The measurement of pressure and vacuum has always been important in dairy processing industry. Such measurements are generally made continuously with standard industrial appliances. Before we study these pressure measuring devices,let us first understand the concept of pressure and vacuum.
i. Concept of Pressure and Vacuum
The pressure exerted by fluid is given in terms of force per unit area; the force exerted in a direction perpendicular to the surface of unit area. Following terms are generally associated with pressure and its measurement.
Atmospheric Pressure (P at ): This is the pressure exerted by the envelope of air surrounding the earth’s surface. Atmospheric pressure is usually determined by a mercury-column barometer. Atmospheric pressure varies with altitude. At sea level,the value of atmospheric pressure is close to 1.013 Kg/cm 2 or 1.01325 bar or 760 mm of mercury column.
Absolute Pressure (P abs ): Pressure has been defined as the force per unit area due to interaction of fluid particles among themselves. Zero pressure intensity will occur when molecular momentum is zero. Such a situation may occur only when there is perfect vacuum, that is, a vanishingly small population of gas molecules or of molecular velocity. Pressure intensity measured from this state of vacuum or zero pressure is called absolute pressure.
Gauge Pressure (P g ) and gauge vacuum (P vac ): Instruments and gauges used to measure fluid pressure generally measure the difference between the unknown pressure ‘P’ and the existing atmospheric pressure P at . When the unknown pressure is more than the atmospheric pressure, the pressure recorded by the instrument is called gauge pressure. A pressure reading below the atmospheric pressure is known as vacuum or negative pressure. Actual absolute pressure is then the algebraic sum of the gauge indication and the atmospheric pressure.
P abs = P at + P g
P abs = P at - P vac
Units of Pressure: The unit used for expressing pressure is ‘Atmosphere’ abbreviated as ‘atm’. One ‘atm’ is simply the pressure in Kg/cm 2 exerted by the atmosphere. Pressure can be expressed in multiples of ‘atm’.The unit of pressure in M.K.S. system is Kg/cm 2 , in S.I. units N/m 2 and in F.P.S.units is pound per sq. inch (psi). One atm is equal to 1.013 Kg/cm 2 It can also be expressed as the height of mercury column. One atm is equal to 76 cm of Hg column at 0°C.
ii. Pressure Gauge
Bourdon tube type pressure gauge is the most common for industrial use. It employs a Bourdon tube element for direct indication of pressure. The bourdon tube element can be of ‘C’ type, spiral type or helical type. Due to its simplicity and ruggedness the ‘C’ type element is commonly employed.
Principle of operation: One end of the ‘C’ type bourdon tube element is sealed at its tip, while the other end is connected to the process pressure which is to be measured. Because of its ‘C’ type shape, there is a difference between inside and outside radii of the tube and the bourdon tube presents different areas to pressure.This causes the tube to tend to straighten up when the pressure is applied to it and results in a motion of the sealed tip end of the tube. The extent of movement of the tip of tube depends upon the amount of applied pressure. The movement of sealed end of the bourdon tube is thus an indicator of the applied pressure or vacuum.
Construction: Construction of a ‘C’ bourdon tube as used in a direct indicating gauge. The ‘C’ tube usually has an arc of 250°. The process pressure is connected to the fixed socket end of the tube while the tip end is sealed.As the pressure is applied there is a movement of the sealed end of the tube. This tip motion is non-linear because less motion results from each increment of additional pressure. This non-linear motion has to be converted into linear rotational pointer response. This is done mechanically by means of a geared sector and pinion movement. The tip motion is transferred to the tail of the movement sector by the connector link. The sector tail is called the ‘traveling angle’. This angle changes with the tip movement in a non-linear fashion and so the movement of the pinion and therefore pointer is linear. This type of pressure gauge is used in all industries and may be obtained in sizes from 5cm diameter up to 35 cm diameter and in many different indicating styles.
Material: The metallic materials used for construction of bourdon tube include brass, bronze, phosphor bronze, beryllium-copper alloy, alloy steel or stainless steel.The non-metallic materials are leather, neoprene and rubber.
Installation: In nearly all pressure gauges, the fluid in which pressure is measured is conducted to the inside of the pressure measuring element and is in direct contact with the element. This creates a problem of handling high temperature, corrosive, sludgy or semisolid materials. Some pressure- gauge elements can be protected by copper-plated, nickel-plated or tinned plated surfaces. But this is always possible especially on the inside of the bourdon tubes. Therefore some other method of excluding the measured fluid must be employed.
One of the effective methods of protecting a pressure gauge element is siphon arrangement. A single coil siphon arrangement This is very effective in protecting the pressure-gauge element from the high temperature of steam. The brass coil traps condensate steam and limits the temperature rise in the gauge. A siphon is necessary on all steam pressure gauges.Another arrangement is diaphragm seal as shown in Fig. 12.6. The unit is usually made up of bronze with a neoprene or thin metal diaphragm. The system is solidly filled with a liquid such as glycerin or oil. The diaphragm is quite flexible, so that the pressure on both sides of the diaphragm is equal. This way the problem of corrosive material can be tackled, as such material would not be in contact with the tube element. The line leading to pressure gauge is always filled with clean oil.
iii. Manometers
One of the oldest means of measurement of pressure is liquid column manometer.It is the simplest, most direct and most accurate of all pressure measuring means.This is a fundamental instrument for detecting the pressure and is used for calibration of other sensors. There are no moving parts, no friction or inertia involved in the measurement and therefore, its accuracy is limited only by the scale visibility.
