Chapter 6 - Instrument System (1).pptx

P & ID DESIGN
CHAPTER 6 -
INSTRUMENT SYSTEM

Chapter 6 – Learning Outcomes
Upon completion of chapter 5, students can:
- Describe operating principles of instruments (sensors)
- Properly select the right type of sensors to use based on
sensors characteristics and process conditions (fluid
properties, pipe sizes, etc.)

1. Moe Toghraei. (2019). “Piping and Instrumentation Diagram
Development”, Wiley.
2. Béla G. Lipták (Editor-in-chief). (2003). “Instrument Engineers'
Handbook, Vol. 1~ Process Measurement and Analysis, 4th
Edition”, CRC Press
3. Cecil L. Smith. (2009). Basic Process Measurements, Wiley.
4. Franklyn W. Kirk, Thomas A. Weedon, Philip Kirk, (2010).
Instrumentation, 5th Ed, American Technical Publishers, Inc.
5. Wayne Seames (2018). “Designing Controls for the Process
Industries”, CRC Press
6. Thomas A. Hughes. (2015). Measurement and Control Basics 5th
Ed, International Society of Automation (ISA)
7. Livelli, Greg, (2013). “Selecting Flowmeters to Minimize Energy
Costs”, Chemical Engineering Progress; Vol. 109, Issue. 5
References for chapter 6

Role of process measurements in process automation

Role of process measurements in process automation
Process measurements fall into two categories:
1 Continuous measurements: An example of a continuous
measurement is a level measurement device that determines the
liquid level in a tank (e g , in meters)
2 Discrete measurements: An example of a discrete measurement is a
level switch that indicates the presence or absence of liquid at the
location at which the level switch is installed (leak detection system)
In continuous processes, most process control applications rely on
continuous measurements. In batch processes, many of the process
control applications utilize discrete as well as continuous
measurements

Components in a measurement system
1 Primary element (sensor). This component produces a signal that is
related in a known manner to the process variable of interest, such as
a voltage, a resistance, mechanical signal (force)...
2. Transmitter: Perform two operations:
 Signal processing. The signal from a sensor is usually related in a
nonlinear fashion to the process variable of interest. If a linear
relationship is desired, linearization is required
 Signal transmission: the measurement output signal need be
transmitted over some distance. The standard analog electrical
output signal range is 4 mA to 20 mA DC. Microprocessor-based
transmitters (smart transmitters) transmit the measured variable
digitally in engineering units
Thus, a temperature transmitter is a measuring element that converts
a thermocouple voltage to a scaled temperature value and transmits
the measured temperature signal to the controller

Characteristics considered in selecting a sensor
 Range is the boundary of the values that identify the
minimum and maximum limits of an element. For example,
a temperature sensor may have a range of –50°F to 200°F
 Range is specified with two numbers representing the lowest
and the highest values. Span is the difference between the
highest and lowest numbers in the range.
 Rangeability (especially used for flow sensors): is the ratio
of the maximum flow to the minimum measurable flow at
the desired measurement accuracy

 Accuracy is the degree to which an observed value matches the
actual value of a measurement over a specified range (accuracy
characterizes accuracy of measurement system)
 Precision is the closeness to which elements provide agreement
among measured values (precision characterizes repeatability of
measurement system)
 Bias is a systematic error or offset introduced into a
measurement system (systematic measurement error)

1. Measurement span
2. Performance: accuracy, repeatability, sensitivity, response time...
3. Cost: initial purchase and installation (capital cost) and recurring
costs (operational expense)
4. Reliability: life cycle time, maintenance requirement
5. Materials of construction: compatible with process conditions ?
(the fluid @ operating temperature & pressure)
6. Prior use: experience in using the sensor
7. Potential for releasing process materials to the environment?
8. Electrical classification? if electrical wiring of sensor requires
enclosures, any preventive measure to protect against possible
hazards during operation ?
9. Physical access: maintenance personnel must have physical
access to the measurement device for maintenance and
calibration
10. Invasive or noninvasive: the insertion of a probe can result in
fouling problems and a need for maintenance.
Factors considered in sensor selection

