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ACTIONS TO IMPROVE THE PREDICTIVE MAINTENANCE IN CRYOGENIC PUMPS IN ENAGÁS

Ricardo Conde Maintenance Manager, Cartagena LNG Terminal

Enagás Transporte, S.A.U. Cartagena, Spain

José A. Lana R&D Project Manager, Direction of Technology and Innovation

Enagás, S.A. Zaragoza, Spain

KEYWORDS: LNG pump, predictive maintenance, condition base maintenance, preventive maintenance

ABSTRACT

In the last few years, Enagás has done a big effort in increasing the efficiency of their infrastructures, in general, and their LNG Import Terminals in particular.

One of the main initiatives has been to optimize maintenance management, keeping the traditional high standards in availability and safety but decreasing the operating cost.

This work shows an example of the actions taken for cryogenic (submerged) pumps, which is one of the most critical pieces of equipment in a LNG terminal.

The work done includes:

• Retrofit of vibration monitoring systems.

• Utilization of process parameters to determine asset condition.

• Validation of new technologies based on the measurement, processing and analysis of electrical parameters signals of the electric motors to trend mechanical failure modes. This work has been done in a series of controlled test.

1.- INTRODUCTION

With over 40 years of experience, Enagás is an international leader in the construction, operation and maintenance of gas facilities. It is the leading natural gas transmission company in Spain and Technical Manager of the Spanish Gas System, and is authorized as an independent Transmission System Operator (TSO) by the European Union.

Enagás owns four of the seven regasification plants in Spain, located in Barcelona, Cartagena, Huelva and Gijón, and is the main shareholder of the BBG regasification plant in Bilbao. It has 10,000 kilometers of gas pipeline, 18 compressor stations and three underground storage facilities. Figure 1 shows the location of the main natural gas infrastructures in Spain. It also holds a participation in TLA Altamira (Mexico) and GNL Quintero (Chile) LNG terminals.

As a consequence of its history, Enagás has a large know-how in all the stages of natural gas infrastructures life cycle: design, construction, commissioning, start-up, operation, maintenance and decommissioning.

The management of such amount of natural gas infrastructure forces to do a big effort in increasing the efficiency of all of them, in general, and its LNG Import Terminals in particular. One of the initiatives has been to optimize maintenance management, keeping the traditional high standards in availability and safety but decreasing the operating cost.

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This paper shows an example of the research activities and actions taken for optimising the maintenance of cryogenic (submerged) pumps.

Figure 1.- Enagás infrastructures in Spain 2.- MAINTENANCE IN THE PROCESS INDUSTRY

In most process industries, traditionally, and following manufacturer recommendation, the mean time between maintenance (MTBM) of different equipment has been done in fixed interval, this is, a preventive maintenance philosophy.

Following this manufacturer recommendation, along the years, Enagás has gained expertise in hundreds of overhaul operations of different plant equipment. In most of the cases, during this work, it was verified that the equipment was in good mechanical condition, so it could have been working, without compromising reliability and safety, during a longer period of time. This finding did Enagás to look the way for extending the maintenance interval to the optimal. In theory, predictive maintenance techniques should allow this.

The extension of MTBM has a direct impact in reducing maintenance cost of a LNG terminal and, as a consequence of this, Enagás has implemented several technologies depending on the plant equipment affected. Figure 2 shows some of them. Anyway, this strategy has advantages and disadvantages (pros/cons), as it is summarized in Figure 3.

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Figure 2.- Predictive maintenance techniques apply to different plant equipment

Figure 3.- Pros&Cons of predictive maintenance techniques

3.- MAINTENANCE OF LNG PUMPS

One of the most critical pieces of equipment in a LNG terminal are the LNG pumps. Their operation conditions are extreme compared to other process industries because they are working under cryogenic conditions (-161 ºC).

Along the years, Enagás has gained expertise in hundreds of pump overhauls, following the preventive maintenance strategy mentioned. As mentioned for other plant equipment, in most of the cases, during this overhaul, it was verified that the pump was in good mechanical condition, so it could have been working, without compromising the reliability and the safety, during a longer period of time.

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The utilization of predictive maintenance technology will depend on the failure mode to be detected. In a LNG pump, these are the followings:

• Mechanical failures: o Imbalance of the shaft. o Misalignment of the shaft. o Damage in bearings. o Mechanical eccentricity in the motor rotor.

• Electrical failures:

o Rotor asymmetry, broken bars. o Rotor insulation loss. o Short-circuit between some motor winding coils.

• Cavitation.

• Combination of more than one failure mode.

