2010:072 CIV

M A S T E R ' S T H E S I S

Improving Availability ofHydraulic Drive Systems

through Knowledge Engineering

Björn Backe

Luleå University of Technology

MSc Programmes in Engineering Mechanical Engineering

Department of Applied Physics and Mechanical EngineeringDivision of Computer Aided Design

2010:072 CIV - ISSN: 1402-1617 - ISRN: LTU-EX--10/072--SE

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Improving Availability of Hydraulic Drive Systems

through Knowledge Engineering

Björn Backe

Luleå University of Technology

Msc Programme in Engineering

Mechanical Engineering

Department of Applied Physics and Mechanical Engineering

Division of Computer Aided Design

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To Tira and Julia

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Abstract This Master’s thesis was developed at the Division of Computer Aided Design (CAD) at Luleå University of Technology [1]. A research project called SSPI (Scalable search of product lifecycle information) [2] was initiated by Luleå University of Technology [3] and Uppsala University (UDBL) [4], where the main goal was to develop software systems for efficient and scalable search of product data and meta-knowledge produced during the entire product lifecycle. The project was intended to develop such software systems based on Data Stream Management Systems (DSMS) and the semantic web model. In the SSPI project the goal was also to build two demonstrators based on case studies in industry. High reliability and availability are important customer demands that are currently increasing in industrial significance. Through better monitoring of critical systems and system parameters, failures can be detected, predicted and avoided before damage occurs. This is, of course, beneficial for the customer. Improving monitoring is of interest for Hägglunds Drives (HDAB) [5], which is why the thesis work was initiated as a part of the SSPI project. Hägglunds Drives participated in this project as an industrial partner in order to learn more about the aforementioned approaches and thereby increase their product availability. In this thesis work a specific drive called a kiln drive was analyzed to determine what can be done from a monitoring perspective to increase availability and reliability. During the thesis work, HDAB conducted a test of a drive, whereby a long-life test of oil was carried out in their laboratory and compared to normal, reduced oil level in the system. The “tank test,” as the system will be referred to, was equipped with sensors to monitor the condition of the system. According to HDAB, this system was comparable to the kiln drive. To verify this, the thesis work included comparison of these two systems to each other with a focus on failures and component configuration and function. The purpose was to find out if failures occurring in the kiln drive could also occur on the tank test. If the same types of failures occur in both systems, then possible tests and monitoring could be performed on the Tank test instead. The benefits of using the tank test for analysis were that no work had to be done on the actual kiln drive and that the system could be run according to different load cases. The tank test was also equipped with more sensors than the kiln drive, so little work was needed to start performing practical tests and measurements. The results from the analysis showed that most failure modes occurring in the kiln drive are to be expected to occur in the tank

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test, due to similarities shared by the systems. The only exceptions were some functions in the kiln drive that do not exist in the tank test. To find common failures that have occurred in kiln drives, interviews with a group of HDAB representatives were conducted. Attempts were made to collect statistical material regarding kiln drives, but no documentation on the system’s failure rate history was available. Instead, the analysis had to be based on the information from interviews. From the interviews, a simplified FMEA (Failure mode and effect analysis) and a FTA (Fault tree analysis) were performed, providing a basis for identifying problems affecting reliability and availability of the kiln drive. The results from the FMEA and FTA analyses were a list of suggested measurement points that are recommended to be monitored to predict, prevent and discover the most critical failure modes. A set of rules was created based on the relations between the measurement points. The purpose of the rules is to show how to use the measurement points in relation to each other to identify some of the important failure modes before they occur. The rules are intended to increase the availability of the drive system by predicting and preventing failures before they occur. In the case of HDAB this means that the downtime of their hydraulic drive systems will be reduced or eliminated when failures can be discovered and remedied at an early stage. The rules were presented to and validated by HDAB within the scope of the thesis work presented here. The rules provide a basis for implementing a software system to perform monitoring. Translations of the rules into computer code are necessary if the system is to be implemented. The rules need to be further tested under real industrial conditions and iterated. Finally, the thesis work provides information, gathered from experienced people within HDAB, regarding the kiln drive system and its specific failure modes. This collected information will be fed into the SSPI project as information about what to search for in data streams from the case study, so as to increase the reliability and availability of the kiln drive.

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Sammanfattning Detta examensarbete har genomförts på Luleå tekniska universitetet, avdelningen för Datorstödd Maskinkonstruktion (DMK) [1]. Ett forskningsprojekt benämnt SSPI (Skalbar sökning av information från hela produktcykeln) [2] var initierat mellan Luleå Tekniska Universitet [3] och Uppsala Universitet (UDBL)[4] där huvudmålet var att utveckla mjukvarusystem för effektiv skalbar sökning av produktdata och meta data producerat under hela produktcykeln. Projektet avsåg att utveckla mjukvaran baserat på Data Stream Management System (DSMS) och den semantiska webbmodellen. I projektet var även målet att bygga två demonstratorer baserat på fallstudier inom industrin. Hög pålitlighet och tillgänglighet är viktiga kundkrav som ökar inom industrin. Genom att ha bättre monitorering av kritiska system och systemparametrar, kan fel förutses, undvikas och upptäckas innan allvarliga skador inträffar. Detta är naturligtvis fördelaktigt för kunden. Att förbättra monitorering är intressant för Hägglunds Drives (HDAB) [5] och var anledningen för initieringen av detta examensarbete som en del av SSPI-projektet. Hägglunds Drives deltog i SSPI projektet som en industriell partner för att lära sig mer om ovanstående nämnda ansatser och därigenom öka deras produkters tillgänglighet. I detta examensarbete är en specifik drift, en Kilndrift, analyserad för att identifiera vad som bör göras ur ett monitoreringsperspektiv för att öka tillgänglighet och pålitlighet. Under genomförandet av examensarbetet hade HDAB i deras laboratorium ett system under pågående test, där livslängden på oljan undersöks då systemet har, jämfört med normalt, reducerad oljevolym. ”Tanktestet” som systemet kommer benämnas, var utrustad med sensorer för att monitorera systemets kondition. Enligt HDAB var tanktestet jämförbart med Kilndriften med avseende på funktion och komponenter, för att verifiera detta genomfördes en analys där systemen jämfördes mot varandra med avseende på feltyper, komponenter och funktion. Syftet var att fastställa om feltyper som inträffar på en Kilndrift också kan förväntas inträffa på tanktestet? Om samma feltyper inträffar på båda systemen, skulle eventuella tester och analyser vara möjliga att genomföra på tanktestet istället. Fördelarna med att använda tanktestet för analyser och tester var att inte behöva göra ingrepp i ett befintligt system ute i fält, dessutom skulle systemet kunna köras enligt önskade driftcykler. Tanktestet var utrustat med fler sensorer än Kilndriften vilket innebar att mindre arbete skulle

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krävas för att börja utföra tester och analyser (baserat på monitorering). Resultatet från analyserna visar att de flesta feltyperna som inträffar i Kilndriften också kan förväntas inträffa i tanktestet pga. likheter mellan systemen. De enda undantagen var några funktioner på Kilndriften som inte fanns på tanktestet. För att finna de vanligaste feltyperna som inträffat på Kilndriften, genomfördes intervjuer med representanter från HDAB. Försök att sammanställa statistiskt material gällande feltyper på Kilndrifter genomfördes, men dokumentation angående felrapportering saknades och var därför inte genomförbart. Alternativet var att basera analyserna på informationen från intervjuerna av representanterna. Med hjälp av informationen från intervjuerna genomfördes en förenklad FMEA (Failure Modes and Effects Analysis) och FTA (Felträdsanalys). FMEA och FTA analyserna fungerade som en bas för att identifiera problem som påverkar pålitlighet och tillgänglighet för Kilndriften. Resultaten från FMEA och FTA analyserna resulterade i en lista på mätpunkter som är rekommenderade att monitorera för att förutse, förhindra och upptäcka de mest kritiska feltyperna. Ett antal regler skapades baserat på relationer mellan mätpunkterna. Syftet med reglerna är att visa på hur mätpunkterna kan användas i relation till varandra för att identifiera att några av de mest kritiska feltyperna. Reglerna ska öka tillgängligheten av HDAB hydrauliska drivsystem genom att förutse och förhindra fel innan de inträffar. I HDAB’s fall betyder detta att driftstopp av deras system kommer att reduceras eller elimineras när felen kan upptäckas och åtgärdas i ett tidigt stadium. Reglerna blev presenterade för och validerade av HDAB inom ramarna för examensarbetet presenterat här. Reglerna fungerar som en bas vid implementering av mjukvarusystem för att utföra monitorering. En översättning av reglerna till exekverbar kod är nödvändig innan implementering. Reglerna behöver testas och itereras mer i verklighet för att bli optimala. Slutligen, detta examensarbete bidrar med information, samlat från erfaren personal hos HDAB gällande Kilndrifter och deras specifika feltyper. Denna information kommer att delas med till SSPI projektet som allmän information om systemet, vad som ska sökas efter i strömmande data från fallstudien, för att öka pålitlighet och tillgänglighet av Kilndriften.

