Linear Flow Orifice Meter

A system of floats and lever arms change the flow rate of alum based on the height of the water in the entrance tank.. The height of water in the entrance tank is based on the outflow of

Linear Flow Orifice Meter

A ThesisPresented to the Faculty of the Graduate School

of Cornell University

In Fulfillment of the Requirements for the Degree of

Masters of Engineering

byLeah Elizabeth BuermanDecember 2008

© 2008 Leah Buerman

BIOGRAPHICAL SKETCHLeah Buermans’s education includes a bachelors of science from Cornell University inBiological and Environmental Engineering Water Purification technologies have been

a focus of her education for two years, during her undergraduate and graduateeducation Leah’s coursework includes Water Quality Analysis, Small ScaleSustainable Water Supply, Watershed Management, Environmental Systems Analysis,and Fluid Mechanics She has attained Engineer in Training Certification and is anactive member of the society of Women engineers Through experiences abroad shehave become aware of the high demand for low cost reliable water purificationtechnologies in the developing world Leah’s graduate work with the Agua Claraproject directly affects the design of water treatment plants in Honduras

Dedicated to my Mom and Dad.

I would like to thank Professor Monroe Weber-Shirk for the immeasurable amount ofhelp that he gave me throughout the project I would also like to thank Professor TomCook for this help during fabrication I would also like to thank Professor Norm Scottfor this invaluable guidance during my undergraduate and graduate education Finally

I would like to thank David Railsback for his innovative idea from which this project

is based

TABLE OF CONTENTS Pages

Access to clean water is a human right that is unattainable by many people The Agua Clara project is working to design and build water treatment plants in Honduras, which is among the poorest countries in the world Major emphasis is placed on the integration of the plants into the communities effectively Careful consideration is made regarding the specific requirements for each community In the neighborhoods with the greatest need for water purification technologies there is often

an intermittent flow of electricity Current water treatment designs control chemical dosing and flow rates with electrically powered pumps and sensors The intermittent electricity supply combined with the current system design would result in intermittentsupply of ‘treated water,’ water sent intermittently through pipelines is no longer considered pure because during shut-down the pipelines will form a vacuum pressure pulling surrounding material into the pipe space contaminating future water flows TheAgua Clara design requires no input of electricity for operation of the plant Water flows through the plant from high elevation to low elevation; elevation change is the energy source for the water treatment The water is treated through 4 processes The first step in water treatment is a grit chamber which removes the large objects such as branches from the water flow using a large spacing metal grid Water is then treated with a coagulant, aluminum hydroxide, and sent through a flocculation tank The coagulant binds to the foreign particles (contaminants) in the water and creates flocs, similar to a snowflake The flocs increase in size as they move through the

flocculation tank Sedimentation follows the flocculation stage, the large flocs are

settled out and the clean water is drawn of the top of the sedimentation tank As a finalmeasure the purified water is treated with chlorine to kill any bacteria remaining the water and provide a residual chlorine concentration for disinfection during transport

My research is focused on the dosing of aluminum hydroxide to the water Currently the operator of the plant manually adjusts the flow rate of aluminum hydroxide as the flow rate through the treatment plant changes Automation of the process is required tomore rapidly and reliably meter alum to the inflow The automation process is

designed using an entrance tank, riser pipe, and float system The contaminated water flows into the entrance tank the through a riser pipe and then empties into the

flocculation tank A system of floats and lever arms change the flow rate of alum based on the height of the water in the entrance tank The height of water in the entrance tank is based on the outflow of water through orifices in a riser pipe located

in the entrance tank The height in the water in the entrance tank with a single outflow orifice is correlated to the square root of the flow rate according to the orifice

equation

h g A

K

Q= orifice orifice 2 ∆

Equation 1 – The orifice equation

This relationship makes metering aluminum hydroxide through a float system nearly impossible Linearization of the head of the water in the entrance tank as a function of flow rate is the goal of my project If the pattern of orifices on the riser pipe can be situated to create a linear relationship between water height in the entrance tank and the flow rate through the plant then the float system can easily adjust the alum flow

rate

1897 achieved the linear relationship but was theoretically based, the width at the bottom of the weir approached infinity.

