Flow Measurement Challenges

Transcription

1 Flow Measurement Challenges Multi-Stage Reverse Osmosis Permeate Stream Standards Certification Education & Training Publishing Conferences & Exhibits Speakers: Chris Caglioti, Andrew Fenske, David Ramsey 2012 ISA Water & Wastewater and Automatic Controls Symposium August 7-9, 2012 Orlando, Florida, USA

2 Presenter Chris Caglioti has over 23 years experience as a class A drinking water treatment plant operator and an ISA level I CCST technician for the City of Cape Coral water production department. He is currently the instrument and controls technician for the North Cape RO Plant. Andrew Fenske has served the City of Cape Coral for over 24 years. He holds dual waster and wastewater licenses and is currently the chief operator for the North Cape RO Plant. David Ramsey is the municipal territory manager for AMJ Equipment Corp and also holds ISA Level 1 CCST certification. David has been in the industry for over 15 years and specializes in process application for all aspects of municipal and industrial process treatment systems including flow, level, pressure, analytical measurement and SCADA systems. 2

3 City Of Cape Coral North Reverse Osmosis Plant 3

4 Presentation Outline Glossary Treatment Plant History Source Water Treatment Process Control Strategy and Instrument Selection We were warned Hydraulics, Laminar Flow, Unexpected Results Solution for the Solutions Questions? 4

5 GLOSSARY Some MLA s used in this presentation MGD-million gallons per day TDS-total dissolved solids PPM-parts per million RO-reverse osmosis umhos-micro mhos (aka millisiemens) 5

6 TREATMENT PLANT HISTORY The City of Cape Coral Southwest Reverse Osmosis (RO) Plant is the oldest and one of the largest municipal RO treatment facilities in the United States. Due to a rapid increase in population and a planned major expansion of the utilities service area, the city began design work in 2005 to expand the existing plant production capacity by 3 MGD and add an additional plant with 12 MGD production capacity. That North RO Plant was completed and on-line in March of

7 SOURCE WATER The City of Cape Coral uses the Upper Floridan Aquifer. Because of its mineral content, brackish water is the term used to describe the quality of the source water. The amount of minerals found in water are expressed as dissolved solids. The city s source water has a total dissolved solids (TDS) content of approximately 200 parts per million (PPM) as opposed to the TDS of seawater which is approximately 35,000 PPM. 7

8 TREATMENT PROCESS In reverse osmosis (RO) treated feed water is fed under pressure across a semi-permeable membrane where mineral separation and concentration occurs. The process stream is then reclassified into permeate and concentrate (also referred to as reject). In a multi stage system, the objective is to recover a much permeate from the feed water and still maintain the minimum required flow of concentrate without causing an over concentration of minerals which will cause scaling of the membrane system. This action is done by using the concentrate stream from the first stage of treatment as the feed water to a second or third stage through of mineral separation allowing for maximum product recovery. 8

9 SPIRAL WOUND MEMBRANE 9

10 Control Strategy and Instrument Selection Control Strategy and Instrument selection A typical RO production unit operates with the goal of maintaining a predetermined recovery ratio via a series of PID control loops and process control measurement inputs including pump speed, concentrate valve position, TDS measurement by means of conductivity analyzers, pressures, concentrate valve controls and process flows both real-time and calculated. Since we are using a two-stage system we can expect that the permeate quality of the second stage will have a higher concentration of minerals than the first stage. In this application, first stage permeate conductivity (expressed as umhos) averages at 80 while the second stage average is 276. Total permeate conductivity of both the first and second stage permeate streams averages at 126. With these values in mind design engineers and staff agreed that magnetic flow technology would be suitable for the application based on most manufacturers minimum requirements for conductivity being around 5 umhos provided that the process stream is stable. 10

11 RO Train Overview 11

12 Mag-Meter Flowmeter Video 12

13 We were warned Having a similar system with a smaller piping scale in place at our South RO plant, when we piloted our first magnetic flow meter we noticed some instability in the readings. Our membrane design engineer had warned us that in some applications, magnetic flow meters had trouble providing a smooth flow signal where a split permeate stream was involved. The manufacturer stepped up with an alternate product which used different coil excitation and pulse width parameters. This solved the erratic readings and the product was specified for our new plant. 13

14 Hydraulics, Laminar Flow and some unexpected results Hydraulics, laminar flow and some unexpected results In the early stages of commissioning, we noticed some instability in conductivity values and total permeate flow for the production units. Our total conductivity fluctuated above and below predicted stable ranges by as much as 40 umhos. Total permeate flows showed similar characteristics of fluctuation by as much as 200 GPM. As our main point of control for permeate recovery was a PID loop for the pump speed via variable frequency drive and based off of the combined flow rates of the production unit, our permeate recovery ratios were not stable. Dosing for post treatment chemicals relied on the total amount of blended permeate calculated through the process control system therefore this process was affected as well. By using advanced diagnostic capabilities of the mag meter, we confirmed suspicions that the flow profile had dissimilar properties of conductance where it passed through the measuring tube. Looking at the possible remedies, the conductivity sensor was relocated downstream of a disc check valve which provided adequate mixing to read a stable signal. However after exhausting all available options within the transmitter functionality such as dampening, signal strength, coil pulsation, electrode measuring period and polarity, the separation of conductance of each individual process stream from the first and second pass was too great for the meter to provide stable signal output. We believe that the hydraulic characteristics up stream of the flow tube provided an extremely laminar flow profile with no mixing. 14

15 Two Stage RO Train Overview 15

16 Two Stage Permeate Header 1 st stage avg. conductivity 90 umho 2 nd stage avg. conductivity 300 umho 16

17 Two Pass Blended Permeate 17

18 Solution for the Solutions! Solution for the Solutions With the city of Cape Coral having success with a large install base of transit time ultra-sonic flow meters, a joint decision between the city, the engineer and manufacturers representative was made to provide a wetted sensor transit time flow meter configured to the same foot print as the magnetic flow meter. Since the ultra sonic technology is largely independent of process fluid characteristics, the non-homogonous flow profile had no effect on the measurement output to the control system. 18

19 Ultrasonic Flowmeter Video 19

20 Questions? Thanks to AMJ Equipment Corp-Lakeland FL Endress-Hauser Mike Humphries-Membrane Systems Inc. Marco Parra-City of Cape Coral Programmer Analyst 20