A Case Study: Monitoring Heat Exchanger Basedon Vibration Sensors and Nondestructive Testing

TechniqueMulan Pang∗, Lei Shu∗, Jianjian Lu∗, Xufeng Zhu∗, Joel Rodrigues†∗Guangdong Petrochemical Equipment Fault Diagnosis Key Laboratory

Guangdong University of Petrochemical Technology, China†Department of Informatics, University of Beira Interior, Protugal

Email: {mulan.pang, lei.shu, jianjian.lu, xufeng.zhu}@lab.gdupt.edu.cn, joeljr@ieee.org

Abstract—Flow-induced tube vibration could cause seriousdamage on large-scale circulating-water heat exchanger in fac-tories, e.g., atomic reactors, nuclear power plants, petroleumand chemical industry. Therefore, efficient inspection technologyis very important for heat exchanger’s blockage and leakage.The existing detection technologies, like temperature test andpressure test, are very easily affected by extraneous factors,e.g., temperature and pressure, while the nondestructive testingtechnology based on vibration sensors is free from above limits.In this paper, we use vibration sensors to detect blockageand leakage of the circulating-water heat exchanger. Accordingto the experimental results, it is found that when detectingheat exchanger’s blockage and leakage with vibration sensors,environmental vibration from the working machines has influenceupon the result, while pump vibration and vibration producedby liquid flowing through regulating-valve make little difference.Nevertheless, fault diagnosis of large-scale circulating-water heatexchanger, which uses nondestructive testing technology basedon vibration sensors, needs to be further improved.

Index Terms—Vibration sensor; Fault diagnosis; Heat ex-changer; Time domain analysis

I. INTRODUCTION

Heat exchanger is a kind of device which is used totransfer heat from one fluid to another, and ensure the specifictemperature that is required by the process of medium andimprove the energy utilization. Heat exchanger is used in alarge variety of industries [1], e.g., power plants, refineries,paper mills, HVAC, food and beverage. As shown in Fig. 1,the tube clearance of the heat exchanger is small, as it hasstrict requirements of the quality of fluid, e.g., the pH levels,deposits, impurities. The heat exchanger transfer condition,directly affects entire equipment stable operation.

Heat exchanger is used in Maoming ethylene plants1, wherethey cool themselves by pumping cooling water, e.g., thesea, river, or factory water, through the tubes [2]. However,water have high levels of beneficial impurities and mineral,which can easily cause blockage in heat exchanger. Once theblockage and fouling occur, the speed of fluid in the tubes willincrease rapidly, and the pressure difference between inner

1Ethylene plant of Maoming, which is situated in the west of GuangdongUniversity of Petrochemical Technology, and the main products and sales ofpolyethylene and SBS, was founded in May 1995.

Fig. 1. The brief diagram of heat exchanger

and outer surfaces of the heat exchanger tubes will lead toobvious vibration. If the blockage problem is not solved intime, it is easy to cause more serious problems, e.g., makingfrequent stop device, decreasing the quality of product, wastingthe producing time, and high cost of purchase. It is foundthat, besides blockage or fouling, the tubes inside the heatexchanger can be seriously damaged by erosion [3] and flow-induced tube vibration [4]. Therefore, it is very important forthe petrochemical industries to inspect and solve the problemof the heat exchanger leakage in time.

To improve the accuracy of the inspection, the key issueis to find the blockage and leakage spot with all possiblehaste, therefore, repairmen of petrochemical industries can beable to maintain the equipment without delay, thus reducingunnecessary losses. To improve the function of the heatexchanger, the speed of fluid should be increased as soon aspossible. However, the faster the rate is, the more easily tubevibration is induced [5]. Considering the vibration when thefluid flows through the tubes, this paper proposes a method todetect heat exchanger tube failures based on vibration sensors.The advantages of this methods are listed as follows:

• Convenience: the usage of a varied vibration sensors andanalysis instruments make it easier to obtain vibration sig-nal and avoid using the complicated and time consumingcomponent analysis method to judge the heat exchanger’sleakage situation.

• Online monitoring: vibration monitoring can be carriedout without stopping heat exchanger. According to thecollected clear data from vibration sensors, the heat

2013 8th International Conference on Communications and Networking in China (CHINACOM)

978-1-4799-1406-7 © 2013 IEEE511

exchanger’s characteristics can be easily analyzed.• Nondestructive testing: the object being measured is not

damaged, during vibration monitoring.Our contribution:• This method can help to realize real-time monitoring and

real-time warning, once processing the data and estab-lishing the database, and applying the vibration sensor tothe wireless sensor networks.

