EXPERIMENT NO: 4

Name Of The Experiment:

Study of Air pressure control with/without air receiver.

Objective:

To demonstrate the effect of the AIR RECEIVER on the response of the process and to

determine the optimum settings for the process controller.

Apparatus Required:

Pressure Process Rig 38-714

Digital Display Module 38-490

Pressure Transmitter 38-461

Process Interface 38-200

Process Controller 38-300

Patch cords

Computer

Theory:

Pressure Process Control System is a single loop pneumatic control system which allows study

of the principles of process control pressure as the process variable to be controlled. The Pressure

Process Rig consists of a low pressure air circuit supported on a bench-mounted panel, making it

suitable for individual student work or for group demonstration.

The addition of the AIR RECEIVER makes the response of the system correspondingly slower.

(Pressure cannot change as quickly in the process pipe when a disturbance occurs or a new set

point is instructed)

Any change in the response of process will necessitates a change in the settings of the

three terms on the controller to achieve optimum control.

Where the response of a process will change in operation, the controller must be

configured to give a stable control under all operating conditions. This may necessitate a

reduction in performance at some conditions.

The circuit includes:

Input supply filter/drier.

Input converter.

Pneumatically operated control valve.

2 Regulators.

4 Manual valves.

6 Gauges.

Sight flow meter.

Orifice Block with changeable Orifice Plates.

Differential Pressure Sensor.

Process Pressure Sensor.

27 liter Air Receiver Tank

20 psi Safety Relief Valve.

Diffusers.

The system includes those pneumatic control components of interest to the process industries.

The design allows the study of component operation and connection to the electrical control

devices through the use of pressure/ current transducers.

The unit consists of a pipeline on which are mounted a Pneumatic Control Valve,

Orifice Block and pressure tapings.

The flow discharges directly to the atmosphere or via an Air Receiver to vary the

process lag. The valve is operated from a Current to Pressure converter, and sensors for direct

and differential pressure facilities measurement of pressure and flow respectively.

Process Rig Controller:

The unit is designed to operate with the 38-200 and 38-300 Process Interface and

Process Controller to configure open or closed loop control circuits. The pneumatic

instrumentation comprises an I/P Converter and Pneumatic Control Valve. The I/P Converter

accepts a 4-20 mA control signal from the 38-200 Interface and converts this to a 3-15 psi

pneumatic signal which operates the control valve.

Rig Control Valves:

The control valve comprises the diaphragm actuator which positions the stem of a plug type

valve. An indicator on the valve shows the actual position on the valve.

A set of manual valves V1,V2 and V3 allow a rear mount air receiver to be connected in

series or in parallel with the process pipe to change the response of the system (to vary the

process lag). The air receiver incorporates a pressure relief valve.

Step changes may be applied to the process by bleeding air through an additional

diffuser by opening and closing the valve V4.

The Discovery software contains an integral Chart Recorder and configuration program.

Procedure:

To perform the practical, setup the equipment using the following steps:

Connect the equipment as shown in figure in patching diagram with the air receiver in

series with the process pipe.

V2, V4 & V6 to close.

Set R1, R2, V1, V3& VS to open.

Adjust R1 to give 25 psi on GI.

Adjust R2 to give 10 psi on G5 with the pneumatic control valve open.

The chart recorder is connected to the pressure transmitter output/ controller input and the

controller output/ current to pressure converter input 4-20 mA loops to provide a record of the

response. The set point of the system has been set to 50%.

Patching Diagram:

Fig: PID control of Air pressure with and without Air receiver

Observation:

Observe the response of the system when valve V4 is open and closed to give a disturbance.

Isolate the AIR RECEIVER from the process pipe by opening V2 and closing V1& V3. Apply a

disturbance to the new configuration & observe the changes in response.

Observation Table:

Condition Set

Point

(%)

Proportional

Band (%)

Integral

Action(TI)

Derivative

Action(TD)

Measured

Value

(%)

Flow

Meter

(Lit/min)

Pneumatic

Control

Valve (%)

Without

Air-

receiver

With Air-

receiver

Precautions:

All the connections should be done properly according to the patching diagram.

Before starting the experiment the valves should be adjusted properly.

Readings should be taken very carefully.

Conclusion:

After observation it has been found that in this practical the PID values have been set such that

the best performance is achieved out of the system.

EXPERIMENT NO: 5

Name of the experiment:

Study of Temperature control of liquid using temperature-sensor

Objective:

To control the temperature of water with the help of P, PI and PID controller.

