Bourdon Tube

PREPARED BY:-

JOYSRI DATTA 09103005059 091030110364 09/05/12 th SEMESTER:- 6

COLLEGE- HALDIA INSTITUTE OF TECHNOLOGY

CONTACT NO.- +91 9475887439

TABLE OF CONTENTS:-

ITEM

Acknowledgements

PAGE NO. 2

Importance of pressure measurement

3

Introduction to bourdon tube

4

Classifications of bourdon tube

5

Types of bourdon tubes

6

C-type Bourdon Tube

7-11

Spiral & helical type Bourdon Tubes

12-17

Differential Gauge

17

Advantages of Bourdon tubes

18

Limitations of Bourdon tubes

18

© JOYSRI DATTA (09/EI/59) 1

Bibliography

19

ACKNOWLEDGEMENT:

Before I delver into the deeper aspects of this project I would like to acknowledge the contribution of the internet & the central library of this institute for providing the relevant information about this project. The report has been written with the active support and assistance of my friends. I would also express my sincere thanks to our Head of the Dept. Prof. DEBADUTTA GHOSH for his assistance in writing this report. Last but not the least I would like to thank my beloved teachers of our dept. who has provided me the opportunity to do this project.

.


IMPORTANCE OF PRESSURE MEASUREMENT Nearly all industrial processes (e.g. metal, chemical, etc) use liquids, GASSES or both. Controlling these processes requires the measurement and control of liquid and gas pressure. So, pressure measurement is one of the most important of all process measurement.

UNDERSTANDING OF PRESSURE:

Pressure is defined as the amount of force applied to a surface or distributed over it.

PRESSURE=FORCE/AREA. METHODS OF PRESSURE MEASUREMENT:

Most pressure instruments measures a difference between two pressures, one usually being that of the atmospheric. The different methods of pressure measurements are: 1. Manometer method. 2. Elastic pressure transducers. 3. Pressure measurement by measuring vacuum. 4. Electrical pressure transducer


INTRODUCTION TO BOURDON TUBE: Bourdon tube is one type of elastic primary sensing element which is used in elastic pressure transducer. The principle of inferring the pressure from the measured deformation of an elastic material is used here. It may be emphasized that any material will be deformed or distorted when any force, no matter how small or great, is applied to it. If all material react in this manner, the amount of deformation or movement of distortion can be used as the measure of the force, and consequently of the pressure which created the force. The pressure range, the no. of operating cycles, formability and the medium which exerts the pressure on the inside of the tube will dictate the material to be used in tube constructions. Phosphor — bronze, bronze, beryllium copper, steel chrome-alloy steels and stainless steels are generally used. For the low pressure ranges (upto 2000 psi) Phosphor — bronze bronze is generally preferred; where corrosion is a problem, stainless steel may be employed. Stainless steel is used for high pressure ranges (above 2000psi). It is most frequently used pressure gauge because of its simplicity and rugged construction. It covers the range from vacuum to very high pressure. It measures the pressure of liquids and gases of all kinds, including steam, water, and air up to pressures of 100,000 pounds per square inch.


The majority of pressure gauges in use have a Bourdon-tube as a measuring element. (The gauge is named for its inventor, Eugene Bourdon, a French engineer.) The Bourdon tube is a device that senses pressure and converts the pressure to displacement. Since the Bourdon-tube displacement is a function of the pressure applied, it may be mechanically amplified amplified and indicated by a pointer.

CLASSIFICATION OF BOURDON TUBE:

Depending

upon the no. of tappings the bourdon tube pressure gauges

are classified as: 

Simplex type



Duplex type 

A simplex gauge has only one bourdon tube and measures only one pressure.



When two bourdon tubes are mounted in a single case with each mechanism acting independently but with two pointers mounted on a common dial, the assembly is called a duplex gauge.


TYPES OF BOURDON TUBES The Bourdon tubes are designed in various forms. These are 1. C-type Bourdon tube 2. Spiral type Bourdon tube 3. Helical type Bourdon tube


C-TYPE BOURDON TUBE CONSTRUCTION:

The bourdon tube pressure instrument is one of the oldest pressure sensing instruments in use today. The bourdon tube (refer to Figure A-1) consists of a thinwalled tube that is flattened diametrically on opposite sides to produce a crosssectional area elliptical in shape, having two long flat sides and two short round

sides. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. The main parts of this instrument are as follows: An elastic transducer that is bourdon tube which is fixed and open at one end to receive the pressure which is to be measured. The other end of the bourdon tube is free and closed. The cross-section of the bourdon tube is elliptical. The bourdon tube is in a bent form to look like a circular arc. To the free end of the bourdon tube is attached an adjustable link, which is in turn connected to a sector and pinion as shown in diagram. To the shaft of the pinion is connected a pointer which sweeps over a pressure calibrated scale. MECHANICAL DETAILS:


Stationary parts: 







A: Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case. B: Chassis plate. The face card is attached to this. It contains bearing holes for the axles.

