proceedings - rochester institute of technologyedge.rit.edu/edge/p13631/public/reviews and...

Multidisciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P13631

GRAVITY FED FLOW PROCESS CONTROL TEACHING DEVICE

James BrinkerhoffChemical Engineer

Christopher KulbagoChemical Engineer

Patrick O’ConnellMechanical Engineer

Theodore RakiewiczElectrical Engineer

Lauren PahlsChemical Engineer

Sarah SalmonChemical Engineer

ABSTRACT A gravity fed flow cart was designed and constructed for the purpose of demonstrating process control

concepts to undergraduate chemical engineering students in a laboratory environment. Inspired by preexisting cart-mounted flow systems being utilized to teach students about fluid dynamics, the gravity fed flow cart was designed to supplement the System Dynamics and Controls course by focusing on the basics of proportional, integral, and possibly derivative control. Requirements for the apparatus include capability for interactive use by a team of three students, as well as ease of storage, transportation, and maintenance. Several process control models were programmed into LabVIEW allowing the students to display and record data, as well as manipulate and practice tuning process control loops. Safety and functionality testing have verified that the finished cart meets all engineering specifications and fully satisfies the customer needs.

NOMENCLATURE Table 1: Definitions of key terminology

Term Definition

LabVIEWLaboratory Virtual Instrument Engineering Workbench – “A graphical programming platform that helps engineers scale from design to test and from small systems to large systems.” [1]

PID Control Proportional, Integral and Derivative Control

Gravity Design Preliminary design concept incorporating an elevated water tank draining to the cart-mounted flow transmitter and single control valve

Pressure DesignPreliminary design concept consisting of an enclosed water tank with head pressure maintained by a pressure regulator flowing to the cart-mounted control valve and flow transmitter

Line Fed Design Final design concept featuring flow supplied by the laboratory faucet feeding to a the cart-mounted flow transmitter and two control valves

Copyright © 2012 Rochester Institute of Technology

Proceedings of the Multidisciplinary Senior Design Conference

Table 2: Meaning of various symbols

Symbol Meaning Symbol MeaningGc(s) Controller transfer function Gd(s) Derivative transfer functionGa(s) Actuator transfer function Gp(s) Process transfer functionGs(s) Sensor/transmitter transfer function D(s) Disturbance outputY(s) Process output Ysp(s) Process setpointKc Controller gain Ka Actuator gainKp Process gain τc Controller time constantτv Valve time constant τi Integration time constantτp Process time constant C.M. Center of massM Mass of a component x Location on the x-axis of a componenty Location on the y-axis of a component z Location on the z-axis of a componentX Location on the x-axis of the C.M. Y Location on the y-axis of the C.M.Z Location on the z-axis of the C.M. P Applied Load

ymax Maximum linear deflection on y-axis V Shear forceM Bending Moment σ Normal Stressc Maximum distance from neutral axis I Second moment of areaτ Shear stress Q First moment of areat Thickness perpendicular to shear θ Angular deflectionE Modulus of Elasticity C1 Integration constant oneC2 Integration constant two

BACKGROUNDAn education from the Rochester Institute of Technology gives students both theoretical training via lectures

and practical experience through laboratory experiments. The process control experiment, conducted during the Chemical Engineering Unit Operations Lab, has the goal of coupling complex process control equations to actual situations in order for students to gain a better appreciation and understanding of the concepts. Previously, the process controls experiment consisted of students warming a beaker of water using an electric heating jacket with thermocouples to monitor the temperature and a LabVIEW control program. However, this procedure was only capable to demonstrating very basic controls principles and bore little resemblance to real-world systems. In the past, the Chemical Engineering Department has used transportable carts to convey concepts such as heat transfer and fluid mechanics. Using the existing carts as inspiration, three senior design teams (P13630, P13631, and P13632) were commissioned to design and build similar carts that demonstrated different process control concepts.

Our customer’s primary needs were students’ safety and learning during cart operation. System Dynamics and Controls professor, Dr. J. Michael Sanchez, requested that students be able to manipulate flow both manually and through LabVIEW in order to gain a better appreciation of control systems. Mr. Paul Gregorious, facilities manager for the Chemical Engineering Department, will lead the maintenance of the cart and cited needs including ease of transport, storage, assembly/disassembly, part replacement, and cleaning. During the lab course, Dr. Christiaan Richter will be guiding the students and requested that the cart allow for collaborative use by three students at a time. In addition of these customer needs, the system was also designed around equipment that was donated by the Eastman Kodak Company in order to minimize the overall cost of the project.

