EXPERIMENT 2: AIR FLOW PROCESS CONTROL (AF 922) 1.0 SUMMARY
The measure of bulk fluid movement known as flow measurement. The equipment uses is the air flow process control training system, Model AF922. The main components consists of a main pipeline, PLI, vessel T90, the process vent manual valve VF, air process control training system (Air Pressure, AP 922), the second pipeline, PLII, orifice plate (FE91) and a differential pressure transmitter with a square squ are foot function (FT91) and a variable v ariable of flow meter (rotameter, FI91). Firstly, the PID controller was set in a manual (M) mode, and SV was set at its set point which is 25kg/hr. Other than that, the control valve FCV91 was fully open with MV was -6.3%. After that, three different trials were allowed to run with different values of PB of 200%, 150% and 100% respectively. The auto mode was then set and waited until the recorder response of the air flow of red pen was fairly steady and disturbance was introduced into the flow to observe the response. There are some errors occurred during conducting this experiment. From the result obtained, the set for trial I resulting the shortest peak of disturbance than set for trial III. The proportional band, PB for trial III is lower compared to trial I and II. Next, the test of PID controller tuning where Ziegler Nichols method by setting the controller. There are some errors occurred that affects the absence of peak. Thus, the PID controller cannot be calculated.
2.0 OBJECTIVE
To identify the important components of the air p ressure control system and to mark them in the P&I Diagram
To carry out the start-up procedures systematically
To study gas volumetric flow rate measurement using orifice plate
To study gas mass flow rate measurement using orifice plate an d perfect gas law to compute the gas density from the pressure and tem perature measurements
To study gas mass flow rate (Fm) control using PID controller
To design a PID controller using Ziegler Nichols closed loop method
3.0 INTRODUCTION
Flow measurement is the measure of bulk fluid movement and is determined through positive displacement meters which collect a fixed volume of fluid, release and refill the fluid, then tally the times the volume is filled to quantify flow. Flow measurement devices which rely on the strength produced by the flowing stream, as it prevails over a known constriction, and indirectly calculates flow. Flow may also be determined by measuring the velocity of fluid over a known area. Both gas and liquid flow can be expressed in volumetric or mass flow rates, and the
quantities can be converted between one another if the substance’s density is known (Steinberge, 2013). Gases and vapours are easily changing their volume under the influences of pressure and temperature. In other words, a gas will yield to an increasing pressure by decreasing in volume as the gas molecules are forced closer together and it will yield to a decreasing temperature by decreasing in volume as the kinetic energy of the individual molecules is reduced. This makes the volumetric measurement more tricky and complex for gases and vapours than for liquids. One cubic meter of gas at high pressure and temperature inside a process vessel will not occupy one cubic meter under difference pressure and temperature condition in the same vessels. This implies that volumetric flow measurement for gas is virtually meaningless without accompanying data on pressure and temperature
(“Volume Flow Rate in Liquid and Gas
Measurement ~ Learning Instrumentation And Contr ol Engineering,” n.d.). Gas/vapour volumetric flow can be measured either by the differential pressure across orifice or by a variable area flow rate. The orifice plate is a device used for measuring flow rate in this experiment. Either a volumetric or mass flow rate may be determined, depending on the calculation associated with the orifice plate. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which states that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa. With the orifice plate mounted in the flow stream, the increase in fluid flow velocities through the reduced area of the orifice produces different pressure across the orifice (“Basics
The Orifice Plate Flow Meter ~ Learning Instrumentation And Control Engineering,” n.d.).
of
Compensation for pressure and temperature variation using Perfect Gas Law can be made to the flow measurement by the differential pressure measurement. Perfect gas also called ideal gas which is defined as a gas that conforms in physical behaviour to a particular idealized relation between pressure, volume and temperature. Gas mass flowrate can be controlled by using PID controller. PID stands for Proportional-Integral-Derivative. These three controllers are combined in such a way that it produces a control signal. PID controller maintains the output such that there is zero error between process variable and set point/ desired output by closed loop operations .The Ziegler-Nichols tuning rules were designed for a ¼ amplitude decay response. This results in a loop that overshoots its set point after a disturbance or set point change. The response in general is somewhat oscillatory, the loop is only marginally robust and it can withstand only small process conditions changes. Zeigler-Nichols proposed closed loop methods for tuning the PID controller. Those are continuous cycling method and damped oscillation method. Procedures for both methods are same but oscillation behaviour is different. In this, first we have to set the p-controller constant, Kp to a particular value while Ki and Kd values are zero. Proportional gain is increased till system oscillates at constant amplitude (Agarwal, 2015).
