control system instrumentation · • the rotating magnets induce a voltage pulse in the coil each...
TRANSCRIPT
Chapter 11
Control System Instrumentation
Measuring Instrumentations
The typical process measuring instrument consists of sensing
elements and transmitters (driving elements) . This combination
of sensor and transmitter is called transducer .
Transducers and Transmitters
Sensor: is a phenomenon that detects process variable and
produces a signal that can be measured like mV, current,
pressure difference, etc.
Transmitter: converts this phenomenon into a signal that can be
transmitted such as current to air (I/P), volt to
current (V/I), volt to pressure (V/P), etc.
Sensor Systems
• Sensor
– temperature sensors
– flow sensors
– level sensors
– pressure sensors
– composition analyzers
• Transmitter
The Control Relevant Aspects of
Sensors
• The time constant/deadtime of the sensor
• The repeatability of the sensor
Sensor Terminology
• Span
• Zero
• Accuracy
• Repeatability
• Process measurement dynamics
• Calibration
Span and Zero Example
• Consider the maximum temperature that is
to be measured is 350ºF and the minimum
temperature is 100ºF.
• The zero is 100ºF and the span is 250ºF
• If the measured temperature is known at
two different sensor output levels (i.e.,
ma’s), the span and zero can be calculated
directly.
Smart Sensors
• Sensors with onboard microprocessors that offer a number of
diagnostic capabilities.
• Smart pH sensors determine when it is necessary to trigger a
wash cycle due to buildup on the electrode surface.
• Smart flow meters use statistical techniques to check for
plugging of the lines to the DP cell.
• Smart temperature sensors use redundant sensors to identify
drift and estimate expected life before failure.
Transmitters
• A transmitter usually converts the sensor output to a signal level
appropriate for input to a controller, such as current signal of
range 4 to 20 mA or pneumatic signal of range 3-15 psig.
• Transmitters are generally designed to be direct acting.
• In addition, most commercial transmitters have an adjustable
input range (or span).
• For example, a temperature transmitter might be adjusted so that
the input range of a platinum resistance element (the sensor) is
50 to 150 °C.
Transmitter/Controller
May need additional transducers for Gm if its output is in
mA or psi. In the above case, Gc is dimensionless (volts/volts).
Measurement / Transmission Lags
• Temperature sensor
make as small as possible (location, materials for
thermowell)
• Current or Pneumatic Transmission lines for
Control Loop
Pneumatic, usually, produces a pure time delay (no
time delays for electronic lines); less common today
compared to electronic transmissions; but it is useful
for place of hot or inductive conditions.
ss
ssM
AU
Cm=
1+s
1
)s(T
)s(T
• Flow Measuring Device:
1. Differential Pressure Meter
- Orifice, Venturi, Nozzle
2. Electromagnetic Flowmeter
3. Vortex Flowmeter
4. Turbine Flowmeter
5. Ultrasonic Flowmeter
FLOW MEASUREMENT
• What is flowrate?? Amount of material passing one point for certain time
How To Choose the Right Flowmeter ??
There are key questions you can ask yourself
when trying to determine which flowmeter is the
best choice for you. The purpose of the
measurement and the physical characteristics of
the fluid being measured are the two main
considerations.
Some specifics to consider are:
• What is the fluid being measured (air, water, gas, etc?)
• If it's not water, what are the properties of the fluid you are measuring?
• What construction materials are acceptable?
• What are the track records of the various technologies you are considering?
• What are the minimum and maximum process pressures and temperatures?
Flow Nozzle
• Consist of an elliptical converging section and cylindrical
throat section.
• Suitable for high-velocity, non-viscous, erosive flows.
• Flow Nozzles have a smooth elliptical inlet leading to a
throat section with a sharp outlet. This restriction in the fluid
flow causes a pressure drop.
• The flow can be calculated from the measured pressure drop
• This device has a greater overall pressure loss or operating
cost in terms of head pressure than a Venturi but offers lower
installation costs.
