DIAGNOSTIC TECHNOLOGY FOR THE VIBRATION OF LNG PUMPS TECHNIQUE POUR L EXAMINATION DE LA VIBRATION DE LA POMPE GNL

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DIAGNOSTIC TECHNOLOGY FOR THE VIBRATION OF LNG PUMPS TECHNIQUE POUR L EXAMINATION DE LA VIBRATION DE LA POMPE GNL Masayuki Ishida Toshiro Sasaki Koichi Ichimura Thermal Power Dept.

1 DIAGNOSTIC TECHNOLOGY FOR THE VIBRATION OF LNG PUMPS TECHNIQUE POUR L EXAMINATION DE LA VIBRATION DE LA POMPE GNL Masayuki Ishida Toshiro Sasaki Koichi Ichimura Thermal Power Dept. Tokyo Electric Power Company 1-3, Uchisaiwai-cho 1-chome, Chiyoda-ku, Tokyo, Japan Yoshiyuki Kitano Masakazu Okubo Kenji Takeuchi Storage System Engineering Dept. NKK Corporation 3-7-1, Nishishinjyuku, Shinjyuku-ku, Tokyo, Japan ABSTRACT LNG pumps are one of the very important parts of the facilities used in liquefaction terminals, transport vessels, and receiving terminals. The bearings of pumps have a limited life span, therefore, it is required to dismantle the pumps every several thousand operating hours and renew its bearings. Although it is a common dilemma among concerned parties to precisely recognize the exhaust condition of the pump s bearings, which is installed within a flammable liquid at a temperature of -162 deg. Celsius, it would be very costly and cause restrictions in operation as well to open up a pump in perfect condition, assuming that the pump is in an abnormal state. At our Futtsu terminal, a vibration sensor has been installed near the pump s bearings. Utilizing the vibration sensor, we have discovered the particular phenomenon of LNG through the analysis of the vibration waves picked up by them, and have developed an original diagnostic technology which makes it possible to recognize the precise exhaust condition of the bearings form the exterior. With this new devise, the number of times of inspection can be reduced, which has, as a result, contributed to the improvement in the pump s loading factor and reduction in maintenance costs. C.1 1

2 In this essay, we will report the details of development in the diagnostic technology for the vibration of LNG pumps at our Futtu Thermal Power Station and its system which has been structured. RESUME La pompe LNG est un appareil important employé à des bases de liquéfaction, bateaux de transport et bases d acceptation, dont le roulement, ayant une vie limitée, se décompose aprés plusieurs milliers d heures d utilisation, nécessitant son rechange. La connaissance precise de la détérioration du roulement de la pompe installé dans du liquide combustible à -162 degrés Celsius est ce qui tourmente les personnes concernées communément. Si l on ouvre une pompe en bon état, en la jugeant anormale, ce causera augmentation du coût ainsi que restriction à l opération. A la base Futtsu on a donc mis un sensor de vibration prés du roulement de la pompe afin de découvrir, à partir de l analyse des formes des ondes vibratoires, le phénomène propre à GNL et de développer une technique originale examinatoire pour la connaissance precise à l extérieur de l état de détérioration du roulement. Cela a rendu possible la réduction de la fréquence des inspections à l amélioration du taux d utilization de la capacité et à la diminution des frais de réparation. Dans ce traité, je fais rapport sur l histoire du développement de la technique pour l examination de la vibration de la pompe LNG et sur le système construit à la centrale thermale Futtsu. 1. Introduction Inspection of the condition of a liquefied natural gas (LNG) pump, an important component in a LNG system, was extremely difficult because it is submerged in liquefied cryogenic gas at a temperature of -162 degrees Celsius. Therefore pump had to be dismantled at regular intervals for inspection, maintenance and servicing under a timebased maintenance system. Even under these circumstances, monitoring systems employing vibration sensors have been applied. Vibration sensors were attached to the pump to detect any defect that might start to affect the component. The sensors were designed to give off early warnings when the vibration exceeded a predetermined acceptance level. In the meantime, diagnostic techniques based on the vibrational characteristics of machines with rotating parts has advanced dramatically. The technique is now so reliable that it is widely applied for vibrational diagnostics to low- and medium-speed rotating machines. The technique now allows the implementation of a condition-based maintenance system, which will contribute to the improved reliability of facilities and dramatically reduce maintenance costs. Attempts were made by a joint effort of the Tokyo Electric Power Company and NKK to apply the conventional vibrational diagnostic technique to the LNG pump. It was revealed, however, that the LNG pump requires a special technique because it possesses unique characteristics of its own. This led to the development of a vibrational diagnostic technique that is unique to the LNG pump. C.1 2