Fluid flow – Reynolds number
A Reynolds number is the ratio between the inertial forces
moving a fluid and viscous forces resisting that movement. It
describes the nature of the fluid flow.
v is the fluid velocity (m/s), d is the inside pipe diameter
(m) and ϑ is the kinematic viscosity (m2/s)

Fluid flow – Reynolds number

Flow measurement - Differential-pressure type
Q 𝑓𝑙𝑜𝑤 = 𝐾 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 ∗
∆𝑃
𝑑(𝑑𝑒𝑛𝑠𝑖𝑡𝑦)

Advantages of differential-pressure (d/p) type flow sensors
- The most popular flow meter type
- Widely used to measure the flow of both gases and liquids,
including viscous and corrosive fluids.
- No moving parts
- Suitably used for measuring practically all flow rates in a wide
variety of pipes and tubes.

Disadvantages of differential-pressure (d/p) type flow sensors
- A square-law relationship between P and Q, which severely
limits their rangeability (typically 3:1, with 5:1 being the
maximum).

Disadvantages of differential-pressure (d/p) type flow sensors
- In addition to the sensor element, several other components are
needed to make a measurement: a transmitter, valves and
fittings to attach the transmitter to the sensor. As a result, the
installation is time consuming and the measurement system
requires relatively high maintenance to eliminate leakage

Flow measurement – Orifice plate
Advantages of orifice plate flow sensors
- The simplest and least expensive flow element within the d/p-
type sensors.
- The total installed cost is relatively independent of pipe
diameter; consequently, the orifice-type installations are
relatively expensive in smaller pipe sizes and rather economical
in pipe sizes over 6 in. (150 mm).
- Orifices can be used in a wide range of applications, because
these plates are available in a variety of materials and in many
designs, such as concentric, segmental, or eccentric.

Disadvantages of orifice plate flow sensors
- Low accuracy and low rangeability of standard orifices,
although substantial improvements have been reported for new
designs.
Total error of an orifice type flow measurement

Disadvantages of orifice plate flow sensors
- High irrecoverable pressure loss (40 to 80% of the generated P)
- Deterioration in both measurement accuracy and in long-term
repeatability as the edge wears or as deposits build up.
- High maintenance to assure no leakage or pressure tap plugging

Concentric Sharp-edged Eccentric
Segmented Wedge design

Straight runs of about 20 times the pipe diameter before and 6 times the pipe
diameter after the orifice plate are recommended to allow the flow disturbances
to die out. Alternatively, the use of a straightening vane upstream of the orifice
plate reduces or eliminates the disturbances

Flow measurement – other d/p type sensors

The annular orifice used to measure the hot and dirty gases
in the steel industry. Here, the process flow passes through
an annular opening between the pipe and a disk-shaped,
concentrically located plate, and flow is indicated by the d/p
In a target flowmeter, a target or
impact plate is inserted into
the flowing stream, and the
resulting impact force is detected
electronically or pneumatically as
an indication of flow.

beta ratio (d/D) is the ratio between the diameter of the orifice
plate (d ) and the internal diameter of the pipe (D)
Pitot tube flowmeters are often
used for very low flow, low
pressure gas flow measurements

Flow measurement – Magnetic Flowmeters
A fundamental principle of electromagnetism states that a voltage is
generated when a conductor moves relative to a magnetic field. As a
flowing conductive liquid moves within the nonconductive tube and
passes through the magnetic field of the coils, a voltage is induced
and detected by the electrodes.

Advantages of magnetic flowmeters:
- Lack of moving parts and no obstruction of flow  no pressure
loss, no wear and tear on their components.
- Indifference to fluid properties including chemical nature,
viscosity, pressure, temperature, and density variations;
- Ability to provide linear analog outputs and to measure
bidirectional flows
- Availability in a wide range of sizes; and ease and speed of
installation on site

Disadvantages of magnetic flowmeters:
- Can be used only on electrically conducive fluids (this
requirement eliminates their use on all gases and on most
hydrocarbon fluids).
- High purchase price and the cost of maintaining the magnetic
field.