Orifice Venturi
Nozzle
Paddle Type Orifice Plate
Sizing an Orifice for a Differential
Pressure Flow Indicator
b is the ratio of the orifice diameter to the pipe diameter.
• 0.2 < b < 0.7
• Pressure drop at minimum flow should be greater than 0.5 psi.
• Pressure drop across the orifice should be less than 4% of the line pressure.
• Choose the maximum value of b that satisfies each of the above specifications.
Electromagnetic Flow meter
•This instrument operates
based on Faraday’s Law.
•It is ideal for liquids that
conduct electricity.
•The magnetic field is
developed by electric coils.
•The conductive liquid forms
an electric conductor as its
move through the magnetic
field established inside the
flowmeter .
•This conductor in the
magnetic filed will generate
an electric voltage that is
proportional to its average
velocity.
Example of a Magnetic Flow Meter
Vortex Flow meter
Also know as vortex shedding
flowmeters or oscillatory flowmeters.
It measures the vibrations of the
downstream vortexes caused by the
barrier placed in a moving stream.
The velocity of vortex flowing in the
stream is directly related to the
stream velocity.
A device that counts the vortices
passing per second will also measure
the flowrate.
Advantage: Low cost installation-do not required impulse tubing and
valve manifold.
Disadvantages: Vortexes are inhibited in viscous fluid at low flow rate.
At high fluid velocity the obstructions may introduce
excessive pressure drop-limited to higher flow rate.
Turbine Flowmeter • Turbine meters have a spinning rotor with propeller-like blades that
is mounted on bearings in a housing on the central longitunidal axis of the pipeline.
• Magnets are embedded in the rotor housing and a pickup coil, isolated from the fluid is placed outside the rotor blades.
• The rotor spins as water or other fluid passes over it.
• The rotating magnets induce a voltage pulse in the coil each time they pass the coils.
• Pulse frequency is proportional to the velocity
• Disadvantages: sensitive to viscosity changes, require maintenance
at their bearing.
Ultrasonic Flow meter •Ultrasonic flowmeters can be categorized into two types based
on the installation method: clamped-on and inline.
•The clamped-on type is located outside of the pipe and there
are no wetted parts. It can easily be installed on existing
piping systems without worrying about corrosion problems.
Clamped-on designs also increase the portablility of the
flowmeter.
•The inline type, on the other hand, requires fitting flanges or
wafers for installation. However, it usually offers better
accuracy and its calibration procedures are more
straightforward.
•Ultrasonic flowmeters measure the traveling times (transit time
models) or the frequency shifts (Doppler models) of ultrasonic
waves in a pre-configured acoustic field that the flow is
passing through to determine the flow velocity.
• Time Transit Model
- A pair of transducers is placed on the pipe wall, one on the upstream and the other on the downstream. The time for acoustic waves to travel from the upstream transducer to the downstream transducer is shorter than the time it requires for the same waves to travel from the downstream to the upstream. The larger the difference, the higher the flow velocity.
Ultrasonic Flow meter
• Doppler Model
- rely on the Doppler effect to relate the frequency
shifts of acoustic waves to the flow velocity. It
usually requires some particles in the flow to reflect
the signals.
Ultrasonic Flow meter
Pressure-Measuring Devices
Most liquid and all gaseous materials in the process
industries are contained within closed
vessels. For the safety of plant personnel and
protection of the vessel, pressure in the vessel is
controlled. Types of pressure sensors are:
• U-Tube Manometer
• Bourdon Tube Sensor
• Bellows-Type Sensor
• Strain Gauge Sensor
U-Manometer
Bourdon, Bellow and Diaphragm Sensor
• Bourdon: A bourbon tube is a curved, hollow
tube with the process pressure applied to the fluid in the tube.
• Bellow: A bellows is a closed vessel with sides
that can expand and contract
• Diaphragm: A diaphragm is typically constructed of two
flexible disks, and when a pressure is applied to one face of the
diaphragm, the position of the disk face changes due to
deformation
• In all, the displacement can be related to pressure.