3 This essay reports the development of the new LNG pump diagnostic technique and provides details of the system developed for the Futtsu Thermal Power Station of the Tokyo Electric Power Company. 2. LNG Pump 2.1. Structure and Features Figure 1 shows a diagram of the LNG pump. The pump has the following structural features: (1) The In-line layout integrates the pump and motor in tandem. (2) The rotating body is supported by bearings at the upper and lower ends of the motor and bushings. The bearings are lubricated by the LNG. (3) The pump is used under an extreme temperature condition of -162 deg. Celsius Problems There were two major problems in the maintenance of the conventional LNG pumps. These were: (1) Warning system reliability: Although alarms based on abnormal vibrations were installed to inform operators of abnormal conditions, the evaluations after actually dismantling the LNG pumps often proved that there was no problem with the pump. (2) Increased maintenance cost: The interval between inspections had to be set shorter than the actual life expectancy because the operational status of the pumps could not be precisely monitored. To cope with these problems, an innovative diagnostic technique needed to be developed to monitor the everyday operational status and provide the possible causes of problems. Upper bearing Vibration sensor Motor Lower bearing Pump extension assembly Bush Impeller Inducer Fig.1 Schematic drawing of LNG pump C.1 3

4 3. Items To Be Considered Items that needed to be considered for the development of a new LNG pump diagnostic technique are described in the following sections Applicability of Conventional Vibrational Diagnostic Technique We investigated the applicability of the conventional vibrational diagnostic method to the following major deterioration problems of the LNG pumps: (1) Bearing damage (2) Unstable operation (3) Contact Between Impeller and Inducer 3.2. Clarifying the Unique Vibration Characteristics of the LNG Pump The unique vibration characteristics of the LNG pump were analyzed, and its relationship to pump malfunctions was clarified Position of the Vibration Sensor The condition of the pump is normally monitored using a vibration sensor mounted on the extension assembly. However, based on the results of vibrational diagnostics in general rotating machinery, abnormal vibration is most effectively detected with the sensor mounted on the bearing. Sensors were therefore mounted on both the extension assembly and the bearing, and a comparison made to determine the most effective position. (See Fig.1.) 4. LN2 Test 4.1. Test Description Malfunctions were simulated in LNG pumps. These were then run in a container of liquid nitrogen to determine the suitability of conventional vibrational diagnostic technology for detecting the various malfunctions and to investigate differences in the ability to detect vibrations with vibration sensors mounted in different positions. The malfunctions that were simulated are listed below. (1) Mild damage to the outer ring of the lower bearing (2) Imbalance of the impeller 4.2. Test Results The test results are shown in Figs. 2 and 3. The following conclusions were drawn on the basis of these results. (1) Damage to the bearing can be detected with the lower bearing sensor. The use of conventional diagnostic technology for rotating machinery is therefore possible. (The vibrational period of the damage to the outer ring is detectable by auto correlation between the acceleration waveforms, See Fig.2). (2) Impeller imbalance is detectable using the upper and lower bearing sensors, and the use of conventional diagnostic technology for rotating machinery is therefore C.1 4

5 possible. (The rotational frequency is detectable from the velocity waveform spectrum. See Fig.3.) (3) Neither bearing damage nor impeller imbalance could be detected with the sensor mounted on the extension assembly. Coefficient of correlation Time lag msec 36 Time lag msec 36 (a) Upper bearing (b) Lower bearing 1 Time lag msec 36 (c) Extension assembly Fig.2 Auto correlation result of LN2 test (Wave from of vibration acceleration) Voltage mv 1 Frequency Hz (a) Upper bearing Frequency Hz (b) Lower bearing 1 1 Frequency Hz (c) Extension assembly Fig.3 Result of LN2 test (Spectrum of vibration velocity) 5. Testing An LNG pump was mounted in an LNG storage tank to clarify the deterioration of the LNG pump during operation and define the pump characteristics Contact Between Impeller and Inducer The waveform of the vibration occurring when the impeller and inducer come into contact due to wear of the bearings and the bushings was measured. The results of these measurements show that the following occurs in the pump under these conditions. (1) The velocity and acceleration of the vibration increase gradually (i.e., vibration does not occur at initial pump). (2) Periodic collision vibration is clearly apparent. The period of this vibration is unrelated to the period of pump rotation, but the collision vibration becomes stronger at certain frequencies. This differs from that occurring when impellers and fans contact housings in conventional rotating machinery and is unique to LNG pumps. C.1 5