• A new method, which breaks through the traditionalmethods affected by various factors, e.g., temperature,pressure and the pH levels, is used to diagnosis heatexchanger failures. To improve the accuracy of heatexchanger fault diagnosis, previous warn and decreasethe cost of enterprise, the technique based on vibrationsensor by analyzing characteristics of vibration to carryout point-to-point comparison method and multi-pointmeasurement.

The rest of this paper is organized as follows: sectionII presents the related work, section III shows the problemstatement, section IV is experimental analysis, and section Vconcludes this paper.

II. RELATED WORK

A. Related work on current inspection methodsOver the decades that the heat exchanger has been in use,

several different inspection methods have gained popularity.During a typical turnaround, heat exchanger is first cleaned toremove sediments, deposits, and corrosion. This is a difficultprocess because some types of deposits are hard to removeand the heat exchanger has a degree of built-in redundancy.

To solve the heat exchanger’s problem, traditional methods[6] analyzed the composition of fluid in heat exchangerbetween import and export. However, in [6], the authorsproposed temperature detection which is more precise, fasterand thriftier than traditional method. The most well-knownmethod is Eddy testing where a probe is physical pushedthrough each tube and pulled back. The probe contains oneor several coils, whose electrical impedance is affected by thesurrounding tube. Another well-known nondestructive methodis based on ultrasound in which a wave is sent through amedium and any reflections caused by discontinuities in themedium are recorded. And other methods [7], including pHvalue detection, COD (Chemical Oxygen Demand) methodand oil content.

However, there are still many problems in current methods.It is not easy to come up with an ideal method, which isfast, accurate, independent of tube materials, and completelyobjective, and so as the exchanger inspection.

The different methods listed earlier score differently on eachof the above points but none of them are prefect, e.g., thedisadvantage of traditional method is hysteretic, low accuracyand costly. The temperature detection is affected by thevolatility of temperature, and it has other many disadvantages,e.g., hardly detection and low efficiency. ET and IRIS’ resultsdepend heavily on subjective interpretation by the technician[2].

B. Related work on fault diagnosis methods based on vibra-tion sensor

Vibration diagnosis occupies the largest proportion in thevarious diagnostic methods, reaching 60% -70% in general.Vibration diagnosis technology has been used in many fields,motorcycles anti-theft system and car anti-theft system, e.g.,[8]. It also has been used in monitoring and diagnosing thevibration fault of mechanical equipment [9], e.g., rotatingmachinery, the shaft, bearings, gears, engines and pumps,e.g., [10]–[15]. In [8], the authors designed a omni-directionalvibration sensor used for car anti-theft system. The car anti-theft system not only can detect car omni-directional vibration,but also comply with intelligent behavior recognition. In[10], the authors used the spectrum analysis method [11] todiagnose the rolling bearings. Since the complicated vibrationfrequency components of the rolling bearings, the specificfrequency components need to be separated by appropri-ate signal processing methods. On this basis, the specificfrequency component will be processed by absolute value.Finally, spectrum analysis can help to affirm the location ofthe heat exchanger failures and the method of inspecting. In[12], [13], the authors used the negative selection algorithmfrom artificial immune system. This is an efficient methodcombining with non-dimension parameter which is insensitiveto external disturbances and has a stable performance. In[14], [15], when the gears are running, complex vibration willoccur because of excitation between internal and external ingear. The major ingredient of vibration is rotating frequency,meshing frequency and its harmonic.

Whether the gears are normal or not, the meshing frequencyand its harmonic will always exist, but the amplitudes ofvibration are different. Therefore, with the change of themeshing frequency and its harmonic amplitude, we judge thegears fault.

To sum up, there is no inspection heat exchanger failurewith vibration sensor at present. Therefore, this paper proposea nondestructive testing method based on vibration sensor.

III. PROBLEM STATEMENT

In this paper, we take the direct reason which causesacceleration of exchangers’ tube into consideration. If theReynolds number reaches a certain level when the fluid flowsthrough heat exchanger, Karman Vortex Street [16] will becaused on both side of heat exchanger tubes. Over the lastdecade, the foreign research on the flow-induced vibration hasmade great progress, and people recognized the flow-inducedvibration as mechanism of induced vibration [17], [18], e.g.,Vortex shedding and Turbulent buffeting. Tubes vibration iscaused by the alternate shedding vortex [19]. The frequencyof vibration is equal to the frequency of vortex shedding,calculated by

fv =SLV

d0(1)

Therefore, when d0 is fixed, the greater the flow is v, thegreater flow-induced vibration frequency fv . Besides, once the

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(a) (b)

Fig. 2. EMT490 data Collector and Node Vibration Sensor

frequency of vortex shedding is equal to the inherent vibrationfrequency, the tube will occur the vortex shedding resonanceor being called vortex resonance.

The first problem of our approach is to detect whether thesurging will cause flow-induced vibration when the environ-mental vibration intensity is big enough.