Most of these failures do not happen suddenly, but it has a development period. During this period some working parameters of the pump deviate from the normal and this can be detected with the appropriate instrumentation. For instance, a LNG pump, as any rotating equipment, is subjected to vibration during its normal operation, but any malfunction produces an increase in the vibration level that can be measured with an accelerometer and used as an indicator of this malfunction.

Below are described several strategies followed by Enagás to find a suitable predictive maintenance technique for LNG pumps. The different approaches were studied in parallel in the last years.

4.- RETROFIT OF VIBRATION MONITORING SYSTEM

Vibration monitoring is a very well-known technology for predictive maintenance of rotating equipment and its performance in many process industry is fully satisfactory.

Vibration is measured with an accelerometer, but a vibration monitoring system is something more complex than just this. On the other hand, it should be taken into account that the system is working in a cryogenic environment (-161 ºC inside the tank), where the sensor (accelerometer), cable, connector… are in direct contact with the flowing LNG in the pump well and, even worse, parallel to a power cable feeding the pump motor, as shown in Figure 4. Additionally, natural gas is a flammable compound and very high containment requirement are needed to avoid a gas leakage to the atmosphere.

Enagás has installed, for years, vibration monitoring system in its LNG pumps directly purchased to the pump vendor, but its performance was not satisfactory, see Figure 5A. The spectrum shows a lot of peak, noise, at low frequencies. This situation was the same for other LNG terminal operators consulted. The conclusion was that bad performance is not related to the technology itself but to this specific application.

As a consequence of this, Enagás decided to develop an own vibrating monitoring system.

The first step was to review the original design and to analyze the fit for purpose of all the parts:

• Accelerometer. • Signal cable. • Cable gland/bulkhead. • Connector. • Signal amplifier.

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• Connection boxes. • Electronic hardware. • Diagnosis software.

As a result of this analysis, it was found that some items were not suitable for measuring vibration in cryogenic condition. In fact, some of these items were not design to work in cryogenic conditions.

The actions taken by Enagás were:

• To look in the market for components suitable for working in cryogenic condition: accelerometer, signal cable & connector.

• To design and manufacture some component very specific for this application: cable gland and bulkhead.

• To test several component/layout in process condition, pressure and temperature, to validate/reject the solution proposed. This was done in the workshop to minimize the risk of failure inside the LNG tank.

Once this process was finished, the chosen option is currently being tested in several pumps installed in LNG tanks and working in real operation condition.

The result after several thousand hours of work and some minor adjustment is that the new system is working satisfactory as it is shown in Figure 5B, where it can be seen the vibration spectrum of the new system, with less noise at low, sub synchronous, frequencies. In conclusion, Enagás considers the design validated and ready to be installed in other LNG pumps.

A first result of this change has been an extension in a 100 % of the MTBM, above the manufacturer recommended value, of the LNG pumps.

Figure 4.- In tank LNG pump. Accelerometer, signal cable and bulkhead

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Figure 5.- Vibration signal (velocity vs. frequency) before & after modifying the vibration monitoring system.

5.- UTILIZATION OF PROCESS PARAMETERS TO DETERMINE ASSET CONDITION

Due to the problems to record good vibration signals seen before, other option to know mechanical state of the pump is the use of process information.

A LNG pump is a highly instrumented process equipment, so we have in every case:

• Suction and discharge pressure. • Suction and discharge temperature. • Flow rate.

It is reasonable to suppose that with all this information it should be possible to check the performance of the equipment without any additional investment in monitoring devices. If the performance of a pump is not optimal, LNG temperature increases or flow decreased, for instance.

When a pump is degraded, the wear produces internal leakage recirculation, so looking to the pump flow-pressure curve the working point is moved from its design one, see Figure 6. This leads to an energy efficiency loss as well, so power consumption rises. All of this malfunction produces an increment in operational cost, so, when the cost of operating in this condition is higher than the maintenance, it is the time for overhaul.

Also, it is known that if there is wear inside the pump, this is traduced in frictions which produces heat. So, measuring temperature at the inlet and outlet of the pump and looking to the trend, it is possible to identify when the wear is increasing. Enagás experience shows that when the difference is more than two degrees Celsius, something wrong is happening inside the pump.

This technique is very useful during commissioning of new equipment in order to detect bad installation issues.

5A.- BEFORE 5B.- AFTER

Only low frequency noise appers but nosignal to the main frequency and multiples.

Frequencies to the main one and multiplesare now observed.

10,2

0,2

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Figure 6.- Original pump curve versus pump curve after operation.

6.- MEASUREMENT, PROCESSING AND ANALYSIS OF ELECTRICAL PARAMETER SIGNALS

Most of the rotating equipment, as any LNG pump, has an electrical motor to drive it.

