CHAPTER 2

PRESSURE MEASUREMENT

PRESSURE DEFINITIONPRESSURE DEFINITION

FORCEPRESSURE is force exerted per unit area of surface.

FORCEP = F/A

P = pressure

F = force

A = surface area exposed to the force

AREAPressure Units:

Pound / Square Inch = P / SI = PSIPound / Square Inch = P / SI = PSI

Newton / Square Meter = N / m2

100 000 N / m2 = 1 Bar100,000 N / m = 1 Bar

14.5 psi = 1 Bar

Inches of mercury “inch Hg” “mm Hg”Inches of mercury inch. Hg , mm.Hg

TYPES OF PRESSURETYPES OF PRESSURE

The pressure at

point X can be

expressed as:

o 10” of mercury

absolute

o 20” of mercury o 20 of mercury

vacuum

20 i h f o -20 inches of

mercury gauge

PRESSURE SENSING ELEMENTS

A basic instrument system consists of three elements:

1. SENSOR or INPUT DEVICE

2 SIGNAL PROCESSOR2. SIGNAL PROCESSOR

3. RECEIVER or OUTPUT DEVICE

The block diagram of a sensor is shown below.

Pressure sensors either convert the pressure into

h i l t i t l t i l t t mechanical movement or into an electrical output.

Complete gauges not only sense the pressure but indicate

them on a dial or scale.

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1) Bourdon TubeBourdon tube is a flattened metal tube has an oval cross-section with one end of the tube connected to the process pressure. The other end is sealed and connected to the pointer or transmitter mechanism. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves.

C-type bourdon

o Range as low as 0 - 15 psi up to 0-1500

, g

psi.

o They are simple and accurate, but they are bulky and get damaged with over-ranging.

Helical bourdon

o Range as low as 0 - 200 psi up to 0 - 6000 ipsi.

o Heavy-duty helical bourdons can sometimes tolerate as high as ten times h ithe maximum range pressure.

Spiral bourdon

o Range as low as 0 10 psi up to 0 100 000

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o Range as low as 0 -10 psi up to 0-100,000 psi.

16/03/2006Designed by Rao KV

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2) Bellows sensor A bellows sensor is an axially

flexible cylindrical enclosure flexible, cylindrical enclosure

with folded sides. When

pressure is applied through an pressure is applied through an

opening, the closed end

extends axially.extends axially.

A bellows sensor can

t l h accurately measure much

lower pressures than a

bourdon tubebourdon tube.

Bellows elements can measure

absolute pressure, gauge

pressure, vacuum, or

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differential pressure.

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3. Diaphragm sensoro Diaphragm is usually metallic and comes in two different

fi ti i l d l configurations; single and capsular.

o The single diaphragm is either flat or with concentric corrugations.

Th l di h i t f t di h ld d t th t o The capsular diaphragm consists of two diaphragms welded together at their perimeters.

o Evacuated capsules are used for absolute pressure reference and single p p gdiaphragms for very sensitive measurements.

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ELECTRONIC PRESSURE SENSORS

This table shows the ranges of pressures and vacuums that various

sensor types are capable of detectingsensor types are capable of detecting.

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4. Strain-Gauge Pressure Sensor

o Strain-gauge pressure sensors are o Strain gauge pressure sensors are used in most types of electronic pressure transmitters. When metallic conductors or semiconductors are subjected to mechanical strain, there is a change in resistance This there is a change in resistance. This resistance is then electrically converted into a 4-20 mA signal proportional to the pressure.

o There are many different designs of strain-gauge pressure sensors. The most common designs use a metallic di h t i l t th diaphragm to isolate the process fluid and exert a force on a force bar.

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bar.

o Most of the strain elements in current use are semiconductor type.

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Strain-Gage Based Pressure Cell

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5. Capacitive Pressure Sensor

o Capacitive pressure sensors are also used in electronic pressure

transmitters. With these devices the change in capacitance

resulting from the movement of an elastic element is

proportional to the pressure applied to the elastic element.

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o The elastic element is usually a stainless steel diaphragm. As shown, a

high-frequency oscillator is controlled by the sensing element. high frequency oscillator is controlled by the sensing element.

Changes in pressure deflect the diaphragm and the resultant change in

capacitance changes the oscillator frequency. p g q y

o The variation in oscillator frequency is converted to a 4-20 mA signal,

proportional to the pressureproportional to the pressure.

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PRACTICAL CONSIDERATIONS

C t i li ti ill b d th Certain applications will be so severe and the

pressure sensor will not remain functional for

reasonable amount of time.

The following protection devices can be used to

protect the pressure sensor.p p

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Di h l d t

1) Diaphragm Seals

o Diaphragm seals are used to

isolate the pressure sensor from

the process fluid.

o This is done when the fluid is

toxic, corrosive, dirty, solidifies at

ambient temperature, or is ambient temperature, or is

extremely cold and may freeze

the instrumentthe instrument..

o The diaphragm seal is a thin,

flexible disk, which separates the

pressure sensor from the process

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media.

