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MECHANICAL ENGINEERING ASPECTS FOR OVERHAULING OF HELIUM COMPRESSOR OF 1.3 KW HELIUM REFRIGERATOR/LIQUEFIER SYSTEM J. C. PATEL Institute for Plasma Research Gandhinagar, Gujarat, INDIA Email: jcpatel@ipr.res.in K. M. PATEL, P. R. SHAH, GLN SRIKANTH, D. R. CHRISTIAN, R. N. PANCHAL, D. P. SONARA, N. BAIRAGI, G. PURWAR, P. N. PANCHAL, R. J. PATEL, G. MAHESURIA, H. D. NIMAVAT, R. SHARMA, A. GARG, V. L. TANNA Abstract Institute for Plasma Research (IPR) has 1.3 kW / 4.5 K helium refrigerator/liquefier (HRL) cryogenics system, which is operational since 2003. The cryogenic HRL plant used to cool down the magnets at of Steady State Super-conducting Tokomak (SST-1). Three identical Mycom make helium screw compressors with Fimet make 315 kW / 3-phase/ 440 VAC / 50 Hz Electrical Motors, Oil removal system operate to supply high-pressure helium gas to HRL cold-box and sub-systems. Heavy duty 315 kW rated electrical motors are an asynchronous induction drive based machines, which is used to run the helium screw compressors. The compressors along with the electrical motors run on round the clock basis for a month long continuous operation. At a time, to facilitate the required mass flow rate of 140 g/s at discharge pressure of 14.0 bar (a) helium gas to the application, two helium screw compressors run with programmable logic control (PLC) software. The third one acts as redundant machine. Hence, its reliability and availability is mandatory. Therefore, it is very essential to maintain and overhaul the Compressor and Motors as per their schedule of operation hours to facilitate the reliable operation of the Tokomak. In this paper, we will describes Mechanical engineering aspects of overhauling experiences of compressors and motors that include their alignment with compressor, online temperature monitoring and motor cooling arrangements and vibrations measurement of compressor, motor and skid.

1. INTRODUCTION

At IPR, we have 1.3 kW at 4.5 K helium cryogenic refrigeration/liquefier (HRL) system is working since 2003 (Supplier: ALAT, France). It is custom based modified Claude cycle, where it produces refrigeration capacity of 650 W at 4.5 K and liquefaction capacity of 200 l/h in a closed loop [1]. The operation features of HRL are, control cool-down, normal operation and warm up mode of superconducting magnets system of SST-1. This process design of HRL plant is optimized for the “mixed mode” operation where refrigeration as well as liquefaction yield can be obtained. This plant has three oil flooded helium screw compressors, oil removal system, main purifier (dual beds) and cold-box, which includes trench of heat exchangers, three turbines (Turbine A, B, C), JT valves, cold circulator and main control Dewar (MCD) to meet the thermodynamic process design requirements in closed loop as shown in Fig 1. For reliable operation of HRL plant, it is mandatory to follow the maintenance procedures and protocols at prescribed schedule by the manufacturer [2].

2. NEED FOR OVERHAULING THE HELIUM SCREW COMPRESSORS

Due to continuous operation on round the clock basis, the helium screw compressors A and B are crossed their operation hours of 24,000 hours. As per operation and maintenance manual from original equipment manufacturer (OEM), it was envisaged to carry out overhauling of both the helium screw compressors. The helium screw compressors overhauling activities have been carried out in collaboration with M/s. Axima Engie, France. Prior to make decision of overhauling of helium screw compressors, review of the records of operation hours 24,000 hours has been confirmed. The present state of health of helium screw compressors has been checked while organizing check up with M/s. Mayekawa India investigation report concluded on vibration and bearings replacement. These helium screw compressors installed and commissioning since 2002. In 2016, we have utilized the total operation hours more than 20,000 hours. Therefore, it was envisaged to carry out the overhauling of these helium screw compressors to ensure more availibity and reliability of the system perspectives. Fig 2 shown the helium screw compressors on a skid.

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FIG. 1. PID of Helium Refrigerator and Liquefier System.

FIG 2. Helium screw compressors skid at IPR cryo facility

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3. MAINTENANCE PROTOCOLS FOLLOWED AS PER OEM RECOMMENDATION

Prior to taking a decision on carrying out helium screw compressors overhauling tasks, we have consulted M/s. Mayekawa India to check and verify the present state of health of helium screw compressors. While organizing check up with M/s. Mayekawa India [3], the test and investigation report have shown that there is critical need of overhauling. The total vibration on compressors skid and noise observed from bearings side. This investigation report concluded that now the time has come to carry out overhauling activities.

4. EXPERIENCE DURING DISMANTLING AND OVERHAULING ACTIVITIES

During overhauling activities by M/s. Axima Engie, France, IPR cryo team has got lot more learning and technical experience. Some of the technical highlights are mentioned below,

Experience of assembly and dismantling of helium gas screw compressors Systematic dismantling procedures of compressors parts according to male and female screws Systematic arrangements of tools and gauges as per dismantling sequence Assembly must be done after required maintenance steps of screws, critical inspection of main bearings,

thrust bearings (step by step), male and female screw slide parts arranged and abnormality study must be done (Refer Fig. 3).

