Pressure Measurement



Pressure is a basic variable in the design, construction and maintenance of industrial processes.

Pressure measurement is the primary variable for a wide range of process measurements, for example:

  • Flow rate (measuring pressure loss through a restriction).
  • Liquid level (measuring the pressure created by a vertical column of liquid).
  • Density of the liquid (measuring the pressure difference across a defined liquid height).


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1. Pressure Concept

Pressure is defined as the ratio of the normal component of the force on a surface to the area of that surface.

Pressure Formula
Pressure Formula

The knife will cut better the sharper it is, because the force exerted is concentrated on a smaller area.


The skier does not sink into the snow because the force is spread over a larger area.






1.1 Pressure - Unit Conversion Table


Pressure - Unit Conversion Table




2. Fluid Concept

In general terms matter can be classified into solids and fluids. The word fluid describes something that can flow (this includes liquids and gases).


The distinction between liquids and gases is not well defined, by varying the environmental conditions it is possible to transform a liquid into a gas and vice versa.


A fluid can be defined as a substance that does not resist, in a permanent way, the deformation caused by a force, therefore it changes its shape (be water my friend).





2.1 Compressibility


We will distinguish 2 types of fluids:

  • Incompressible: They are little affected by pressure changes. Their density is constant.

    Most liquids are incompressible.

    Gases can be considered incompressible when the pressure variation is small with respect to the absolute pressure.

  • Compressible: They are affected by pressure by varying their density. Most of the gases are compressible.
Compressibility
Compressibility




2.1.1 Incompressible Fluids


According to Bernoulli, we can decompose the pressure of an incompressible fluid into 2 components:


Pressure of an incompressible fluid
  • Static pressure:

    All points at the same depth will be subjected to the same pressure, regardless of the shape of the vessel in which the liquid is contained.


  • Dynamic pressure:


According to Euler's continuity equation (mass balance):

Euler's continuity equation
Euler's continuity equation




2.1.2 Compressible Fluids


As we know gases are highly compressible fluids.

Compressed gas
Compressed gas

From a mechanical point of view, the fundamental difference between liquids and gases is that the latter can be compressed. Their volume, therefore, is not constant and consequently neither is their density.

Considering the fundamental role of this physical quantity in fluid statics, it is understandable that the equilibrium of gases must be considered separately from that of liquids.

Boyle's Law

“The volume of the gas contained in a container is reduced if the pressure is increased.”


Boyle's Law
Boyle's Law

The theoretical study of the flow of a gaseous fluid is beyond the scope of this presentation, as a note we indicate that the propagation of pressure within a compressible fluid is performed at speeds close to the speed of sound.





3. Pressure Types

Pressure Types

You can find more detailed information about Pressure Types in Absolute Pressure vs Gauge Pressure vs Differential Pressure Measurements..





3.1 Atmospheric Pressure


It is caused by the weight of the atmosphere. It depends on climatic changes, the reference value is taken at sea level which is equal to 1013 mbar or 760 mmHg.


It is caused by the weight of the atmosphere. It depends on climatic changes, the reference value is taken at sea level which is equal to 1013 mbar or 760 mmHg.





3.2 Differential Pressure


It is defined as the difference between two pressures, also known as dP, pressure drop, etc....


APPLICATIONS:

  • Level measurement in closed tanks
  • Density Measurement
  • Flow Measurement
  • Interface Level Measurement
  • Distillation Col. monitoring Flooding
  • Filter monitoring
  • Pump and valve monitoring
  • Fire System Monitoring - Sprinklers
  • Process Feed Pressure Monitoring





3.3 Absolute Pressure


It is defined as the pressure referred to absolute zero pressure. It is distinguished by the subscript abs. Another way of defining it is by adding the atmospheric pressure to the relative / gauge pressure indicated by the measurement.


APPLICATIONS:

  • Applications where high accuracy in vacuum measurement is required
  • Low pressure or vacuum applications where it is necessary for pressure control to measure the influence of atmospheric pressure.
  • - Low pressure measurement in vacuum distillation columns
  • - Vacuum reactors
  • Control of leaks in tanks and circuits





3.4 Relative Pressure


It is determined by an element that measures the difference between absolute and atmospheric pressure.

It is distinguished by the subscript reg or g.

They make up 95% of the types of meters installed in the chemical industry. It can be positive (P1>Patm) or negative also known as empty (P1<Patm).


APPLICATIONS:

  • Level measurement in atmospheric tanks.
  • Pressure measurement in circuits and pressure equipment where the working pressure suffers variations greater than those caused by variations due to the atmosphere.
  • Air conditioning and control of air and corrosive gases - Clean rooms.