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Preface This Master’s thesis is a final project for the MSc degree in Mechanical

Engineering at the Division of Computer Aided Design at Luleå University of Technology. I would like to thank Bengt Liljedahl, Arne Byström, Berth-Ove Byström, Christer Eberger, Gunnar Ivarsson and Anders Westerlund from Hägglunds Drives AB for their ideas and support throughout the project. Professor Lennart Karlsson and Lecturer Magnus Löfstrand of Luleå University of Technology are also acknowledged for their coaching and feedback.

Björn Backe Luleå, January 2010

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Table of Contents 1 Challenges in Condition Monitoring .........................................14

1.1 Introduction ......................................................................14 1.2 SSPI project......................................................................14 1.3 Hägglunds Drives AB ......................................................14 1.4 Master’s thesis work description ......................................15 1.4.1 Background ......................................................................15 1.4.2 Goal ..................................................................................16 1.4.3 Purpose.............................................................................16 1.4.4 Scope ................................................................................16

2. Method .........................................................................................17 3. Literature study ..........................................................................20

3.1 Pelletizing kiln..................................................................20 3.2 FMEA, Failure Mode and Effect Analysis .......................21 3.2.1 Defining and analyzing the system function ....................22 3.2.2 Performing the FMEA analysis ........................................22 3.2.3 Reviewing the FMEA worksheet .....................................23 3.3 FTA, Fault Tree Analysis .................................................23 3.4 Angular pumps and motors ..............................................25 3.4.1 Hydraulic pumps ..............................................................25 3.4.2 Hydraulic motors..............................................................26 3.5 Cooling.............................................................................29 3.6 Filtration...........................................................................31 3.7 Tribology in hydraulic systems ........................................32 3.8 Sensors .............................................................................32

4. Analysis and description of hydraulic drive systems ...............35 4.1 Overview of system components......................................35 4.2 Tank test ...........................................................................35 4.2.1 Cooling circuit..................................................................36 4.2.2 Hydraulic motor Compact CA100....................................36 4.2.3 Hydraulic Pump Bosch-Rexroth SP355 ...........................37 4.2.4 Electric motor...................................................................38 4.3 Kiln drive .........................................................................38 4.3.1 PEC unit ...........................................................................39 4.3.2 Flushing circuit.................................................................40 4.3.3 Hydraulic motor Marathon MB800..................................40 4.3.4 Hydraulic Pump Bosch-Rexroth SP500 ...........................40 4.3.5 Electric motor...................................................................41 4.4 Spider control system .......................................................41 4.5 Summary of kiln drive and tank test analysis...................41

5 Identifying system failure modes and new measurement points ............................................................................................43

5.1 FMEA Analysis................................................................43 5.2 FTA fault tree analysis .....................................................44

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5.3 Suggestions for new measurement points ........................45 5.4 Rule design for system monitoring...................................45

6. Analysis of results and rules for system monitoring ................46 6.1 Summarizing the FMEA ..................................................46 6.1.1 Shaft seal Hydraulic pump. ..............................................46 6.1.2 Water valve ......................................................................47 6.1.3 Accumulator / Swing down Function...............................47 6.1.4 Sensors .............................................................................48 6.1.5 Control of displacement on hydraulic pump ....................48 6.1.6 Spider ...............................................................................49 6.1.7 Tubes and Hoses...............................................................49 6.1.8 Electric motor...................................................................50 6.1.9 The oil in the system ........................................................50 6.1.10 Motor damage ..................................................................50 6.2 FTA ..................................................................................51 6.3 Suggestions for new measurement points and rule design for system monitoring ...........................................52

7. Discussion and Conclusions .......................................................59 8. Future work.................................................................................59 9. References....................................................................................61 Appendix A. ......................................................................................63 Appendix B........................................................................................64 Appendix C. ......................................................................................65 Appendix D. ......................................................................................66 Appendix E........................................................................................69 Appendix F........................................................................................74 Appendix G .......................................................................................75

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Nomenclature Variable Description Unit ω Angular speed rad/s M Torque Nm V Displacement cm3/rev p Pressure bar η Efficiency - Q Flow l/min P Power W

Q& Heat transfer W

T Temperature °C A Area m2 D Characteristic length m α Heat transfer number W/°C m2 λ Heat conductivity number W/m°C

Nu Nusselt’s number - GrPr Grashof Prandtl’s number - σ Stefan Boltzmann’s constant W/ m2K4

ε Emissivity - n Shaft speed rpm f Frequency Hz l Length m F Force N

Subscripts

m Hydraulic motor p Hydraulic pump

loss Power loss in Input

conv Convection v Vertical h Horizontal

Tmean Mean temperature real Adjusted number surf Surface env Environment rad Radiation tot Total

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1 Challenges in Condition Monitoring An introduction, short description of the SSPI project and Hägglunds

Drives is described in the following sub chapters, together with the

description of the Master’s thesis background, goal and purpose.

1.1 Introduction

High reliability and availability are important customer demands that are currently increasing in industrial significance. Through better monitoring of critical systems and system parameters, failures can be detected, predicted and avoided before damage occurs. One way of increasing the reliability and availability is by monitoring and analyzing data streams from products in use. There is, however, a challenge in how to search for and use data from products in use. These issues are of interest in the SSPI project [2] at Luleå University of Technology (CAD) [1] and Uppsala University (UDBL) [4] and also for companies like Hägglunds Drives (HDAB) [5]. HDAB were monitoring their systems but they had a need to further increase the availability and reliability of their systems. This Master’s thesis describes the work done with HDAB to meet the above mentioned need.

1.2 SSPI project

In the SSPI project, Luleå University of Technology (LTU), Division of Computer Aided Design [1] and Uppsala Database Laboratory (UDBL) [4] are collaborating to develop software systems for efficient and scalable search of product data and meta-knowledge produced during the entire product lifecycle. The intention is to develop such software systems based on Data Stream Management Systems (DSMS) and the semantic web model. The role and interest of the SSPI partners in this thesis was to gain information from this work regarding the case study at HDAB. They also contributed their knowledge concerning searching data streams.

1.3 Hägglunds Drives AB

Hägglunds Drives AB develops and manufactures hydraulic motors and complete hydraulic drive systems and is world leading in their high-torque, low rotation speed segment [5]. The company’s production, research and development facilities are located in Mellansel, Sweden. HDAB has a global sales organization with its own sales companies in sixteen countries.

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The company’s drive systems are typically used in the mining, recycling, paper & pulp, marine, rubber & plastic, chemicals and sugar industries. Hägglunds Drives participated in the SSPI project as an industrial partner in order to gain more knowledge in the field of Condition Monitoring through the SSPI [2] and iStreams [6] projects.

1.4 Master’s thesis work description

1.4.1 Background

The aim of this Master’s thesis was to gather and analyze knowledge and information to increase the reliability and availability of a hydraulic drive system from Hägglunds Drives . One of HDAB’s existing drives, a kiln drive, was used as the main industrial application. The physical drive was located at LKAB’s plant in Svappavaara. Further monitoring of the kiln drive was of specific interest to HDAB, since this system is a critical application for their customer. In Fig. 11, a kiln drive for transporting iron ore pellets is presented.

Figure 1. Kiln drive in China. Transportation of iron

ore pellets. A test rig, similar to the kiln drive system, was set up in HDAB’s

laboratory, Fig. 2. HDAB used the test rig, or the “tank test” as the system will be referred to, as a case study to investigate the effect on the oil during a long-life test with reduced (compared to normal) oil tank level.

1 Hägglunds Drives AB, “Kiln drive in China”, accessed 2010-01-31, from

http://www.hagglunds.com/Upload/20060907101535A_kilnDrives.pdf

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Figure 2. The tank test at HDAB’s laboratory. Left: the

PEC unit and tank. Right: the hydraulic motor/pump CA100.

