Instrumentation – Page 7

Pressure Gauges are pressure measuring instruments. Pressure and Temperature are two very important parameters for all chemical industries. So, pressure measurement is of utmost importance. There are various pressure measuring instruments available to perform this task. Pressure measurement basically means the analysis of the fluid forces that are imparted on a surface. Accuracy of pressure measuring instruments are very important for proper operational control. In this article, we will explore various pressure measuring devices or pressure gauges used across industries. What is Pressure? Pressure (P) is defined as Force (F) per Unit Area (A).The pressure is the action of one force against another force.The pressure is the force applied to or distributed over a surface.P = F/A F: Force A: Area Fig. 1: Units of Pressure Absolute pressure Measured above total vacuum or zero absolute.Zero absolute represents a total lack of pressure.Range: 0-1 Kg/cm^2 (a)/ 0-1 Bar (a)/ 0-760 mm Hg (a) Atmospheric pressure: The pressure exerted by the earth’s atmosphere. Barometric pressure: Same as Atmospheric pressure. Vacuum Pressure: Pressure below atmospheric. Fig. 2: Absolute and gage Pressure Differential pressure: It is the difference in pressure between two points of measurements. In a sense, Absolute Pressure could be considered as a differential pressure with a total vacuum or zero absolute as the reference. Gauge/Gage pressure: The pressure above atmospheric is gauge pressure. Represents a positive difference between measured and existing atmospheric pressure. For example – Blood pressure. Pressure Measurement The measurement of pressure is considered the basic process variable for measurement of:- Flow (difference of two pressures).Level (Head or backpressure).Temperature (Fluid pressure in a filled thermal system). The Pressure Measurement Systems consist of two basic parts:- A primary element, which is in contact, directly or indirectly, with the pressure medium and interacts with pressure changes.A secondary element, which translates this interaction into appropriate values for indicating, recording, and/or controlling. Classification of Pressure Measuring Instruments or Pressure Gauges Pressure measuring devices can be classified on the basis of : A: Pressure ranges:- Vacuum gage, Draft gage, Low range, compound gage, Medium range, high range, etc.. B: Design principle involved in their operation: Mechanical movement of the sensing element – e.g. Bourdon gages, Diaphragm gagesElectronic Sensors: e.g. Strain gages, Capacitive, Potentiometric, Resonant wire, Piezoelectric, Magnetic, Optical, etc. C: On the basis of their application: Local pressure indication,Remote pressure indication,Corrosive servicePulsating serviceDifferential pressure measurement Methods of Pressure Measurement Fig. 3: Pressure Measurement Basic Measurement – U Tube Manometer A manometer is one of the oldest pressure measuring instruments and widely used even in recent times. Their operation is simple and provides accurate results. As the name suggests it is a "U" shaped tube made of glass and partially filled with liquid. The principle of the manometer is given as P= HEIGHT*DENSITY Where “P” is Pounds /sq. Inch; “HEIGHT” in Inch “ DENSITY” in pounds /Cu. Inch Advantages of Manometer Fluids simple & time-provenHigh accuracy & sensitivityWide range of filling Disadvantages of Manometer No over-range protectionLarge & bulkyMeasured fluids must be compatible with the manometer fluidsNeed for leveling Fig. 4: U-tube Manometer and sensing element Pressure Sensing Elements Pressure Sensing Elements are basically mechanical elements like plates, shells, and tubes. On the application of pressure, these elements deflect which is then converted into physical movement. They are a very important part of pressure measuring devices. The main types of sensing elements are Bourdon tubes, diaphragms, capsules, and bellows.All the above devices, except diaphragms, provide a fairly large displacement. Mechanical gauges use that displacement which is further used by electrical sensors that require a significant movement. Bourdon Tube Pressure Gauges The pressure measuring device, Bourdon tube is the most common. They measure medium to high pressures. Bourdon tube is basically a curved tube with a circular, coiled, or spiral shape. It is the twisted tube whose cross-sectional isn't circular.Bourdon tubes types are c-type, helical type and spiral type.They should be filled with oil to limit the damage caused by vibration.Range: (-)1 to 1600 Kg/cm^2 Advantages of Bourdon Tube Low cost & simple constructionWide rangeabilityGood accuracyAdaptable to transducer designs Disadvantages of Bourdon Tube Low spring gradient below 50psigSubject to HysteresisSusceptible to shock & vibration Fig. 5: Bourdon tubes Bellows type Pressure Gauge Bellows are also used as a pressure measuring instrument. Their main features are Made of Bronze, S.S., BeCu, Monel, etc..The movement is proportional to a number of convolutions sensitivity is proportional to size.In general, a bellows can detect a slightly lower pressure than a diaphragm.The range is from 0-5 mmHg to 0-2000 psiAccuracy in the range of 1% spanIt is a series of a circular parts so formed or joined that they can be expanded axially by pressure. A wide range of spring is employed to limit the travel of bellows.The measurement is limited from .5 to 70 psi.It is greatly used as receiving elements for pneumatic recorders, indicators & controllers & also as a differential unit of flow measurement. Advantages of Bellows Pressure Gauge High force deliveredModerate costGood in the low to moderate pressure gauge Disadvantages of Bellows Pressure Gauge Need ambient temperature pressure compensationRequire spring for accurate characteristicsLimited availability Fig. 6: Bellows Diaphragm Pressure Gauge The diaphragm can be used to measure the pressure of both liquids and gases. One circular diaphragm is clamped between a pair of flanges to constitute the pressure measuring element. The deflection of a flexible membrane is used for pressure measurement. For known pressures, the deflection is repeatable. Hence, calibration is possible.The pressure difference between its two faces dictates the deformation of a thin diaphragm.The reference face can be open to the atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. Mechanical, optical, or capacitive techniques are used to measure the deformation. Ceramic and metallic diaphragms are used.Range: (-) 10000 to (+) 10000 mm-WC Advantages of Diaphragm pressure gauge Small size & moderate costLinearityAdaptability to slurry services & absolute & diff. press.High over range characteristics Disadvantages of Diaphragm pressure gauge Limited to low pressureDifficult to repairLess vibration & shock resistance Fig. 7: Diaphragm Capsule Pressure Gauge Capsules as pressure measuring devices are used normally for low pressure applications. A capsule is formed by joining the peripheries of two diaphragms through soldering or welding.Used in some absolute pressure gages. Fig. 8: Capsules Range of Elastic-Element Pressure Gages Fig. 9: Range of Elastic-Element Pressure Gages Pressure Measuring Accessories Fig. 10: Pressure Measuring Accessories Diaphragm seals: These are designed to isolate the sensing element of pressure gauges from process fluids. Gauge Saver: Gauge Savers also known as overpressure protectors are applicable where pressures exceed the maximum pressure rating of the pressure gauge. Pulsation dampener: Dampeners considerably reduce the pulsations and make the gauge reading easier and also improves the life of the gauge. Siphon: This connection between the pressure gauge & process in applications, where high temperatures like steam, vapors or fluids are present. It acts as a cooling coil and protects the gauge from high temperatures and also helps in dissipating heat. Precautions: When first installed the siphon should be filled with water or any other suitable separate liquid. U Type – For Horizontal pressure tappingQ Type - For vertical pressure tapping Needle valve: The large round handle offers maximum ease and precise control to throttle the pressure to the gauge. Block & Bleed Valve: Equipment Isolation with automatic pressure bleed for safety Manifolds These are fluid distribution devices.These are used in conjunction with pressure gauges, differential pressure gauges & differential pressure transmitters.They combine instrument isolation & equalizing in one block.The manifolds are available in 2way, 3 way & 5-way types with remote & direct mounting styles Gauge cack: It is used in conjunction with the siphon as an isolation valve. It is not recommended for pressure over 100 psi. Fig. 11: Different types of Pressure Measuring Instruments Few more resources for you.Understanding Pressure and Temperature in the context of Pressure Vessel Design Pressure Tests of Piping systems-Hydrotest Vs Pneumatic Test Temperature Measurement by Filled Thermal Systems FUNDAMENTALS OF TEMPERATURE: An article Few Points for High Temperature and High-Pressure Piping