An Experimental Study of Clogging Fault Diagnosis
in Heat Exchangers Based on Vibration Signals
The water
circulating heat exchangers employed in petrochemical industrials have attracted
great attention in condition monitoring and fault diagnosis. In this paper, an
approach based on vibration signals is proposed. By the proposed method,
vibration signals are collected for different conditions through various high
precision wireless sensors mounted on the surface of the heat exchanger. Furthermore,
by analyzing the characteristics of the vibration signals, a database of fault
patterns is established, which therefore provides a scheme for conditional
monitoring of the heat exchanger. An experimental platform set up to evaluate
the feasibility and effectiveness of the proposed approach, and support vector
machine based on dimensionless parameters is developed for fault classification.
The results have shown that the proposed method is efficient and has achieved a
high accuracy for benchmarking vibration signals under both normal and faulty
conditions.
Heat exchanger
is a common device used to transfer heat. In modern refineries, the cost of
heat exchangers is around 40% of the total investment in a facility; the
corresponding maintenance workload can be as high as60% to 70% of the total
maintenance workload. Therefore it is extremely important to make sure that
heat exchangers operate safely and economically. Instead of purified water, the
cooling medium used in a head exchanger is industrial grade circulating water, which
continues to evaporate and effloresce during use; thus its salt content
constantly increases. Meanwhile, carbon dioxide in the water turns into its
gaseous form and escapes from the cooling tower, leading to the continuous
formation of calcium carbonate scales within the heat exchanger. Further, when
the circulating water is exposed to the air, a large amount of dust as well as
mud pellets and microorganisms dissolve into the water. Much of this material
is then deposited on the inner walls of the heat exchangers tubes, forming
scales. According to a survey by Steinhagenet al. [1], of 3000 heat exchangers
of different types used by 1100 companies in New Zealand, more than 90% showed
varying degrees of scaling within their tubes.
Over time,
scaling on the inner walls of the heat exchangers tubes will cause the fluid
inside to change its direction of flow, thus generating a transverse force that
causes the tubes to vibrate. If some of these tubes are heavily scaled or blocked
given that the velocity of fluid at the inlet is fixed the flow rate and
pressure in the other tubes will be increased and the heat exchanger will
vibrate violently. If this condition is not treated in a timely manner, leakage
will ensue, leading to a significant financial cost. According to the statistical
data, some 30% of heat exchangers break down because of vibration problems,
indicating that the vibration of tube bundles is a major cause of damage [2]. This
issue has become a key cause of concern in petrochemical industries; that is,
it is considered vital to monitor, in a timely manner, the clogging conditions
in heat exchangers during use so as to reduce the frequency of their maintenance
and ensure the safe operation of these devices by analyzing their vibration
signals. Major facts discussed in this paper include the following:
•Starting with
the problem of monitoring, the vibration mechanism of the water circulating
heat exchangers described and the corresponding research reviewed. Finally a condition
monitoring approach based on the heat exchanger’s vibration signals is proposed.
•The following procedure is described: An experimental platform for a water circulating heat exchanger was established and a support vector machine (SVM) used to classify the dimensionless parameters of its vibration signals. This procedure verified the divisibility of the vibration signals generated by the heat exchanger under both normal and fault conditions.
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- CONDITION MONITORING OF HEAT EXCHANGERS
The fault diagnosis of heat exchangers has posed a major challenge
in the industry because faults such as leakage and clogging within the heat
exchange tubes are not visible. The fault monitoring techniques used in todays
refineries are relatively outdated [3]. , the major methods being the monitoring
of PH values, oil content, chemical oxygen demand, residual chlorine, the
functioning of specific instruments and etc. For example, in the Maoming
Ethylene Factory, the main fault monitoring method used with its water
circulating heat exchanger is to determine the difference in temperature at the
water inlet versus the water outlet. However, the accuracy of this method is
influenced by many external factors. For instance, the temperature of the
medium entering the exchanger may not be stable; it can fluctuate to some
extent, and this will have a direct effect on the temperature difference. However,
by developing an online device to monitor the heat exchanger, sensors can be
installed in different parts of it so as to monitor changes in the circulating
water continuously, thus enabling online detection and identification. This
method has achieved considerable success [4], with commercial products
available for use in countries including the United States, Germany, and Japan
[5]. As described in reference [6], a single chip microcomputer (SPCE061A) was
utilized to construct an online monitoring and controlling system designed to
detect the temperature at the inlet and outlet as well as that of the water
vapor; it can also measure the flow rate and control the flow of both water and
vapor. Stojan Persin et al. [7] applied analytical fault detection techniques
to do real time fault diagnosis for an industrial scale pilot heat exchanger. An
online monitoring and prediction system is proposed in reference [8], which explores
the way in which this device monitors and predicts a heat exchangers
performance, resulting in improved production efficiency of each individual unit,
lower costs of production and maintenance, as well as a higher level of safety during
production. The online performance monitoring of a shell and tube type heat
exchanger using steam and water has been developed on the basis of a
theoretical model to monitor input and output process variables [9]. Adaptive
observers used to estimate the overall heat transfer coefficient and detect a
performance degradation of the heat exchanger [10]. Fuzzy models based on
clustering techniques are used to detect leaks in a complex heat exchanger [11].