The thesis work was performed in order to investigate the current

monitoring techniques used on the kiln drive system and to suggest suitable ways of increasing the reliability and availability of the system. Since the tank test was already set up in HDAB’s laboratory, the rig was also considered to be a suitable case study for this thesis work. In the test rig, 24 sensors were mounted, which facilitated monitoring and analysis of data. Since the tank test was to be used to perform measurements and practical tests, the systems needed to be analyzed and compared from a functional and failure mode point of view. The question was whether the tank test could serve as a case study and represent the kiln drive. Would the same failures occur in the tank test as in the kiln drive?

1.4.2 Goal The goal of the Master’s thesis was to gather and analyze information

and knowledge, and to suggest how to improve the availability of HDAB’s hydraulic drive systems.

1.4.3 Purpose By investigating common failure modes and HDAB’s present

monitoring techniques for the kiln drive, the aim was to analyze data and recommend suitable methods that HDAB can use to improve the reliability and availability their kiln drive system. The analysis results will also help HDAB to gain more knowledge concerning the monitoring of their systems and, subsequently, to be able to extend and offer more services based on monitoring.

1.4.4 Scope Since the system was to be monitored and measurement of many

different parameters were of potential interest, it was realized early on that certain areas had to be excluded due to the restricted time available for the thesis work. Measuring vibrations, for example, is an extensive

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area of which HDAB had little experience and therefore it was excluded from the scope of the thesis. The cost for monitoring equipment of the systems has not been considered in the thesis work. The aim was to explore the possible monitoring alternatives first, before considering costs. Since the system in the test rig at HDAB was similar to a kiln drive, logged data from the sensors in the tank test system were collected and saved for possible analysis. The similarity between these systems and the access to real data from the tank test meant that it was not necessary to perform any operations on the actual kiln drive. 2. Method

The chapter contains a description of the method used throughout the

thesis work presented here.

At the start of the thesis work it was decided that the guidelines from

the Sirius Masterplan, developed at Luleå University of Technology [3], would be used (Appendix A). The Sirius Masterplan is a method similar to the one developed by K.T. Ulrich, and S.D. Eppinger [8]. The different stages in the process are given in Fig.3 [8].

Figure 3. Stages of the Ulrich and Eppinger process

In the Planning phase, a Gantt chart was made to estimate the time

needed for each step of the process. The author has during the thesis work followed the Gantt chart, but changes had to be made to the plan, since

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the time needed for the stages was found to be insufficient. The final Gantt chart is presented in Appendix B. After the planning phase a literature study that would enhance the author’s technical knowledge was performed. In Chapter 3 a summary of the literature study is presented.

In the Design space exploration phase, the needfinding was carried

out. In order to properly understand what increased reliability and availability meant to Hägglunds Drives AB, an initial meeting with a group of HDAB representatives was held. The group consisted of: Manager Design, Bengt Liljedahl; Manager Development Controls, Arne Byström; After Market Business Development, Christer Eberger, and Anders Westerlund. During the meeting the scope of the thesis work was discussed. HDAB presented what they thought to be important outcomes of the thesis work. By interpreting statements made during the meeting, the author found that the meaning of availability for Hägglunds was the need for better dependability, i.e. to keep the system performance high, despite errors or other interference. One of HDAB’s ideas was to improve their monitoring technique in order to increase the reliability and availability of their hydraulic motor systems. HDAB suggested a specific scenario in which one of their existing drives, a kiln drive, was chosen as the case study. The goal was to gather and analyze knowledge and information that could be used to improve the availability of the case study system. HDAB also suggested another system, the tank test, which is similar to the kiln drive and could be used to perform tests on if necessary. The tank test, situated at their laboratory in Mellansel, Sweden, was supposed to represent the kiln drive case, so no actual tests would have to be performed on the actual kiln drive.

In Chapter 5, which corresponds to the Concept Design and

Prototyping phase, the task of finding the failure modes which affect the reliability and availability of kiln drives is described. The methods used were an FMEA analysis and an FTA analysis, which were based on interviews with the group of representatives from HDAB. The method for developing the set of rules and identifying the measurement points needed for improving the availability of the kiln drive is also presented.

Concurrently with and subsequent to the interviews, an analysis of the

kiln drive and the tank test at HDAB’s laboratory was conducted (Chapter 4). The purpose was to investigate whether the tank test could represent the kiln drive case and be used to perform practical tests to verify possible suggested methods. In this chapter the biggest differences in function and component configuration between the systems are also explained in more detail.

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The results of the Failure Mode and Effect Analysis (FMEA) and the Fault Tree Analysis (FTA) are described in Chapter 6. The Detail and Design & Prototyping, Pre-Launch and the Product Launch stages were not considered, since the goal was not to produce a product, but to gather and analyze knowledge and information that could be used to improve the availability of the case study system.

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3. Literature study In this chapter, the areas requiring further study to increase the

author’s overall technical knowledge are documented.

3.1 Pelletizing kiln

The rotary pelletizing kiln in Svappavaara is a step in the process of producing iron ore pellets. It is an inclined rotating oven which heat treats and transports iron ore pellets [9]. Before the pellets enter the kiln the material must undergo several heat treatments in preparation for the final treatments. At one end of the oven, a coal burner that heats the oven up to about 1250 degrees Celsius, which is just below the melting temperature of the constituents of the pellets, is mounted. At this temperature the pellets undergo a chemical reaction that improves the mechanical properties of the pellets. The rotary pelletizing kiln (Fig. 5)2 in the Svappavaara plant (Fig. 4) is 43 metres long and 6metres in diameter.

Figure 4. Svappavaara plant.

2 Metso,”Rotary kiln technologies”, http://www.metso.com/miningandconstruction/mm_pyro.nsf/WebWID/WTB-041108-2256F-95C90, accessed 2010-01-31.

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Figure 5. Rotary kiln in Svappavaara plant.

Due to the high temperature inside the kiln, it is necessary to it

rotating. If it stops, it will sag between the steel rollers that hold it up. This phenomenon is called bananaing.

3.2 FMEA, Failure Mode and Effect Analysis

The failure mode and effect analysis (FMEA) method is a systematic process for evaluating failure modes that occur in a system [10]. The objective is to identify the items that affect the system and may need to be adjusted, to reduce the severity, or fix the specific failure mode. Performing the FMEA gives an overview of the problems that affect reliability. The FMEA analysis depends and builds upon available knowledge of the analysis group; therefore, the composition of the group is import. People with different knowledge of the product should be selected, i.e. people from different departments; e.g., Research and Development, Aftermarket or Service. The FMEA method is used in industry to aid analysis of reliability and availability. It is a qualitative analysis tool for finding relations between a component or a function’s failure mode and the systems failure effect. This method also includes analysis of how to take action to prevent or reduce damage consequences caused by failure modes. There are two basic approaches [11]: the basic Functional FMEA and Hardware FMEA. In the Functional approach, which is used in more complex systems (top-down structure), the system is successively decomposed down to subsystems and equipment level, depending on available information and the objective of analysis. In the

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Hardware approach (bottom-up structure) the system components are considered in isolation in order to establish likely effect on the system. The procedure for the analysis can be performed in the following steps [10].

1. Defining and analyzing the system function 2. Performing the FMEA analysis 3. Reviewing the FMEA worksheets

3.2.1 Defining and analyzing the system function The system definition is an important step to ensure that the engineers

understand the system before proceeding to the analysis of the system’s failure modes and effects. A functional description that contains a definition of the system operational modes should be constructed. Also, the scope of the analysis should be defined.

3.2.2 Performing the FMEA analysis Completing a set of FMEA worksheets involves analyzing the effect

of failure modes on the system. FMEA worksheets can take a variety of forms, depending on the analysis requirements. Severity rankings and failure rates of the failure modes are entered and the criticality of each failure mode on system reliability is evaluated. An example of an FMEA Worksheet is given in Fig. 6. In the left columns the component, function and failure modes are defined. The next steps will be to identify the cause of failure, failure effect and failure detection method. In the right columns the failure rates (Po), severity (S), probability of discovering failure (Pd) are estimated. The risk priority number (RPN) is retrieved by multiplying the values of Po, S and Pd.

Figure 6. Example of FMEA worksheet.

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The estimation of Po (Probability of Occurrence), S (Severity) and Pd (Probability of discovering failure) is based on the following criteria in Table 1.