Displacement is convert to electrical signal ~ required
secondary element.. • Used elastic material
Bourdon Tube Sensor
Bourdon Tube Sensor
Bellows-Type Sensor
Diaphragms Sensor
Strain Gauge Sensor
• The electrical resistance of a metal wire depends on
the strain applied to the wire. Deflection of the
diaphragm due to the applied pressure causes strain
in the wire, and the electrical resistance can be
measured and related to pressure.
• Fluid – Force – Diaphragm Displacement (detected
by strain gauge sensor) –
and the resistance change (detected by Wheatstone
bridge).
Strain Gauge Sensor
Strain Gauge Sensor
Temperature control is important for separation and reaction
processes, and temperature must be maintained within limits to
ensure safe and reliable operation of process equipment.
Temperature can be measured by many methods; several of the
more common are described in this subsection. You should
understand the strengths and limitations of each sensor, so that you
can select the best sensor for each application.
In nearly all cases, the temperature sensor is protected from the
process materials to prevent interference with proper sensing and to
eliminate damage to the sensor. Thus, some physically strong,
chemically resistant barrier exists between the process and sensor;
often, this barrier is termed a sheath or thermowell, especially for
thermocouple sensors.
Temperature Sensing Systems
There are several methods used to measure temperature. The followings are just
few of these methods, which may be employed.
The capillary tube (fluid thermometer) A capillary tube is a very thin tube. This type of thermometer has a bulb filled
with mercury. When the temperature rises, the mercury in the bulb expands.
This expansion pushes the mercury higher in the capillary tube.
Temperature Sensing Systems
Thermocouple
• Consist of two dissimilar metal and connected ~ voltage generated
• Hot junction ~ measure temperature
• Cold junction ~ reference (known temperature)
• E1 = voltage generated by T1 (hot junction)
• E2 = voltage generated by T2 (cold junction)
• Et = E1 – E2
E1 E2
Hot
junctio
n
Cold
junctio
n
• When the junctions of two dissimilar metals are
at different temperatures, an electromotive force
(emf) is developed
• The emf is calculated using the following
equation:
emf (volts) = (Th – Tc)oC x b
b = constant (V/K) , T (K)
• Different combinations of metals result in
different voltage- temperature characteristics. In
industry, thermocouples are usually classified by
a one-letter type designation that describes their
response.
Standard thermocouples
_____________________________________________________
Type Materials Normal Range
J Iron-constantan -190oC to 760oC
T Copper-constantan -200 oC to 371 oC
K Chromel-alumel -190 oC to 1260 oC
E Chromel-constantan -100 oC to 1260 oC
S 90% platinum +
10% rhodium- platinum 0 oC to 1482 oC
R 87% platinum +
13% rhodium- platinum 0 oC to 1482 oC
•Thermocouple are often insulated electrically with ceramic
material (high temperature) and sheathed in stainless steel
•Used thermowell for effectively seal off the process fluid or gas.
•Advisable to use thermowell to prevent heat loss and personnel
injury
Temperature sensor
without thermowell
Temperature sensor
with thermowell
RTD (Resistance Temperature Detector )
• The electrical resistance of many metals changes
with temperature, metals for which resistance
increases with temperature are used in RTDs
• Temperature is determined from the change in the
electrical resistance of the metal wire.
• Linear relationship using equation RT= Ro(1+aT)
RT = the resistance at temperature, T
R0 = the resistance at base temperature of 0 °C
T = the temperature of the sensor (to be determined
from RT)
a = the temperature coefficient of the metal.
R100 = Resistance at 100oC (steam point)
R0 = Resistance at 0oC (ice point)
• Limitation: 0 - 100 oC
R100/ R0 - 1
100 a =
• RTD sensitivity, a can be noted from typical
value of metal used,
Platinum = 0.004 / oC
Nickel =0.005 / oC
• The effective range of RTDs principally
depend on the type of wire used
Platinum RTD = -100 to 650 oC
Nickel RTD = -180 to 300 oC
Thermistor:
This sensor is similar to an RTD, but applies metals for which the resistance
decreases with increasing temperature. The relationship is often very nonlinear,
but thermistors can provide very accurate temperature measurements for small
spans and low temperatures.