Flow measurement – Ultrasonic Flowmeters
 A Doppler ultrasonic meter is a flowmeter that transmits an
ultrasonic pulse diagonally across a flow stream, which reflects off
turbulence, bubbles, or suspended particles and is detected by a
receiving crystal.
 The measured signal (Doppler frequency shift) is proportional
to the velocity of the flowing stream

 The liquid velocity slows the upstream signal and increases the
received frequency while speeding up the downstream signal and
decreasing the received frequency.
 The difference in the measured frequencies is used to calculate the
transit time of the ultrasonic beams and thus the liquid velocity

 The Doppler meter:
• Frequently used in a “clamp-on” design, which can be attached
to the outside of existing pipelines.
• Low cost, which does not increase with pipe size.
• Not suitable for the measurement of clean fluids or clean gases
 The transit-time type ultrasonic flowmeters:
• Often used in water treatment and chemical plant applications.
• Considerably more expensive than the Doppler version, but it
offers better accuracy.
• Usable only on relatively clean fluid applications.
• Introduce no restriction or obstruction to flow, so its pressure
drop is low.
Ideally suited to measure the flow of corrosive liquids (both
types)

Flow measurement – Vortex Shedding Meters
Vortices are formed and travel downstream at a frequency that is
linearly proportional to velocity. The frequency of release of the
vortices can be measured with ultrasonic sensors or other type of
sensors.
Vortex shedding
meters create
disturbances in ﬂow
that are
measured to
calculate ﬂow.

Flow measurement – Vortex Shedding Meters
Advantages of vortex shedding flowmeters:
- Have no moving components and can measure the flow of
gas, steam, or liquid.
- Good accuracy and repeatability, high rangeability, low
maintenance
- Vortex shedding meters can be general-purpose, economically
competitive alternatives to the orifice plate, and they
have better accuracy and rangeability than the orifice plate

Flow measurement – Turbine Meters
A turbine meter is a ﬂowmeter consisting of turbine blades
mounted on a wheel that measures the velocity of a stream by
counting the pulses produced by the blades as they pass an
electromagnetic pickup

Advantages of turbine flowmeters:
- Can be used in both liquids and gases, and for a wide range of
applications (low and high flowrates, low and high pressure and
temperature…) with suitable designs of inserted turbines
- Liquid turbine meter is one of the most accurate meters
available for low- to medium-viscosity products (widely used in
blending applications, product sales)
- Rangeability of single turbine meters is around 10:1, for dual-
turbine meters, it exceeds 100:1.
- They are easy to install and, relative to the pipe diameter,
are also small in size and weight.

Disadvantages of turbine flowmeters:
- High cost
- Not suitable for viscous or dirty liquids
- Potential for being damaged by over-speeding if slugs of gas or
vapor are sent through the liquid meter
- The installation of upstream filters is often recommended

Flow measurement – Variable-area Meters
A variable-area flowmeter is a meter that maintains a constant
differential pressure and allows the flow area to change with
flow rate. The most common type of variable-area flowmeter is
the rotameter.

Variable-area Meters for low-flow applications

Advantages of variable-area flowmeters:
- Widely used for applications in which small flow rates are to be
measured or where local indication is required
- Are self-contained in nature (meters with clear tubes), which
eliminates the need for power supplies
- Low cost, low pressure loss, direct flow indication, and the
ability to detect very low flow rates of both gases or liquids,
including viscous fluids

Disadvantages of variable-area flowmeters:
- Require vertical mounting and they are available only in smaller
sizes
- Meters with clear tubes: low accuracy, used in low pressure
applications, limited availability of transmitters
- Meters with metallic tubes: can be used in applications with
larger pipe sizes and higher pressure than the clear tube units,
but they are limited to use with clean fluids

Flow measurement – Positive-displacement (PD) Meters
The PD meters trap a fixed volume of fluid and transfer
it from the inlet to the outlet side of the meter. The number of
such calibrated “packages” of fluid is counted as a measure of
volumetric flow

Advantages of positive-displacement (PD) flowmeters:
- Often used when accurate quantities need to be delivered, either
for reasons of recipe formulation in batch processes or for
accounting purposes during sales
- Liquid PD meters offer good accuracy and rangeability
(>10:1) and are particularly suited to measure the flow of
high-viscosity fluids.

