FAILURE ANALYSIS OF A PRESSURE CONTROL SYSTEM
STEVEN ANAK TUMI
UNIVERSITI MALAYSIA SARA WAR
'IS
2003
176
S843 2003
•
I
BORANG PENYERAHAN TESIS
Judul FAIUIR£ ANALYSIS OF A PRESSIIRE CONTROL SYSTEM SESI PENGAJIAN: 2002
Saya STEVEN AN AK TT 1MI
mengaku membenarkan tesis ini disirnpan di Pusat Khidmat Maklumat Akademik, Universiti Malaysia Sarawak dengan syarat-syarat kegunaan seperti berikut:
I. Hakmilik kertas prQjek adalah di bawah nama penulis melainkan penulisan sebagai projek bersama dan dibiayai oleh UNIMAS, hakmiliknya adalah kepunyaan UNIMAS .
2. Naskhah salin an di da'am bentuk kertas atau mikro hanya boleh dibuat dengan kebenaran bertulis daripada penulis.
3. Pusat Khidmat Maklumat Akademik, UNIMAS dibenarkan membuat salinan untuk pengajian mereka.
4. Kertas projek hanya boleh diterbitkan dengan kebenaran penulis. Bayaran royalti adalah mengikut kadar yang dipersetujui kelak.
5. * Saya membenarkanltidftk membenftlkftn Perpustakaan membuat salinan kertas projek ini sebagai bahan pertukaran di an tara institusi pengajian tinggi.
6. ** Sila tandakan (J)
(mengandungi maklumat yang berdarjah keselamatan atau
ID SULIT
Kepentingan Malaysia seperti yang termaktub di dalam AKT A RAHSIA RASMI 1972).
DTERHAD (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasilbadan di mana penyelidikan dijalankan).
D
TIDAK TERHADDisahkan oleh
(TANDATANGAN PENULIS) Alamat Tetap: Lot 311, Lorong B7,
R.P.R. Bau 12, lalan KuchingfSerian,
!2..
11BI YAM g-r. H'J· I3FHH I93250 Kuching, SARA W AK. Nama Penyelia
Tarikh: 20 lANUARI 2003 Tarikh: ~
/0
I /-rrJ0 3• 1
Potong yang tidak berkenaan
CATATAN: *
lika Kertas Projek ini SULIT atau TERHAD, sila lampirkan surat daripada
**
pihak berkuasa/organisasi berkenaan dengan menyertakan sekali tempoh kertas projek. Ini perJu dikelaskan sebagai SULIT atau TERHAD.FAILURE ANALYSIS OF A PRESSURE CONTROL SYSTEM
P.KHIDMAT MAKLUMAT AKADEMIK UNIMAS
11111111111111111111111111111 0000118383
by
STEVEN AK TUMI
This project report is submitted in partial fulfillment of the requirements for the degree of Bachelor of Engineering (Hons.) Mechanical Engineering
&Manufacturing System from the Faculty of Engineering, Universiti Malaysia Sarawak
March 2002
This project report attached hereto, entitled "Failure Analysis Of A Pressure Control System", prepared and submitted by Steven anak Tumi as a partial fulfillment of the
requirement for the degree of Bachelor of Engineering with Honours (mechanical Enginering and Manufacturing System) is hereby read and approved by:
Supervisor Date
To all my colleagues and staffs ofEngineering Faculty who have walk
along with me through all these winding turns in engineering studies.
ACKNOWLEDGEMENTS
I would like to thank Ms. Rubiyah bt Hj Baini for her professionalism in supervising and guiding me in the completion of this thesis. Thanks also to her for sacrificing her precious time giving advises and guidance to me. Without these, I may not be able to complete this thesis and therefore may not be able to gain this valuable knowledge and experience of a lifetime.
I would also like to acknowledge the technicians of the mechanical lab, Mr. Rhyer and Mr. Masri, for giving full cooperation during the experiment, thus made the task easier for me.
Last but not least, I would like to thank my family and friends for their full support. Thank you all.
I
ABSTRACT
The awareness of safety in the workplace has been growing rapidly for the past few years. In manufacturing industry, especially, humans are facing various possible risks or hazards while performing their duties in a fast pace working environment. In order to avoid undesired disaster, highest safety precaution should be taken into account. In collaboration to this effort, this study focused on the preventive measures that may reduce the risks related to the hardware already implemented in the manufacturing world.