Figure 1: The Ziegler-Nichols Closed-Loop Tuning Method
4.0 METHODOLOGY
This experiment had been divided into several categories. Firstly was the Start Up Procedure.
Firstly, switch the "PANEL, SCADA/DDC" selector switch at the front of the
cubicle to "PANEL,SCADA"position and switch ON the main power supply until all the panel instruments lit up. Fully close the manual by-pass valve around the control valve FCV91. Open the vent VF and close the valve MV901. Place the panel controller FIC91 in the Manual (M) mode. Open fully the control valve FCV91 by setting the MV of FIC91 to -6.3%. Adjust the manual valve MV900A till the air flow rate is about 50 kg/hr. Set MV = 106.3% at FIC91 to fully shut the control valve FCV91. Set the MV = 50% at FIC91 and check control valve FCV91 (in the plant) is 50% opened. Observed the data required based on the table of result given in the manual. Another sets of readings for MV = 50% and MV = 70% was repeated and record the result. Next, the control of air flow system experiment. Set the FIC91 in Manual (M) mode, open the control valve FCV91 fully with MV = -6.3%. FIC91 still in Manual mode, adjust its setpoint, SV = 25 kg/hr. Access the PID parameters of FIC91 and set the first (I) PID trial values: PB =
200%, TI = 6 s, TD = 0 s. Start the recorder by pressing “RCD” pushbutton at FPTR91. Switch FIC91 to Auto (A) mode and watch the recorder response (i.e. Air flow response) until the air flow (red pen) is fairly steady. Repeat step (i.e. Introducing pulse disturbaance) with the second (II) and third (III) sets of PID trial values; PB = 150%, TI = 6 s, TD = 0 s and PB = 100%, TI = 6 s, TD = 0 s. Repeat the experiment for the above 3 sets of PID trial values with step change in setpoint for the mass flow rate by changing the present setpoint of FIC91 from SV= 25 kg/hr to SV = 30 kg/hr. Observe and record the response till it becomes fairly steady. Switch back the set point SV = 25 kg/hr. Use the first (I) PID trial value but now with TD = 20 s. Mark the PID values on the chart paper simultaneously. The disturbance was applied by opening and shutting fully the by-pass valve around control valve FCV91 and observes the response. Lastly is the shutdown procedures. The RCD button was turn off. Switch FIC91 to Manual mode with MV = 0% and switch off the main power supply. Shut off the process air supply at AR900 and shut off the instrument air supply
4.0 RESULT
Table 1 : Quarter Amplitude Damping Response ( Blue Pen ) Instrument
I MV = -
II MV =
III MV
6.3%
50%
=70%
Readings
Controller
At the PANEL
FIC91, I/O Data
FT91, √h
X1 - % of 0-100 mm H20
60.2
44.0
15.8
TIT911, T
X2 - % of 0-120°C
27.6
27.7
27.7
PT911, P
X3 - % of 0-60 psia
69.8
89.9
78.0
53.1
32.3
10.6
25.1
11.2
4.0
FIC91, Main Face Plate, Fm
PV, kg/hr
FIC90, PT Register Fv
PO1, M3/Hr
Recorder FPTR91 Channel 1 (Red)
Fm, kg/Hr
32.0
22.2
10.6
Channel 2 (Green)
Fv m3,Hr
15.4
11.3
4.0
By Calculation
Fvb, Nm3/Hr
39.3
32.61
10.9
Fm, kg/Hr
50.84
42.16
14.10
FI911, Nm3/Hr
48
23
10
PG900, psig
32
42
34
At the Plant
Table 2 : Air Flow measurement Data Instrument Readings
PANEL
CALCULATION
Mass flow rate, kg/hr
MVI (-6.3%) = 32.0
MVI (-6.3%) = 50.84
MVII (50%) = 22.2
MVII (50%) = 42.16
MVIII (70%) = 10.6
MVIII (70%) = 14.1
MVI (-6.3%) = 48
MVI (-6.3%) = 39.3
MVII (50%) = 23
MVII (50%) = 32.61
MVIII (70%) = 10
MVIII (70%) = 10.9
Volumetric flow rate, Nm3/hr
Table 3 : Set of trial value Trial I
Trial II
Trial III
PB1 (%)
200
150
100
TI1 (sec)
6
6
6
TD1 (sec)
0
0
0
5.0 DISCUSSION
This air flow process control experiment was done to identify the important components of the air pressure control system, to study gas volumetric flow rate measurement by using orifice plate, to determine the gas mass flow rate measurement using orifice plate and perfect gas law to complete the gas density from the pressure and gas measurements, to study the gas mass flow rate (Fm) control using PID controller and also to design a PID controller by using Ziegler Nichols closed loop method. In this experiment, in order to detect the differential pressure throughout the flow the orifice plate was used and the differential pressure measurement was calculated. The volumetric flow rate and mass flow rate was recorded in the chart by the air flow process control training system, where the red color in the chart showed the mass flow rate and the green color showed the volumetric flow rate throughout the experiment. This experiment was conducted by control the air flow