Chapter 4 Process Control

Process Control MeasurementFinal Control Element

7. Process Control Measurement

Considered as measuring element in closed-loop terminology.

Measurement classification:1. Temperature2. Level3. Pressure4. Flow5. Analysis – pH, turbidity, chlorine

2

Temperature measurement

Thermocouple

-very rugged and cheap

-operating in wide range of measurement (0 - 1700°C)

-consists of a pair of wires from dissimilar metal & joined at one end.

-the other end are connected to a meter or a circuit as shown in the following figure

3

Cont.

When the solder joint is heated, a measurable voltage is generated at the other joint where it is connected to a meter or a circuit.

The generated voltage is increased with the increment of the measured temperature.

+

-

4

Resistance Temperature Devices (RTD)

RTD responds to temperature by changing their electrical resistance.

It rely on the principles that the resistance of a metal varies with temperature.

5

Thermistor (Thermal + Resistor)

Thermistors are special solid temperature sensors that behave like temperature-sensitive resistors.

There are basically two broad types: NTC - Negative Temperature

Coefficient, used mostly in temperature sensing (-100°C to 450°C). It is constructed of ceramics composed of oxides of transition metals (manganese, cobalt, copper, and nickel).

-high temperature will cause lower value of resistance.

PTC - Positive Temperature Coefficient, used mostly in electric current control.

-resistance increase with temperature

6

Level Measurement

Covers a lot of level tank such as in fuel tanks, water tank as well as powder tank.

Varies in principle which determine the cost, accuracy and performance.

Among the types are:1. Float system2. Pressure method3. Capacitive devices4. Ultrasonic level gauge5. Radar method6. Laser method

7

Float system

Using potentiometer and commonly used in vehicle fuel tanks as following figure.

Another type known as tape/ tank gauge used pulley and counterweight/ counterspring.

8

Pressure method (hydrostatic system) The hydrostatic pressure due to a liquid is directly proportional to its

depth and hence to the level of its surface. In the case of open-topped vessels (or covered ones which are

vented to the atmosphere), the level can be measured by inserting an appropriate pressure transducer at the bottom of the vessel, as shown in Figure (a). The liquid level, h, is then related to the measured pressure, P, according to:

Where, is the liquid density and g is the acceleration due to gravity.

9

Cont.

When liquid-containing vessels are totally sealed, the liquid level can be calculated by measuring the differential pressure between the top and bottom of the tank.

The liquid level is related to the differential pressure measured, according to:

10

Cont.

This uses a dip pipe which reaches to the bottom of the tank and is purged free of liquid by a steady flow of gas through it.

The rate of flow is adjusted until gas bubbles are just seen to emerge from the end of the tube.

The pressure in the tube, measured by a pressure transducer, is then equal to the liquid pressure at the bottom of the tank.

It is important that the gas used is inert with respect to the liquid in the vessel.

11

Capacitive device

Suitable for use in extreme conditions measuring liquid metals (high temperatures), liquid gases (low temperatures), corrosive liquids (acids, etc.) and high-pressure processes.

Two versions are used according to whether the measured substance is conducting or not.

For nonconducting substances, two bare-metal capacitor plates in the form of concentric cylinders are immersed in the substance, as shown in Figure.

In the case of conducting substances, exactly the same measurement techniques are applied, but the capacitor plates are encapsulated in an insulating material.

12

Ultrasonic level gauge

The ultrasonic method uses a transmitter and receiver.

The transmitter emits pressure waves at approximately 20kHz.

The ultrasonic waves reflect off the material the level is being measured.

The level is determined from the time it takes for the emitted waves to be reflected and detected by the receiver.

The higher the level, the quicker the waves are reflected to the receiver.

The use of ultrasonic level measurement can be used for liquids (including corrosive) and solids

13

Radar method

Basically, all types operate on the principle of beaming microwaves downward from a sensor located on top of the vessel.

The sensor receives back a portion of the energy that is reflected off the surface of the measured medium.

Travel time for the signal (called the time of flight) is used to determine level.

For continuous level measurement, there are two main types of noninvasive systems, as well as one invasive type that uses a cable or rod as a wave guide and extends down into the tank’s contents to near its bottom.

14

Laser method

The laser operates on the same general principles as the radar method but uses laser-generated pulses of infrared light directed at the liquid surface.

It is immune to environmental conditions and can be used with sealed vessels with a glass window is provided in the top of the vessel.