Therefore, it is of particular interest that the object of study is a pressure control system.
The pressure contro1 system has been well established and acknowledged in its usage, from the most sophisticated natural gas plant to the simplest and most common house equipment. Realizing this, it is the intention of the study to completely understand the operation of a pressure control system. Following that, failure analysis is conducted in order to evaluate the performance of the system under real life situation. In this analysis, some well-known methods such as HAZOP, FTA, and FMEA, are used. From these results, recommendations are presented to improve the performance and reliability of the pressure control system.
ABSTRAK
Kesedaran terhadap kese1amatan di tempat kelja semakin meningkat barn-baru ini. Di dalam industri pembuatan khasnya, manusia berhadapan dengan pelbagai risiko kemalangan ketika melakukan tugas mereka dalam suasana kelja yang mencabar. Dalam usaha mengelakkan kejadian yang tidak diingini berlaku, langkah-Iangkah keselamatan yang ketat harns dikuatkuasakan. Sehubungan dengan itu, kajian ini menitikberatkan langkah-langkah pencegahan yang sebaiknya bagi mengurangkan risiko kemalangan
khasnya yang berkaitan dengan mesin serta peralatan dalam industri pembuatan. Bagi memenuhi keperluan ini, maka sistem kawalan tekanan dijadikan bahan kajian bagi tesis
lDl.
Sistem kawalan tekanan telah pun dikenali kegunaan serta kepentingannya, dari loji gas asli yang tercanggih hinggalah ke peralatan rumah yang paling biasa. Menyedari hakikat ini, kajian ini meletakkan matlamat untuk memahami sepenuhnya operasi sistem kawalan tekanan ini. Seterusnya, analisis kegagalan dilakukan bagi menilai kemampuan sistem tersebut dalam situasi sebenar serta mengkaji faktor-faktor yang mungkin menyebabkan kegagalan sistem ini berfungsi secara normal.. Analisis ini dilakukan dengan menggunakan kaedah-kae~ah seperti HAZOP, FT A dan FMEA. Akhimya, beberapa cadangan dikemukakan bagi memperbaiki sistem ini.
LIST OF FIGURES
FIGURE PAGE
Figure 1.1 Basic System Schematic 3
Figure 1.2 Control System Element and Configuration 3
Figure 2.1 Absolute, Gage and Vacuum Pressure 15
Figure 2.2 UCP-P Pressure Control Unit 35
Figure 2.3 Schematic Diagram of Pressure Control Unit 40
Figure 4.1 Adjusting Valve Analysis 61
Figure 4.2 Pneumatic Valve Analysis 62
Figure 4.3 Pressure Transducer Analysis 63
Figure 4.4 Differential Pressure Transducer Analysis 64
Figure 4.5 Flow Meter Analysis 65
LIST OF TABLES
TABLE PAGE
Table 3.1 HAZOP Result Table 46
Table 3.2 FTA Gates 49
Table 4.1 Result of System Test Run 55,56
Table 4.2 Result of FMEA 57,58
Table 4.3 Result of HAZOP 59,60
Table 5.1 Summarization of Equipment Analysis 71
TABLE OF CONTENT
ACKNOWLEDGEMENTS ABSTRACT
LIST OF FIGURES LIST OF TABLES
CHAPTER 1 INTRODUCTION ...(1- 13)
1.1 Control System ... " ... '" ... 1
1.2 Basic Concepts ...3
1.2.1 Open Loop Dynamic ...6
1.2.2 Feedback ControL ...7
1.3 Control of Pressure...9
1.4 Objectives ... 12
CHAPTER 2 LITERATURE REVIEW ...(14 - 40) 2.1 Fundamentals of Pressure... 14
2.2 Pressure Ratings ... 16
2.2.1 MiDimum and Maximum Pressure ... 16
2.2.2 Pressure Accuracy ... 17
2.2.3 Pressure Leakage ... 17
2.3 Flow Ratings ... 19
2.3.1 Minimum a;{d Maximum Flow ...22
2.3.2 Flow Rate Leakage ... 23
2.4 Instrumentation...24
2.4.1 ValveRatings...24
2.4.1.a Minimum and Maximum Flow ...25
2.4.1.b Minimum and Maximum Pressure ...25
2.4.1.c Flow Versus Pressure Drop ...26
2.4.1.d Response Time ...27
2.4.1.e Internal Leakage ...27
2.4.1.f Temperature.... ... 28
2.4.1.g Viscosity...28
2.4.2 Pressure Control Valve ...28
2.4.3 Flow Control Valve ... '" '" ...29
2.4.4 Pneumatic Valve ... .....30
2.4.5 Pressure Gauges ... '" ... '" ... ...31
2.4.5a Gauge Accuracy ...32
2.4.5b Failure Modes ...33
2.4 System of Study... 34
2.5.1 Description of System... .35
2.5.2 Components...37
2.5.2a Adjusting Valve ...37
2.5.2b IIP Converter...37
2.5.2c Pneumatic Valve ... 38
2.5.2d Piezoelectric Pressure Sensor ...38