Overall, the primary goal of the cart is to teach undergraduate Chemical Engineering students about process control through a hands-on demonstration. Process control is an engineering discipline that utilizes mathematical algorithms programed into computer systems in order to change a process variable to desired value with the assistance of physical devices such as sensors, transmitters, and actuators. Chemical production industries rely heavily on process control systems in order to maintain product quality, increase throughput, and reduce energy usage. Therefore, process control is a key-learning objective for Chemical Engineering students.

The completed gravity fed flow cart can demonstrate process control to students in both manual and automatic forms. Manual control, which does not employ any software, will be exhibited when students physically open or close a valve in order to cause a change to occur in the system. Through their attempts at manually controlling the

Project P13631

Proceedings of the Multi-Disciplinary Senior Design Conference

system, students will learn the inherent difficulties of manual control and gain a greater appreciation of why automatic control is so crucial to successful operation. Transitioning to automatic control, students will utilize a LabVIEW program to manipulate the system with both Proportional (P) and Proportional-Integral (PI) control. Proportional control is a linear feedback process control system where a set point is defined for a variable (tank level, for example), the deviation from the set point is calculated, and another variable (valve position, for example) is manipulated with the intention of achieving the set point. Proportional-Integral Control also transmits a control output (opening or closing a valve), but additionally controls how much offset is introduced into P Control. For most processes, P Control will result in oscillatory under/over shooting offset from the set point and the process will either take a very long time, or never stabilize. PI control tries to eliminate this oscillation by introducing integrating process control, which although greatly increasing the complexity of the controls algorithms needed, also results in a more stable process. By testing the flow cart under both manual control, P control, and PI control, students will be able to witness the benefits and drawbacks of each type of process control first hand.

METHODOLOGYConcept Selection and Detailed Design

Three distinct designs were developed and evaluated based upon their ability to satisfy all customer needs and engineering specifications. The original system design featured an elevated water tank outputting gravity induced flow to the cart below. A secondary design concept centered on the use of a pressurized vessel to control flow rates by manipulating head pressure. Lastly, the final design uses a sink faucet to produce flow that is then controlled through a series of two control valves. A Pugh Chart (Table 3) was constructed in order to perform a side-by-side comparison of the three designs.

Table 3: Pugh Chart developed to evaluate design concepts

Criteria Baseline WeightDesign Concepts

Gravity Pressure Line Fed1 Programming Complexity 0 3 2 -1 -22 Student Safety 0 3 0 -1 03 Setup Safety 0 3 -1 0 04 Head Pressure Control 0 3 -2 2 15 Risk of System Failure 0 3 -2 2 16 Ease of Implementation 0 3 -2 -1 17 Fluid Reservoir Cost 0 2 -1 0 28 Overall System Cost 0 2 -1 -2 19 Transportability of Cart 0 2 0 -1 010 Simplicity of Adding Water 0 2 -2 -1 211 Simulation Capabilities 0 2 0 2 212 Run Location Flexibility 0 2 -2 0 -113 Ease of Maintenance 0 2 -1 -2 014 Feasibility of Recycle Loop 0 1 2 1 -215 Structural Complexity 0 1 -2 -1 0Initial Total -12 -3 5Weighted Total -29 -5 13

Table 4: Legend for Pugh Chart (Table 3)

Baseline Weighting Scale Scoring Scale

Preexisting Flow Carts

3 – Extremely Important2 – Somewhat Important1 – Not Very Important

+2 – Much Better+1 – Somewhat Better 0 – Equal/Unchanged -1 – Somewhat Worse -2 – Much Worse

The team’s primary concerns were student safety, risk of failure, ease of implementation, and programming complexity. After significant discussion, the line fed system was chosen despite its increased risk due to programming complexity because of its ability to ensure safety and ease of implementation. Later in the design



process, a fluid reservoir and level transmitter were added to the design so as to bring the system closer to the original gravity fed flow proposal and allow for greater visual learning opportunities. On the constructed cart (CAD rendering shown in Figure 1), water flows from the faucet through plastic tubing to the cart where it first encounters the Coriolis flow meter and then travels to the first pneumatic control valve. After exiting the control valve, the water flows into the reservoir and the fluid level in the reservoir in monitored by a level transmitter. The second control valve regulates drainage out of the bottom of the reservoir, and then the water exits the system and is deposited to a floor drain. The reservoir level is the process variable that students will manipulate during the experiment by changing the valve positions either manually or through LabVIEW automation.