Disadvantages of positive-displacement (PD) flowmeters:
- Applicable to clean fluids, because their operation depends on
close meshing surfaces.
- Require regular recalibration and maintenance, particularly
when used to measure the flow of nonlubricating liquids.
- Bulky and heavy.
- High cost (purchase cost and installation cost): in addition to
block and bypass valves, they also require filters and air
releases for proper operation.

Flow measurement – Coriolis Mass Flowmeters
A Coriolis flow meter contains a
tube which is energized by a fixed
vibration. When a fluid passes
through this tube the mass flow
momentum will cause a change in
the tube vibration, the tube will
twist resulting in a phase shift. This
phase shift can be measured and a
linear output derived proportional
to flow.

Advantages of Coriolis mass flowmeters:
- Coriolis meters can operate at process flow velocities from
0.061 to 6.1 m/sec and therefore can provide a rangeability of
100:1
- Accuracies are in the range of 0,2% for mass flow and around
1% for density
- Pressure and temperature ratings are acceptable
- Can be used on virtually any liquid or gas flows where the mass
rather than the volume of the process flow is of interest
- Very popular in the measurement of fuel flows and reactor feed
flows

Disadvantages of Coriolis mass flowmeters:
- Relatively small sizes (up to 6 in. [150 mm])
- Inability to handle high-temperature process fluids (over 400°F
[205°C])
- Relatively high cost

Flow measurement – Thermal Mass Flowmeters
A thermal mass meter is a mass flowmeter consisting of two
resistance temperature detector (RTD) temperature probes and a
heating element that measure the heat loss to the fluid mass.
Thermal mass meters are predominantly used for measuring gas
flows.
The heated probe loses heat to
the stream by convection. The
electric circuitry is designed to
maintain a constant difference in
temperature between the two
probes by varying the power to
the heating element.
The power becomes the
measured variable of the system.
The variations in power are
proportional to the variations in
mass flow rate

Flow measurement –Flowmeters Selection

Flow measurement – Symbols for flowmeters in P & ID

Flow measurement – Symbols for flowmeters in P & ID
Use reducer / enlarger to increase fluid
velocity when passing through the flow
meters
Use strainer to protect the flow meters
Use by-pass line if frequent inspection
/ maintenance of flow meters is needed

Temperature measurement
Temperature is the degree or intensity of heat measured on a
definite scale. Temperature is an indirect measurement
of the heat energy contained in molecules. When molecules have a
low level of energy they are cold, and as energy
increases they get warmer. The energy is in the form of molecular
movement or vibration of the molecules.

Temperature measurement – Nonelectric sensors
LIQUID-IN-GLASS
THERMOMETERS
The volume of a liquid
changes when the
temperature changes. When
liquid is placed in a glass
tube, the top of the liquid
moves with a change in
temperature.

INDUSTRIAL
THERMOMETERS
With this type of
thermometer, the glass
tube is not marked or
scaled.
Both the tube and the
scales are enclosed
in a metal case.

