Filter systems are widely used in petrochemical plants for removing solid impurities from hydrocarbon oils. The backwash is the cleaning process used to remove the impurities on the sieves of the filters without a need to interrupt the operation of the entire system. This paper presents a case study based on the actual project of a filter system in a petrochemical plant, to demonstrate the significant effect of vibration on the structural integrity of piping. The induced vibration had led to the structural fatigue failure of the pipes connecting the filter system. A preliminary assessment suggested that the vibrations are caused by the operation of backwashing of the filter system. A process for solving the vibration problem based on the modal analysis of the filter system using the commercial finite element software for simulation is therefore proposed. The computed natural frequencies of the system and the vibration data measured on site are assessed based on the resonance effect of the complete system including the piping connected to the filters. Several approaches are proposed to adjust the natural frequencies of the system in such a way that an optimal and a reasonable solution for solving the vibration problem is obtained.
A filter system plays an important role in petrochemical plant for removing solid particles and impurities from hydrocarbon oils. In the system, the pipes of various geometrical properties are connected to pumps and filters to transport oil and oil products for treatment [
One of the earliest studies of vibration problems in pipes was conducted by Ashley and Haviland [
In a petrochemical plant, the filter system is used to filter solid particles and impurities from hydrocarbon oils. The solid particles will clog the sieves after some period of time of operation and thus will affect the flow of oils in pipes. However, it is impossible to replace the sieves or to remove the filter for cleaning while the filter system is in operation. Therefore, a backwashing process is used to clean the sieves of the filters. With full particles clogging the sieves and reduced speed of oil flows, a pressure difference between inlet and outlet of the filter system will be generated. The backwashing process will then be activated due to the difference of these working pressures. The backwashing process uses the product oil as a backwashing liquid to flush the filter nets at high pressure so that the solid particles on the sieves could be removed. Once the backwashing liquid hits the filter nets and pipes after it passes through the filters, it causes the filter and the piping system to vibrate. In general, the magnitude of vibration is insignificant and cannot be observed. However, with the increasing production of oil products to meet the demand, the petrochemical company redesigned the system with one additional filter system (Group F) installed next to the existing filters (Groups A to E) as shown in Figure
The exciting and addition filter system.
An internal stress analysis of the pipes was thereafter performed to rectify the unexpected vibration problem that resulted in fatigue cracking on pipes. The internal stress analysis is a conventional approach commonly adopted for analyzing pipe abnormality under various load conditions based on structural mechanics [
In the current study, the filter and the piping system were connected and modelled as a complete system. A modal analysis is conducted to determine the natural frequencies of the system. The exciting forces of the vibration include the pulses of the oil in the pipes and the interaction effect among the oil, the pipes, and the filter system. The response vibration of the complete system due to backwash was measured and presented in a form of time spectrum. The time spectrum was then transformed into the frequency domain by means of fast Fourier transformation (FFT). The FFT spectrum allowed the response frequencies that contributed to vibration to be determined. The peaks of the spectrums presented the frequencies to the natural frequencies of the complete system, where the unexpected vibration is due to the effect of resonance. With the comparison of the FFT spectrums of response vibration to the natural frequencies of the complete system, the region of resonance could be identified [
The filter and the piping system are modelled using commercial software SolidWorks and ANSYS, as shown in Figure
A filter and piping system model.
There are 6 groups of filter and piping system. Each group comprises 8 filter elements. The detail of each filter system is shown in Figure
Details of each filter system.
The filters were made of stainless steel. Each single filter comprised 28 filter elements with diameter 25 mm and length 814 mm. The filter elements were threaded into a common flange. The pipes are made of mild steels with geometrical properties as shown in Table
Outer diameter and thickness of pipes.
Pipe number | Outer diameter (mm) | Thickness (mm) |
---|---|---|
|
150 | 7 |
|
100 | 7 |
|
100 | 7 |
|
100 | 7 |
|
100 | 7 |
|
50 | 4 |
Cross section view of bracket (mm).
A complete filter and piping system.
The modal analysis was carried out to determine the natural frequencies of the complete system. In the analysis, there were
To obtain the natural frequencies of the system in the modal analysis, the exciting force vector was set to 0. The matrix representation of transformation after the combination of (
On the basis of equations of motion, the natural frequencies of the system could be obtained from the FEA of the model using the commercial software ANSYS. The natural frequencies of the system below 15 Hz are listed in Table
Natural frequencies of existing system.
Modal order | Nature frequency |
---|---|
|
5.4104 |
|
5.503 |
|
5.6867 |
|
7.7515 |
|
9.5334 |
|
10.436 |
|
10.64 |
|
10.918 |
|
11.337 |
|
11.37 |
|
11.413 |
|
12.153 |
|
12.673 |
|
12.941 |
|
13.504 |
|
14.106 |
|
14.354 |
|
14.729 |
|
14.749 |
|
14.865 |
Vibration mode of the 1st modal order.
Vibration mode of the 2nd modal order.
Vibration mode of the 3rd modal order.
Vibration mode of the 4th modal order.
Vibration mode of the 5th modal order.
After the system was redesigned with one additional filter and the piping system (new system), the unexpected vibrations occurred at Group F during the backwashing process. The main exciting force of the vibrations was the pulses of the oil and the interaction effect of the oil and the pipes. The locations of vibration measurement of the system of Group F are illustrated in Figure
Locations of vibration measurement.