Table 1. Rating criteria [10] Criteria for assessing probability of occurance (Po) Rating

Unlikely for failure to occur 1

Very low probablitiy for failure to occur 2-3

Low probability for failure to occur 4-5

Some probability for failure to occur 6-7

High probability for failure to occur 8-9

Very high probability for failure to occur 10 Criteria for assessing degree of severity (S) Rating

Unlikely for failure to occur 1

Very low probablitiy for failure to occur 2-3

Low probability for failure to occur 4-6

Some probability for failure to occur 7-9

High probability for failure to occur 10 Criteria for probability of discovering failure mode (Pd) Rating

Indication for failure is always dicovered 1

High probablitiy for failure to be discovered 2-4

Some probability for failure to be discovered 5-7

Low probability for failure to be discovered 8-9

Unlikely for indication to failure will be discovered 10

3.2.3 Reviewing the FMEA worksheet To ensure the usefulness of a performed FMEA, both for the actual

analysis object and for future usage, e.g. as a basis for new FMEA analysis of similar products/processes, the analysis must be summarized and documented. The FMEA worksheets must be reviewed to identify problems that affect reliability and need to be adjusted, and summarized. Recommendations for improvements should be made and areas that need further analysis highlighted. Afterwards, a follow up of the improvements that have been implemented should be performed to ensure that the problems identified have been remedied.

3.3 FTA, Fault Tree Analysis

The Fault Tree Analysis is a logical way of describing relations between an unwanted event in the system and the causes of this event [10]. The development of a fault tree starts by defining an unwanted top event. Then, by analyzing and thinking through what the direct causes of this failure are, the causes are connected by a logical gate. The work

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continues gradually downwards to the basic events at component or detail level. An example of a Fault Tree Analysis is given in Fig. 7.

Figure 7. Example of FTA. (Fault tree analysis).

The different symbols used to represent the causal relationships are

the gates and events defined in Fig. 8.

Figure 8. Logical gates.

The size of the tree is set by the chosen boundary of the system. For

example, is it necessary to extend the analysis to the subsystem level or beyond this to the component level? It is important to define these boundaries with care, so the analysis will not be too broad or narrow to

“Top event”

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satisfy the study objective [11]. The amount of data available to describe the relationships between failure modes and the causes for the failure modes also limits the tree sizes.

The benefits of using fault three analysis are many. Some of them are [11];

1. Attention is focused on a specific, most-critical failure mode instead of total system function. 2. The tree diagram can be used to help communicate the results of the analysis to others involved, such as clients or customers. 3. Once the analysis is performed, the system will be described both quantitatively and qualitatively.

3.4 Angular pumps and motors

In these subchapters, theories describing the function of hydraulic

axial and radial piston pump/motors are documented.

3.4.1 Hydraulic pumps Piston pumps are separated into two main categories, radial and axial

piston pumps. [12]. The function of a hydraulic pump is the direct opposite of a hydraulic motor. The pump can be used as a motor and vice versa. One big difference between radial and axial piston pumps is the orientation of the pistons inside the pump. The radial piston pump has radially oriented pistons and the axial piston pump, Fig. 9 3, has axially oriented pistons. Generally, the piston pumps are suitable for high-pressure applications; this is accomplished with a small piston diameter. The volumetric efficiency of piston pumps is high due to minimal leakages and the pumps operate very well at low rotational speeds. In axial piston pumps, the moment of inertia is in general low, which is beneficial for rapid changes in the rotational speed.

3 O. Isaksson, ”Presentation – Pumps and motors”, Luleå tekniska universitet, 2008

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Figure 9. Axial piston pump.

3.4.2 Hydraulic motors

The same types of pumps as mentioned in Chapter 3.4.1, can also be used as motors. A hydraulic motor, Fig. 10 4, is a mechanical machine that converts hydraulic pressure and flow into torque and angular displacement [14] (p.37-65). A hydraulic motor is usually designed to work with pressure at both sides of the motor [14]. Therefore, the usage of a hydraulic motor is beneficial in areas where the environment is harsh, for example, in marine applications. Due to the variable torque transfer and angular displacement of the motor, a gearbox is not necessary.

4 O. Isaksson, Grundläggande hydraulik, Luleå tekniska universitet, Luleå, 1999

27

Figure 10. Radial piston motor.

Important parameters that describe the performance of a hydraulic motor or pump are the torque, Mm , angular speed, ωm, and the efficiency, ηm of the machine [12] (p. 30-33). A fundamental observation regarding hydraulics is that the pressure is directly proportional to the load. The load of a hydraulic motor gives rise to the torque of the driven shaft. The output torque of the motor is dependent on the pressure caused by the load, the displacement Vpm, and the hydromechanical efficiency and is therefore determined by

hmmmmm VpM ηϕ ⋅⋅∆= . (3.1)

The angular speed, ωm, is dependent on the hydraulic flow, displacement and the volumetric efficiency of the motor. The displacement, Vpm is the amount of fluid that is forced to pass through the machine for every revolution of the shaft [12] (p. 30). The required flow into the motor to obtain a certain angular speed is described by

vm

mm

m

VQ

η

ωϕ ⋅= . (3.2)

The total efficiency of a hydraulic motor,

28

mm

mmtm

Qp

M

⋅∆

⋅=

ωη , (3.3)

consists of two components, the volumetric

m

mm

vmQ

Vϕωη⋅

= , (3.4)

and the hydromechanical efficiency,

mm

mhmm

Vp

M

ϕ

η⋅∆

= . (3.5)

Relations given by Eq. 3.6 to 3.10. can be applied to describe the performance of a hydraulic pump [12] (p. 33). Since the function of a pump is the direct opposite of a motor, the interesting parameters are the flow out of the pump and the torque needed to drive the pump.

mp

pp

p

VpM

η

ϕ⋅∆= (3.6)

vpppp VQ ηωϕ ⋅⋅= (3.7)

The efficiency of the pump is given by

pp

pp

tpM

Qp

ωη

⋅

⋅∆= , (3.8)

pp

p

vpV

Q

ϕωη

⋅= , (3.9)

p

pp

mpM

Vp ϕη

⋅∆= . (3.10)

In a motor the output power is called the mechanical power and is described by

29

mmMP ω⋅= (3.11) The input power is the fluid power

mm QpP ⋅∆= (3.12)

For a pump, the situation is reversed, the mechanical power is the input and the fluid power is the output.

Disregarding the quality of the oil and other external parameters, the lifetime of the motor is related to the pressure and speed. A cubical mean value of the pressure can be calculated with respect to the lifecycle. By using the mean value it is possible to determine the lifetime of the motor,

3

332211

333

3322

3211

31

%...%%%

%...%%%3

nn

nnn

mean

ppppp

⋅++⋅+⋅+⋅

⋅⋅∆++⋅⋅∆+⋅⋅∆+⋅⋅∆=

ωωωω

ωωωω

(3.13) Equation 3.13 denotes the percentage of time at the specific operating pressure according to the lifecycle, where n denotes the number of operating points. For example, the first operating point where n is 1 can correspond to a pressure of 300bar, a speed of 3rpm and 5% of the total lifetime.

3.5 Cooling

In hydraulic drive systems, efficiency losses occur due to mechanical friction and viscous losses, the efficiency loss generates an increase in the oil temperature in the system [14] (p. 170-171). The temperature keeps rising until equilibrium is reached between accumulated heat due to efficiency losses and heat conducted by radiation and natural convection. To increase the lifetime it is necessary to cool the system. Vital components in the system can have limitations, for example, seals, which are often not able to handle temperatures above 90oC. When most hydraulic oils are exposed to temperatures above 60oC, the oil degrades and loses its lubricant properties and has to be changed often. The viscosity of the lubricant decreases with elevated temperature, which gives lower film thickness and lower viscous efficiency. These reasons are important to consider at the design stage to achieve the appropriate cooling system. The cooling effect is achieved by using a surrounding medium like a gas or a liquid with lower temperature than the oil.