Figure 1: 3D rendering produced in CAD of cart sans wheels.

A concern that the group had with the line fed design concept was its feasibility in regards to the use of sink faucets as a source of flow. To eliminate this question, the team obtained a device to test the water pressure of the laboratory’s sinks with the help of team guide, Steve Possanza. Results from the test indicated that even if multiple sinks were running concurrently, the water pressure of a fully open faucet remained between 46 and 48 psi that was deemed to be an acceptable operating range for the system.

Figure 2: Faucet water pressure testing; results varied from 46 to 48 psi.

Modeling of Process Control DynamicsFor this cart, a non-integrating (or self-regulating) process is present in the level function of the draining tank. If

a level in the tank was to be kept constant, the inlet flow will ideally be set to match the outlet flow via the control valves on the cart. This is a learning methodology that is to be taught via this process control cart. From the main equations for process control, our main controller transfer function and overall regulating process transfer function were developed and programmed into LabVIEW for the control of the process. These equations are below. [2]

Project P13631


Gc=Kc

τ c τv s2+s (τ c τ v )+1+ Kp

τ i τ c τv s3+ τ i s2 ( τc+ τ v )+τ i s

Where Gc is the controller transfer function, Kc is the controller gain from tuning (KaKp is equal to Kc; the actuator and process gains), τc is the controller time constant, τv is the valve time constant, s is the transfer function variable in the Laplace transform space, τI is the integration time constant, and Kp is the process time constant.

Y (s)Y sp(s)

=Gd (s )D(s)+Gc (s)Ga(s)G p(s)

Gc ( s )Ga (s )G p ( s ) Gs(s)+1

This equation is the entire transfer function for the process control apparatus. Where Y(s) is the output of the process, Ys(s) is the set point of the process, Gd is the derivative transfer function, D(s) is the disturbance output, G c

is the controller transfer function, Ga is the valve transfer function, Gp is the process transfer function, and Gs is the level transmitter or flow transmitter transfer function.

RESULTSDetailed Safety Testing

The center of mass (C.M.) was calculated to validate the static stability of the assembled apparatus, as the avoidance of the use of a counter weight was a mechanical design criterion. The C.M. location was estimated via the physical spatial measurement of estimated component C.M.s with respect to a Cartesian coordinate origin. Components such as wiring, tubing, zip-ties, tape, fasteners, and other small items are considered to have a negligible mass and were not used in the C.M. calculation of the assembled apparatus.

The origin was taken to be, from a top-down perspective, the front left corner table surface of the top shelf. The points representing the wheelbase were measured as: P1 (1.75 in, 1 in, -30 in), P2 (27.25 in, 1 in, -30 in), P3 (27.25 in, 13.25 in, -30 in), and P4 (1.75 in, 13.25 in, -30 in). The coordinate point representing the assembled apparatus C.M. is then found via the equations below.

X=∑ M x

∑ M;Y=

∑ M y

∑ M;Z=

∑ M z

∑ M;

These results were found to be:X=13.49 in ;Y=8.90 in ; Z=−4.64 in

Due to the C.M. location being well within the wheelbase, it is clear that the assembled apparatus is statically stable.

The structural validity of various components of custom design was analyzed to check for compliance with design criteria. In the case of the collapsible table, the table needed to hold 20 pounds at its free end and not deflect more than 1 inch. The table design was then checked for normal and shear stresses, as well as deflection.

The table was modeled as a beam with two fixed points, one at the screw location on the hinge, and one at the table brace.

P = 20 lbf – Design maximum loadymax = 1 inch – Design maximum allowable deflection

Using singularity functions the equations of shear and bending moment were obtained.

V=−1713

P<x−0>+ 3013

P<x−6.5>M =−1713

P<x−0¿1 + 3013

P<x−6.5¿1

From the shear and bending moment diagrams yield the absolute maximum shear to be 26.15 lbf, and the absolute maximum bending moment to be 170 lbf-in. Material properties for the high density polyethylene (HDPE) table were taken from McMaster-Carr.com. [4] The maximum allowable normal stress is 410,000 psi, and the maximum allowable shear stress is assumed to be 2/3 of this value. The design margins for each case are:

σ= McI

=170 lbf∗in∗0.25 in16

in4=3100 psi

Design Margin=12.16 for maximum bending moment



τ=VQ¿ =

26.13 lbf (0.5 in3 )16

in4 (16 in )=2067 psi

Design Margin=421.8 for absolute maximum shearUsing the singularity functions above and the design criteria, the maximum deflection was calculated.