The computed time spectrums were transformed into the frequency domain by FFT. The FFT spectrums showed the most power response frequency. The FFT spectrums of Locations 1 to 3 are shown in Figures
The FFT spectrums of Location 1.
The FFT spectrums of Location 2.
The FFT spectrums of Location 3.
From the displacement spectrums, it can be seen that the peaks were located at the frequencies between 5 Hz and 10 Hz. The comparison with the results of the modal analysis indicated that several natural frequencies were found smaller than 10 Hz. It can be deduced from the observation that the system was in the region of resonance. On the other hand, it can be seen from Figure
Critical locations of vibration.
From the modal analysis and the vibration measurement, the natural frequencies of the system should be increased above 10 Hz to avoid the effect of resonance. From the observation of the system performance on site, it can be seen that the vibration actually occurred on free span long pipes without any supports. These pipes were located in the critical vibration of the system. Thus, adding adequate constraints to stiffen these pipes was the first approach to solving the vibration problem.
The first approach is to add adequate constraints by connecting the pipes together. This was carried out by connecting the long pipes (Location 4) using 50 mm diameter pipes. After the pipes are connected, the natural frequencies of the system slightly increase, as shown in Table
Natural frequencies of the system (1st approach).
Modal order | Natural frequency |
---|---|
|
7.1962 |
|
7.5748 |
|
9.3668 |
|
10.426 |
|
10.643 |
|
10.918 |
|
11.347 |
|
11.413 |
|
11.72 |
|
12.312 |
|
12.822 |
|
13.144 |
|
13.517 |
|
14.292 |
|
14.715 |
|
14.751 |
Vibration mode of 1st modal order.
Vibration mode of 2nd modal order.
The second approach is to connect the long pipes above the passage with 2 additional supports at both sides, as shown in Figure
Natural frequencies of the system (2nd approach).
Modal order | Natural frequency |
---|---|
|
7.6771 |
|
9.7916 |
|
10.425 |
|
10.558 |
|
10.759 |
|
10.918 |
|
11.349 |
|
11.414 |
|
12.327 |
|
12.834 |
|
13.139 |
|
13.261 |
|
13.551 |
|
14.308 |
|
14.401 |
|
14.715 |
Additional constraints (2nd approach).
The 3rd approach is to add additional constraints at Location 1 by connecting the long pipes with 50 mm diameter pipes, as shown in Figure
Natural frequencies of the system (3rd approach).
Modal order | Natural frequency |
---|---|
|
10.558 |
|
11.323 |
|
13.261 |
|
19.462 |
|
19.984 |
|
20.137 |
|
20.491 |
|
20.534 |
|
20.678 |
|
20.928 |
Additional constraints (3rd approach).
The process of Solution 2 was identical to that of Solution 1. The locations of additional constraints are similar to those in Solution 1, as shown in Figure
Natural frequencies of the new system.
Modal order | Natural frequency |
---|---|
|
12.156 |
|
16.414 |
|
17.301 |
|
18.083 |
|
18.367 |
|
18.444 |
|
18.541 |
|
18.636 |
|
19.723 |
|
20.321 |
Additional constraints in Solution 2.
For Solution 3, the pipes were connected using spring bumpers to control the increasing stress of the structure system with reduced vibration. The bumpers also absorbed part of the vibration energy. However, the spring bumpers did not increase the natural frequencies of the system effectively. The natural frequencies of the new structure with additional filter and piping system are listed in Table
Natural frequencies of the new structure.
Modal order | Natural frequency |
---|---|
|
5.6967 |
|
7.3319 |
|
8.6949 |
|
10.512 |
|
10.534 |
|
11.01 |
|
11.048 |
|
11.428 |
|
11.488 |
|
12.241 |
The natural frequencies of Solutions 1, 2, and 3 are summarized in Table
Natural frequencies of original and new system.
Modal order | Original | Solution 1 | Solution 2 | Solution 3 |
---|---|---|---|---|
|
5.4104 | 10.558 | 12.156 | 5.6967 |
|
5.503 | 11.323 | 16.414 | 7.3319 |
|
5.6867 | 13.261 | 17.301 | 8.6949 |
|
7.7515 | 19.462 | 18.083 | 10.512 |
|
9.5334 | 19.984 | 18.367 | 10.534 |
|
10.436 | 20.137 | 18.444 | 11.01 |
|
10.64 | 20.491 | 18.541 | 11.048 |
|
10.918 | 20.534 | 18.636 | 11.428 |
|
11.337 | 20.678 | 19.723 | 11.488 |
|
11.37 | 20.928 | 20.321 | 12.241 |
The structural modification combined Solutions 1 and 2 based on site condition. The long pipes in Location 4 in Solution 1 were connected with each other. The pipes above were supported and fixed onto a large bracket. As the pipes in Location 5 were very close to the foundation of the system, they were fixed to the short brackets on the floor (Figure
Structural modification.
The FFT spectrums of original and new structure at Location 2.
The current study provides three (
All the 3 proposed solutions will result in the increment of natural frequencies of the system. However, from the simulation, it can be seen that the fixed constraint is more effective than the elastic restraint in changing the natural frequencies to reduce the vibration of the structure system. The proposed Solution 3, with spring bumpers to constrain the pipes, has a longer construction time and is less effective in increasing the natural frequencies. With the consideration of analysis results and the site conditions, Solutions 1 and 2 are the optimal solutions for solving the resonance problem of backwashing system. The combined Solutions 1 and 2 provide the best solution based on the site situation.
The authors declare that they have no conflicts of interest.