30

Commonly used liquids are water or oil and cooling with gas is mostly done with air. The cooling capacity in, for example, an air-oil cooler depends on the air and oil flow through the cooler and also the temperatures of the media. Air cooling systems often include a fan to increase the cooling effect. The cooling capacity is specified in W/°C, since the amount of heat exchange is proportional to the temperature difference. The cooling capacity increases with the size of the cooling element. By studying the total efficiency of the system, a good approximation of the needed cooling capacity is gained. The cooling effect is described by:

( ) intloss PP ⋅−= η1 (3.14) In Eq. 3.14 [12] (p. 167-168) of the cooling effect, the heat dissipation through natural convection and radiation are not included, but must be considered in order to achieve an effective cooling system. Natural convection is a theoretically difficult area to study and when complex area structures are considered, this can be seen as an approximation. One approach to approximating natural convection and radiation of energy is presented by Hermansson [15] and follows below. Heat transfer due to natural convection is given by

TAQconv ∆⋅⋅= α& (3.15)

where α is the heat transfer number. α is dependent on the conductivity number, the characteristic length and Nusselt’s number (dimensionless) according to

D

Nu⋅=

λα (3.16)

Nusselt’s number for vertical areas is determined by

33.0)Pr(13.0 realV GrNu = (3.17) and for horizontal cylindrical areas by

33.0)Pr(11.0 realH GrNu = (3.18)

Grashof Prantdl’s real number, realGr Pr , is geometry and temperature dependent and is calculated by

31

3PrPr DTGrGr Tmeanreal ⋅∆⋅= (3.19) where T∆ is the temperature difference between the device surface and the environment, i.e.

envsurf TTT −=∆ (3.20)

D is the characteristic length and TmeanGr Pr is a tabulated dimensionless number which can be found in fundamental thermodynamic literature, e.g. Hermansson [15] and is determined by the mean temperature between the surface and the environment:

2

envsurf

mean

TTT

+= (3.21)

Radiation of energy is given by

4TAQrad ⋅⋅⋅= σε& (3.22)

where ε is emissivity of the surface, σ is the Stefan Boltzmann constant, T is the surface temperature and A the surface area. When both natural convection and radiation of energy are calculated they are summed to determine the total heat dissipation without forced cooling:

radconvtot QQQ&&& += (3.23)

When designing a mechanical device it is suitable to write a script in, for example, the Mathworks program Matlab to simplify the calculations over a wide span of surrounding and surface temperatures and compare the heat dissipation with the power losses to be able to dimension the cooling system correctly.

3.6 Filtration

Hydraulic systems operate at considerable pressure, and in order to provide the efficiency for such pressures, pumps and motors have narrow tolerances [16]. The reliability of a hydraulic system depends a lot on the cleanliness level [12] (p. 163-166) of the oil. The objective with a filter is to reduce the contamination and protect the included components against

32

wear. Different variables influence the degree of wear and important factors to control are the type and size of the contamination and the number of particles. The size of the particles and the filter mesh size are measured in µm and refer to the dynamic clearance. The absolute filtration level is where 100% of a certain size of particle (and larger) is retained by the filter [16]. Different components in the system require different cleanness and the system should be analyzed to determine the allowed size of particles that are not harmful for the system. Filter elements are made of different materials like paper and metal. The filter elements are folded to achieve a high filtration area and minimize the size of the filter. Three common types of filters are suction filters, pressure filters and return filters. Variables like dynamic clearance, pressure drop, ease of mounting and placement in the hydraulic system affect the choice of filter. The contamination level of the filter is indicated optically or optically-electrically.

3.7 Tribology in hydraulic systems

There are three main types of oils used in the industry; for example, transmission oils, motor oils and hydraulic oils [12] (p. 1-13). Each type of oil has specific properties that make it suitable for a certain application; for example, transmission oil is optimized to withstand high pressure and motor oil is developed to reduce friction. The fluid in a hydraulic system has three main functions: to transfer the power, to conduct the heat and contaminations away and to lubricate the components in the system. One of the most important properties is viscosity. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. The viscosity is strongly dependent on the temperature and decreases (thus making the oil thinner) with increased temperature [12] (p. 6-10). If the viscosity is to low, it will lead to a higher wear rate and premature failure. The hydraulic fluid can be either a mineral oil or synthetic oil. Mineral oil is the most commonly used because of its more simple composition and low cost [17]. Most industrial oils have additives such as anti-oxidants [17], VI-improver and anti-foam additives to mention the most common [17]. The type of lubricant chosen has significant impact on the lifetime performance and reliability of a machine [17].

3.8 Sensors

Electronic control systems have long played a key role in industry [18]. These control systems use electrical sensors to monitor processes. By analogue or digital electrical signals from the sensors the control systems are fed with information from different parameters in the system.

33

When monitoring is performed on a hydraulic system some important parameters to measure are commonly temperature, pressure and angular speed. The measurements of these parameters are mostly performed by analogue sensors, but digital sensors are also now widely used. A sensor can be defined as three function blocks, as in Fig. 11 [18].

Figure 11. Sensor.

The probe is the part of the sensor that is directly affected by the parameter. In a pressure sensor, for example, it can be a membrane that deforms due to the pressure from some media. The transducer converts the function of the probe into an electrical signal. For example, in a pressure sensor the deformation of the membrane affects a strain sensor that converts it into a change in resistance. The internal signal treatment is electronic equipment placed in direct contact with the transducer. This may include amplifiers that are needed to produce a useful output signal. The sensor has a protective cover that surrounds the function blocks and protects against harsh environments, but it has to have a “window” to allow the sensor probe to be in contact with the media to be measured. This window should also have the ability to protect against the environment. Several different words are more or less synonymous with sensor. Other often used names for sensor are transmitter, or detector. Transmitters are those sensors that contain an internal signal treatment circuit and produce one of the industry standardized outputs. Detectors are those sensors that are commonly used for measuring sound or light and the output is often a binary output (I/O) for a certain action, for

Input

“Window”

Transducer Probe Internal signal treatment Output

Feed supply Current/Voltage

Sensor

34

instance, in a glass breaking detector in an alarm system. Sensors that produce a binary output signal are also often called switches.

35

4. Analysis and description of hydraulic drive systems

This chapter provides an overview of the parts and a description of

the functionality of the systems.

4.1 Overview of system components.

According to a written statement from Bengt Liljedahl, Manager Design at HDAB, the hydraulic kiln drive system from Hägglunds was comparable to the system tested in the HDAB laboratory with regard to failure modes. The only differences would be the number and size of motors and pumps. To verify this statement, an analysis of both systems, mainly at function level, was performed. Hydraulic schemes and technical specifications of both systems were analyzed and compared. In the following chapter the analyses of both systems were performed to give an overview of the most important differences between the drives.

4.2 Tank test

The tank test at the HDAB laboratory was built as two separate and connected systems. One system worked as a pump and motor circuit, as in the kiln drive case. The other circuit had the opposite function in order to simulate different load cases, and to return energy back to the system, i.e. to make the system regenerative. An overview of the components in the test rig can be seen in Fig. 12, and in Appendix C, a hydraulic scheme is shown.

Figure 12. Tank test at HDAB laboratory.

CA100 (Motor) CA100 (Pump)

Reservoir 100L Reservoir 300L

Electric motor

SP355 (Motor) SP355 Pump

PEC Unit

36

In the PEC unit, Fig.12, two tandem mounted pumps from Bosch-Rexroth were driven by an electric motor from ABB [20]. The system was a closed loop system, which means that the outlet oil from the motor was led back to the inlet of the pump. In an open loop system the oil outlet from the motor is fed back to the tank [16]. The system was regenerative, which means that some of the power required to drive the simulated load was returned to the system by one of the tandem mounted pumps. As mentioned, one of the pumps had the opposite function, i.e. it worked like a motor. The power that the CA100 motor produces forces the other CA100 motor to work as a pump, which in turn powers the SP355 motor. This gives less waste of power than working against a load that dissipates the achieved energy outside the system. The purpose of this lab test in Hägglunds case was to perform a long-life test of the oil [21]. HDAB wanted to investigate the effect on the oil properties at reduced oil tank level. Therefore, the pump-motor circuit had a down-scaled oil tank of 100 litres and a full-scale 300-litre tank on a corresponding circuit. To monitor the condition of the system a total of 24 sensors were mounted. The sensors that belong to the load circuit are not considered, since this circuit was not of importance for monitoring in the scope of this thesis. In the hydraulic scheme in Appendix C, the positions of the sensors can be seen.

4.2.1 Cooling circuit The cooling circuit in the tank test was designed to use some of the

exchange fluid from the closed loop circuit. The oil from the closed loop circuit was drained by the flushing valve on the pump and also by normal internal leakage in motor and pump. The closed loop was refilled by the feed pump with cooled oil. The flushing valve delivered the cooling flow to the cooler and from there, a constant cooling flow, also called flushing flow, was distributed to both motor and pump casing. The cooler was an oil-water cooler and the coolant flow was activated by a water valve. The water valve was controlled by the cooled oil temperature directly after the cooler and also by the temperature inside the motor house. The aim was to keep an even temperature inside the motor house, regardless of power usage.

4.2.2 Hydraulic motor Compact CA100 The hydraulic motor used in the tank test was a CA100 [20], which is

a radial piston motor manufactured by HDAB. This CA100 motor is a small, lightweight motor with the same durability as other Hägglunds motors. The power to weight ratio is comparable to other Hägglunds motor concepts [22]. The specifications for this motor can be seen in Table 2 and a CAD model of the motor is shown in Fig.13.