BIMETALLIC THERMOMETERS
The principle of differential thermal expansion is the basis of
operation for some thermometers such as bimetallic expansion
thermometers. When one material has a greater coefficient of
thermal expansion than another material, the difference in
expansion can be used as a measure of temperature by
direct reading or by connection to a mechanical linkage.
A bimetallic thermometer is a thermal expansion thermometer that
uses a strip consisting of two metal alloys with different
coefficients of thermal expansion that are fused together and
formed into a single strip, and a pointer or indicating mechanism
calibrated for temperature reading

Liquid-Filled Pressure-Spring Thermometers
A liquid-filled pressure-spring thermometer is a pressure-
spring thermometer that is filled with a liquid under pressure.
When the bulb is immersed in a heated substance, the liquid
expands. This causes the pressure spring to unwind. The
indicating, recording, and controlling mechanisms are attached
to the pressure spring and are actuated by its movements

Bistate/Phase Change Sensors
These low cost nonelectric sensors are made from heat-
sensitive fusible crystalline solids that change decisively from a
solid to a liquid with a different color at a fixed temperature
depending on the blend of ingredients. They are available as
crayons, lacquers, pellets, or labels over a wide range of
temperatures from 100 to 3000°F (38 to 1650°C).

Bistate/Phase Change Sensors

Temperature measurement – Electrical thermometers
Thermocouples (TC)
A TC is an assembly of two wires of unlike metals joined at one
end designated the hot end. At the other end, referred to as the
cold junction, the open circuit voltage is measured. Called the
Seebeck voltage, this voltage (electromotive force) depends on
the difference in temperature between the hot and the cold
junction and the Seebeck coefficient of the two metals

RESISTANCE TEMPERATURE DETECTORS (RTDs)
A resistance temperature detector (RTD) is an electrical
thermometer consisting of a high-precision resistor with
resistance that varies with temperature, a voltage or current
source, and a measuring circuit. RTDs are accurate and
reliable temperature sensors especially for low temperatures
and small ranges. They are generally more expensive than
thermocouples and are not used for high temperatures or
corrosive measuring environments.

THERMISTORS
A thermistor is a temperature-
sensitive resistor consisting of
solid-state semiconductors
made from sintered metal oxides
and lead wires, hermetically sealed
in glass. They are available in
several shapes such as rods,
disks, beads, washers, and flakes

Temperature measurement – Infrared radiation thermometers
Bodies that are at thermal equilibrium must balance the energy
entering that object, such as heat or light, with the energy
leaving that object. The energy leaving the surface of an
object is often emitted as electromagnetic radiation. An IR
thermometer is a thermometer that measures the infrared
radiation (IR) emitted by an object to determine its temperature.
IR thermometers generally have very quick response times.
They can typically make many measurements per second. An IR
thermometer can be used in areas where it is very difficult to use
a contact thermometer

General IR thermometers system

Temperature measurement – Thermometers selection

Pressure measurement
Pressure is the easiest and most responsive parameter to
measure in the process plant. Devices with accuracy adequate
for the vast majority of process applications are economical
and require very little maintenance. As such, it is usually
justified to include pressure measurements in most process
streams, even when not needed for controls.

Pressure gauges and sensors are usually configured to measure
the pressure in one of three ways: the absolute pressure relative
to a vacuum, the gauge pressure relative to the surrounding
atmosphere, and the differential pressure between two sensing
points. Absolute pressure measurement has units of kPa, psia,
or atm. Gauge pressure measurement has units of kPag, barg, or
psig. Differential pressure has units of kPa or psi (with no “a”
or “g” designator).

lb per sq in. = pound force per square inch = psi

Pressure measurement - Manometer
A manometer is a device for
measuring pressure with a
liquid-filled tube. A
manometer is the simplest
device for measuring pressure.
In a manometer, a fluid under
pressure is allowed to push
against a liquid in a tube. The
movement of the liquid is
proportional to the pressure

Pressure measurement - Pressure-sensing Elements

In a Bourdon tube
pressure gauge, a spring
is the elastic element that
changes shape with
changes in the force (the
pressure) exerted upon it.
Mechanical linkages
convert the spring’s
motion to that of a
pointer

A typical bellows-type pressure gauge is manufactured by forming
many accordion-like pleats into a cylindrical tube; the sides expand
with increasing pressure and contract with decreasing pressure.