Principle of operation: The pressure exerted by a column of liquid of height ‘h’ and density ‘r’ at the base of the cylinder containing it, is equal to ‘hrg’, where ‘g’ is the acceleration due to gravity. A manometer is shown in Fig. 12.5 below.
Tube Manometer It has two vertical tubes known as legs, which are connected at the base. The assembly is partly filled with manometer liquid, which may be water, oil or mercury.When the legs of the manometer are vertical, then:
The pressure at the bottom of the right leg = P 1 + h 1 rg
The pressure at the bottom of the left leg = P 2 + h 2 rg
When the fluid is under static balance condition, then
P 1 + h 1 rg = P2 + h 2 rg
Therefore P 1 -P 2 = (h2-h 1 ) rg
A manometer can thus be used for measurement of pressure differential, that is,the difference in pressures P 1 and P 2 . Further, if one of the legs of the manometer is connected to the vessel in which the fluid pressure or vacuum is to be measured and other leg is kept open to atmosphere then this manometer would be able to read the pressure difference between the vessel and the atmosphere.
Construction: Liquid manometers are the simplest differential pressure or vacuum detectors. A simple U-tube Manometer It has two vertical tubes known as legs which are connected at the base through a metal fitting. The assembly is partly filled with manometer liquid, which may be water, oil or mercury.An easier-to-read scale can be is attached to the manometer. In industrial installations,the use of glass tube manometers is limited to locations where tube breakage will not create hazardous conditions for the operator.
Materials: The performance of any manometer is largely a function of the indicating fluid selected. Amongst various manometer liquids, most commonly employed are water, oil or mercury. The filling fluid has to be chemically inert and compatible with the process media and produce a clear, visible interface. The fluid should not coat the glass tube and should not be corrosive to standard material such as copper,aluminum and steel. The fluid should not freeze due to low ambient temperature be capable to maintain its density unaffected by temperature.
Installation: Glass manometer is usually mounted on the equipment or pressure vessel at that place where the operator could easily read the manometer scale. In industrial installations, the use of glass tube manometers is limited to locations where tube breakage will not create hazardous conditions for the operator. At critical locations the gauge glass tube is provided with a thick guard glass cover to protect against flying glass pieces in case of accidental breakage of tube.
i. Concept of Pressure and Vacuum
The pressure exerted by fluid is given in terms of force per unit area; the force exerted in a direction perpendicular to the surface of unit area. Following terms are generally associated with pressure and its measurement.
Atmospheric Pressure (P at ): This is the pressure exerted by the envelope of air surrounding the earth’s surface. Atmospheric pressure is usually determined by a mercury-column barometer. Atmospheric pressure varies with altitude. At sea level,the value of atmospheric pressure is close to 1.013 Kg/cm 2 or 1.01325 bar or 760 mm of mercury column.
Absolute Pressure (P abs ): Pressure has been defined as the force per unit area due to interaction of fluid particles among themselves. Zero pressure intensity will occur when molecular momentum is zero. Such a situation may occur only when there is perfect vacuum, that is, a vanishingly small population of gas molecules or of molecular velocity. Pressure intensity measured from this state of vacuum or zero pressure is called absolute pressure.
Gauge Pressure (P g ) and gauge vacuum (P vac ): Instruments and gauges used to measure fluid pressure generally measure the difference between the unknown pressure ‘P’ and the existing atmospheric pressure P at . When the unknown pressure is more than the atmospheric pressure, the pressure recorded by the instrument is called gauge pressure. A pressure reading below the atmospheric pressure is known as vacuum or negative pressure. Actual absolute pressure is then the algebraic sum of the gauge indication and the atmospheric pressure.
P abs = P at + P g
P abs = P at - P vac
Units of Pressure: The unit used for expressing pressure is ‘Atmosphere’ abbreviated as ‘atm’. One ‘atm’ is simply the pressure in Kg/cm 2 exerted by the atmosphere. Pressure can be expressed in multiples of ‘atm’.The unit of pressure in M.K.S. system is Kg/cm 2 , in S.I. units N/m 2 and in F.P.S.units is pound per sq. inch (psi). One atm is equal to 1.013 Kg/cm 2 It can also be expressed as the height of mercury column. One atm is equal to 76 cm of Hg column at 0°C.
ii. Pressure Gauge
Bourdon tube type pressure gauge is the most common for industrial use. It employs a Bourdon tube element for direct indication of pressure. The bourdon tube element can be of ‘C’ type, spiral type or helical type. Due to its simplicity and ruggedness the ‘C’ type element is commonly employed.