Apparatus required:

Temperature Process Rig. 38-600

Process controller 38-300

Process interface 38-200

Digital display module 38-490

Thermistor temperature transmitter 38-441

Computer

Patch cords

THEORY:

The basic of this experiment is to control the temperature of the process with the use of heat

exchanger. This process contains temperature process rig. The temperature rig has two isolated

water circuits. The primary circuit which is used normally as heat source comprises:

• A heater

• A circular pump

• A servo valve for flow control

• A pulse flow meter

• A header tank

• A heat exchanger

The secondary circuit contains a heat exchanger and a cooler.

The primary circuit is self contained and has to be filled before the system is used. The

secondary circuit is normally supplied via flexible hoses, from the Basic Control Rig which is set

up to provide a controlled flow. An alternative arrangement is to use a Temperature Auxiliary

Control Pack 38-480 to provide a control flow from a mains water tap. Alternatively if we have a

38-610 forced Air Cooling Unit, the water from the Temperature Rig can be cooled and re-

circulated.

Role of Thermistor:

• The Thermistor Temperature Transmitter is a device which takes temperature

informationfrom the thermistors (T1 - T5) and transmits it to the Process Interface (PI).

• A thermistor is a device, the electrical characteristics of which alter in a predictable way

with a change of temperature.

• The resistance of a thermistor is a function of the temperature around it, or 'ambient'

temperature.

• The Thermistor Temperature Transmitter reads theresistance value and converts it to a 4-

20 mA signal with respect to actual temperature.

• By converting to the 4-20 mA current signal format, communication is no longer

restrictedto short distances, a concern when dealing with large process plants. Also by

using this format signals and equipment become standardized, removing the need for

special interfaces.

• When using the thermistor and transmitter combination, temperature measurements are

carried out to monitor a process parameter. This parameter is monitored and used to

determine the control effort that should be applied to control the process correctly.

• In this experiment Thermistor Temperature Transmitter (TTT) is used. TTT is calibrated

against the (38-490) Digital Display Module (DDM). Once calibrated, the TTT can be

used to accurately monitor the temperaturemeasured by two different thermistors.

There are five such devices included with the Temperature Process Rig. They are

positioned to measure the temperature at five points around the secondary and primary flows.

• In the primary flowthey are positioned before (T1) and after (T2) the heat exchanger. This

is obviously crucial in observing the cooling effect of the heat transfer.

• In the secondary flowthey are also positioned before (T3) and after (T4) the heat

exchanger. The fifth device is placed at the output (T5) of the radiator in order to show

the temperature of the flow before and after cooling has taken place.

Heat Exchanger:

A major element in the topic of process control is the heat exchanger. These devices can be

found in so many configurations that a person who has been simply introduced to the science of

heat exchangers can be quite perplexed in trying to determine which of the almost limitless types

available, many apparently satisfying the required heat transfer duty, should be used. For

example, designs which incorporate tubes are only a subset of the many heat exchangers

available. However, often the most critical step in the analysis of a heat exchanger is the

determination of the overall heat transfer coefficient, U. This, in turn, involves the application of

convection and (or) phase change correlation's to find the surface coefficients, h and uses these

with the areas, A1 and A2 and wall resistance Rw, to find the result of the following equation :

The determination of pressure drop should be evaluated, as this also is an important

design aspect. Some heat exchangers that perform extremely well thermally may however

require a very high pumping power. Therefore it is a compromise between these constants when

satisfying the specification.

The Secondary Flow:

• Domestic heating systems often consist of a series of radiators designed to extract energy

from hot water being pumped through them. The situation sometimes occurs whereby

one or more of the radiators is partly filled with air instead of water.

• The air does not transfer heat to the metal of the radiator as effectively as the water. This

can be demonstrated by the time taken for the element of an electric kettle to become too

hot in the absence of water. The air around the element does not remove the energy from

it fast enough to prevent overheating.

• The cooling radiator supplied as part of the Temperature Process Rig can sometimes fall

victim to the same problem. Air can be introduced into the system in a number of ways

through pumps and joints. This air can find itself trapped in the upper part of the cooling

radiator, where it will remain until bleeding can be carried out.

• Bleeding involves the removal of air from a fluid system by whatever method. The type

of domestic system mentioned earlier is usually bled from a small 'tap' on the offending

radiator.

• Air is pushed out under the system pressure until water begins to be expelled. The tap is

closed and the radiator is free of air.

Operation of the Cooler:

• The main reason for the cooler on the Temperature Process Rig (TPR) is to drop the

temperature of the heated return fluid (secondary flow).

