Instrument Selection Criteria Installation Cost Engineering Essay

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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:








Bourden Tube

C type

Upto 100 Mpa


Low cost

Reasonable accuracy


Helical type

Upto 100 Mpa


Wide limits of application

Affected by vibration

Spiral type

Upto 100 Mpa



Vacuum to 500 kpa


Low cost

Temperature compensation needed


Upto 60 kpa


Very small span

Strain Gauge

Upto 100 MPa


Large range of pressures


Capacitance pressure transducer

High vacuum to 10000 psig




Sensitive to temperature changes

Very fast

Resonant Wire

5 to10000



Capacitance Manometer

Upto 1 militorr


McLeod Gauge

1-10-6 torr

Calibration of other gauges

Bulk modulus Cell

200000 psig


Button Diaphragm Repeater

Upto 10000 psig

Level Detectors

3.1-Selection Criteria

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


3.1.1-For Solids

Bulk Density

Flow Characteristics

Particle Size Distribution

Moisture Content

Corrosive Nature Of solids

3.1.2-For Liquids

Pressure Range

Temperature Range

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

Level Detector









Cannot be used in pressurized vessels

Low cost

Easy to use

Sight Glass


Not for high pressure vessels




Resistance Tape


Digital signal

Conductivity Measurement


For conducting liquids only

Low cost



Sediments may block probe

Not for pressurized tanks

Tanks should be freely vented

Easily installed

No moving parts

Good accuracy

Hydrostatic Level measurement


Megnetic Level



Can be used in pressurized vessels



Surface foam may absorb signal

Temperature and pressure reduce speed of signal

Capacitence Measurement


Electrode coating

Low sensitivity

Shielding &noise

Can be used for solids

Flow Detectors

4.1-Selection criteria

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


4.2-Process/instrument suitability

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

Beta ratio

Impulse tube tap locations

Tap finish

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.

Installation 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.

Market analysis

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.