201309 CLD 20202 Chapter 2-Pressure Measurement.ppt

8/14/2019 201309 CLD 20202 Chapter 2-Pressure Measurement.ppt

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CLD 20202

Process Instrumentation

CHAPTER 2

PressureMeasurement


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Objectives

After finish this lesson, you will be able

to:

Define the pressure

Differentiate the types of pressure

Understand the characteristic of liquidpressure & factor that affected


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Understand the characteristic of gaspressure

Apply the different unit of pressure

measurement Understand the principle, operation,

advantages & disadvantages of pressure

measurement device


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Pressure

Defined as normal force exerted by a fluid per

unit area (gas & liquid)

Units of pressure N/m2 (Pascal, Pa)

Other units = bar, atm, kgf/cm2, lbf/in2 = psi

Ama

AFP


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Unit of Measurement

English unit -pound per square inch(psi)

(SI) unit - Newton per square meter

(Pascal) Common units

Kilograms per square centimeter (kg/cm2)

Kilopascals (kPa)Bar ( 1 bar = 100,000 Pa)


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referenced to column of fluid

inches of water (in H2O)

millimeter of mercury (mm Hg)torr


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indicated pressure includes or excludesatmospheric pressure

psig psia


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Example


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Absolute, Gauge & Vacuum

Pressure Several common ways to measure pressure

1. Zero absolute pressure

2. Standard atm pressure

Gauge pressure (psig)refer to atm pressure

(14.7psia)

Absolute Pressure (psia)refer to zero abs pressure(vacuum)

Vacuum Pressure (-50mmHg)refer atm pressure

& measured inve direction


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Most pressure measuring devices calibrated to

read zero in the atmosphereGauge Pressure

Pressure below atmospheric

pressure

Vacuum Pressure

atmabsgauge PPP

absatmvac PPP


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Example 1


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Solution


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Example 2


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Solution


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Hydrostatic Pressure

Pressure a fluid exerts on an object or

container walls

Applied to determine LEVEL of liquid in tanks

P = hydrostatic pressurerho = density of liquid

H = height of the liquid

G = gravitational force

gHP


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Variation of pressure with depth

P in a fluid at a rest

does not change in the

horizontal direction

In the presence of a

gravitational field, P

increases with

depthmore fluid rests

on deeper layers


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Liquid Pressure Characteristic

Influencedby :

Position within static

fluid but at a given

position (height)

Unaffectedby :

Shapeof the confining

boundaries


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Factors Affecting LiquidPressure

Density

Depth

SurfacePressure

P proportionalto density

P proportionalTo depth

Any P acting on theSurface (P atm)


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Gas Pressure Characteristic

Gas expand to fill entire area thatcontains them.

2 factors that affect a gas

Volume Temperature

BOYLES LAWdescribes the inversely

proportional relationship

between the absolute

PRESSURE and VOLUME of

gas, if the TEMPERATURE

kept constant

CHARLES LAWfor a fixed mass of gas at

constant volume, PRESSURE

directly proportionalto the

TEMPERATURE


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BOYLES LAW

Volume of gas inversely proportional to pressure

V 1 / PV = constant / P

P*V = constant 1/P

V

P

V@ single T

P1*V1= constant = P2*V2

P1V1= P2V2


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CHARLES LAW: (1746-1823)

Pressure is proportional to absolutetemperature (assuming V and n constant)

P T (Temp in kelvin)

P = constant * T

T

P

Also,Volume of gas is proportional to the

amount of gas present

V n v = constant * n

@ConstantV and n


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Now with the equations

Liquids do not obey the ideal gas lawsVirtually no change with P (hydraulics), small change with T

P = constant x T

v = constant x n

P x V = constant

P1V1 = P2V2

P x V = constant x T x constant x n

P x V = constant x n x T

P x V = R x n x T

PV = nRT


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Typical Pressure Transmitter consists 2 parts:

1. Primary Element2. Secondary Element

Pressure Measurement Devices


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Pressure Measurement Devices

Primary element- pressure sensor / pressure element- converts to: physical motion that is proportion toapplied pressure (mechanical /electrical value)

- read by the secondary element.

Secondary element- pressure measuring instrument

- electronics (transducer)- convert output (primary element) tosignal such as 4-20mA.


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electronic-based sensor (strain gages)have better response and higheraccuracy than mechanical-based types

(bourdon) pressure measuring instruments -

pressure transducer


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THANK YOU

Documents

201309 CLD 20202 Chapter 2-Pressure Measurement.ppt