2.5.2e Differential Pressure Sensor ... 39
2.5.2f Flow Meter ...39
CHAPTER 3 METHODOLOGy .... ...(41 - 53) 3.1 Stage 1: Understanding of The System ...41
3.1.1 Experimental Setup ... .41
3.2 Stage 2: Implementation of Failure Analysis ...42
3.2.1 HAZOP (Hazard And Operability) Method ...44
3.2.1 a Keywords ...45
3.2.1 b HAZOP Study Methodology ... .46
3.2.1c HAZOP Procedure ... .47
3.2.2 Fault Tree Analysis (FTA) ... 47
3.2.2.a Fault Tree Gates ... , ... .48
3.2.2.b FTA Analysis Steps ... ... 51
(i) System Definition ... 51
(ii) Fault Tree Construction ...51
3.2.2.c Summary of Fault Tree Analysis .... ... 51
3.2.3 Failure Modes and Effect Analysis (FMEA) ... 52
4 RESULTS ... ... ... (54 - 65) 4.1 Result of System Test Run ... 54
4.2 Result of Failure Modes and Effect Analysis .... ... 57
4.3 Result ofHAZOP And Operability Study ... ... 59
4.4 Result ofF'mlt Tree Analysis ... 61
5 DISCUSSION ... ... ... (66 -71) 6 CONCLUSION AND RECOMMENDATION... (72 - 75) 6.1 Conclusion... ...72
6.2 Recommendation ...73
REFERENCES
APPENDICES
L
Introduction
CHAPTER 1
INTRODUCTION
1.1 Control System
In every day life we encounter control systems in operation and actually perform many controlling actions ourselves. Take, for example, picking up a cup of coffee from the table, which involves the movement of our hands in a coordinated fashion. This movement is made in response to signals arising due to information obtained from our senses - in this case sight in particular. The action of picking up the cup of coffee, is an example of the human being behaving as an intelligent control system exhibiting, in this case, signs of accuracy and balance - both of which are difficult properties to quantify and study.
Certain properties are common between many different fields, examples being distance, height, speed, flow rate, voltage, etc., and it is the mere fact of this common ability that has given rise to the fi~ld of control system. In its most general sense a system can be virtually any part of life one cares to consider, although it is more usual for a system to be regarded as something to which the concepts of cause and effect apply. A control system can then be thought of as a system for which we manipulate the cause
element in order to arrive at a more desirable effect. In terms of engineering it therefore follows that the study of control systems is multi-disciplinary and is applicable equally
1
Introduction
well in the fields of chemical, mechanical, electrical, electronic, marine, nuclear, etc., engmeenng.
A system is defined as a combination of components that act together and perform
a certain objective. A system is not limited to physical ones. The concept of the system can be applied to abstract, dynamic phenomena such as those encountered in daily life.
Control, as defined by Matsushika Ogata (1990), are means of measuring the value of the controlled variable of the system and applying the manipulated variable to the system to correct or limit derivation of the measured value from a desired value. By combining both definitions, a control system can be defined as an interconnection of components fonning a system configuration that will provide a desired system response. A great variety of components may be a part of a single control system, whether they are electrical, mechanical, hydraulic, pneumatic, or human, or any combination of these. The desired result is a value of some variable in the system, for example, the temperature of a room, the level of liquid in a tank, or the pressure in a pipe. In terms of the system under control, an actuator is used to apply a signal as input to the system whereas a suitable measurement device is employed to witness the response in the form of an output from the system. Schematically, a systeIJ,1 can therefore be depicted as a block that operates on an mput signal in order to provide an output signal. The characteristics of that system are then contained within the block, as shown in Figure 1.1.