4. Pressure Gauges





4.1 PI - Indicators





4.1.1 Pressure Gauges

Process control in modern industry

Pressure gauges are primary standards of most national standards bureaus.

The main advantages of these instruments are their high accuracy, low cost and simplicity, minimum span with inclined ranges from 0 to 2.5mm H2O; maximum span is 4bar.

The accuracy is 0.2 mmH2O for vertical and 0.05 mmH2O for inclined.

The major limitations are possible rupture and problems related to the contained fluid.







4.1.2 Bourdon Tubes

In 1852 E.Bourdon patented a curved tube which, when held and pressurized at its open end, produces a movement at its closed end.


Bourdon Tubes

Bourdon Tube

Bourdon tubes are used to detect high pressures, not good at low pressures or vacuum, span from 1bar to 6900bar.

Their accuracy varies from 0.25 to 5% of span. They are used as PI standard in many process industries.

The main advantages are low cost, easy replacement.






4.2 PT - Transmitters


Transmitters

These instruments are composed of two functional units:

  • Primary Unit: The pressure transducer (primary unit) consists of the interface to the process, the sensor and the primary electronics.

    In the primary unit, the process fluid (L,G,V) exerts pressure on the sensor through an insulating, flexible, corrosion-resistant diaphragm. When the sensor detects pressure changes, it induces variations in the primary physical value, which depends on the technology used (piezoresistive or capacitive).


    Transmitters

    The primary electronics converts these variations into a digital signal that feeds a microcontroller. This performs a linearization of the primary output, compensating for sensor non-linearity, static pressure and temperature changes.

    The measured values and sensor parameters are transferred to the subunit, where the communications card is located. The output data value is converted to a 4-20 mA signal.


  • Secondary Unit: It consists of the rest of the electronics, the terminal block and the enclosure.

    Transmitters





4.2.1 Piezo-Resistive

It is the most popular pressure gauge.

It consists of the diffusion of resistive elements inside a single silicon chip, acting as a diaphragm. The silicon wafer is safer than the metal diaphragm because after being deformed it returns to its original shape due to its excellent elasticity. The thickness of the wafer determines the pressure range of the sensor.


These sensors are small, inexpensive, accurate, highly repeatable and have a wide measuring range from 0.08bar to 14000bar. Turndown 100..25 to 1. Its accuracy varies from 0.2 to 0.5% of span.


They have good and quick response and good resistance to overpressure and vibration.Their major limitation is the high sensitivity to temperature variations.





4.2.2 Capacitive

A capacitor consists of two parallel conducting plates separated by a small gap. Pressure displaces one of the plates that acts as a diaphragm, changing the capacitance of the circuit.The principle of operation is the measurement of the change in capacity resulting from the movement of an elastic metallic element (diaphragm).


Transmitters

The inlet pressure acts on the diaphragm causing a change in capacity. There is a linear relationship between capacity and pressure.



These sensors have an excellent response, accuracy, linearity, repeatability and stability.

Measuring range from 0.0008bar up to 350bar. Rangeability 20 to 1.Its accuracy varies from 0.1 to 0.2% of span. They are elatively high cost compared to piezoresistive (10:1).

They can be sensitive to magnetic fields and are affected by temperature, vibration and variations in the dielectric constant of the process fluid.







4.3 Selection Criteria


Selection Criteria




4.4 Process Connection


Process Connection




5. Pressure Suppliers List


  • ABB Instrumentation (www.abb.com/us/instrumentation)
  • Barton Instruments (www.barton-instruments.com)
  • Brooks Instrument (www.brooksinstrument.com)
  • Dresser Instrument (www.dresserinstruments.com)
  • Endress + Hauser Inc. (www.us.endress.com)
  • Fisher controls (www.fisher.com)
  • Foxboro/Invensys (www.foxboro.com)
  • Honeywell (www.iac.honeywell.com)
  • Marsh Bellofram (marshbellofram.com)
  • Moore Industries (www.miinet.com)
  • Rosemount/Emerson (www.rosemount.com)
  • Siemens (www.sea.siemens.com)
  • United Electric (www.ueonline.com)
  • Youkogawa Corp. of America (www.yca.com)




6. References


  • ANSI/ISA-50.1-1982 (R1992) formerly ANSI/ISA-S50.1-1982 (R1992) Compatibility of Analog Signals for Electronic Industrial Process Instruments.
  • LIPTAK,B. (2003) Instrument Engineers Handbook.Process Measurement and Analysis.
  • BEAMEX (2012) Ultimate Calibration.



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