θ= 1EI [−17

26P < x−0 >2+30

26P< x−6.5 in >2]+C1

ymax=1

EI [−17104

P< x−0 >3+ 30104

P < x−6.5 in >3]+C1 x+C2

Where, C2=0 ;C1=17 P

104 EIThe free end deflection of the table was calculated to -0.109 in.

Design Margin=9.183 for maximum deflectionThe deflection is therefore the driving criterion, and is satisfied with a design margin of 9.183.

All electrical work implemented was checked by a Master Electrician from Kodak. All wires were confirmed as grounded and the electrician approved the design and implementation.

Functionality VerificationIn order to program the first order process model into LabVIEW, the gains and time constants of the control

valves and the overall process had to be obtained. These results are what students will calculate during the lab.

Table 5: Experimentally derived constants for control valves 1 and 2 to be used for lab instructor’s reference.Gain (inches/%) Time Constant (min)

Control Valve 1 0.9 15.37Control Valve 2 1.6 7.45

Design VerificationOne of the first steps in the initial planning phase of this project was to convert customer needs into

engineering specifications around which we could base our technical drawings. The specifications were divided into major categories that included Safety Testing, Fixture System, Flow System, and LabVIEW Control Interface, and Safety. These were verified prior to commissioning of cart.

Table 6: Results of safety testing

Description Measure of Performance

Engineering Units

Marginal Value

Ideal Value

Validation Method

Result

Safe and Ergonomic Design

Operationally Binary No Yes Operational Study Pass

Electrical devices protected from physical and chemical interference

Operationally Binary No Yes Operational Study Pass

Static stability analysis Spatial Measurement

Inches Outside wheel base

Inside wheel base

Center of gravity calculations

Pass

Stress and deflection analysis of table

Factor of Safety Ratio

No Units <1 ≥2 Stress/deflection calculations

Pass

Table 7: Flow system functionality test results


Engineering Units

Marginal Value

Ideal Value

Validation Method Result

Project P13631


Minimum Operating Flow Rate

Flow Rate Measurement

gr/min 500 1000 Simulate minimum flow rate and confirm with a beaker

Pass

Maximum Operating Flow Rate

Flow Rate Measurement

gr/min 1500 1000 Simulate minimum flow rate and confirm with a beaker

Pass

Manual Operation of Instruments

Operationally Binary No Yes Successful manual manipulation by student

Pass

Table 8: Fixtures system design verification results


Engineering Units

Marginal Value

Ideal Value


Design for user assembly

Time Minutes 30 <15 Time study with lab technician

Pass

Minimum Space Requirements

Area Feet2 8 6 Area Measurements Pass

Mobility and Adaptability in Lab

Operationally Binary No Yes Transport from storage to lab to assembly

Pass

All fittings sealed Operationally Binary No Yes Leak check before commissioning

Pass

Design for chemical engineering student use

Time Minutes 210 180 Time study of procedure with 4th year chemical engineers

Tentative Pass*

*Design for chemical engineering student use has not been officially tested according to the validation method. Based on speculations and estimates from the lab manual [3], the lab should be within the specified time limits. 4 th

year students will be contacted to complete this validation in the future.

Table 9: LabVIEW control interface design verification results


Engineering Units

Marginal Value

Ideal Value


Automated operation of instruments

Operationally mA <4 >20 4-20 Measurements with multimeter

Pass

Automated data collection

Operationally Binary No Yes Successful LabVIEW implementation by students

Pass

Instrument and Controller Power Supply

Voltage Volts 110 120 Measurements with multimeter

Pass

Proportional and Integral control

Operationally Binary No Yes Successful control operation from LabVIEW

Pass

DISCUSSION The final product is a gravity fed flow process controls teaching device that can be utilized by future

students to increase understanding of controls theory and applications. Students can manipulate the level in the tank manually or through a proportional or integral control through LabVIEW. Two control valves present interaction when simultaneously used to demonstrate the complicated process of cascade control.