C: Secondary chassis plate. It supports the outer ends of the axles. D: Posts to join and space the two chassis plates.

Moving Parts:

1. Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block. 2. Moving end of Bourdon tube. This end is sealed. 3. Pivot and pivot pin. © JOYSRI DATTA (09/EI/59) 8

4. Link joining pivot pin to lever (5) with pins to allow joint rotation. 5. Lever. This is an extension of the sector gear (7). 6. Sector gear axle pin. 7. Sector gear. 8. Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Due to the short distance between the lever arm link boss and the pivot pin and the difference between the effective radius of the sector gear and that of the spur gear, any motion of the Bourdon tube is greatly amplified. A small motion of the tube results in a large motion of the indicator needle. 9.

Hair spring to preload the gear train to eliminate gear lash and hysteresis.

OPERATING PRINCIPLE:

The pressure to be measured is connected to the fixed open end of the bourdon tube. The applied pressure acts on the inner walls of the bourdon tube. Due to the applied pressure, the bourdon tube tends to change in cross – section from elliptical to circular. This tends to straighten the bourdon tube causing a displacement of the free end of the bourdon tube. This displacement of the free closed end of the bourdon tube is proportional to the applied pressure. The resulting movement of the free end of the tube causes the pointer to move over the scale. The displacement of this end is amplified and is calibrated in terms of pressure. The movement of the tube at the free end is called tip travel. Connecting link connects the tip of the bourdon tube of the linear rotary motion unit for transmitting the motion of the tip to the linear rotary motion unit.


The movement of the tube at the free end is called tip travel. Connecting link connects the tip of the bourdon tube of the linear rotary motion unit for transmitting the motion of the tip to the linear rotary motion unit. Since, the resulting tip motions non-linear because less motion results from each increment of additional pressure. It has to be converted into linear rotational pointer response. This is done mechanically by means of a geared sector and pinion movement In practice, a flattened thin-wall, closed-end tube is connected at the hollow end to a fixed pipe containing the fluid pressure to be measured. As the pressure increases, the closed end moves in an arc, and this motion is converted into the rotation of a (segment of a) gear by a connecting link which is usually adjustable. A small diameter pinion gear is on the pointer shaft, so the motion is magnified further by the gear ratio. The positioning of the indicator card behind the pointer, the initial pointer shaft position, the linkage length and initial position all


provide means to calibrate the pointer to indicate the desired range of pressure for variations in the behavior of the Bourdon tube itself. The angle between the connecting link and the sector tail is called the traveling angel. This angle changes with tip movement compensating for the nonlinearity of the tip movement itself. Differential pressure can be measured by gauges containing two different Bourdon tubes, with connecting linkages. Bourdon tubes measure gauge pressure, relative to ambient atmospheric pressure, as opposed to absolute pressure; vacuum is sensed as a reverse motion. Some aneroid barometers use Bourdon tubes closed at both ends (but most use diaphragms or capsules, see below). When the measured pressure is rapidly pulsing, such as when the gauge is near a reprocating pump, an orfice restriction in the connecting pipe is frequently used to avoid unnecessary wear on the gears and provide an average reading; when the whole gauge is subject to mechanical vibration, the entire case including the pointer and indicator card can be filled with an oil or glycerin. Typical high-quality modern gauges provide an accuracy of ±2% of span, and a special high-precision gauge can be as accurate as 0.1% of full scale .


SPIRAL AND HELICAL BOURDONS: The displacement of tip varies inversely as the wall thickness and depends upon the cross-sectional form of the tube. It also varies directly as the length of arc which in turn depends upon the angle subtended by the arc through which it is 0

bent. Thus in a tube having an arc of 180 , the displacement of the tube will be 0

twice that of a similar tube having an arc of 90 .Therefore The displacement of the free end (tip) may be increased by increasing the length of the arc of the tube without changing the wall thickness. 0

When the angle through which the arc is bent reaches 360 , its length can be increased further in two ways: I.The tube can be made in the form of a spiral. II.The tube can be made in the form of a helix. An increased displacement of the free end can be obtained by increasing the no. of turns in the spiral or helix avoiding the need for further magnification. It has been mentioned earlier that magnification is obtained through the use of geared sector and pinion is used in C-type Bourdon tube. But while using spiral or helix type bourdon tubes there is no need for further amplification and hence the geared sector and pinion arrangement is not used in their case. The absence of geared sector and pinion arrangement eliminates the backlash which tends to occur when they (geared sector and pinion) become worn owing to continued use.