SVM and relevance vector machine (RVM) have been widely used in the condition
monitoring and fault diagnosis of machinery [12]. The particle swarm
optimization algorithm is applied to estimate the parameters of a SVM which is
used to predict faults in a heat exchanger [13]. The Dynamic Principal
Component Analysis (DPCA) method and a set of Diagnostic Observers (DO) were
designed to online detect and isolate faults related to sensor or actuators malfunctions
in a shell and tube industrial heat exchanger [14], [15]. Dragan [16] presented
a two stage modeling procedure based on prior knowledge and recorded data to detect
faults for a heat exchanger in an incineration unit. Ingimundardóttir and Lalot
[17] presented a method for the detection of fouling in a cross flow heat
exchanger using wavelets. Paper [18] proposes a new method for fouling
detection in a heat exchanger. It is based on the modeling of the system in a
fuzzy TakagiCSugeno representation. With this representation, the design of a
fuzzy observer with unknown inputs of polynomial types is obtained via a LMI formulation.
Shuet al. [19] and Zhuet al. [20] report a fault diagnosis method in
petrochemical water circulating heat exchangers based on vibration and sound. Preliminary
results show that it was difficult to identify faults by sound and that further
research is required concerning vibration signals. From these reports it is
clear that several researchers have studied the online monitoring and fault
diagnosis system of water circulating heat exchangers; however, these
investigations are limited to the monitoring of parameters such as temperature,
pressure, and flow rate, with little progress in the fault diagnosis of heat
exchangers. Besides, since a heat exchanger is a static device and vibrations
generated by the circulating water within the tubes is far less than that
generated by mechanical rotation, little research has focused on condition
monitoring utilizing vibration or sound parameters.
- RESEARCH ON VIBRATIONS IN HEAT EXCHANGERS
The movement of fluid in a heat exchanger is very complicated,
including cross flow, axial flow, and bypass flow in tube bundles; moreover,
there are stagnant areas at the inlets and outlets of the tube bundles. The
rate and direction of flow of various fluids continue to change irregularly, creating
heterogeneous force field for the heat transfer tube. Then the heat transfer
tube vibrates under the influences of several forces initiated by the flowing
fluid. When the frequency of induced vibration comes close to the machines inherent
frequency, the heat exchanger will vibrate violently, causing damage to the
baffles and the joint between the tube and baffle. Further, the resonant
vibrations of tube bundles, pumps, and compressors; the direct pulsating impact
generated by rotating machinery; and the frequent on off switching of the heat
exchanger all induce the tube bundles to vibrate, finally causing them to fail.
Investigations of vibration conditions in heat exchangers have been conducted
by several research groups. Feng et al. [21] utilized Fluent software to
simulate the characteristics of flow induced vibrations in a singe straight
tube, two parallel tubes, two tandem tubes, and tube bundles, obtaining
corresponding dynamic responses and characteristics of the flow field. Considering
aspects of modal shape and dynamic responses, Zheng et al. [22] analyzed the influence
of vibrations induced by pulsating flow on the vibrations of heat exchangers. Wang
et al. [23] studied the inherent vibration characteristics of planar elastic
tube bundles and vibration patterns of the same elastic tube bundles under the
influence of pulsating flow. They used Fluent software to achieve a simulated
calculation of the pulsating flow generator; as a result, a vibration frequency
in agreement with the real frequency was obtained. Chen et al. [24] fulfilled
equivalent simulation analyses of vibrations in a heat exchanger using Ansys
software and studied various factors that caused vibration of tube bundles. According
to them, vibration of the tube bundles can be effectively avoided if the excitation
frequency is kept away from the inherent frequency. Nevertheless, all this
research was carried out based on the inherent vibration characteristics of the
heat exchanger without any analysis of vibration conditions with regard to
fault diagnosis. In the present paper, the flow rate of the fluid is considered
to be the direct factor that causes the tube bundles in a shell and tube heat
exchanger to vibrate. When fluid runs through the tube and the Reynolds number
reaches a certain value, asymmetrical alternately shed vortex waves namely Karmans
vortex streets appear periodically on both sides and the back of tubes. In the
past couple of decades, studies off low induced vibrations have made
significant progress in other countries and a highly accepted mechanism of flow
induced vibration has been proposed, e. g. , vortex shedding and turbulent
buffeting. The alternate formation and shedding of vortex generates a periodic
exciting force perpendicular to the direction of fluid flow on both sides of
tubes and causes the tubes to vibrate. The erotically, different vibration
signals will be generated at distinct flow rates and under varying fault
conditions in a heat exchanger, thus laying the foundation for this research.
A condition monitoring and fault diagnosis method based on wireless vibration sensors in water circulating heat exchangers has been proposed. By establishing an experimental platform for a water circulating heat exchanger and performing a series of experiments that simulate clogging faults, it has become clear that vibration signals generated by heat exchange under normal and clogging conditions are significantly different. Based on dimensionless parameters in an SVM classification model, it is clear that the difference between vibration signals collected under normal conditions versus those collected under fault conditions is greater than 85%. Therefore it is feasible to set up a condition monitoring system for a water circulating heat exchanger based on vibration signals. However, although this SVM model provides relatively high accuracy in identifying whether the heat exchanger is clogged, further research is required to improve its diagnostic efficiency and accuracy (e. g. , with regard to the exact number of clogged tubes). In the future, an improved multilevel fault classification device can be designed based on the number of clogged tubes, thus providing better monitoring.