2
Introduction
INPUT SIGNAL SYSTEM 1 - - - -OUTPUT SIGNAL
Figure 1. 1 Basic System Schematic [Bateson, 1999]
1.2 Basic Concepts
In order to study the characteristic and behavior of the control unit, the basic concepts of control systems must be understood first. The basic elements of control systems are illustrated in Fig. 1.2. They are generic in that every control system will have at least these elements. More complex systems with mUltiple loops and interacting controls are combinations of the basic elements defined below. The elements derive from the goal of synthesizing a feedback loop to control a physical process to a desired output.
External Power Disturbanc e
r---J---
I II
-7
ActuatorP-7
Law I
:
I
State /
I
/
Output-"'.
Process
Measurement
I Synthesized Elements of
:
1
Desired I
Output
----7
Comparatorr7
ControI I
: T
I
' - - ---if
_ _ _ _ _ _ _ _ _ _ _ _ _ I Control System
Fig 1.2 Control System Element and Configuration [Bateson, 1999]
3
Introduction
Block Diagrams
A block diagram is a schematic representation of a system that depicts the major elements in a control system and how they are connected. Fig. 1.2 is an example of a block diagram. A block diagram illustrates power and information flow as well as interconnections. In their strictest form, for mathematical relationships between inputs and outputs are given in the blocks of the diagram.
Compensation
Compensator is the filter or device (and the accompanying design procedure) that
compensates for one or more poor response areas in the frequency domain of a process to be controlled.
Controller
The name given to that element of the control system that operates on the comparison of the desired and actual output of the system and provides inputs to the process actuator to bring the output closer to the desired response. The controller incorporates the intelligence of the control system. Synthesis in a control system is
...
primarily concerned with defining the controller.
Dynamic System
A dynamic process is a process that requires differential equations for its model with time as an independent variable. A dynamic system has energy storage that is out of
4
Introduction
phase with its motion. In order to synthesize a control, then, infonnation on the time relationship of the energy storages and the motions must be understood. This knowledge is embodied in the solutions of time-dependent differential equations.
Disturbance
An influence on a process or control that is generally not predictable except statistically. A control system compensates for such disturbances by either measuring them directly or detecting their influence on the output of the system and countering their effects through the controller and actuator.
Error is defined as the difference between the desired value of an output of a controlled system and the actual value. The resulting signal is most of the used as an input to the controller, whose output is used to correct the error.
Feedback
Measuring the process output and transmitting back to the originating input location the results of a controller and actuator and the disturbances changing the behavior. This is often described as 'closing the loop'. An open-loop system is one where there is no feedback. Negative feedback is the common type; positive feedback is used occasionally for increasing the gain. The feedback signal is basic to automatic control,
5
Introduction
and the accuracy of the required measurement transduction is the limiting factor in the accuracy ofa control system.
Gain
Gain is the multiplier of the error in a controller to produce the required unit of actuation per unit change in the error. High gain, then, denotes a quick-responding system.
Inputs
Inputs are the variables that are the causal factors in changing a system's outputs.
Inputs vary with time in the same frequency range as the system response.
Outputs
Outputs are the variables defined as those of primary interest to be measured and controlled. These variables are defined as part of the development of the model.
Experiments with the actual system will indicate which of the many variables are important and essential in characterizing the system's response.
-.
1.2.1 Open Loop Dynamic
Studying the non-stationary situations is systems dynamic, and it always previous to the appropriate control design which prevents from the irreversibility of some
6
Introduction
diversions of the stationary state: during the process, the manipulated variables change the value of the controlled variables, in a way in which even the most simple cases can be easily predicted with a mathematical model. For all this is it essential a deep knowledge of the process [Edibon, 2001].
The dynamic simulation of a process consists on developing the equations of mass and energy balance, as well as physic laws or other different sources of information, on selecting the figures from the required physica~-chemical values and from the prefixed variables, and finally on obtaining the difference from the stationary regime from the initially defined state.
