02 Presure Measurement

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Oil Pipeline Design, Operation and Construction Metering Instruments

7.2 Pressure Measurement

For a fluid at rest pressure can be defined as the force exertedperpendicularly by the fluid on a unit area of any bounding surface. It isbasically a mechanical concept which can be fully described in terms of the primary dimensions of mass, Length and time.

The whole of the pressure measurement is encompassed on theabove definition and the following three observations:

1. The pressure is strongly influenced by position within astatic fluid but, at a given position, it is quite independent of direction. (This fact has to be taken into account inmanometry).2. Pressure is unaffected by the shape of the confiningboundaries. (Thus a great variety of fluid pressure transducersis available).3. A pressure applied to a confined fluid via, a movablesurface is transferred undiminished throughout the fluid to allbounding surfaces. (Principle employed in dead-weighttesters).

The zero gauge pressure corresponds to atmospheric pressure andis approximately 14.696 psi or 1 kg/cm 2. The pressure can also bemeasured in terms of liquid columns which can be supported by them Interms of mercury column atmospheric pressure will be equal to 29.92inches or 760 mm.

In practice we may require to measure gauge which is the differencebetween the measured and atmospheric pressure or absolute pressurewhich is equal to the gauge pressure plus the atmospheric pressure or pressures below atmospheric, which is termed as, vacuum or draft. Thevacuum or draft forms a very small portion of the total measurementrange.

The Bourdon, diaphragm and bellow are the common elements usedin pressure measurements because of their flexibility for variouspurposes such as indicating, recording, controlling and operation of various protection devices. Manometric elements using water andmercury measuring liquid are also in use for measurements in lower

ranges.There are a lot of types of pressure measuring instruments but not all

of it in commonly and practically used in pipe line field.In the next section we are going to present some of the commonly

used measuring instruments, one classic of type and two modern types.We introduce types of pressure measuring instruments that is going

to be used in pump station and along the line to serve the SCADAsystem collecting data from monitor points by fiber optics, microwaves

John Samir Zaky Mina Safwat Sobhy Samir Fouad Nasif Usama Boshra Habib


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and satellite communications technology and transmit it to controlstations.7.2.1 Mechanical Pressure Transducers

Bourdon Tube, The main feature of the Bourdon tubes is their largedeflection.

The bourdon tube consists of an elastic element (a small-volumetube of oval cross-section bent in a circular arc) fixed at one end whichis open to accept the applied pressure, but free at the other end, whichis closed to allow displacement under the deforming action of thepressure difference across the tube walls. When pressure is applied, theoval shaped tube tends to become circular, with a subsequent increasein the radius of the circular arc. The movement of the free end of thetube is transmitted to a sector through an adjustable connecting link.The sector is in mesh with pinion on which is fixed a pointer, whichrotates over a calibrated scale to give a mechanical indication of thepressure. As the reference pressure in the case containing the bourdontube is usually atmospheric, the pointer indicates gauge pressure.Bourdon tubes are susceptible to vibration and acceleration.

Because of the simplicity and ruggedness, Bourdon tube is the mostfrequently used pressure gauge. It can be used for ranges from zero to1 kg/cm 2 (zero to 15 psig) to zero to 700 kg/cm 2 (zero to 100,000 lb /square inch) On vacuum side measurements from zero to 760 mm (zeroto 29.92 inches) are possible

Figure 7.2.1 Bourdon tube transducer



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7.2.2 Passive Electrical Pressure Transducer The various types of electro-mechanical transducers available work

on the following principles:1. Unbonded resistance2. Bonded strain gauges with flat diaphragms3. Other types

Unbonded resistance elements, The unbonded resistance elementscan be employed for determining the pressure by straining theresistance elements by pressure. The device employs a four-leggedflexure linkage. Two resistance elements of very small diameter (about0125 mm) are wound around the legged flexure. A diaphragm is placedat the top, centre of the device and its movement due to application of pressure is transmitted to flexure thereby stretching the resistanceelements. These two active elements, along with two inactive elementswound around fixed posts are connected to form a conventionalresistance bridge. This device can be used for absolute, differential or gauge pressure ranges from 0-0.1 to 0-35 kg/cm 2 for temperatureranging from -65 to 300 C

Bonded strain gauges with flat diaphragms, In this device straingauges are applied directly to a diaphragm surface either at the centreson diametrically opposite faces or as shown in Figure 7.2.2. In both the

arrangements one gauge is subjected to tension while the other gaugesenses compression. The two gauges are used in adjacent bridge arms,thereby adding their individual outputs and simultaneously providingtemperature compensation. It is generally found that the area for mounting the strain gauges is very small. Therefore, spiral grids have tobe used, unfortunately whose performance is not satisfactory.



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Figure 7.2.2 Bonded strain gauges with flat diaphragms

Other types, One method utilizes the change of capacitance due toflexing of the diaphragms.

Another method is based on electro-mechanical resonance principle.In this method, a fine wire under tension vibrates at its natural frequencyand one end of it is connected to the centre of a pressure-sensingdiaphragm, which varies the wire tension, depending on the appliedpressure. The wire vibrates in a magnetic field provided by smallpermanent magnets. Due to the vibration of wire in magnetic field, an

AC potential is developed in the wire. This E.M.F. is amplified and fedback to energize driving coils which maintain the vibration. The pressureis measured by the output Frequency.



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7.2.3 Active Electric Pressure Transducers

Piezoelectric crystals (quartz, Rochelle salt, barium titarate,lead-,zinconate-titanate) are the elements which provide basis for theactive electric pressure transducers. These produce a surface potentialdifference when stressed in appropriate directions. Pressure pick-upsdesigned around such active elements have the crystal geometryoriented to give maximum piezoelectric response in a desired directionwith little or no response in other directions. In piezoelectric transducers,although the E.M.F. developed is proportional to pressure, it is generallyvery difficult to calibrate it by normal static pressures. The latesttechnique is to calibrate these by exciting them by an electric field rather than by an actual physical pressure.


02 Presure Measurement

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