37

Table 2. Motor specifications [15].

Figure 13. Cad model of CA100 hydraulic motor.

4.2.3 Hydraulic Pump Bosch-Rexroth SP355 The pump used was a SP355 axial piston pump from Bosch-Rexroth,

seen in Fig.12. It had a variable displacement and the maximum displacement was 355 cubic centimetres [20]. The pump had a built-in boost pump that fed the system, which compensates for system leakage fluid flow in the closed loop circuit. The boost pump also fed the flushing circuit. Flushing of the pump casing and bearings are required when the pump operates under high power usage. Displacement is controlled by an electrohydraulic control with proportional solenoids. The pump displacement is proportional to the solenoid current, so the required flow can be adjusted variably [23].

38

Figure 14. Bosch-Rexroth axial piston pump SP355.

4.2.4 Electric motor A 315 kW, 4-pole electric motor from ABB was mounted to power

the two tandem-mounted pumps [20]. The motor runs at constant speed, 1485 rpm [20].

4.3 Kiln drive

A drive system for a rotary pelletizing kiln can be built in different configurations, depending on the system requirements. As an example, a specific drive situated in Svappavaara was chosen. This drive had an installed power of 640 kW [24] [25]. The operating cycle of the drive can be approximated to be smooth. The kiln drive mainly runs at constant speed (20-22 rpm), and pressure (120-140 bars), [26], and the only stops are for planned maintenance work. The kiln drive had the same function as the tank test, except that the kiln drive had more components, since the requirements for the system are different, i.e. greater output. In Fig.15 a simplified image that represents the kiln drive system is seen. In Appendix D, hydraulic schemes over one of the PEC units, the flushing circuit and a hydraulic assembly circuit of the entire system are presented.

39

Figure 15. Simplified image of Kiln Drive system.

4.3.1 PEC unit The kiln drive system had three PEC units (power units) mounted

[24][25], of which one was an auxiliary PEC unit. Inside the PEC units the electric motors, pumps, tank and filters are mounted (see Fig. 16).

Figure 16. PEC Unit.

Oil cooler

Hydraulic pump

Electric motor

Filtrations units

MB800

Auxiliary PEC

PEC Unit 1

PEC Unit 2

Rotary Pelletizing Kiln

Connection block High pressure

Auxiliary PEC Connection block. Low pressure

40

For each hydraulic motor there were two electric motors and two

hydraulic pumps delivering the required flow and pressure to the motor. The auxiliary drive was connected to the hydraulic system in case of sudden power loss. The auxiliary unit starts up as soon the main drive shuts down, and was also to be used to run the kiln during maintenance work. The kiln oven is a critical application; therefore, if for some reason the drive shuts down, there has to be an auxiliary drive to empty the kiln of iron ore pellets and to keep it rotating while cooling down. If this doesn’t work, the kiln will be damaged, and an unplanned stop is catastrophic to the customer, since downtime is costly. Due to the heat inside the kiln, as mentioned before, the pellets will clog up and the whole tube will bend.

4.3.2 Flushing circuit The flushing circuit for the motor casing was a separate circuit [24],

Appendix D, which consisted of two parallel mounted pumps that delivered a constant oil flow from the reservoir, passing only through a cooler and directly to the motor casing. Here, the coolant flow through the cooler was activated by a water valve that was controlled by the tank temperature. The coolant media in this case was water. The hydraulic pump casings were not flushed in the kiln drive case, due to low power usage.

4.3.3 Hydraulic motor Marathon MB800 Two Marathon MB800 [19] motors where used in the Svappavaara

kiln drive. This motor has a robust design and is suitable for harsh environments and high-torque applications. The specifications for the motor are given in Table 3.

Table 3. Motor specifications MB800 [27].

4.3.4 Hydraulic Pump Bosch-Rexroth SP500 The hydraulic pumps used in the kiln drive were of the same model

and had the same functions as in the tank test. The displacement of the pumps in this drive was 500 cubic centimetres [24][25].

41

4.3.5 Electric motor For each pump, one 160 kW electric motor from ABB is mounted [24]

[25]. The system has four pumps and therefore four electric motors. The electric motor runs at constant speed 1486 rpm during operation.

4.4 Spider control system

The Spider is a control system used to control the drive system [28]. The control system has been developed by Hägglunds Drives AB and is used in many of their applications. The Spider can be configured by the front panel or via a PC interface. The compact design of the Spider makes it easy to mount on or close by the hydraulic system. Sensors mounted on the hydraulic system provide the Spider with the necessary information to be able to control the stroke of the hydraulic pump and thereby the angular speed or pressure. The sensors, together with the Spider, make it possible to control and log events in the hydraulic system.

4.5 Summary of kiln drive and tank test analysis

Due to the output requirements of the systems, main differences between these two systems were mainly the size and number of components. The differences in the size of the motors and pumps would, according to HDAB, not affect the probability for the same type of failures to occur in the tank test as in the kiln drive.

Failure rates may differ due to the difference in number of

components, but this was not considered, since the aim was to investigate whether a specific failure mode would occur or not. The kiln drive and tank test analyses where performed in parallel with the FMEA performed in Chapter 5.1. Therefore, to keep the scope of the analysis at a feasible level, the focus was on identifying and comparing the parts of the system that were included in the reported failure modes.

The biggest differences between the tank test and the kiln drive can be

distinguished as follows. 1. In the tank test the cooling/flushing flow for the motor casing was fed from the main pump. The flushing flow for the motor casing was taken directly from the loop circuit, but the cooling of the closed loop circuit worked in the same way for the tank test as for the kiln drive. In the kiln drive, only the motor casings were flushed, and this was done by the

42

separate cooling circuit. Due to the low power usage in the kiln drive case, the main pumps casings did not have to be flushed. 2. The kiln drive had an extra mounted auxiliary drive as an emergency circuit, which can also be used for running the kiln during maintenance work. 3. The kiln drive had also an emergency function, a so-called swing-down function. This function prevents the motors from cavitating if the kiln starts to rotate in the wrong direction for some reason. The auxiliary drive and the swing-down function did not exist on the tank test and failure modes occurring in these applications on the kiln drive can therefore not occur in the tank test. Although the cooling/flushing circuits between the two systems are not similar, some failure modes occurring in the kiln drive can be expected to occur in the tank test, for example, dysfunctional water valves. When comparing the monitoring equipment of the two systems, the tank test had more sensors mounted to monitor the condition of the system.

43

5 Identifying system failure modes and new measurement points

This chapter describes the work done to identify the kiln drive system

failure modes and the work in finding necessary measurement points and

rules for controlling the system in a way that will increase reliability and

availability.

5.1 FMEA Analysis

The FMEA work was performed in parallel with the analysis of the systems with regard to differences in component configuration and function. The aim with the FMEA was to find the failure modes occurring in the kiln drive. Information was collected by interviewing representatives from HDAB who were in some way involved and had knowledge regarding the function of the kiln drive. Attempts were made to collect statistical material regarding failure reports on the kiln drive, but no documentation was available. Instead, the interviews had to provide a basis for the FMEA.

Before the interviews, a questionnaire was sent to the participants. The

questionnaire was supposed to support and prepare the participants for the interviews. The questions were aimed at identifying the most common errors that occur in the kiln drive, and were formulated in such a way that they could afterwards be used in completing the FMEA. To gain more information, the questions invited the participants to bring their own material to the interview. An example of the questionnaire can be found in Appendix E. The interviews performed were semi-structured, which opened up the possibility for the participants to discuss the subject. The FMEA was intended to identify and investigate existing problems rather than a full system analysis at detail level. The interviews were conducted individually with each participant. The interviewees were representatives from different departments within HDAB: Manager Design, Bengt Liljedahl; Manager Development Controls, Arne Byström; Manager Application Development, Gunnar Ivarsson; After Market Business Development, Christer Eberger; Design Engineer/PhD student, Daniel Nilsson, and Service Engineer, Lennart Johansson.

After the interviews the completion of the FMEA was initiated. By

reviewing the recorded interviews the material was analyzed, interpreted and compiled into a FMEA worksheet. The ranking of the failure modes, both severity (S) and probability of occurrence (Po) were based on the information from the interviews. When the worksheet was completed the next steps were to analyze and summarize the failure modes and make recommendations for improvements, which were done for each specific

44

failure mode. Each failure mode was also analyzed to find which new measurement points are necessary to predict, discover and prevent failures before any damage occurs.