Figure 1: Linear Proportional Weir

Sutro modified the design by Stout in 1908 to create a practical design The Sutro weir has a rectangular base and the flow through the weir is proportional to the height

of the water through the curved portion of the weir plus 2/3 of the height of the

rectangular base

Figure 1: Sutro Weir

Implementation of sutro weirs in currently operating water treatment facilities is infeasible due to restrictions of shape and space Grit chambers receive the inflow firstand transmit the water to the rest of the plant through pipes If a sutro weir shaped holewere cut into a pipe the pipe would become unstable and require skilled labor for construction Therefore we are approximating the sutro weir with a riser pipe added onto the pipes that connect the initial grit chamber (entrance tank) and the flocculationtank The pipe would be easily introduced into previously constructed plants and new plants at low cost Holes would be drilled into the riser pipe that would mimic the sutro weir basics of design The drilling of holes as specified heights doesn’t require skilled labor The water is forced to flow through the riser pipe in order to leave the entrance tank and enter the flocculation tank

Sutro Weir Variables

W = base of rectangular weir

s = height of rectangular weir

h = weir height above rectangular weir

K= Vena contracta area ratio, average value is 0.62

g = acceleration due to gravity

Sutro Weir Equations

Total Flow Rate Through Weir

The constant of proportionality, Co :

Flow Through Rectangular Base of Weir

Rectangular Base Width

Rectangular Base Height

PiSutro

Profile of Curved Portion

of the Agua Clara water purification system with a maximum flow rate of 120L/min

A prototype LFOM was created to accommodate a maximum flow rate of 125 L/min,

in order to avoid over flow during surges in flow rate The prototype has a head loss of

20 cm Unfiltered water from Cascadilla Creek flows into a 5 gallon entrance tank through the LFOM orifices and then freefalls from the bottom of the riser pipe into thestart of the flocculation tank shown in figure 3

Figure 3: Design including the linearized flow tank

A flange connects the riser pipe to the entrance tank to avoid leakages The riser pipe

is made out of three inch white PVC pipe with a consistent orifice diameter of three eighths inches Holes in the riser pipe were spaced over 20 cm and every centimeter was evaluated for necessary holes as shown in figure 4

Figure 4: The riser pipe with the necessary holes for a flow rate of 125 L/min

The water from the plant flows into the entrance tank, a five gallon bucket, and through the riser pipe into the flocculation tank

Construction of the prototype was undertaken with the goal of reduced cost The overall budget for the LFOM was eighteen dollars, table 1

Budget

5 gallon pale, transparent 1 $5

3" PVC Pipe 4.1' $16.64 for 8'

stainless steel screw 4 $2.00

Total: ~$18.00

The completed entrance tank and riser pipe are shown in figure 5

Figure 5: close up of completed bucket

The riser pipe design’s accuracy was tested in Ithaca, NY The water from the plant flows into the entrance tank, a five gallon bucket, and through the riser pipe into the flocculation tank, the set-up is shown in figure 6

Figure 6: Completed LFOM set-up.

In-line with the water inflow pipe a flow meter, siemens model SITRANS F M MAG

3100, is installed The device provides flow measurements in GPM with an accuracy

of +/- 0.25% of flow rate Before each trial the riser pipe is cleaned and all holes obstructions are removed Performing the experiment requires reading the flow rate from the flow meter and measuring water height in the entrance tank with a ruler Flow rates were varied between 5 GPM and 36 GPM by 1 GPM increments The flow meter requires acclimation time to report accurate readings After every manual change in flow rate there was a rest period of five minutes before water height and flow rate were measured The recorded water height is adjusted by an offset of 3.5 inches The offset accounts for the distance between the bottom of the bucket and the center of the first row of holes Three trails were run on three different days

The difference between the experimental results and the predicted results is available

in figure 8 The percent deviation for all flow rates is shown in figure 9 The equation used to calculate the percent deviation is shown in equation 2

Equation 2: Percent deviation equation

Figure 8: The difference between the predicted height and the recorded height as a function of the flow rate.

