Basic of measurement instruments

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Basics of Measurement Instruments

1

– DP measurement includes orifice plates, venturi tube, flow nozzle

& pitot tube.

Orifice plate

- Types Concentric, eccentric, segmental

- Commonly used concentric

- Calculations are run to ISO 5167 etc

Software employed

- Type of tapping on pipe is important for

calculations Can be flange taps, radius taps,

corner taps, etc

- Beta ratio (orifice Dia to pipe ID) should be

between 0.3 & 0.7

- Typical permanent pressure loss = 0.7 of DP

at normal flow

- Straight lengths is important consideration

If not possible, flow straighteners should be

provided

- Recommended installation is on horizontal

lines If on vertical lines it is preferred to

have flow from bottom to top

- Taps for liquids from bottom half of tap &

for gases from top half of pipe

- Flange location - Tap location 1 inch upstream and 1 inch downstream from face of orifice Most commonly used

- Radius taps – 1 pipe diameter (D) upstream & 0.5D downstream from orifice plate

- Corner taps taps located at the face of the orifice plate Normally used in the line sizes smaller than 2"

- Pipe taps – 2.5 pipe diameter (D) upstream & 8D downstream from orifice plate

*Flow element is introduced

in the flow path to produce a

DP This can be orifice plate, venturi, flow nozzle etc

*Based on Bernoulli’s theorem, flow rate is proportional to the square root of DP.

*This DP is measured by a

DP transmitter that is extracts the square root & is calibrated to flow units.

Head meters Principle

Bernoulli’s theorem

In a fluid stream the sum of pressure head, velocity head & elevation head at a point is equal to their sum at any other point in the direction of flow plus the losses due to friction between these two

points”

- Rotameters (also known as variable-area

flowmeters) are typically made from a tapered glass tube that is positioned vertically in the fluid flow.

- A float that is the same size as the base of the glass tube rides upward in relation to the amount

of flow Because the tube is larger in diameter at the top of the glass than at the bottom, the float resides at the point where the differential

pressure between the upper and lower surfaces balance the weight of the float.

- Variations : Glass tube & Metal tube

- A measuring scale is provided as a reference & the position of the float is measured off against this scale

- Can be equipped with switch contacts &

transmitters.

Rotameter

instrument, amplified & indicated as flow

*Min fluid conductivity : 20uS/cm Special

Acoustic signal (ultrasonic) is transmitted from one sensor to the other.

If sent in the direction of flow, the signal will be received earlier If sent against the direction of flow, the signal will be received later In effect the resultant time difference is directly

proportional to the flow rate

This time difference is used as an indication of the flow rate

Ultrasonic

Doppler effect

When wave energy like sound or radio waves travels from two objects, the wavelength can seem to be changed if one or both of them are moving This is called the Doppler effect

Particles in fluid stream which is moving cause the frequency shift between the transmitted signal & received signal This frequency shift indicates the actual flow rate

Ultrasonic

Vortex flowmeter – Von Karman effect

when a non-streamlined object (bluff body) is placed in the path of a fast-flowing stream, the fluid will alternately separate & form vortices from the object on its two downstream sides

Vortex shedding frequency is directly proportional to the velocity of the fluid in the pipe, and therefore to volumetric flow rate

The shedding frequency is independent of fluid properties such as density, viscosity,

conductivity, etc., except that the flow must be turbulent for vortex shedding

Nowadays the vortex is sensed using pressure sensing by piezoresistive or capacitive sensors

Vortex

Level measurement

Flat glass gauges Transparent type Reflex type

Transparent gages have a vision slot on both

sides of the chamber Light enters the gage

from the side opposite the observer so that both

the level of a liquid and its characteristics can be

seen For easier liquid observation in dark

environments, illuminators are available for use

with transparent gauges.

*Reflex gages have a single vision slot in which light can enter the gage chamber to determine liquid level.

*Light striking the area of glass covered by transparent liquid is allowed to pass through to the interior of the gage along the entire length of the covered prism The light is reflected from the back of the chamber to the observer Typically the chamber appears dark.

*Light striking the area of the glass above the liquid level is reflected instead of passing through to the back of the chamber This produces a silver appearance in the glass The liquid level in the chamber is

represented where the silver and dark/opaque area meet.

Level gauges

Tubular gauge glass Magnetic gauge glass Flat glass gauges

Level measurement - Switches

Float type

Floats on fluid surface

Displacer type – Up to 4 points.

– Different materials

•Liquid level movement changes the buoyancy force on a displacer suspended from a range spring The force change on the spring causes

it to expand or contract moving an attraction sleeve into or out of the field of the switch magnet, actuating the switch.

Level Transmitters

Differential pressure type

•Uses a DPT to measure the pressure at the bottom of the tank

•If tank is atmospheric, the low side of transmitter is open to atmosphere

•If tank is pressurised, the low side of transmitter is connected to the top of the tank

•Diaphragm seals with capillary could be used if the transmitter are to be mounted at viewable locations.

•Transmitters use signal processing algorithms to filter out unwanted noise Echo suppression technique.

•Installation is very critical Install the instrument so that the ultrasound waves are not blocked in any condition.

•Ultrasonic sensors have a blanking distance which is typically 250 to 300mm The sensor cannot respond to any level which is blanking distance away from the sensor.

•Typical cone angle 11-12 Deg Use the cone angle to ensure that the installation does not block the ultrasound waves.

Level measurement

" Ultrasonic"

• Open channel flow measurement using

ultrasonic level transmitter.

• This sketch indicates how an ultrasonic sensor

is used for measurement of flow through an open channel using a weir / flume

• Level build up before the weir plate is

measured by the ultrasonic level sensor & then equated to flow

• Most transmitters manufacturers have standard algorithms available for the regular weir

profile normally used.