system by using the different set of PID parameters. In manual (M) mode, the SV value was set at its set point which is 25kg/hr and the control valve FCV91 was fully open with MV was -6.3%. This experiment was run in different trial values. For the first set, the PB was set at 200%, T1 at 6s and TD is 0s. The auto mode was set and waited until the recorder response of the air flow (red pen) was fairly steady. Then, a pulse disturbance was introduced by quickly opening and shutting fully the by-pass valve around control valve FCV91. The same steps were used by using set II and set III with the same values of TI and TD but different values of PB which were 150% and 100% respectively. Based on the result in table 1 and 2, the value of mass flow rate and volumetric flow rate were taken from the panel and also were calculated at different values of MV. From the results obtained, the reading of the mass flow rate from the panel showed that, the increased the values of MV, the increased the mass flow rate. However, from the calculation result, the values of mass flow rate were decreased when then MV values of MV increased. On the other hand, the reading of volumetric flow rate on the panel decreased when the values of MV increased. The values of volumetric flow rate also decreased in calculation when the values of MV increased. The proportional control mode is the main driving force in a controller. It changes the controller output in proportion to the error. When the error is bigger, the control action will become bigger since more control action needs to correct the large errors. If the proportional band, PB is set too high, the control loop will begin oscillating and become unstable. If the PB is
set too low, it will not respond adequately to disturbance changes (F. Smuts, 2011). Based on the result recorded in figure 2, the set for trial I showed the shortest peak of disturbance while the set for trial III showed the longest peak for the disturbance. The proportional band, PB for trial III is lower compared to trial I and II. Supposedly, the disturbance peak for trial I is longer than the others two trial because it PB is the highest which is 200%. There are some errors occurred during conducting this experiment.
Figure 2: Result for trials I, II and III at SV = 25 Lastly, the last testing of this experiment is PID controller tuning where Ziegler Nichols method by setting the controller as below: PB = 100%
TI = 9999s
TD = 0s
The set point pulse disturbance was given to the process by increase the SV from 25 kg/hr to 28 kg/ hr in a few seconds and change back to its original SV. To make sure the tuning process done by the system itself, this was done in Auto (A) mode. The PB was decrease from 100% to 70% and last to 30% in order to get the uniform oscillation. The response form is shown in Figure 3. The result showed no peak due to some errors. The new value of PID controller cannot be calculated.
Figure 3: The increasing of SV from 25kg/hr to 28kg/hr with PB decrease from 100% to 30%. Ziegler-Nichols method has two methods in order to tuning the process control that was developed in 1940s. The first method is being used in this experiment. It is a direct measurement on the controller parameters or closed loop tuning method. The integral and derivative gain, TD is set to zero. The proportional gain is decreased until the system start to form the uniform oscillatory response. Based on oscillatory response, the critical value of Kc = Kcu and the period of oscillation, Tc = Tn are formed. Figure below shows the example of closed loop tuning method response:
Figure 4: Closed loop tuning method response. Next, the second method of Ziegler Nichols method is based on determination of the open loop step response of the process. By applying the step input to the process and recording
the process, the step response is measured. The response is scaled to correspond to a unit step input and characterized by parameters a and T del . T del is the time delay of the system and a/ T del is the steepest slope of the step response. Figure 5 showed the example of open loop tuning method response:
Figure 5: Open loop tuning method response. There were a few errors that occurred during conducting the experiment. The flow rate of the experiment was set at the controller FIC91. Supposedly, it should be read at the flow rate gauge F1911. Then, the by-pass valve on the pipeline PLI and PLII was not fully open while conducting the experiment.