Figure 9: The percent deviation between the predicted height and the recorded height

as a function of water height

The results were divergent from the predicted values with larger percent deviations at the lower flow rates and a stabilizing error of approximately 10% The design algorithm was changed after the experiment to reduce the errors at low flow rates These changes re-evaluate the orifice pattern at the first two rows of orifices Another postulated source of error was the accuracy of the flow meter installed in-line

at the pilot plant The readings may be inaccurate though the specifications guaranteedaccuracy up to 0.25% of the flow rate Data taken manually to determine actual flow compared to the flow meter is not significantly from the flow meter to suggest

problems with the flow meter

The riser pipe with a pattern of orifices was used to restrict the flow so the height of water in the entrance tank of a water treatment plant directly correlates to theflow through the riser pipe Three trials were undertaken to determine if the system of orifices accurately approximates a weir As long as a system can be devised to preventthe clogging of holes during operation the riser pipe is an effective tool The riser pipe entrance tank system provides data on flow with an accuracy of approximately ten percent

predicted failure conditions The point of failure is predicted assuming conservation ofmomentum A pipe diameter was calculated to be 1.5 in to achieve failure at 62.5 L/min, half of the maximum flow rate of the plant where the prototype will be tested The head loss through the LFOM is 20cm and there are twenty rows of orifices, the orifice pattern used with the LFOM is the same design used in the approximation experiment The riser pipe is shown in figure 10.

Figure 10: Point of failure riser pipe and entrance tank.

Methods

The riser pipe design’s accuracy was tested in Ithaca, NY The water from the plant flows into the entrance tank, a five gallon bucket, and through the riser pipe into the flocculation tank, the set-up is shown in figure 6 The original entrance tank was used and a series of reducers were used to connect the smaller diameter pipe to the existing hole (that was created for the 3" diameter riser pipe) Before each trial the riser pipe is cleaned and all holes obstructions are removed Performing the experiment requires reading the flow rate from the flow meter and measuring water height in the entrance tank with a ruler, shown in figure 11

Figure 11: Measuring the water height in the entrance tank.

Flow rates were varied between 8 GPM and 22 GPM by 1 GPM increments The flow meter requires acclimation time to report accurate readings After every manual change in flow rate there was a rest period of five minutes before water height and flow rate were measured The recorded water height is adjusted by an offset of 4.875 inches The offset accounts for the distance between the bottom of the bucket and the center of the first row of holes The device was in evident failure at the upper flow rates as shown in figure 12

Figure 12: The entrance tank and riser pipe in evident failure.

trials and the predicted data.

The difference between the experimental results and the predicted results is available

in figure 14 The percent deviation for all flow rates is shown in figure 15 The equation used to calculate the percent deviation is shown in equation 2

Figure 14: The difference between the predicted height and the recorded height as a function of the flow rate

Figure 15: The percent deviation between the predicted height and the recorded height

as a function of water height

the LFOM failed over the entire range of flows.

Future Research

The perplexing results may be due to the fact that there we were not witnessing a point

of failure, a flow rate at which a LFOM will fail, but a complete failure If the pipe is sized too small to accommodate the flow rate which the orifice pattern is designed to support then the LFOM will fail for all flow rates This hypothesis would agree with the results It would be beneficial in future research to test the LFOM created above with a diameter of 1.5 inches with an orifice pattern designed to handle the maximum flow rate for the pipe, 62.5 L/min It would also be interesting to apply a flow rate in excess of the 62.5 L/min and watch the system for evidence of failure

CHAPTER 4

MathCAD DESIGN CODE

Introduction

Research is being done to determine the optimal process for determining the diameter

of the orifices on the LOFM The drill size was previously determined using the assumption of 5 holes in the first row of orifices and then applying that drill size to therest of the LFOM The first row of orifices in the linear flow meter is approximating a rectangular base When Sutro altered the linear proportional weir, which has a base width that approaches infinity as the height approaches zero, by adding the rectangularbase he added a design restriction that the linear relationship only occurs above 1/3 theheight of the rectangular base Through trial and error an orifice number on the initial row was found to optimally be 5 The five orifices were chosen as optimal because when five orifices are used on the initial level then the top rows had single orifices Thus 5 was the minimum number of orifices in the bottom row that resulted in non zero values for orifices in all of the rows The new method will calculate the optimal drill size based on the flow rates through the top row of orifices

Methods

The design characteristics of the LFOM were modeled in the MathCAD program The design requires three user inputs to create a design The design is based upon a plant flow rate, the desired number of rows of orifices, and the maximum allowable head