5.2 FTA fault tree analysis

The purpose of using and performing the FTA together with the FMEA as an evaluation method was to give the motor-specific failure modes greater attention. Another benefit of constructing the fault tree was to gain better material to use in discussing the actions needed to monitor the system. The FTA was performed on the motor-specific failures, which are of great importance to HDAB, since these types of failures are critical. The FTA was based on the information from the interviews and the complementary material [29] provided by HDAB regarding failure types on their hydraulic motors. Four fault trees were constructed. The process started by defining the unwanted “top event” and was worked down to the point where the necessary measurement points were identified. An example can be seen in Fig.17.

Figure 17. FTA. Finding new measurement points

“Top event”

45

5.3 Suggestions for new measurement points

Based on the FMEA review and FTA, the parameters necessary to monitor were identified. The failure modes that were expected to occur in both systems where compiled into a list together with the present and suggested measurement points. For each failure mode, a note was made for every measurement point that was needed to prevent, predict or discover the failure mode. A second list was thereafter compiled, containing the most frequently identified parameters to be monitored on the system. The parameters most frequently used were given the highest rank and the second list was used to show which parameters were the most important to monitor in the kiln drive case. A simplified hydraulic scheme for the kiln drive was then developed for noting the locations of the measurement points and corresponding sensors.

5.4 Rule design for system monitoring

Certain relations between measurement points were found during the work. The idea was to use these to discover some of the failure modes at an early stage; these could be used to predict the occurrence of a specific failure mode. The rules where developed by analyzing the system’s failure modes and discussing further with the representatives at HDAB. The list of the new measurement points determined from the interviews was added together with a complementary questionnaire that was sent out and iterated with HDAB. Since all the interviewed representatives had considerable experience of HDAB hydraulic systems, their knowledge was considered to be of greatest importance when designing the rules. The complementary questionnaire can be found in Appendix E.

When gathering knowledge from the participants, the rules needed to be written down in a specification which explains what these rules will do and how they are meant to work if implemented in a monitoring system. The explanation of the rules was chosen be written as non-executable simplified computer code to achieve a common understanding, i.e. to be able to communicate more easily with the participants, regardless of their prior knowledge in programming.

The aim was to show the structure, how the parameters will be used

and what actions will be taken. The final result was presented to the representatives at HDAB and discussed and iterated for validation.

46

6. Analysis of results and rules for system monitoring

In this chapter the results of the FMEA and FTA are presented. The list of suggestions for new measurement points and the list of rules for

predicting and preventing some of the failure modes are also presented.

6.1 Summarizing the FMEA

A review for each stated failure in the FMEA worksheet was

conducted. The following subchapters describe each failure mode from the FMEA. Recommendations were made as to what actions to take to solve or alleviate the specific problem. The FMEA worksheet can be seen in Appendix F. The ratings of the probability of occurrence and severity in the FMEA worksheet are based on the information from the interviews and are, due to lack of documented statistical information, only estimated values.

6.1.1 Shaft seal Hydraulic pump.

The shaft seals on the hydraulic pumps on the specific kiln drive have had to be exchanged approximate 20 times during a 6-year period. A small amount of oil (compared to the tank level) has leaked out inside the PEC unit. This type of leakage does not affect the drive critically, but is not good from an environmental perspective.

Recommendation: When a small oil leak occurs this is hardly

noticeable on the oil level in the tank, since the oil volume that has leaked out in comparison to the tank volume is small. Therefore, a complementary way of discovering this type of problem is to measure the temperature inside the house of the pump and also the house pressure, since, according to HDAB, this affects the life length of the seal. Excessively high temperature in the house contributes to the ageing of the seal. The idea is to prevent this type of problem at an early stage, since the failure of these seals is often a result of some other problem, for example, high pressure or temperature. The cleanliness of the oil is also important, since particles may wear down the seals. The surrounding environment is also of importance, but this must be considered when the system is designed. Different types of seals have been developed for different types of surrounding environment. According to the service engineers at HDAB, the seals leak differently from one drive to another. By complementing the system with sensors for measuring the above mentioned parameters, indications of impending failure could be given at an early stage. Otherwise, measuring moisture in the bottom of the PEC

47

unit could be taken into consideration. A more thorough investigation of why these seals fail on the kiln drive should be performed.

6.1.2 Water valve A common failure on the kiln drive is water valves that do not open. If

the valves do not open, the cooling of the system fails. As a consequence of a valve not opening, an alert will eventually be issued, and if the problem continues, the drive will eventually shut down due to high temperature in the system. One of the reasons for this failure, according to HDAB, is the quality of the coolant water, which leads to malfunction of the control of the valve. Bad connections and internal errors can also cause the valve to malfunction.

Recommendation: To discover the failure the temperature of the oil

before and after the cooler should measured. Placing the temperatures before and after cooler in relation with the surrounding temperature and the coolant temperature, a measure of the cooling effect could be achieved. In the kiln drive case the oil flow is constant through the separate flushing circuit. By investigating the relations between the above mentioned parameters, other considerable failures could be detected. For example, if the cooler is about to clog up. Placing higher demands on the coolant quality would probably reduce these types of problems.

6.1.3 Accumulator / Swing down Function Failures have been detected on two occasions at kiln drives; in one

case the failure was due to incorrect mounting of the accumulator. The accumulator was connected to the wrong port of the pump, which led to a complete failure of the motor. The accumulator’s task is to pressurize the inlet at unplanned stops and prevent the drive from uncontrollably running in the opposite direction. The pressure prevents cavitation in the motor. Otherwise, there is a chance that the piston in the motor hit the cam ring with too much force, which leads to damage or complete failure.

Recommendation: According to HDAB, a condition for start-up of

the system is enough pressure in the accumulator. But this pressure can drop over time and the function will cease to work. The question as to how often this pressure is to be measured remains. Perhaps periodical inspection of the pressure should be routinely performed.

48

6.1.4 Sensors Some problems with, e.g. level sensors have been encountered. The

sensors have stopped working or have been sending incorrect signals. The cause can be sensors which are not capable of withstanding the environmental conditions. Due to vibrations, dirt or heat the sensor becomes defective. A probable scenario for this failure can also be incorrect mounting.

Recommendation: If a sensor becomes defective the result is either

no signal or an incorrect signal. The consequences can be unplanned stops, depending on the task of the sensor. Discovering if a sensor is not working correctly is problematic, since no complete diagnosis of sensor functionality is performed by, e.g. the Spider. If the signal is lost completely can be detected by the Spider, but determining whether the signal is incorrect is more difficult. If a diagnosis performed by the Spider where the current is measured to discover possible short-circuiting, then it would be easier to discover the above mentioned problem.

6.1.5 Control of displacement on hydraulic pump The hydraulic pump has a variable displacement, which means that

the flow and therefore the angular speed of the hydraulic motor are controlled by this. By controlling the angle of the swash plate in the pump the speed is set. If there are disturbances to the solenoid which control the swash plate, this leads to an incorrect or default control of the pump. Vibrations and incorrect mounting can cause this type of failure. The result can be reduced speed or a complete stop of the drive.

Recommendation: Measuring contact resistance would indicate bad

connections, which is done by the Spider. The Spider gives an alert when there is high resistance and the current consumption reaches 2 times the required. Furthermore, predicting this type of failure can be difficult, as there is a probability of these failures occurring sporadically.

By measuring the current consumption to the solenoid versus the angular speed on the hydraulic motor, and placing them in relation to constant high pressure and stable temperatures, this could give a hint that this type of failure has occurred.

49

6.1.6 Spider If the Spider is subject to physical impact, there is naturally a

possibility that the system will shut down. Other failures which have occurred are overload when controlling too many sensors or valves, but this is usually not a problem. These failures are affected by the human factor; but here, the same failures for sensors can also be applied in this case. There are always possibilities that electrical components break or circuit boards become defective. In China, for example, transients on the electricity grid are common. Therefore, sensitive electrical components are subject to a greater probability of breakage. A check of the net before installation would probably predict and prevent these problems. If the Spider stops working for some of these reasons, the system will also shut down. This doesn’t damage the system, but the drive stops, which is critical.

Recommendation: These are rare failures and occur mostly at start-

up of new installations. One way of detecting a failure with the Spider can be to measure the current consumption to the Spider. This failure will also be detected by the loss of information from the sensors. How the monitoring of the Spider should be done depends a lot on the design of the total system.