6 5.2. Excessive Vibration Unique to LNG Pumps Experience shows that LNG pumps are sometimes subject to excessive vibration, the cause of which is undetermined. Causes such as the rotating stall and cavitation have been suggested, but it appears that in most cases the phenomenon is unique to the structure of LNG pumps. The two patterns noted below have been observed. (1) Pattern 1 (a) Effective acceleration increases (b) Commonly occurs in the upper bearing (c) Not continuous (2) Pattern 2 (a) Periodic with high peak acceleration (b) Commonly occurs in the lower bearing (c) Not continuous It is now possible to determine the causes of the excessive vibration unique to LNG pumps, as characterized by these two patterns, through vibrational data analysis and experimental verification. While there are other causes of excessive vibration such as rotating stall, they are not due to deterioration of the pump and do not hinder operation provided they are not sufficient to cause damage Summary of Experimental Results The experimental results, including those of the LN2 test, are summarized below. (1) Approval of Conventional Vibrational Diagnostics Conventional vibrational diagnostic technology was found to be applicable for damage to bearings and imbalances. Contact between impeller and inducer exhibits characteristics that differ from the conventional case, but waveform measurement and analysis provided clarification of this phenomenon. (2) Vibration Unique to LNG Pumps The excessive vibration is of a type unique to LNG pumps; however, clarification of the phenomena has, in turn, clarified the relationship to pump malfunctions. (3) Sensor Position It was found that the best sensor position is at the bearings, as the sensors need to be positioned at places where they can detect, with good sensitivity, the vibrations that indicate pump deterioration, and where they are not affected by factors irrelevant to pump deterioration (e.g., tank fluid level). 6. LNG Pump Diagnosis System Specialized logic was developed for LNG pump diagnostics that was based on the experimental results described above. This logic is employed in an on-line vibrational diagnostic system that includes vibration monitoring functions in addition to those required for LNG pumps. The configuration of this system is shown in Fig.4. C.1 6

7 LNG pump On-line monitor Control unit (Flow rate of LNG pumps) Operation control CPU (Running time of equipments, Tank fluid level) LNG pump (high pressure) Host computer (Work station) Printer Other equipments Ethernet LAN Fig.4 Configuration of diagnosis system Personal computers 6.1. System Functions The primary functions of the system are as follows. (1) Monitoring of LNG pumps, LNG high pressure pumps, suction drums and drain pumps, and BOG compressors is continuous and automatic. (2) Immediate warning of a malfunction and automatic print-out of a precise diagnostic report for use in analyzing the problem are provided. An example report is shown in Fig.5. (3) Trend management and life-span prediction are possible. These allow prediction of the time at which caution and limit values for the equipment are reached. (4) Data between different equipment can be compared. (5) Historical changes in precise diagnostic data can be viewed. C.1 7

8 (a) Facilities diagnosis report 6.2. Diagnostic Examples Fig.5 Example of precise diagnosis report (b) Cause of abnormality analysis report (1) An Example of Excessive Vibration in an LNG Pump The acceleration waveform of excessive vibration that developed in the upper bearing of an LNG pump is compared to a normal acceleration waveform in Fig.6. Acceleration G Acceleration G (a) Excessive Vibration (b) Normal Vibration Fig.6 Wave form of vibration acceleration.1 sec.1 sec Transitions in the acceleration associated with this excessive vibration are shown in Fig.7. This excessive vibration does not occur continuously, but takes the form of a sudden increase in vibration, followed by a return to normal levels. C.1 8

1 8 Acceleration G 6 4 2 1996/2/16 19:37:2 1996/2/2 12:44:39 1996/2/24 5:52:16 1996/2/27 22:59:53 1996/3/2 16:7:3 Fig.

approximately 11 months after beginning operation.

9 1 8 Acceleration G /2/16 19:37:2 1996/2/2 12:44: /2/24 5:52: /2/27 22:59: /3/2 16:7:3 Fig.7 Transition of vibration acceleration 1996/3/6 9:15:8 (2) An Example of Detection of a Malfunction in High Pressure LNG Pump Bearings Abnormal vibration was noted in this high pressure LNG pump approximately 11 months after beginning operation. A precise diagnosis revealed that shock vibration was occurring that had a frequency associated with damage to the inner ring of the bearing, and it was determined that continuous operation of the pump was no longer possible. The vibration waveform is shown in Fig.8. Acceleration G Fig.8 Wave form of bearing fault pump.1 sec The pump was taken out of operation and dismantled, and checks revealed flaking on the bearing surface in the inner ring and on the rollers. See Photo 1. Photo 1. Damage of inner ring of bearing C.1 9

10 7. Conclusion The use of this system has allowed an extension in the time between inspections of LNG pumps from 6 hours to 19 hours, an increase in the equipment reliability, and a major reduction in maintenance costs. There are, however, problems associated with the multi-stage pump shafts of weak stiffness, that have yet to be resolved, and the accumulation of further data is required to develop the technology for this application. C.1 1

Evolution of an LNG Terminal: Senboku Terminal of Osaka Gas. Toshiro Otsuka Senboku LNG Terminal Osaka Gas Co., Ltd.

Evolution of an LNG Terminal: Senboku Terminal of Osaka Gas Toshiro Otsuka Senboku LNG Terminal Osaka Gas Co., Ltd. Osaka Gas service territory Himeji LNG terminal Senboku 2 Senboku 1 Senboku LNG terminal