Bourdon Tubes

Bourdon Tubes are known for its very high range of differential pressure measurement in the range of almost 100,000 psi (700 MPa). It is an elastic type pressure transducer.

The device was invented by Eugene Bourdon in the year 1849. The basic idea behind the device is that, cross-sectional tubing when deformed in any way will tend to regain its circular form under the action of pressure. The bourdon pressure gauges used today have a slight elliptical cross-section and the tube is generally bent into a C-shape or arc length of about 27 degrees. The detailed diagram of the bourdon tube is shown below.

As seen in the figure, the pressure input is given to a socket which is soldered to the tube at the base. The other end or free end of the device is sealed by a tip. This tip is connected to a segmental lever through an adjustable length link. The lever length may also be adjustable. The segmental lever is suitably pivoted and the spindle holds the pointer as shown in the figure. A hair spring is sometimes used to fasten the spindle of the frame of the instrument to provide necessary tension for proper meshing of the gear teeth and thereby freeing the system from the backlash. Any error due to friction in the spindle bearings is known as lost motion. The mechanical construction has to be highly accurate in the case of a Bourdon Tube Gauge. If we consider a cross-section of the tube, its outer edge will have a larger surface than the inner portion. The tube walls will have a thickness between 0.01 and 0.05 inches.

Working

As the fluid pressure enters the bourdon tube, it tries to be reformed and because of a free tip available, this action causes the tip to travel in free space and the tube unwinds. The simultaneous actions of bending and tension due to the internal pressure make a non-linear movement of the free tip. This travel is suitable guided and amplified for the measurement of the internal pressure. But the main requirement of the device is that whenever the same pressure is applied, the movement of the tip should be the same and on withdrawal of the pressure the tip should return to the initial point.

A lot of compound stresses originate in the tube as soon as the pressure is applied. This makes the travel of the tip to be non-linear in nature. If the tip travel is considerably small, the stresses can be considered to produce a linear motion that is parallel to the axis of the link. The small linear tip movement is matched with a rotational pointer movement. This is known as multiplication, which can be adjusted by adjusting the length of the lever. For the same amount of tip travel, a shorter lever gives larger rotation. The approximately linear motion of the tip when converted to a circular motion with the link-lever and pinion attachment, a one-to-one correspondence between them may not occur and distortion results. This is known as angularity which can be minimized by adjusting the length of the link.

Other than C-type, bourdon gauges can also be constructed in the form of a helix or a spiral. The types are varied for specific uses and space accommodations, for better linearity and larger sensitivity. For thorough repeatability, the bourdon tubes materials must have good elastic or spring characteristics. The surrounding in which the process is carried out is also important as corrosive atmosphere or fluid would require a material which is corrosion proof. The commonly used materials are phosphor-bronze, silicon-bronze, beryllium-copper, inconel, and other C-Cr-Ni-Mo alloys, and so on.

In the case of forming processes, empirical relations are known to choose the tube size, shape and thickness and the radius of the C-tube. Because of the internal pressure, the near elliptic or rather the flattened section of the tube tries to expand as shown by the dotted line in the figure below (a). The same expansion lengthwise is shown in figure (b). The arrangement of the tube, however forces an expansion on the outer surface and a compression on the inner surface, thus allowing the tube to unwind. This is shown in figure (c).

Expansion of Bourdon Tube Due to Internal Pressure Like all elastic elements a bourdon tube also has some hysteresis in a given pressure cycle. By proper choice of material and its heat treatment, this may be kept to within 0.1 and 0.5 percent of the maximum pressure cycle. Sensitivity of the tip movement of a bourdon element without restraint can be as high as 0.01 percent of full range pressure reducing to 0.1 percent with restraint at the central pivot.

Types of Bourdon Tube

There are 3 main types of elastic elements for pressure measurement, namely

Bourdon Tubes,
Bellows, and
Diaphragm.

C-Type Bourdon Tube

This instrument is by far the most common device used to indicate gauge pressure throughout the oil gas industry.

A bourdon tube obey Hookes Law, that is within elastic limits. Its free end will experience a movement that is proportional to the fluid pressure applied. The measuring element named for bourdon is partially flattened metal tube formed in a 250° Arc. The tube is sealed at one end (the tip ) and connected to the pressure at the other end (socket).

Any pressure inside the tube exceeding the pressure on the outside cause the tube to become more circular in cross section. As a result, the tip moves in an arc. This movement is connected through a level, quadrant and pinion to a pointer which moves round a scale to indicate the pressure.

The amount of movement of the free end of the tube is directly proportional to the pressure applied ( providing the tube elastic limit is not exceeded ).

Where greater sensitivity is required, the bourdon tube may be constructed in the form of a Spiral or Helix.

Spiral Bourdon Tube

Spiral Bourdon Tube is made by winding a partially flattened metal tube into a spiral having several turns instead of a single C-bend arc.

The tip movement of the spiral equals the sum of the tip movements of all its individual C-bend arcs.

Therefore it produces a greater tip movement with a C-bend bourdon tube. It is mainly used in low- pressure application. Spiral bourdon tube is shown in figure.

Helical Bourdon Tube

Helical is a bourdon tube wound in the form of helix. It allows the tip movement to be converted to a circular motion.

By installing a central shaft inside the helix along its axis and connecting it to the tip, the tip movement become a circular motion of the shaft.

Advantages of the Spiral and Helical Tubes over the C-Type Bourdon Tube

Both the spiral and helical tubes are more sensitive than the C-Type tube. This means that for a given applied pressure a spiral or helical tube will show more movement than an equivalent C-Type tube, thus avoiding the need for a magnifying linkage.
Spiral and helical tubes can be manufactured in very much smaller sizes than the equivalent C-Type tubes. Hence, they can be fitted into smaller spaces, such as inside recorders or controller cases where a C-Type would be unsuitable because of the size.

Application Of Bourdon Tube Elements

Before using a Bourdon tube on a particular process application, a number of questions need to be considered. We need only to consider two here.

What is the maximum operating pressure likely to be encountered by the tube? Manufacturers recommend that the normal operating pressure should not exceed 60% of the maximum scale reading. For example, if the normal working pressure were 6 bar, we would select a bourdon tube instrument ( pressure gauge) having full-scale deflection of 10 bar.
Is the process fluid corrosive or non corrosive? Material for the bourdon tubes must be able to handle the process fluid. Therefore, selection of pressure gauge must take into account the corrosivity of the line fluid.

Bourdon tube Working principle and Applications