Thermisters are made from oxides of metals such as copper, nickel, cobalt and
lithium which are blended to produce the required resistance-temperature
characteristics. Most Thermistors have negative non-linear temperature
coefficients. Thermistors are produced in many shapes and sizes such as beads,
discs and probes and may be coated in a glass or steel sheath for added
protection and strength.
Filled systems:
A fluid expands with increasing temperature and exerts a varying
pressure on the containing vessel. When the vessel is similar to a
bourbon tube, the varying pressure causes a deformation that
changes the position detected to determine the temperature.
The tube in a filled-system temperature indicator can be filled with a
liquid and vapour. When the temperature rises, more liquid is changed
to vapour. The increase vapour pressure straightens the spiral
bourdon tube pressure instrument. The figure below show how the
movement of the bourdon tube, caused by a pressure change, is read
as a temperature change
Sensor Type Limits of
Application (°C)
Accuracy1,2 Dynamics:
t (s)
Advantages Disadvantages
Thermocouple
type E:
chromel-constantan -100 to 1000
±1.5 or 0.5%
for 0 to 900 °C
see note 3
-good reproducibility
-wide range -minimum span of 40
°C
-temperature vs. emf
not exactly linear
-drift over time
-low emf corrupted by
noise
type J:
iron-constantan 0 to 750 ±2.2 or 0.75%
type K:
chromel-nickel 0 to 1250 ±2.2 or 0.75%
type T:
copper-constantan -160 to 400
±1.0 or 1.5%
for -160 to 0 °C
RTD -200 to 650 0.15 + 0.2|T| see note 3
-good accuracy
-small span possible
-linearity
-self-heating
-less physically rugged
-self-heating error
Thermister -40 to 150 ± 0.10 °C see note 3 -good accuracy
-little drift
-highly nonlinear
-only small span
-less physically rugged
-drift
Bimetallic - ± 2% - -low cost
-physically rugged -local display
Filled system -200 to 800 ± 1% 1 to 10 -simple and low cost
-no hazards
-not high temperatures
-sensitive to external
pressure
Table: Summary of temperature sensors
• Level = a measurement of the height of the free surface of the liquid from a fixed datum or reference.
• Level accuracy:- – Smoothen the process operation.
– Comply the custom and taxing regulation.
• Level can represent the amount of asset and related to money matter
• Tank contains liquid and sometime the solid.
• Sometimes the liquid solidifies, very corrosive, and vaporizes – create difficulties.
LEVEL MEASUREMENT
The difference in pressures between to points in a vessel depends
on the fluids between these two points. If the difference in
densities between the fluids is significant, which is certainly true
for a vapor and liquid and can be true for two different liquids, the
difference in pressure can be used to determine the interface level
between the fluids. Usually, a seal liquid is used in the two
connecting pipes (legs) to prevent plugging at the sensing points.
Differential Pressure Level Measurement
Differential Pressure Level Measurement
DPT
Vapor
Diaphragm
Lower Tap
Upper Tap
Liquid
Buoyancy Sensor
• Buoyancy = displacement
• By Archimedes principle, a body immersed
in a liquid is buoyed by a force equal to the
weight of the liquid displaced by the
body. Thus, a body that is more dense than
the liquid can be placed in the vessel, and
the amount of liquid displaced by the body,
measured by the weight of the body when in
the liquid, can be used to determine the
level .
Buoyancy Sensor
Buoyancy Sensor
Float Sensor
The float of material
that is lighter than
the fluid follows the
movement of the
liquid level.
The position of the
float, perhaps
attached to a rod,
can be determined to
measure the level.
Capacitance Sensor
A capacitance probe can
be immersed in the
liquid of the tank, and
the capacitance between
the probe and the vessel
wall depends on the
level.