• The overall effect of this process is to prevent the secondary flow circuit (water in the

tank of the BPR) from heating up too quickly.

• This is achieved using a cooler, which consists of a radiator and a fan unit, commonly

known as an Air blow Water Cooler. The radiator itself comprises an aluminum structure

of heat dissipating fins, whereby the fluid to be cooled passes behind.

• In order to increase the cooling efficiency, a fan is attached to the rear of the radiator to

draw air through the radiator dissipating the heat from the fins.

• It must be noted that coolers of this type can only reduce the temperature to a minimum

degree equal to the ambient air temperature. However with respect to the TPR due to the

size of the cooler this would actually take a considerable amount of time depending upon

the temperature of the BPR fluid.

• It is therefore shown that the cooler is only intended to provide a degree of cooling to the

BPR (if connected).

• However in industrial applications, a cooler may be the primary source (only source) for

cooling a process, in which case its specification would be critical to the dissipation

required. Coolers of this type tend to be relatively large with respect to their function.

• In this particular case the cooler is switched on to demonstrate its efficiency in cooling

the secondary flow before returning to the sump tank of the BPR.

In the overall controlling of the process, the roles of the controllers are very important. The

functions of the controllers and their role in the process are described briefly described here.

P,I,D control action:

For satisfactory control of processes having large distance velocity and transfer lags,and where

load changes may be sudden and/or sustained,the controller must incorporate P,I and D

action.Such a controller is known as a three term controller.

The derivative component of the control effort enable, a controller to recognize a rapidly

changing error and take extra action to account for it. By applying a control effort that it is not

simply directly proportional to the error,the response of the plant has been improved.This ability

to recognize a rapid change in the rate of change of error in a system is very important in many

situations.

A massive increase in the core temp of a nuclear reactor caused by a failure elsewhere in a plant

could result in meltdown.By applying a very important large control effort,the time taken to

reverse the direction of the system towards failure can be reduced.It is producing an

overcompensation for the rapidly changing error to halt its progress.

But it is not only overcompensation that a derivative action offers to a system.As the measured

value of a system approaches its set point,the rate of change of error will decrease as the

proportional action reduces.This reducing error rate will produce a negative control contribution

from the derivative term,reducing the control effort further.This applies to a breaking effect,the

chance of overshoot.

The derivative action will pull a system away from failure by producing an overly large control

effort,and slow down its approach to the set point with the aim of preventing overshoot.

However for a process with flow load changes such as the temp figure, the addition of derivative

action to proportional+integral,does not produce any real advantage.

We can vary proportional band(changing the gain which changes the contribution of the

proportional action term);Integral action time(or reset time,determining the contribution of the

reset term)and derivative action term(determining the contribution of the derivative action term) .

Patching diagram:

Fig.: Basic Process Rig Patching

Fig: Temperature Process Rig Patching

Procedure:

Assemble hardware as shown in the patching diagram.

Switch on the TPR PI and allow approx 15 minutes for the water in the tank to heat up.

Ensure that reverse control action has been set.

Once the water has had sufficient time to heat,switch on the TRR pump and cooling fan.

Switch the TTT to display temp at T2.Set the TPR manual valve to be 25% open.

The practical begins in P+I control mode,as met in the last practical and the derivative action is

introduced with the onscreen button.When the derivative component is on,time is variable with

the control bar.In this way we can compare PI and PID control with the click of a virtual switch.

We will probably have to set a low PB and a low integral timeto notice any effect of D.

Derivative action improves the response of the controller to a rapidly changing error,and provides

a breaking effect when the measured value approaches the set point.

Vary parameters available onscreen and observe their effects.

Try auto tuning of this process.

Observation:

Sl. No Sensor Set point

(°C)

Proportional

band (%)

Integral

Action(TI)

Derivative

action(TD)

Opening

of servo

valve (%)

Measured

value

(°C)

1.

2.

3.

Precautions:

All the connections should be done properly according to the patching diagram.

Before starting the experiment the valves should be adjusted properly.

Readings should be taken very carefully.

Conclusion:

After observation it has been found that in this practical the PID values have been set such that

the best performance is achieved out of the system.

EXPERIMENT NO: 8

Name of the Experiment:

Study of Air Temperature Control in a Process Tube with Different Control

Actions.

Objective:

To control the Air Temperature in a Process Tube by using different control actions.