15

Pressure

Question?What is basic equation of pressure calculation?Answer:

Pressure = Force/AreaQuestion?Name pressure unit.Answer:Bars, Pascal, Kg/m2, N/m2, Psi, torrs

Type of pressure measurement:1. bourdon tubes2. bellow elements3. diaphragm elements4. capsules

16

Bourdon tubes

One end tube is closed & another is connected to process pressure.

When pressure is applied, tube oval cross-section will become circular & results tube to straight hence moving the end tip.

This motion is detected by linkage and will move the pointer to indicate measurement.

17

Bellows elements

Accurate in the range between zero and 350 kPa gage. Suitable for low pressure application

18

Diaphragm elements.

A diaphragm is actually a flexible disc that can be flat or have concentric corrugations

19

Capsules

Two diaphragm are welded together around their circumference (capsule)

Measured pressure is applied inside capsule & forced developed is balanced by spring action.

20

Flow

Question?Give flow SI units.Answer:M3/s, liter/s Type of flow measurement:

1. Differential pressure2. Turbine (vortex)3. Electromagnetic4. Ultrasonic5. Coriolis

21

Differential pressure

Differential pressure meters involve the insertion of some device into a fluid-carrying pipe which causes an obstruction and creates a pressure difference on either side of the device.

Such devices include the orifice plate, the venturi tube, the flow nozzle and the Dall flow tube.

22

Turbine (vortex) flowmeter

Flow of fluid that passes the wheel cause it to rotate and where it proportional to the volume flowrate of the fluid

23

Electromagnetic flowmeter

Suitable for conductive fluids. Magnetic field in the tube is

generated by field coils in both side of the tube.

Voltage induced in the fluid is detected by 2 electrodes in opposite side of the tube.

Using Faraday’s Law, the velocity of the fluid can be determine,

E=BLvwhere E = induced voltage, B = flux density, L = distance of electrodes & v = flow velocity

24

Ultrasonic flowmeter

Not restricted to conductive fluid but also for corrosive fluid and slurries.

Due to clamp-on mode, this type of flowmeter is cheaper compare to the rest.

Divided into 2 types:1. Doppler shift2. Transit time

25

Doppler shift

The presence of scattering elements (solid particles, gas bubble) within flowing fluid will deflect the ultrasonic energy

26

Transit time

Suitable for clean liquids or gases.

Flowing fluid inside the tube causes a time difference between the transit time of the beams travelling upstream and downstream, and measurement of this difference allows the flow velocity to be calculated.

27

8. Final Control Element

For a typical process-control application, the conversion of a process-controller signal to a control function can be represented by the steps shown in Figure.

The input control signal may take many forms, including an electric current, a digital signal, or pneumatic pressure.

28

Signal conversion

The principal objective of signal conversion is to convert the low-energy control signal to a high-energy signal to drive the actuator.

Controller output signals are typically in one of three forms: electrical current, usually 4- to 20-mA; pneumatic pressure, usually 3 to 15 psi or 20 to 100

kPa; digital signals, usually TTL-level voltages in serial or

parallel format.

29

Actuators

The actuator is a translation of the (converted) control signal into action on the control element.

Thus, if a valve is to be operated, then the actuator is a device that converts the control signal into the physical action of opening or closing the valve.

Actuators take on many diverse forms to suit the particular requirements of process-control loops.

They can be electrical, pneumatic, or hydraulic types.

30

Control element

The final control element is designed as an integral part of the process.

If flow is to be controlled, then the control element, a valve, must be built directly into the flow system.

Similarly, if temperature is to be controlled, then some mechanism or control element that has a direct influence on temperature must be involved in the process.

This could be a heater/cooler combination that is electrically actuated by relays or a pneumatic valve to control influx of reactants.

31

Cont.

In figure, a control system is shown to control the degree of baking of crackers, as determined by the cracker color.

The optical measurement system produces a 4- to 20-mA conditioned signal that is an analog representation of cracker color (and, therefore, proper baking).

The controller compares the measurement to a setpoint and outputs a 4- to 20-mA signal that regulates the conveyer belt feed-motor speed to adjust baking time.

The final control operation is then represented by a signal conversion that transforms the 4- to 20-mA signal into a 50- to 100-V signal as required for motor speed control.

The motor itself is the actuator, and the conveyer belt assembly is the control element.

32

Mechanical - Solid-Material Hopper Valves The control system is to maintain the flow of grain from

the storage bin to provide a constant flow rate on the conveyor.

33

Mechanical – Paper thickness

The paper is in a wet fiber suspension and is passed between rollers.

By varying the roller separation, paper thickness is regulated.

The mechanical control element shown is the movable roller.

34

Electrical – Temperature control

Temperature often is controlled by using electrical heaters in some application of industrial control.

35