The second problem is to detect the areas in heat exchangerwhich are most likely to occur vibration. Among those areas,the high velocity often produces at the heat exchanger importand export, which will cause vibration.

The third problem of this paper is to detect the existence ofturbulent buffeting of the heat exchanger. When the frequencyof turbulent buffeting is equal to the frequency of tubesvibration, vortex resonance will occur.

The fourth problem is to detect whether the heat exchangeris affected by the vibrations from a working pump.

The fifth problem is to detect whether the surging willcause flow-induced vibration when water goes through thecirculating-water regulating valve.

We propose experiment scheme to solve these five problemsin the following section.

IV. EXPERIMENTAL ANALYSIS

A. Methodology

The experimental hardware: we use the EMT490 datacollector. The system is easy to carry and use, especially forthe rapid detection of vibration. i.e., Fig. 2.

The experimental software: we use EMT490 system whichis powerful, flexible and convenient, and this system cansimultaneously analyze vibrate of multiple channels or otherinput signal, i.e., Fig. 3.

We conduct experiments at Maoming petrochemical refin-ery2, where noise is serious and vibration is strong. There area lot of working machines around heat exchanger, and theymake severe vibration which can be regarded as environmentalvibration. To detect whether the environmental vibration hasan impact on this experiment, we need to detect a stopped heatexchanger vibration signal, and treat use its data as the controlgroup. As shown in Fig. 4, it is the testing heat exchanger. Theexperiment scheme is shown as follows:

2Maoming petrochemical refinery has 13.5 million tons of oil refining, 1million tons of ethylene production capacity, realized profits of 4.646 billionyuan in 2010, is one of the best benefits of Sinopec refining enterprise.

Fig. 3. The EMT490 System

Fig. 4. The testing heat exchanger

• To detect whether the fluid flows through the circulating-water regulating valve influence this experiment, wecollect the vibration signal of the two working heatexchanger’s circulating-water regulating valve. We collectdata and mark those data as RV1, RV2 in the document.

• To detect whether the turbulence has an effect on ourexperiment, we collect vibration signal of the middleof the two working heat exchangers, where the heatexchanger’s above, below, front and behind. Collect dataand mark those data as A1, B1, F1, H1, A2, B2, F2, H2in the document.

• To detect whether the flow-induced vibration influencedthe tube, we collect the vibration signal of the import andexport of both a stopped heat exchanger and two workingheat exchangers. We collect data and mark those data asHX0, HX1 and HX2 in the document.

• To detect that whether the vibration of the working pumphas influenced on the tube, we collect the vibration signalof the pump and the neighboring heat exchanger. Wecollect data and mark those data as Pump and heatexchanger in the document.

• Use EMT490 data collector to collect vibration data,and then the vibration waveform data gathered will beuploaded to the computer. We use EMT490 software toanalyze vibration data.

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B. Vibration signal feature extraction methods: time-domainanalysis of the signal

Time-domain analysis uses time-domain parameters to mon-itor and diagnosis heat exchanger early fault. In [20], theapplications of the RMS, peak, crest factor and kurtosiswere introduced by authors, and the author pointed out thattime-domain analysis is a good way to simple diagnosis. In[21], the authors pointed out that the peak is the valid wayto identify the damaging component on the surface fault.However, for wear fault, the RMS is a valid method to detectthis type of problem. What is more, these types of faultcan be identified by the crest factor. In [22], the authorspointed out that when equipment parameters are not completeand spectrum analysis method is not applicable, the time-domain waveform and kurtosis are feasible to preliminarilydetect the equipment failure. Some scholars improved thetraditional vibration information’s time-domain analysis, andput forward the non-dimensional parameter diagnosis method[23]. The character of the non-dimension parameters used forfault diagnosis was introduced by author in [24]. In this paper,we take the early faults’ sensitivity and stability into account,then monitor heat exchanger fault by comparing the vibrationsignal non-dimensional parameter. Time-domain parameter isshown as follows:

RMS: the RMS shows the size changing of vibrationsignal (Xi, i =1- N , N for the sampling points), when theinstantaneous amplitude value changes over time. The RMSis denoted as

XRMS =

√√√√ 1

N

N∑i=1

x2i (2)

The RMS reflects the strength of the vibration. When themachine move abnormally, the vibration inevitably increases.

Peak and Crest factor: in mechanical fault diagnosis, it isvery likely to possess stable peak XPEAK . The crest factorC is denoted as

C =XPEAK

XRMS(3)

The Crest factor value is between 2.5 and 3.5, with a normalvibration signal. Otherwise, it will be more than 3.5, whichindicates that the machine has been damaged. Sometimes, itcan be recorded to the crest factor value up to 7 before themachine failure.