Safety was a key concern through out the projects. This cart is safe to use when operated by the procedure outlined in lab manual. [3] Rigorous mechanical analysis was implemented to determine the static stability as well as the stress and deflection limits of the metal bracketing and the plastic table, respectively. As for electrical components, an electrical box was installed to protect the majority of electrical components from water. Other electrical components, such as the tips of the level transmitter, were protected by tape that is only to be removed by a lab instructor as specified by the lab manual. [3] All wiring and electrical work was checked by a Master Electrician from Kodak.

Additionally, the final design of the cart fulfilled customer needs of mobility and assembly. The cart and all its components weigh only 136.5 pounds and are easily transported on wheels. There is also a brake system in place for when it is time to assemble or disassemble the cart. The cart can be used anywhere in the lab, provided there is a



source of water and instrument air at sufficient pressures and a drain available. A detailed bill of materials will ensure ordering parts when they need to be replaced is simple. Two level transmitters were purchased since they are delicate components susceptible to damage from bending.

CONCLUSIONS AND RECOMMENDATIONS Though this cart fulfills the majority of customer needs, further refinements could be made by a future team in

order to conserve energy and water in the lab. Currently, the cart has a line feed connected to a sink and a line output connected to a drain. If the lab runs as specified in the lab manual [3], then the water could be potentially wasted for 3+ hours per week at around 1.62 liters per minute. To save water and increase cart mobility in the lab, a simple pump could be installed to route the water back to the sink instead of the drain. This way, the cart would not have to be tied to a drain and the water would be at least somewhat recycled. One more way to recycle even more water would require an additional large tank and a recirculation pump. The same water could be reused. This could cut down on convenience of maintenance for those having to replace or move the large tank.

Before handing over the cart to the lab for student use, time trials with 4 th year chemical engineering students should take place to verify that the three labs can be completed within three hours. If the lab runs for too long or too short, then the lab manual should be adjusted to fit the lab within the time constraints. If possible, the labs should be coordinated to what the students are learning in the process controls class.

The first level transmitter purchased often gave erroneous readings after being clamped to the tank on the cart. Purchasing a new level transmitter and using electrical tape to hold it in place solved this problem. As a safety and stability precaution, the lab manual [3] specifies that only the lab instructor can remove the tape from or adjust the level transmitter. If the level transmitter is to become bent again despite the precaution, it is recommended that alternatives for sensing the height in the tank be explored. A pressure sensor placed on the bottom of the tank is one viable alternative.

REFERENCES [1] http://www.ni.com/labview/ [2] Riggs and Karim. Chemical and Bio-Process Control, 3rd Edition. [3] Liquid Level Control of Gravity Drained Tank.[4] http://www.mcmaster.com/#8574kac/=phbh79

ACKNOWLEDGMENTS First and foremost, we would like to thank Steve Possanza, our senior design guide, for sharing his

technical expertise and providing direction in order to ensure smooth and timely completion of the project. The construction of this project would not have been possible without the generous donation of critical

equipment including the control valves and flow transmitter, as well as many smaller components by the Eastman Kodak Company. Addition to the physical donation, several Kodak employees volunteered their time to assist with fabrication and provide technical advice for which we are very grateful.

From the Rochester Institute of Technology, both the Multidisciplinary Senior Design (MSD) program and the Department of Chemical Engineering were both integral in the success of our project. We would like to thank the MSD team for teaching us many useful design techniques and project management skills. For presenting us with our project concept and financially supporting its development, the Chemical Engineering Department has our utmost appreciation.

In addition to the Chemical Engineering Department as a whole, we would like to extend our thanks to several faculty members for providing further assistance. With the help of Dr. Sanchez, we were able to develop a lab procedure congruent with the learning objectives in the System Dynamics course. We would like to thank Dr. Christiaan Richter for offering constructive criticism throughout the design process, enabling us to better meet our customer’s needs. Lastly, for allowing us to utilize Department facilities and tools in order to assembly our project, as well as answering our questions about lab resource availability and cart robustness needs.

From outside of the Chemical Engineering Department, we would like to thank Professor Slack and Professor Indovina, of RIT’s Electrical Engineering Department, for bestowing us with invaluable advice and guidance for the design and assembly of our electrical systems.

Project P13631

http://www.mcmaster.com/#8574kac/=phbh79

http://www.ni.com/labview/

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