SPIRAL TYPE BOURDON TUBE

Bourdon tubes also may be made in the form of a spiral uses a long length of flattened tubing to provide increased tip travel. This does not change the operating principle of the Bourdon tube, but produces tip motion equal to the sum of the individual motions that would result from each part of the spiral considered as a Cshape. Small-diameter spirals can be manufactured to provide enough motion to drive the indicating pointer directly through an arc up to 270° without having to use a multiplying movement. Alternatively, they may be manufactured to be used in conjunction with a multiplying movement. In this case, the required motion is distributed over several turns, resulting in lower stress in the Bourdon material. This improves fatigue life when compared to a C-shaped C -shaped Bourdon tube in the same pressure range. 

It is essentially a series of C-bourdon tube joined end to end.



When the pressure to be measured is applied to the spiral, this flat spiral tends to uncoil producing a greater movement of the free end requiring no mechanical amplification.



Hence the geared sector and pinion arrangement is not used here.




The spiral tubes reduce work hardening at elevated pressures by spreading the higher degree of stress over more than 1,000 degrees of coil, thereby minimizing the possibilities of failure.



This increases the sensitivity and accuracy of the instrument because no lost motion or friction is introduced through the links and levers.



The absence of geared sector and pinion arrangement eliminates the backlash.



The accuracy of spiral tube is higher than that of C-type elements on account of absence of friction.



The accuracy is typically about 0.5%


HELICAL TYPE BOURDON TUBE:

Another mechanical high pressure sensor uses a helical Bourdon element. This device may include as many as twenty coils and can measure pressures well in excess of 10,000 psig. The standard element material is heavy-duty stainless steel, and the measurement error is around 1% of span. Helical Bourdon tube sensors provide high over range protection and are suitable for fluctuating pressure service, but must be protected from plugging. This protection can be provided by highpressure, button diaphragm-type chemical seal elements. An improvement on the design shown in Figure detects tip motion optically, without requiring any mechanical linkage. This is desirable because of errors introduced by linkage friction. In such units, a reference diode also is provided to compensate for the aging of the light source, for temperature variations, and for dirt build-up on the optics. Because the sensor movement is usually small (0.02 in.), both hysteresis and repeatability errors typically are negligible. Such units are available for measuring pressures up to 60,000 psig. 

It is wound in the form of a helix. A helical element is shown in the figure.



The displacement of the tip is larger than that of a spiral element.



Usually a central shaft is installed within this element and a pointer is driven from the shaft by connecting links.



This system transmits only the circular motion of the tip to the pointer which is directly proportional to the changes in pressure.




One advantage of this design includes the high over range capabilities which may be in the ratio as high as 10: 1.



It is suitable for pressure measurement on continuously fluctuating services.



It is adaptable for high pressure services.




High pressure elements might have as many as twenty coils, while low-span sensors can have only two or three coils.



The no. of coils employed in helix elements depends upon the pressure to be measured.



The accuracies obtainable from helical elements may vary from +



The helical tubes reduce work hardening at elevated pressures by spreading the higher degree of stress over more than 1,000 degrees of coil, thereby minimizing the possibilities of failure.

DIFFERENTIAL GAUGE: Another version of bourdon tube instruments is the differential pressure gauge, which consists of two tubes that actuate one measuring element jointly but in opposite directions, thus indicating difference btw. two measurements.


ADVANTAGES OF BOURDON TUBE PRESSURE GAUGES: 1. These Bourdon tube pressure gauges give accurate results. 2. Bourdon tube cost low. 3. Bourdon tubes are simple in construction. 4. They can be modified to give electrical outputs. 5. They are safe even for high pressure measurement. 6. Accuracy is high especially at high pressures. 7. Their greatest advantage is that they are easily adapted for designs for obtaining electrical outputs.

LIMITATIONS OF BOURDON TUBE PRESSURE GAUGES: 1. They respond slowly to changes in pressure. 2. They are subjected to hysteresis. 3. They are sensitive to shocks s hocks and vibrations. 4. Amplifications is a must as the displacement of the free end of the bourdon tube is low. 5. It cannot be used for precision measurement.


BIBLIOGRAPHY 1. A

course

in

ELECTRICAL

MEASUREMENTS

AND

AND

ELECTRONICS

INSTRUMENTATION

by

A.K.SAWHNEY, PUNEET SAWHNEY 2. INDUSTRIAL INSTRUMENTATION INSTRUMENTATI ON AND CONTRON by S.K.SING

3. http://www.engineersedge.com/instrumentation 4. http://en.Wikipedia.org/wiki/pressure


Bourdon Tube

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