Kurtosis: kurtosis K is a statistical parameter to characterizevibration signal. In essence, kurtosis provides a measure ofthe “peakedness” of a random signal, and kurtosis is thedomain processing most commonly used in the vibration signaldimensionless parameter indicators. Kurtosis is defined asfollows:

K =

1N

N∑i=1

(|xi| − x)

X4RMS

(4)

TABLE ICOMPARE OF THE BREADTH PARAMETER TO THE SENSITIVITY AND THE

STABILITY OF THE FAULT

The breadth parameter Sensitivity bfseries Stability

Shape Factor poor Good

Crest Factor General General

Impulse Factor Better General

Clearance Factor Good General

Kurtosis Good Good

The amplitude obeys normal distribution law. When the heatexchanger is normal, kurtosis value is about 3. While kurtosisvalue is closer to 4 or more, it indicates impulsive vibrationhas been occurred.

Other time-domain parameters: the common of non-dimensional parameters are the impulse factor, clearance fac-tor, shape factor, etc..

Comparison of non-dimensional parameters is shown inTable I.

The RMS, peak, crest factor, kurtosis, impulse factor, clear-ance factor and shape factor are the most common parametersto analyze the signal vibration. In addition, these indicators canbe used directly in detecting real-time vibration signal, withoutany need for additional signal processing and transformation.Therefore, detecting real-time vibration signal can avoid thedefects of signal, e.g., malformations and problems. The mostimportant feature is that these parameters are sensitive enoughto detect the faults and defects, nevertheless, they are notsensitive to the amplitude and frequency of signal. In otherwords, these parameters are independent of the machine oper-ating condition, and they just depend on the probability densityfunction of amplitude. With the variation of these parameters,the failure characteristics of mechanical equipment can bereflected directly. All these parameters mean good diagnosticcapacity of early fault.

C. Data process

We analyze three heat exchangers’ parameters. The variousparameters of value are small. As shown in Fig. 7 and Fig. 8,the kurtosis value is about 3 when the three heat exchangersbehave normally, while the crest factor fluctuates and the RMSis small. Obviously, in the range of the permitted errors, thevalue of kurtosis, crest factor and RMS of the three heatexchangers are very close.

The results of data analysis as follows:• As shown in Fig. 5 and Fig. 6, owing to the interference

of environmental vibration, it cannot be detected whetherthe turbulence induced vibration on the middle of the heatexchanger or not, and we measured the normal vibrationsignals. Within the error range allowed, vibration signalsaround the middle of the heat exchanger is very close.

• As shown in Fig. 7 and Fig. 8, the sensitivity of kurtosisperform well. In according with the collected data ofthe heat exchanger’s both ends, those heat exchangersare normally working (all kurtosis value are about 3).

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(a) Above and Bottom

(b) Front and Hind

Fig. 5. The trend curve of the kurtosis, crest factor and RMS of the middleof the first heat exchanger

(a) Above and Bottom

(b) Front and Hind

Fig. 6. The trend curve of the kurtosis, crest factor and RMS of the middleof the second heat exchanger

Fig. 7. The trend curve of the kurtosis, crest factor and RMS of the inlet ofheat exchanger

Fig. 8. The trend curve of the kurtosis, crest factor and RMS of the outletof heat exchanger

Fig. 9. The trend curve of the kurtosis, crest factor and RMS of the RV ofheat exchanger

Fig. 10. The trend curve of the kurtosis, crest factor and RMS of the pumpand heat exchanger

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Calculated values are close to the three heat exchangers’parameters, because the two working heat exchangers arelargely affected by the outside, and the vibration datameasured by sensors is almost stimulated by environmen-tal vibration.

• As shown in Fig. 7 and Fig. 9, vibration signals detectedin stopped heat exchanger is aroused by the environment.In other words, the vibration from the external environ-ment has influence on detecting data. The flow-inducedvibration has an impact on the heat exchanger.

• As shown in Fig. 10, the data of pump are larger thanthat of the heat exchanger, and the vibration arouse bypump has no impact on detecting heat exchanger.

V. CONCLUSION

By conducting extensive experiments on heat exchangers inMaoming ethylene plants, we analyze the collected vibrationsignal from one stopped heat exchanger and two workingheat exchanger. And we compared the vibration signals’ time-domain parameters by time-domain analysis. The deducedresults from these experiments are not obvious for diagnosingthe tube failure. It is difficult to analyze heat exchanger’stube failure by the vibration diagnosis method with currenttechnologies. To sum up, it needs to be further improved.

ACKNOWLEDGMENT

Lei Shu is the corresponding author. This work is supportedby the Guangdong University of Petrochemical Technology’sInternal Project No. 2012RC0106.

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