A diaphragm is usually a flexible metal disk. Increasing pressure causes the
diaphragm to deform. The diaphragm is the pressure-sensing mechanism of
choice in electronic instrumentation because of the following characteristics:
 Compact arrangements are possible.
 Diaphragms can sense gauge pressure, absolute pressure, or differential
pressure.

Pressure measurement - Pressure Sensors
 A pressure transducer is used to convert the mechanical
displacement of a diaphragm caused by a change in applied
external force into an electrical signal (the other two types of
pressure sensing elements, Bourdon tubes and bellows, are rarely
used in pressure sensors)
 A pressure transmitter is a pressure transducer with a power
supply and a device that conditions and converts the transducer
output into a standard analog or digital output.
Pressure sensor = pressure-sensing element +
pressure transmitter

Resistance pressure transducers (strain gauges) are the most
widely used electrical pressure transducers. A strain gauge is a
transducer that measures the deformation, or strain, of a rigid
body as a result of the force applied to the body.

A piezoelectric pressure
transducer:
This type of transducer
produces an electrical
output proportional to the
pressure on the
diaphragm.

Pressure measurement – Sensors Selection

Pressure measurement – sensors arrangement

Level measurement
Measurement of liquid level in process vessels is very important
to prevent overﬂow and potential safety problems due to
excessively high liquid levels and pump damage due to very low
levels. For some process operations, the liquid level must be
tightly regulated in order to have a specified residence time in
the vessel (e.g., some chemical reactors)

Level measurement - Sight-type Instruments
Tubular sight glass gauges

Level measurement - Sight-type Instruments

Level measurement - Pressure-type Instruments
 There are many applications where it is more convenient to
measure the pressure at the bottom of a tank than to measure
the actual location of the top of the liquid. For example, a
tank may be sealed to prevent the escape of volatile or toxic
fluids.
 Common methods of using pressure to measure level are
hydrostatic pressure and bubbler systems
 About 75% of level measurements are based on either
pressure or differential pressure

Hydrostatic pressure variations
present at the base of a liquid
column provide the means of
determining liquid level in a
storage vessel. As long as the
liquid in the tank has a
constant density, variations in
pressure are caused only by
variations in level

When you use bubbler systems to measure liquid level, you install
a dip tube in a tank with its open end a few inches from the
bottom. A gas is slowly fed into the tube until the pressure is equal
to the hydrostatic head of the liquid in the tank. At that point, the
gas flow bubbles out of the end of the bubble tube.

Level measurement - Float-type Instruments
A float is a point level measuring instrument consisting of a hollow
ball that floats on top of a liquid in a tank and is attached to the
instrument. The float is connected by a lever to an ON/OFF switch
actuated by the movement of the float.
A float activates an ON/OFF
switch when the level exceeds
the alarm level.

Level measurement - Float-type Instruments

Level measurement - Displacers
Displacer level gauges operate on Archimedes’ principle; they use the change
in buoyant force acting on a partially submerged displacer. A displacer consists of
a buoyant cylindrical object, heavier than the liquid, that is immersed in the
liquid and connected to a spring or torsion device that measures the buoyancy of
the cylinder. The measured level ranges match the displacer lengths

Level measurement - Ultrasonic sensor
An ultrasonic sensor is a
level measuring instrument
that uses ultrasonic sounds
to measure level. The
transmitter generates a
high-frequency sound
directed at the surface of
the material in the vessel.

Level measurement – (noncontact) Pulsed radar sensor
A pulsed radar level sensor is a level measuring sensor consisting of a
radar generator that directs an intermittent pulse with a constant
frequency toward the surface of the material in a vessel

A guided wave radar is a level measuring detector consisting of a cable
or rod as the wave carrier extending from the emitter down to
the bottom of the vessel and electronics to measure the transit time
Level measurement – Guided wave radar sensor

Level measurement – Sensors selection

Level measurement – sensors representation in P & ID

Summary: On-Line Measurement Options for Process Control

Chapter 6 - Instrument System (1).pptx

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