Level measurement – Displacer & capacitance

Capacitance

• Varying liquid level varies the capacitance.

• This is sensed by electronic circuits & indicated as

level.

• The dielectric constant of the service liquid should be

constant Of Vacuum the constant is 1

• Varying liquid level causes a change in the

capacitance & this change is sensed and indicated in

Pressure measurement Gauges

Pressure gauges

Bourdon tube, Capsule, Bellows, Diaphragm

• Diaphragm type pressure gauges are designed for measuring low/very low pressures Comprises of a diaphragm that pushes a piston thereby moving a pointer over a scale.

• Bourdon tube The Bourdon Tube is a thin walled tube of oval cross section which may be of ‘C’ form or spirally wound This tube expands when pressure is applied internally; this expansion is converted into rotation of a concentric pointer with a gear movement The reading indicated on a dial by the pointer is

proportional to the pressure applied

• More than 1 Barg.

• Case Liquid filling posible (Typically glycerine)

• Capsule Pressure gauges are designed for measuring low/very low pressures and vacuum The pressure medium needs to be gaseous The capsule consists of two diaphragms joined and sealed around their circumference.

Pressure Gauges

• Pressure gauges can be equipped with diaphragm seals This is done to protect the internals of the PG from process fluid Also used on applications that tend to clog normal pressure gauge

• Generally Pressure gauge mounted directly on

diaphragm seal

• Filled capillary also possible

• Filling liquid is normally silicon oil, glycerine, halocarbon, vegetable oil etc

• Wetted parts of different materials – Hastelloy,

Monel, SS with PTFE lined etc

Pressure Transmitters

• Generally pressure transmitters work on either

capacitive principle or piezoresistive principle which

is resistance that changes with stress

• PT are generally 2 wire instruments

• OT can be equipped with diaphragm seals with or without capillary

• Displays are optional

• Devices available for a host of communication

protocols – HART, Foundation fieldbus, Profibus etc

Temperature Gauges

Generally used Temperature gauges

1) Bimetal thermometers & 2) Filled capillary systems – gas as well as liquid

Bimetallic devices take advantage of the difference in

rate of thermal expansion between different metals Strips

of two metals are bonded together When heated, one side

will expand more than the other, and the resulting

bending is translated into a temperature reading by

mechanical linkage to a pointer

Filled capillary devices are filled either with gas or liquid This fluid expands with temperature

This expansion moves a pointer

on the scale

Temperature Transmitters

Though a lot of technology is available for temperature

measurement normally used versions are

1)RTD

2)Thermocouples

Others being radiation methods (Infrared), Pyrometry etc

• Thermocouple Temperature Measurement Sensors

Works on the principle that current flows through a closed circuit made up of two dissimilar metals & if the two junctions are kept at different temperatures

• Categorized as different types – Type T, E, K etc Each type defines the different metals used & the temperature range that it can be used for

• Resistance Temperature Devices(RTD)

Resistive temperature devices capitalize on the fact that the electrical resistance of a material changes as its temperature changes Two key types are the metallic devices (commonly referred to as RTDs), and thermistors

• Infrared Temperature Measurement Devices

Infrared sensors are noncontacting devices They infer temperature by measuring the thermal radiation emitted by a material Generally used on portable instrument

Temperature Thermowells

Thermowells are used so that the temperature sensing

instrument can be isolated from the process (piping, tank etc)

without interrupting the process

Available in various materials

Generally in SS 316

Insertion length of the thermowell is important To be

sufficient to allow proper sensing of the temperature Generally

for pipelines, the length should be two thirds of the pipe (See

sketch) If this is not possible, to be at least till the centre of the

I=0.66DU

Temperature Transmitters

There are two basic types of thermistors,

PTC and NTC A PTC (positive temperature

coefficient) device will increase resistance

with an increase on temperature

An NTC (negative temperature coefficient)

device will decrease resistance with an

increase in temperature

Both PTC and NTC devices are

manufactured in a broad selection of styles

and resistance values

However, NTC thermistors are more

commonly used than PTC thermistors,

especially for temperature measurement

applications

Conductivity

Measurement

Conductivity is the ability of a material to conduct electric

current The principle by which instruments measure

conductivity is simple - two plates are placed in the sample, a

potential is applied across the plates (normally a sine wave

voltage), and the current is measured.

The basic unit of conductance is the siemens (S), formerly

called the mho Since cell geometry affects conductance

values, standardized measurements are expressed in specific

conductivity units (S/cm) to compensate for variations in

electrode dimensions In other words conductivity is the

conductance as measured between the opposite faces of a

1-cm cube of the material This measurement has units of

siemens/cm

The cell constant, K, is equal to the area normal to the current

flow in centimeters squared divided by the length in

centimeters between the electrodes

Cell Constant Optimum Conductivity Range 0.01 0.055 - 20 μS/cm

0.1 0.5 - 200 μS/cm

1 0.01 - 2 mS/cm

10 1 - 200 mS/cm

pH measurement

Measuring pH involves comparing the potential

of solutions with unknown [H+] to a known reference potential pH meters convert the

voltage ratio between a reference half-cell and a sensing half-cell to pH values.

In the sketch the volt meter we refer to is

actually the pH meter The pH meter measures the potential between pH sensing electrode

(which is sensitive just to Hydrogen ions [H+]) and a reference electrode (which provides a constant potential, independent of the

concentration of Hydrogen ions [H+]).

Analyser – Cond, pH etc

XXX

To Nearest Drain Point

( Maintain a loop in this line by raising the inlet & outlet

tube over the cell holder This will ensure that the cell is

in the lowest point & will prevent the cell from getting

dry).

Sensor Transmitter