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2) Siphons

• Siphons are generally used to isolate a hot-

process media from the pressure sensor. p p

• The siphon is a metal, tubular device shaped

l h b f ll d h h hin a loop. It can either be filled with a high-

boiling-point liquid or process condensate

which acts as a barrier to the heat contained

in the hot gases or steam.

• The path the hot vapor takes to the pressure

sensor is relatively long and narrow with a sensor is relatively long and narrow with a

lot of surface area for cooling siphons.

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3) Throttling Devices

o Throttling devices are commonly used to dampen high-frequency

pressure fluctuations by putting a restriction in the inlet to the

pressure sensor.

a) Throttling Screws

o Throttling screws are the simplest

means of providing a restriction.

o Throttling screws have several o Throttling screws have several

orifice sizes.

Th i t d i t t d o They are inserted into a tapped

hole in the base (socket) of the

pressure sensor to provide a flow

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pressure sensor to provide a flow

restriction

o Pressure Snubbers are very common for

b) Pressure Snubbers

o Pressure Snubbers are very common for

reducing pressure fluctuations and filtering

the media. Snubbers have a porous the media. Snubbers have a porous

element, which restricts the velocity and

filters the fluid.

o The pulsation dampener is also sometimes

called a pressure Snubbers but does not called a pressure Snubbers, but does not

have a filtering element.

o The most common design has a bar-stock

fitting. As the pressure pulse comes

through the dampener, the piston is forced

up and restricts the flow from the large

h b b l i th tl t f th

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chamber by closing the outlet of the

chamber.

c) Pressure-Limiting Valves

o Pressure-limiting valves protect the pressure

sensor from overpressure by blocking the

process fluid at a preset limit.

o There are several designs of pressure-o There are several designs of pressure

limiting valves. One has the fluid coming in

the inlet passing through a filter and out to the inlet, passing through a filter, and out to

the pressure sensor.

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PRESSURE MEASUREMENT DEVICES

1 MECHANICAL DEVICES1. MECHANICAL DEVICES

In mechanical pressure measurement devices the measuring

elements (sensors) can never separate it from the rest of the

system (signal processor and receiver/output device) as in many y ( g p / p ) y

cases they are all integral (e.g. a mechanical pressure gauge

incorporates all of these elements). incorporates all of these elements).

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PRESSURE GAUGE

Precautions

Tubes for gauges to be o Tubes for gauges to be

used on Acetylene must

b d f t l be made of steel.

o Associated Fittings for use

on oxygen must be kept

entirely free of oil.y

o Gauges used on

Hydrogen plants must be

gold plated.

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Gauge errors

oA gauge with a zero error will always read high or low by a oA gauge with a zero error will always read high or low by a constant amount.

A gauge with a span error has an internal magnification error oA gauge with a span error has an internal magnification error, where the error increases as long as the measured pressure increases.c eases

oA gauge with a linearity error may read correctly at 0 and 100% but will not follow a linear path between these points. 100% but will not follow a linear path between these points.

PRESSURE TRANSMITTER

a) Pneumatic transmitter

Supply: 20 Psi OR 1.4 Bar

Output: 3 to 15 Psi OR 0.2 to 1.0 Bar

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b) Electronic transmitter

Supply: 24 V DC

Output: 4 to 20 mA

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1. Pressure is applied to the sensor.

2. A change in pressure is measured by a change in the sensor

outputoutput.

3. The sensor signal is conditioned for various parameters.

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4. The conditioned signal is converted to an appropriate analogue

output (i.e. 4 – 20 mA)

c) Smart Transmitter

HHC (Hand Held

o Smart Transmitter can be programmed with a HHC (Hand

(Communicator).

o Smart Transmitter can be programmed with a HHC (Hand

Held Communicator).

o They come with addressable and communication facility.

o Transmitter can be re-ranged and zero checked without

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o Transmitter can be re ranged and zero checked without

disconnecting from the line.

Electronic Transmitter Wiring Connections

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PRESSURE SWITCHES o An electric pressure switch senses pressure and opens or l l t i l it h closes an electrical switch

element at a set pressure to signal another electrical device.

o Most pressure switches trip at a pressure above atmospheric, and are called gauge pressure or g g psimply pressure switches.

o Switches can also be manufactured to trip at pressure manufactured to trip at pressure referenced to a complete vacuum and this is called absolute pressure switches absolute pressure switches.

o Pressure switches are set to trip at a certain point with rising or f lli falling pressure.

o The difference in the trip point and the set point is called dead

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and the set point is called dead band or reset or switch differential.

Electrical Switch Contacts

The electrical switch is

usually:y

single-pole double-

throwthrow

or

double-pole double-

throw.

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PRESSURE SWITCHESSwitch Terminology

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PRESSURE REGULATOR

o Pneumatic regulators are used o Pneumatic regulators are used

with every pneumatic

Instrument Instrument.

o Regulator maintains a steady

outlet pressure irrespective of

variation in the upstream

pressure.

o Regulator comes with a filter to o Regulator comes with a filter to

remove wet the dust.

o A drain valve remove any

condensation of air.

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