated09.06.2017

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