6.0 CONCLUSION & RECOMMENDATION
As a conclusion, this experiment was being conducted by controlling the air flow system by using the different set of PID controller to measure the volumetric flowrate as well as the mass flowrate measurement which the result can be seen through the chart printed from the air flow process control training system. Initially, the PID controller was set in a manual(M) mode, and SV was set at its set point which is 25kg/hr. Other than that, the control valve FCV91 was fully open with MV was -6.3%. After that, three different trials were allowed to run with different values of PB of 200%, 150% and 100% respectively. The auto mode was then set and waited until the recorder response of the air flow of red pen was fairly steady and disturbance was introduced into the flow to observe the response. Based on the result, trial I has the shortest peak of disturbance and trial III has the longest peak for the disturbance and the PB showed for trial III has the lowest peak compared trial I and II due to some errors. Lastly, the Ziegler Nichols method was run to obtained new values of the PID system which was the close loop system and also open loop system. However, due to some errors, there was no peak appeared on the chart. Hence, the PID controller cannot be calculated. The recommendation to be proposed to overcome the errors occurred during the experiment is that every valve and control system should be set accordingly and should be double inspection so that the control valve FCV91 would be fully open and set the F1911 to -6.3% . Besides that, the by-pass valve on the pipeline PLI and PLII should be fully open so that the flow of gas will not be interrupted.
7.0 APPENDICES
The mass flow rate measurement is given by: Assuming perfect gas law, Fm = k m Where,
ℎ⁄
Fv
= Volumetric flow rate, m3/hr
Fvb
= Volumetric flow rate, Nm3/hr
Fm
= Mass flow rate, kg/hr
h
= Differential pressure, mm H2O
P
= Absolute pressure, psia
T
= Temperature, K
k 1
= 0.256
k vb
= 1.356
k m
= 1.753
a) Obtain the actual value of h from FT91 : For (I) :
√ ℎ = (60.2) h = (60.2)2 h = 3624.04 mm H20 For (II) :
√ ℎ = (44.0) h = (44.0)2 h = 1936 mm H20
For (III) :
√ ℎ = (15.8) h = (15.8)2 h = 249.64 mm H20
With P/T compensation:
Fvb = K vb
ℎ⁄ ,
Given h =3624.04 mm H20, P = 69.8 psia , T= 300.75 oK
= 1.356
3624.04(69.8)⁄300.75
= 39.3 Nm3/hr
Perfect gas law,
ℎ⁄ 3624.04(69.8)⁄300.75 = 1.753 Fm = k m
= 50.84 kg/hr
8.0 REFERENCES
Agarwal, T. (2015). How Does a PID Controller Work? - Structure & Tuning Methods. Retrieved August 27, 2017, from https://www.elprocus.com/the-working-of-a-pid-controller/
Basics of The Orifice Plate Flow Meter ~ Learning Instrumentation And Control Engineering. (n.d.). Retrieved August 27, 2017, from http://www.instrumentationtoolbox.com/2013/03/basicsof-orifice-flow-meter.html#axzz4qvQY5yFf
F. Smuts, J. (2011). PID Controllers Explained | Control Notes. Blog.opticontrols.com. Retrieved 25 August 2017, from http://blog.opticontrols.com/archives/344
Steinberge, B. (2013). Gas mass flow rate units of measure. Retrieved August 27, 2017, from https://sagemetering.com/back-to-basics/gas-mass-flow-rate-units-of-measure/
Volume Flow Rate in Liquid and Gas Measurement ~ Learning Instrumentation And Control Engineering. (n.d.). Retrieved August 27, 2017, from http://www.instrumentationtoolbox.com/2013/05/volume-flow-rate-in-liquid-andgas.html#axzz4qvQY5yFf