Principle of operation: One end of the ‘C’ type bourdon tube element is sealed at its tip, while the other end is connected to the process pressure which is to be measured. Because of its ‘C’ type shape, there is a difference between inside and outside radii of the tube and the bourdon tube presents different areas to pressure.This causes the tube to tend to straighten up when the pressure is applied to it and results in a motion of the sealed tip end of the tube. The extent of movement of the tip of tube depends upon the amount of applied pressure. The movement of sealed end of the bourdon tube is thus an indicator of the applied pressure or vacuum.
Construction: Construction of a ‘C’ bourdon tube as used in a direct indicating gauge. The ‘C’ tube usually has an arc of 250°. The process pressure is connected to the fixed socket end of the tube while the tip end is sealed.As the pressure is applied there is a movement of the sealed end of the tube. This tip motion is non-linear because less motion results from each increment of additional pressure. This non-linear motion has to be converted into linear rotational pointer response. This is done mechanically by means of a geared sector and pinion movement. The tip motion is transferred to the tail of the movement sector by the connector link. The sector tail is called the ‘traveling angle’. This angle changes with the tip movement in a non-linear fashion and so the movement of the pinion and therefore pointer is linear. This type of pressure gauge is used in all industries and may be obtained in sizes from 5cm diameter up to 35 cm diameter and in many different indicating styles.
Material: The metallic materials used for construction of bourdon tube include brass, bronze, phosphor bronze, beryllium-copper alloy, alloy steel or stainless steel.The non-metallic materials are leather, neoprene and rubber.
Installation: In nearly all pressure gauges, the fluid in which pressure is measured is conducted to the inside of the pressure measuring element and is in direct contact with the element. This creates a problem of handling high temperature, corrosive, sludgy or semisolid materials. Some pressure- gauge elements can be protected by copper-plated, nickel-plated or tinned plated surfaces. But this is always possible especially on the inside of the bourdon tubes. Therefore some other method of excluding the measured fluid must be employed.
One of the effective methods of protecting a pressure gauge element is siphon arrangement. A single coil siphon arrangement This is very effective in protecting the pressure-gauge element from the high temperature of steam. The brass coil traps condensate steam and limits the temperature rise in the gauge. A siphon is necessary on all steam pressure gauges.Another arrangement is diaphragm seal as shown in Fig. 12.6. The unit is usually made up of bronze with a neoprene or thin metal diaphragm. The system is solidly filled with a liquid such as glycerin or oil. The diaphragm is quite flexible, so that the pressure on both sides of the diaphragm is equal. This way the problem of corrosive material can be tackled, as such material would not be in contact with the tube element. The line leading to pressure gauge is always filled with clean oil.
iii. Manometers
One of the oldest means of measurement of pressure is liquid column manometer.It is the simplest, most direct and most accurate of all pressure measuring means.This is a fundamental instrument for detecting the pressure and is used for calibration of other sensors. There are no moving parts, no friction or inertia involved in the measurement and therefore, its accuracy is limited only by the scale visibility.
Principle of operation: The pressure exerted by a column of liquid of height ‘h’ and density ‘r’ at the base of the cylinder containing it, is equal to ‘hrg’, where ‘g’ is the acceleration due to gravity. A manometer is shown in Fig. 12.5 below.
Tube Manometer It has two vertical tubes known as legs, which are connected at the base. The assembly is partly filled with manometer liquid, which may be water, oil or mercury.When the legs of the manometer are vertical, then:
The pressure at the bottom of the right leg = P 1 + h 1 rg
The pressure at the bottom of the left leg = P 2 + h 2 rg
When the fluid is under static balance condition, then
P 1 + h 1 rg = P2 + h 2 rg
Therefore P 1 -P 2 = (h2-h 1 ) rg
A manometer can thus be used for measurement of pressure differential, that is,the difference in pressures P 1 and P 2 . Further, if one of the legs of the manometer is connected to the vessel in which the fluid pressure or vacuum is to be measured and other leg is kept open to atmosphere then this manometer would be able to read the pressure difference between the vessel and the atmosphere.
Construction: Liquid manometers are the simplest differential pressure or vacuum detectors. A simple U-tube Manometer It has two vertical tubes known as legs which are connected at the base through a metal fitting. The assembly is partly filled with manometer liquid, which may be water, oil or mercury.An easier-to-read scale can be is attached to the manometer. In industrial installations,the use of glass tube manometers is limited to locations where tube breakage will not create hazardous conditions for the operator.
Materials: The performance of any manometer is largely a function of the indicating fluid selected. Amongst various manometer liquids, most commonly employed are water, oil or mercury. The filling fluid has to be chemically inert and compatible with the process media and produce a clear, visible interface. The fluid should not coat the glass tube and should not be corrosive to standard material such as copper,aluminum and steel. The fluid should not freeze due to low ambient temperature be capable to maintain its density unaffected by temperature.
Installation: Glass manometer is usually mounted on the equipment or pressure vessel at that place where the operator could easily read the manometer scale. In industrial installations, the use of glass tube manometers is limited to locations where tube breakage will not create hazardous conditions for the operator. At critical locations the gauge glass tube is provided with a thick guard glass cover to protect against flying glass pieces in case of accidental breakage of tube.
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