Inspection for slide parts must be done for wear and tear. Decisions must be taken after examination of parts and if required consumable spare parts must be replaced. Respecting the proper maintenance schedule with critical care is mandatory for the life of the compressor.

FIG 3. Overhauling activities and bearing replacement

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5. ROLE OF SPARE PARTS DURING COMPRESSORS OVERHAULING ACTIVITIES

In order to have reliability and availability of helium cryogenic system at IPR, the spares inventory plays a crucial role. There are two types of spares i.e. Mandatory spares and Global spares. The mandatory spares are consumed during the maintenance as well as overhauling activities whereas the global spares are required to co-op with any kind of emergency failure. Such global spares inventory allows helium cryo system to be operational. As per manufacturer guidelines, every 24,000 hours of operation hours demands overhauling activities on helium screw compressor. During overhauling activities, we have replaced essential and mandatory spare parts viz. all gaskets, seals, O-rings, balance piston sets, thrust bearing sets, oil seals, mechanical shaft seals, sleeve bearing sets, etc. have been replaced. As on today, we have kept one set of mandatory spares with us to make sure that the compressors station to be made operational all the time.

6. ASSURANCE ON COMPRESSORS (A & B) ALIGNMENT WITH ELECTRICAL MOTORS While carrying out activities on alignment of helium screw compressors A & B after overhauling activities, it was found that the parallel and angular alignment of both the compressors have been found below 100 µ (micron) level. These data on alignment are in the acceptable limit as per OEM and Axima Refrigeration (ENGIE), France [4]. After achieving the acceptable level of alignment as mention above, we have authorized the compressor run as following sequences: (a) First the stand along run test of overhauled compressor (at least for 1 hour) during this time, we need to

measure vibration, temperature rise on Motor etc. observe any kind of abnormal noise from compressor.

(b) After completion of (a) activity, we have run the compressor in different sequences Viz. A-C-B, B-C-A and C-B-A etc. to inspect its sequential performance (at least each sequence for 1 hour test run).

Activities wise the relevant photographs and video were taken for future reference.

FIG 4. Temperature and Vibration Measurement during the test run

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TABLE 1. MEASURED PARAMETERS FOR COMPRESSOR-A & B AFTER OVERHAULING

7. CONCLUSIONS

Alignment of electrical motors with compressors skid in horizontal / vertical was achieved within 50 ± 10 using classical dial-gauge method. The assembled compressor on skid, levelling of electrical motors done within 10. The full load compressors test run was carried out different possible sequences using PLC commands. During the test run, we have monitored the maximum flow rate of helium screw compressors, suction and discharge pressure, electrical motors current, phase imbalance current measurement, bearing side temperature rise and vibration have been measured and found within the satisfactory level. During this task, IPR cryo crews got better exposure on learning preventive maintenance of helium screw compressors and control mitigation of vibration, noise and alignment issues.

ACKNOWLEDGEMENT

The authors would like to pay their sincere thanks to M/s AXIMA ENGIE (FRANCE) expert, who participated during the helium screw compressors overhauling activities. Technical discussions and debates were very interesting and learning in a way during this task. The cryo division also extends their gratitude to electrical utilities as well as water cooling divisions (IPR) for their continuous round the clock supports and help while taking the test run of compressors one by one.

REFERENCES

[1] C.P. Dhard, B. Sarkar, R. Misra, A.K. Sahu, V. L. Tanna, J. Tank, et al., “Commissioning and operational experience with 1 kW class helium refrigerator/liquefier for SST-1”, (Proceedings, Transactions of the Cryogenic Engineering Conference (CEC 2003), Anchorage, Alaska, September 22-26, 2003), AIP Conf. Proc. Volume 710, pp160–167(2004). [2] Mahesuriya G et al, “Operational and Troubleshooting Experiences in the SST-1 Cryogenic System”, (Proceedings, Transactions of the Cryogenic Engineering Conference (CEC 2013) 17-21 June 2013. Anchorage, Alaska, USA), AIP Conf. Proc., Volume -1573-59B, pp. 1645-1651(2014). [3] Technical support from M/s. Mayekawa India, www.mayekawa.co.in/ [4] Technical support from M/s. Axima Engie, France, https://engie-axima.fr/en/

Parameters Compressor-A Compressor-B Suction Pressure 1.0 bar (a) 1.05 bar (a) Discharge Pressure 14.8 bar (a) 13.92 bar (a) Oil Pressure 18.07 bar (a) 17.72 bar (a) Oil Differential Pressure 3.90 bar (a) 3.77 bar (a) Suction Temperature 32.5 degree C 30.3 degree C Discharge Temperature 72.8 degree C 75.2 degree C Current 426 A 441 A Slide Valve 100% 100%