6.1.7 Tubes and Hoses Hoses age with time, tubes suffer fatigue, and the chance for

fracturing increases. Wear on hoses can arise if they are incorrectly mounted and pulsations or vibrations occur in the system. This also subjects the tubes to fatigue. A fracture can be discovered by measuring the pressure. Today, this is done by pressure switches in several places in the system that alert when it reaches a defined limit.

Recommendation: Fractures on tubes and burst hoses are failures

which are rare on kiln drives. Pressure will be lost in the part of the system that suffers the fracture. Apart from the pressure switches, this type of problem will probably be detected by the other pressure sensors. Otherwise, periodic visual inspection of the hoses and tubes could prevent fracture.

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6.1.8 Electric motor Common failure in an electric motor is due to overload; the motor

circuit’s temperature will become too hot, resulting in damage. Some other common reasons can be high power usage in combination with dirt or excessively high ambient temperature.

Recommendation: By measuring the temperature in the circuits,

action can be taken when the alert limit is exceeded. Measuring temperature in the electric motor’s circuit is standard on most motor brands today.

6.1.9 The oil in the system If the oil temperature in the system has for some reason been too high,

the oil starts to degrade and loses its lubricating properties. If the maintenance is not done properly, this is dangerous. It is important that the filtration of the oil works well, so that particles are removed before they damage components in the system. By taking oil samples and analyzing the particles present in the oil, the location of damage in the system can be revealed. Cleanliness when performing repair or maintenance work is important. Insufficient maintenance will, with time, result in abnormal wear and eventual motor failure.

Recommendation: Measuring the pressure before and after the

filtration unit would indicate when the filter is about to clog up. But, if the filter is about to clog up, excessive wear has already occurred. Completing the pressure drop measurement over the filter with a particle counter mounted parallel to the filter would indicate whether particle content in the system has increased. If these measurements are taken, motor wear could be detected at an earlier stage. It is also important to measure and keep the system temperatures and the oil saturation within recommended levels. The temperature needs to be measured to compensate for pressure drop due to viscosity.

6.1.10 Motor damage Common damage on a hydraulic motor is mostly due to normal wear.

But in the kiln drive case there are no reported cases of motor damage. However, this is still an important consideration, as there is always a possibility that such damage will occur. More critical damage such as

51

fatigue damage, i.e. pitting in the cam ring, fatigue fracture of cylinder liner and seizure are often related to high pressures, high temperatures and contamination or material defects (inclusions). Failures associated with material defects (inclusions) are critical and hard to detect. If fatigue damage occurs, the outcome is usually catastrophic. Temperature-dependent failures can be easier to predict by controlling the temperature in the system and keeping the power usage within allowed limits. Seizure and risk for leaking shaft seals are partly dependent on the angular speed. Particles in the oil are an issue when it comes to fatigue, as they create indentations in the material surface. In these areas there will be a stress concentration that initiates pitting.

Recommendation: By measuring several system temperatures, for

example, drainage, loop and surrounding temperature, the system can be monitored so that it does not exceed the temperature limits in the circuits, motors or pumps. Excessively high temperature affects the oil. The oil will become too thin, the oil carrying capacity becomes weaker and it will break through the oil film. Looking at the relation or trend between the drainage temperature and surrounding temperature and loop temperature could indicate possible wear in the motor, since an internal leakage gives rise to an increase in temperature inside the motor. But this means that other parameters must be constant, for instance the flushing flow and temperature. Leakage could also give rise to increased stroke angle on the pump, since it has to deliver larger flow to retain the same speed. If internal leakage occurs, the oil will be fed out into the motor or pump house and the drainage flow will increase. This increase in flow could give rise to a higher house pressure, which is harmful to seals. To have a reliable system the relations between these parameters should be further investigated.

6.2 FTA

The fault tree analysis conducted produced four fault tree diagrams (Appendix G). Each FTA describes the motor-specific failure modes, where the aim of the diagrams is to find necessary measurement points to increase the availability of the kiln drive.

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6.3 Suggestions for new measurement points and rule design for system monitoring

From the FTA and the FMEA a list of the parameters to be measured was created. These measurement points were marked out in the simplified hydraulic scheme, representing the kiln drive system (Fig. 18) to more clearly show the placement of the measurement points. Figure 18. Simplified hydraulic scheme kiln drive

M

MM

1. A

ngula

r S

pe

ed

2. P

art

icle

coun

ter

3. M

eta

l S

CA

N

4. P

um

p h

ouse tem

pera

ture

5 M

oto

r hou

se tem

pera

ture

6. T

em

p o

il, b

efo

re c

oole

r

7. T

em

p o

il, a

fte

r coole

r

8. T

em

p c

oolin

g m

edia

,befo

re c

oole

r

9. T

em

p c

oolin

g m

edia

, after

coole

r

10.

Surr

oundin

g tem

pera

ture

11.

Hig

h p

ressure

12.

Filt

er

indic

ato

r sw

itch

13. C

urc

uit tem

pera

ture

14. T

ank tem

pera

ture

.

15. P

um

p h

ouse p

ressure

16. M

oto

r house p

ressure

17. T

em

p e

lectr

ic m

oto

r

18. T

ank level

19. Low

pre

ssure

20. O

il sa

tura

tion

21. S

troker

angle

MS

Reserv

oir

Surr

ound

ing tem

p

11

13

5

16

98

3

2

CS

19

12

12

7

6

14

18

15

4

21

17

Symbols

Tem

pera

ture

sensor

Pre

ssure

ga

uge

Check v

alv

e

Pum

p v

ariable

dis

pla

ce

m.

Moto

r fixed d

ispla

cem

.

Filt

er

with c

onta

min

ation c

trl

Coole

r

Part

icle

counte

r

Meta

l S

CA

N

Ele

ctr

ic m

oto

r

Tan

k le

vel sw

itch

MCS

MS

Ta

chom

ete

r1

Hyd

raulic

moto

r

Hydra

ulic

pum

p

Simplified Hydraulic Scheme

Kiln Drive

Flu

shin

g c

urc

uit

Loop. H

igh p

ressure

sid

e

Loop. Low

pre

ssure

sid

e

Dra

inage c

urc

uit

Charg

e

pum

p

10

Coolin

g w

ate

r in

let

Coolin

g w

ate

r outlet

Flow control

20

53

A ranking list was also created in order to show which identified measurement points were to be most frequently used to predict, prevent and discover the failure modes. The rules developed from the complementary questionnaire and other relations found from the FTA, FMEA and system analysis where presented to and validated by engineers at HDAB: Manager Design, Bengt Liljedahl; Manager Development Hydraulic Systems, Berth-Ove Byström; Manager Development Controls, Arne Byström, and Manager Hydraulic Laboratory, Anders Westerlund. The ranking list is presented in Fig. 19.

1 Angular speed 7 x

2 Particle counter 6 x

3 Metalscan 5

4 Temp oil, before cooler 5 x

5 Temp oil, after cooler 5 x

6 Temp cooling media, before cooler 5

7 Temp cooling media, after cooler 5 x

8 Surrounding temp 5

9 High pressure 5

10 Filter indicator 5 x

11 Temp house pump 4

12 Temp house motor 4 x

13 Temp in the loop 4 x

14 Temp tank 2 x

15 House pressure pump 2 x

16 House pressure motor 2 x

17 Low pressure 2 x

18 Tank level 1 x

19 Temp electric motor 1 x

20 Oil saturation 1

21 Stroke angle 1 x

22 Current electric motor 1 ?

23 current spider 1

Most used parameter to predict and discover failures. | Rank| |Tank test|

The x represents what

is measured at the

tank test today Figure 19. Ranking list. The rules for system monitoring listed below are to be translated into the stream query language of the software implemented. The rules must be tested in reality and iterated to be optimized.

54

1. Outer leakage, wear.

Rule 1: IF Thusp OR Thusm increases from setpoint. % setpoint for thusp and thusm AND Tomg and Tloop are constant. AND The angular velocity decreases but Phög and Ställvinkel are constant. % Depends on the type of feedback. OR angular speed and Phög are constant but Ställvinkel has

increased. % Depends on the type of feedback.

**AND there are an increase of particles in the system. **AND deltaP over the filter and Phus has increased. **AND Phus has increased. THEN a outer leakage is possible, depending on where the temperature increased. Inside pump or motor?

Rule 2: IF the cooling effect increases with time (Deviates from setpoint) % setpoint for cooling effect. AND Tomg is constant, AND Thusm or Thusp has increased. THEN an efficiency loss is possible. This efficiency loss can be

due to outer leakage (from high pressure/low pressur