Apparatus used:

Process control Trainer Kit 37-100

PID Unit (PID 150Y)

Function Generator cum power supply SM 5076-1

Theory:

Process control Trainer Kit 37-100 is a heating element controlled by a thyristor circuit feeds

heat into the air-stream circulated by an axial fan along a polypropylene tube. A thermistor

detector, which may be placed at one of three points along the tube length, senses the

temperature at that point.

The volume of air flow is controlled by varying the speed of the fan via potentiometer, a change

in setting represents a supply side disturbance and the effects are easily demonstrated.

To control the process temperature without extensive operator involvement, a temperature

control system relies upon a controller, which accepts a temperature sensor such as a

thermocouple, RTD or thermistor as input. It compares the actual temperature to the desired

control temperature, or set point, and provides an output to a control element. The controller is

one part of the entire control system, and the whole system should be analysed in selecting the

proper controller.

There are three basic types of controllers: On-Off, Proportional and PID. Depending upon the

system to be controlled, the operator will be able to use one type or another to control the

process.

On-Off Control:

An on-off controller is the simplest form of temperature control device. The output from the

device is either on or off, with no middle state. An on-off controller will switch the output only

when the temperature crosses the set point. For heating control, the output is on when the

temperature is below the set point, and off above set point. Since the temperature crosses the set

point to change the output state, the process temperature will be cycling continually, going from

below set point to above, and back below. In cases where this cycling occurs rapidly, and to

prevent damage to contactors and valves, an on-off differential, or “hysteresis,” is added to the

controller operations. This differential requires that the temperature exceed set point by a certain

amount before the output will turn off or on again. On-off differential prevents the output from

“chattering” or making fast, continual switches if the cycling above and below the set point

occurs very rapidly. On-off control is usually used where a precise control is not necessary, in

systems which cannot handle having the energy turned on and off frequently, where the mass of

the system is so great that temperatures change extremely slowly, or for a temperature alarm.

One special type of on-off control used for alarm is a limit controller. This controller uses a

latching relay, which must be manually reset, and is used to shut down a process when a certain

temperature is reached.

PID Control:

The third controller type provides proportional with integral and derivative control, or PID. This

controller combines proportional control with two additional adjustments, which helps the unit

automatically compensate for changes in the system. These adjustments, integral and derivative,

are expressed in time-based units; they are also referred to by their reciprocals, RESET and

RATE, respectively. The proportional, integral and derivative terms must be individually

adjusted or “turned” to a particular system using trial and error. It provides the most accurate and

stable control of the three controller types, and is best used in systems which have a relatively

small mass, those which react quickly to changes often and the controller is expected to

compensate automatically due to frequent changes in set point.

Patching Diagram:

Fig: Patching Diagram of PID control of Air temperature in a process tube

Circuit Diagram:

Procedure:

For two-step control:

Power supply is switched on.

Bridge is balanced by rotating the screw (that is zero set point gives zero measured

value).

Set point is changed to adesired value.

Point „X‟ and „Y‟ is shorted on the kit.

All the toggle switch is adjusted for two-step control.

By adjusting the maximum heating power, adjustment of heater output is done.

Throttle control is selected for fan speed control.

Overlap is selected for adjusting the differential gap.

For continuous control:

All the toggle switch for continuous control is adjusted.

Set proportional band, integral time and derivative time.

The internal disturbance is introduced and accordingly proportional band, integral time

and derivative time is adjusted by trial and error method.

Now an external square wave (frequency=0.1-0.5 Hertz) is introduced from a function

generator.

Now the value of proportional band, integral time and derivative time is adjusted

accordingly to get maximum accuracy.

The external set value (frequency of 0.308 Hz and Vrms=3.216 volts) is applied from

function generator.

Observation Table:

For two-step control:

SET POINT OVERLAP MAXIMUM

HEATER

TRANSFER

THROTTLE

CONTROL

MEASURED

VALUE

For continuous control:

SET POINT

RANGE

PROPORTIONAL

GAIN(Kp)

INTEGRAL

TIME(Ti)

DERIVATIVE

TIME(Td)

MEASURED

VALUE RANGE

Precaution:

(1) All the connections should be done properly accordingly.

(2) The function generator should be handled properly.

(3) We should take the readings very carefully.

Conclusion:

By this experiment we measure the temperature of air in a process tube by thermistor type

temperature sensor and control the temperature using with (for continuous mode) and without

(for two-step control) the PID controller. By changing the values of proportional band, integral

action and derivative action; the best performance is achieved with respect to set point changes.

We can also compare the system performance achieved by P, PI and PID control action.