This predictive maintenance technology is based on the measurement, processing and analysis of the electrical parameters signals of the electric motor to trend mechanical and electrical failure modes in the unit, this is, to use the motor as the sensor to control de state of the unit, understanding as unit the pump itself and the motor.

Currently, there are in the market some commercial systems that claims, according to the manufacturer/vendor information, to be able to monitor a pump, detect any malfunction and, even, to recognize the pump component faulty. Figure 7 shows a sketch of this technology.

Basically, the electrical measurement (voltage and current) monitors are intelligent condition-monitoring devices which continuously monitor the three phase alternating current (AC) motors, of all sizes and power levels. It is supposed to provide clear and unambiguous indications when the performance of the motor, or even the machinery that it is connected to, begins to degrade.

Each monitor system takes current and voltage signals from the three phases. The units are networked to a workstation running an special software where they all can be viewed together. The software collects and manages information from all the units, provides enhanced diagnostic capability, and allows remote operation of the complete system. This software (features depending on the supplier) analyses the measurement and gives the information on the pump and motor condition, triggering an alarm if the value of any parameter is outside the settings. Two different philosophies were seen in the systems tested:

- The analysis software has embedded some fix trigger level of monitored parameters. - The software has a learning period to know, under right operation condition, what the normal value

of the different parameters are. Once these are set, the software is able to detect any deviation and trigger an alarm.

In order to check these technologies, Enagás carried out a testing program of several systems in collaboration with the Mechanical Engineering Department of the Technical University of Cartagena, Spain.

The work was done in a series of controlled tests in a test bench designed by the University. Figure 8 shows the main component of the test bench.

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The systems were tested with a water centrifugal pump, connected to a 3 kW AC motor. The bench allows to produce artificially all the electric, mechanical and operational failure modes in this type of equipment (listed previously). Figure 9 shows how the mechanical unbalanced of the shaft was produced and Figure 10 the rotor asymmetry by broken bars.

The main conclusions of this work have been:

• This technology has shown that, in most of the cases, is able to detect when something is wrong in the unit.

• Although the technology detects the fault, in many cases is not able to discriminate it, so a generic message appears. This makes the system not valid for the required purpose. For instance, we cannot disassemble a pump thinking we had a problem in it and, afterwards, it is seen that the issue was in the electrical power feed.

• The software developed by the manufacturers is quite powerful and when it is carefully analyzed by an expert a lot information can be obtained, but not in an automatic way.

• This technology is very promising but needs additional development for a general use in the industry.

Figure 7.-Sketch of the condition monitoring system

based on monitoring electrical parameters of the AC motor

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1.- Support structure

2.- Water storage

3.- AC motor, 3kW, connected to the pump though a rigid coupling

4.- 1 stage, centrifugal pump

5.- Rotameter and pressure transductor

6.- Suction & discharge valve

7.- Electric supply board

8.- Data acquisition unit

9.- Monitoring software

10.- Water cooling interchanger

Figure 8.- Test bench at the Technical University of Cartagena

Figure 9.- Simulation of unbalance in the pump shaft

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Figure 10.- Rotor asymmetry, two broken bars 7.- CONCLUSIONS

Enagás has carried out in the last years a great effort in research for finding new tools and/or methods for implementing an effective predictive/condition based maintenance strategy in the LNG pumps installed in its terminals.

Three different approaches were followed:

• Reconditioning of vibration monitoring systems.

• Utilisation of process parameter to determine asset condition.

• Validation of a new technology based on the measurement, processing and analysis of the electrical parameter signals of electric motor to trend mechanical failure modes.

The main conclusions reached are:

• Retrofit of vibration monitoring systems allows to use this technology.

• The use of process parameters complements vibration monitoring.

• The use of electrical parameters is a technology not mature enough but it is the future because of its simplicity and cost.

The first results of these works are allowed to extend the MTBM of the LNG pumps and, consequently, a reduction in maintenance cost of this asset. It is expected that the results of on-going research will finish in additional cost reduction without any compromise to the reliability, availability and safety in the operation of Enagás’ LNG terminals.

ACKNOWLEDGEMENTS

The Authors want to thank to Enagás for getting permission for presenting this paper at the LNG’17 Conference.

We also want to thank to our colleagues F. Manobel, J. Pérez and G. García from the maintenance staff of Enagás Transporte S.A.U., and to Professors G. Munuera, J.L. Aguirre and A. Valverde from the Technical University of Cartagena, Spain, for their contribution to develop and testing all the predictive maintenance techniques and their help in the preparation of this paper.