Open loop dynamic represents the performance of the process in the absence of controllers. That way, the speed and the tolerance of the answer of the process facing disturbances can be studied, and the needs of control systems can be justified.
1.2.2 Feedback Control
Feedback control is based on measuring the controlled variable, on contrasting it with the wanted value (ordered value), and, depending on the margin of error, on acting
...
on the control element of the manipulated variable. In a feedback control system, the correction effect persists as long as the error is not null; the corresponding signals are spread backwards through the circuit, creating comparison, calculation and correction (closed loop) cycles with a special time of answer [Edibon, 2001].
7
Introduction
The elements of a control system in closed loop are: the sensor, the transducer, the comparer, the controller and the final controlling element. These elements are described briefly below:
Sensor - Any mechanism that is able to measure the value of the controlled variable (also called primary measuring element).
Transducer - It converts the obtained figures into normalized, pneumatic or electric equivalent signals according to the distance between the process and the control room, and these signals are sent to the comparer (also called signals transducer element).
Controller - It receives the error of the comparer, interprets it and acts on the final control element according to the three types of correcting actions:
~ Proportional the signal sent to the final control element is proportional to the error (if the proportion behaves as alVnothing)
~ Integral - the corrector signal is proportional to the error accumulated with the time (error integral).
~ Derivative If the signal is proportional to the speed of error variation.
The two last options are normally combined with the main one (proportional) in
order to improve the control quality; in more complex processes the three of them are combined (PID).
8
Introduction
Final Control Element - It is a mechanism that acts on the manipulated variable according to the signal of the controller, and regulates the inlet flow of mass or energy to the process (valve, volumetric bomb, compressor, rheostat, etc.). The control systems are effective when dealing with small disturbances, as otheIWise the final element would be completely open or closed, and it would not work properly over a certain figure (saturated); the correction should be done manually by modifying the parameters of the unit.
1.3 Control of Pressure
Applications of pressure measurement equipments are found on a wide variety of equipment used in industry and at home. For example, typical equipment includes the compressor, fire extinguishers, sprinkler systems for fire protection, domestic hot-water heating systems, medical equipments, and so on. There are many reasons why it may be necessary to measure the pressure of fluid. With these pressure-measuring devices, it helps to indicate whether the devices or equipments are in good condition or not. For example, a pressure loss in a fire ;extinguisher indicates that the device cannot be used anymore. When the conditions of the equipments are known, precautions may be taken whenever the equipments are not functioning well. In some applications, only a rough indication of the pressure is needed, while in others the pressure may be critical, requiring
an accurate measurement in order to avoid endangering personnel and equipment. Several reasons are explained below.
9
Introduction
Providing Operating Information
Many products from the petrochemical industries require careful control of pressure during the manufacturing process. While the actual control will be probably automatic, a pressure gauge will give the operator a constant indication of the pressure so that adjustments can be made to the control loop.
In some applications the normal pressure of the process may not be highly critical, but it is necessary to know when the pressure exceeds some set limit. If the pressure becomes too high due to an abnormal condition in the process, it may cause damage to instrumentation, pumps, or other equipment or even burst the pressure vessel. A continuous indication of the pressure will allow the operator to shut down the process or vent the system before any damage is done.
Providing Test Data
In the shop or laboratory, pressure measurements are often required as part of the
evaluation and testing of materials or equipment. Generally such measurements must be ~.
made with a high degree of accuracy, requiring the use of a special class of gauge called test gauges.
10
Introduction
Measuring Quantity
The quantity of a gas stored in a tank is proportional to the pressure of the gas.
Therefore a pressure gauge when installed on a tank of known volume can be calibrated in tenns of quantity, enabling the user to determine how much gas is consumed by a particular process.
The quantity of liquid in a tank of known volume can be calculated by measuring the pressure of the liquid at the bottom of the tank. Gauges are furnished calibrated in tenns of gallons of a specified liquid.
Measuring Force
The force generated by a piston and cylinder can be calculated by mUltiplying the area of the piston by the pressure acting on the area. Thus gauges for use with hydraulic presses may be supplied with dials calibrated in terms of tons of force.
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![Figure 1. 1 Basic System Schematic [Bateson, 1999]](https://thumb-ap.123doks.com/thumbv2/123dok/3876527.3973887/16.892.2.868.19.1232/figure-basic-system-schematic-bateson.webp)
