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On a fundamental level, pressure is universal. Regardless of the fluid in question; liquid or gas, hot or cold, corrosive or inert, pressure is nothing more than the amount of force exerted by that fluid over a unit area:
It should come as no surprise, then, that the common mechanical sensing elements for measuring pressure (bellows, diaphragm, bourdon tube, etc.) are equally applicable to all fluid pressure measurement applications, at least in principle. It is normally a matter of proper material selection and element strength (material thickness) to make a pressure instrument suitable for any range of process fluids.
Fill fluids used in pressure instruments whether it be the dielectric liquid inside a differential capacitance sensor, the fill liquid of a remote or chemical seal system, or liquid used to fill a vertical section of impulse tubing must be chosen so as to not adversely react with or contaminate the process.
Pure oxygen processes require that no system component have traces of hydrocarbon fluids present. While oxygen itself is not explosive, it greatly accelerates the combustion and explosive potential of any flammable substance. Therefore, a pressure gauge calibrated using oil as the working fluid in a deadweight tester would definitely not be suitable for pure oxygen service! The same may be said for a differential pressure transmitter with a hydrocarbon-based fill inside its pressure-sensing Capsule.
Pharmaceutical, medical, and food manufacturing processes require strict purity and the ability to disinfect all elements in the process system at will. Stagnant lines are not allowed in such processes, as microbe cultures may nourish in such dead end" piping. Remote seals are very helpful in overcoming this problem, but the fill fluids used in remote systems must be chosen so that a leak in the isolating diaphragm will not contaminate the process.
Manometers, of course, are rather limited in their application, as their operation depends on direct contact between process fluid and manometer liquid. In the early days of industrial instrumentation, liquid mercury was a very common medium for process manometers, and it was not unusual to see a mercury manometer used in direct contact with a process fluid such as oil or water to provide pressure indication:
Upto 100 Mpa
Upto 100 Mpa
Wide limits of application
Affected by vibration
Upto 100 Mpa
Vacuum to 500 kpa
Temperature compensation needed
Upto 60 kpa
Very small span
Upto 100 MPa
Large range of pressures
Capacitance pressure transducer
High vacuum to 10000 psig
Sensitive to temperature changes
Upto 1 militorr
Calibration of other gauges
Bulk modulus Cell
Button Diaphragm Repeater
Upto 10000 psig
Selection of level sensors depend upon following factors
Open Or Closed Tank
Direct contact Or Indirect Contact
Continuous Or Point Measurement
Direct Or Indirect Measurement
Characteristics Of Measured Fluid Or Solid
Operating Temperature Range
Operating Pressure Range
Material Of Construction
Density Of Medium
Particle Size Distribution
Corrosive Nature Of solids
The primary consideration for selecting a proper temperature sensing element for any application is the expected temperature range. Mechanical (bi-metal) and filled-system temperature sensors are limited to relatively low process temperatures, and cannot relay signals very far from the point of measurement.
Thermocouples are by far the most rugged and wide-ranging of the contact-type temperature sensors. Accuracies vary with thermocouple type and installation quality. RTDs are more fragile than thermocouples, but they require no reference compensation and are inherently more linear.
Optical sensors lack the ability to measure temperature of fluids inside vessels unless a transparent window is provided in the vessel for light emissions to reach the sensor. Otherwise, the best an optical sensor can do is report the skin temperature of a vessel. For monitoring surface temperatures of solid objects, especially objects that would be impractical or even dangerous to contact (e.g. electrical insulators on high-voltage power lines), optical sensors are the only appropriate solution.
Chemical reactivity is a concern for contact-type sensors. If the sensing element is held inside a thermo well, that thermo well must be selected for minimum reaction with the process fluid(s). Bare thermocouples are particularly vulnerable to chemical reactions given the nature of most thermocouple metals (iron, nickel, copper, etc.), and must be carefully chosen for the particular process chemistry to avoid reliability problem later.
3.2-Comparison of different level detectors
Cannot be used in pressurized vessels
Easy to use
Not for high pressure vessels
For conducting liquids only
Sediments may block probe
Not for pressurized tanks
Tanks should be freely vented
No moving parts
Hydrostatic Level measurement
Can be used in pressurized vessels
Surface foam may absorb signal
Temperature and pressure reduce speed of signal
Can be used for solids
The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation. Here are some key questions which need to be answered before selecting a flow meter:
Type of fluid e.g. Air or Water
Required measurement is rate or total.
Viscosity of the liquid.
Local or remote display.
Maximum and minimum flow rate.
Minimum and maximum pressure
Minimum and maximum process temperature.
Compatibility of the fluid with the process
Size of the pipe
Type of signal required
Every flow-measuring instrument exploits a physical principle to measure the flow rate of fluid stream. Understanding each of these principles as they apply to different flow measurement technologies is the first and most important step in properly applying a suitable technology to the measurement of a particular process stream flow rate. The following table lists the specific operating principles exploited by different flow measurement technologies.
A potentially important factor in choosing an appropriate flow meter technology is energy loss caused by pressure drop. Some flow meter designs, such as the common orifice plate, are inexpensive to install but carry a high price in terms of the energy lost in permanent pressure drop. Energy costs money, and so industrial facilities would be wise to consider the long-term cost of a flow meter before settling on the one that is cheapest to install. It could very well be, for example, that an expensive venturi tube will cost less after years of operation than a cheap orifice plate.
In this regard, certain flow meters stand above the rest: those with obstructionless flow tubes. Magnetic and ultrasonic flow meters have no obstructions whatsoever in the path of the flow. This translates to (nearly) zero permanent pressure loss along the length of the tube, and therefore. Thermal mass and straight-tube Coriolis flow meters are nearly obstructionless, while vortex and turbine meters are only slightly worse.
Instrument Installation Procedure
Following points are considered in installation of an instrument
Selection of the right place for the installation of the specific instrument.
Checking of the static and dynamic characteristics of the instrument.
Checking the calibration of the instrument.
Fit the instrument properly at that place.
Attach the inlet and discharge lines with the instrument.
Protect the instruments from environmental impacts.
5.1-Installation of the flow meter
The following list shows some of the details one must consider in installing a pressure-based flow meter element:
Necessary upstream and downstream straight-pipe lengths
Impulse tube tap locations
Transmitter location in relation to the pipe
Cost Analysis for instruments
Material of Construction
The cost of the instrument is directly related with material of construction from which it is made. Higher the cost of the material, higher the cost will be the instrument.
Reliability and Quality
Reliability and quality of instrument is also related with its price directly more the reliable the instrument more will be its cost. Less reliable instrument is usually low cost.
To installation the instrument we need experienced contractor and we have to pay, for its installation.
Accuracy and precision
More accuracy requires more cost to apply the necessary parts to increase accuracy and precision of the instrument.
The comparative prices of instruments in market should be up to date. Trade with a specific company then there will be some advantage can be gain by trading efficiently.