By measuring the
capacitance of the
liquid, the level of the
tank can be determined
Capacitance Sensor
• The term analyzer refers to any sensor that measures a physical property of the process material. This property could relate to purity (e.g., mole % of various components), a basic physical property (e.g., density or viscosity), or an indication of product quality demanded by the customers in the final use of the material (e.g., gasoline octane or fuel heating value).
• Used to measure the physical properties
• Standalone in laboratory or installed near to equipment.
ANALYTICAL MEASUREMENT
pH Meter
Discuss and answer all the questions
• What is the purpose of pH meter??
• How it’s measured??
• Explain standard hydrogen electrode.
• List and describe types of electrodes.
Conductivity Meter
Discuss and answer all the questions
• What is the purpose of conductivity meter ??
• Explain polarization effect.
• How to measure conductivity?
Called Actuator System
• Control Valve
– Valve body
– Valve actuator
• I/P converter
• Instrument air system
Pneumatic Control Valve
Cross-section of a Globe Valve
Typical Globe Control Valve
Air to Close Valve, Fail Open
Pneumatic Control Valve
Pneumatic Control Valve
Air to Open Valve, Fail Close
Types of Globe Valves
• Quick Opening- used for safety by-pass
applications where quick opening is desired
• Equal Percentage- used for about 90% of
control valve applications since it results
in the most linear installed characteristics
• Linear- used when a relatively constant
pressure drop is maintained across the valve
Inherent Valve Characteristics
0
0.5
1
0 20 40 60 80 100
Stem Position (% Open)
f(x)
=%
QO
Linear
Control Valve Design Procedure
• Choose a control valve so that the average
flow rate results when the valve is 2/3 open.
• After the valve has been sized, check to
ensure that the maximum and minimum
flow rates will be accurately metered.
Additional Information Required to
Size a Control Valve
• CV versus % open for different valve
sizes.
• Available pressure drop across the valve
versus flow rate for each valve.
• Note that the effect of flow on the upstream
and downstream pressure must be known.
Valve Sizing Example
• Size a control valve for max 150 GPM of
water and min of 50 GPM.
• Therefore, choose the valve size so that
valve is approximately 67% open at 100
GPM.
Determine CV at 100 GPM
• Use the valve flow equation (Equation 2.1) to
calculate Cv
• For DP, use pressure drop versus stem position
(e.g., Table 2.2)
/)(
PK
FxC m
vD
Cv versus % Valve Travel for
Different Sized Valves
• Body % Valve Opening
• Size in 50 60 70
• 1 3.63 5.28 7.59
• 1.5 4.30 6.46 9.84
• Cv 2 11.1 20.7 32.8
• 3 21.7 36.0 60.4
• 4 31.2 52.6 96.7
Check Max and Min Flows
• Ensure that the flow rate will be accurately
controlled at the maximum and minimum
flow rates.
• At minimum flow rate valve should be at
least 10-15% open.
• At maximum flow rate the valve should be
at most 85-90% open.
Valve Deadband
• It is the maximum change in instrument air
pressure to a valve that does not cause a change in
the flow rate through the valve.
• Deadband determines the degree of precision
that a control valve or flow controller can provide.
• Deadband is primarily affected by the friction
between the valve stem and the packing.
Valve Actuator Selection
• Choose an air-to-open for applications for
which it is desired to have the valve fail
closed.
• Choose an air-to-close for applications for
which it is desired to have the valve fail
open.
Optional Equipment
• Valve positioner- a controller that adjusts the instrument air in order to maintain the stem position at the specified position.
• Greatly reduces the deadband of the valve.
• Positioners are almost always used on valves serviced by a DCS.
• Booster relay- provides high capacity air flow to the actuator of a valve.
• Can significantly increase the speed of large valves.
Control Relevant Aspects of
Actuator Systems
• The key factors are the deadband of the actuator
and the dynamic response as indicated by the
time constant of the valve.
• Control valve by itself- deadband 10-25% and a
time constant of 3-15 seconds.
• Control valve with a valve positioner or in a flow
control loop- deadband 0.1-0.5% and a time
constant of 0.5-2 seconds.
See ex 9.1 for A to O and A to C