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Liverpool to Ashfield Pipeline – An Innovative, User Friendly Control System For Energy Savings And Odor Control

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Purpose: This presentation will describe how the control system for the Liverpool to Ashfield Pipeline (LAP) was developed, tested and commissioned and the lessons that were learned during the process that can be applied to future applications.

Benefits

Benefits: The LAP was designed to transfer treated and raw sewage from two contributing pump stations over a wide range of flows and a pipeline route with two intermediate high points. The control system is designed to save energy in pumping while avoiding air entrainment in the pipeline. This was achieved by means of a modulating Pressure Control Valve (PCV) at the end of the pipeline and a lookup table programmed into the telemetry control system.

Entraining air into a large wastewater force main such as the LAP has the following disadvantages:

Air entrained in the pipeline will come out of solution at high points in the pipeline leading to odor and corrosion issues at these high points;


The sewage exiting the pipeline will be more odorous and affect the receiving sewer; and


The entrained air could have a detrimental effect on the pipeline' s hydraulic performance as well as amplifying water hammer effects on the pipeline.


The operating philosophy developed for this project allows for the operator to remotely select the flow at each pumping station. The system automatically adjusts the number of pumps and the speed of these pumps at each station as well as the opening percentage for the PCV in order to maintain these flows. This allows the pumps to operate at the minimum possible speed and therefore pressure while still maintaining pipe full conditions. The system uses around 650kW less power at full pumping capacity than a similar static system would use.

Scope of Work: The Liverpool to Ashfield Pipeline (LAP) project comprised the design and construction of the 15 mile long, 40 inch diameter Liverpool to Ashfield Pipeline, Liverpool Sewage Treatment Plant (STP) Amplification Stage 5B and Fairfield Storm Sewage Treatment Plant (SSTP) Diversion Scheme located in South West Sydney, Australia. The aim of the project was to transfer flow away from the Northern Georges River Submain (NGRS), an existing brick sewer that required rehabilitation, to the Western Branch Main Sewer (WBMS) at Ashfield. The scheme is also intended to serve after the rehabilitation of the NGRS is completed as a conduit for secondary treated effluent from Liverpool STP and the nearby Glenfield STP to two treatment plants along the pipeline route that will treat the effluent to recycled water quality for non-potable uses.

This is the largest sewerage force main in Sydney. One of the hallmarks of the project is the innovative and complex control philosophy that was developed for the project. The complexity of the control philosophy was due to three main factors:

The requirement for two pumping stations to be able to transfer a wide range of flows from 7 to 40 mgd made up of varying flows from each station;


The undulating profile of the pipeline route with two intermediate high points;


The requirement to maintain pipe full conditions to eliminate air entrainment in the pipeline and the desire to do so while minimizing the pumping energy required.


In order to address the above conditions the following methodology was developed:

An eccentric plug valve was installed at the downstream end of the pipeline which acts as a pressure control valve (PCV). The PCV is electrically actuated and connected to and controlled by the Sydney Water IICATS (telemetry) system.


The position of the PCV is set based on the flow in the pipeline to ensure that the hydraulic grade line is always above the high points in the system and therefore the pipe is full while minimizing pumping head and hence minimizing pumping energy. The different flow combinations and PCV opening percentage are programmed into a lookup table in the Sydney Water telemetry control system.


The operator can remotely select the required flow from the Liverpool and Fairfield Pumping Stations and the system automatically selects the number and speed of pumps required and the opening percentage for the PCV. The pumps then vary speed to match the required flow rate.

The PCV position is set to make sure that a head xp is maintained at Potts Hill and a head xe is maintained at Enfield (the two intermediate high points).


The benefits of this approach include:

A simplified, intelligent operating system that eliminates the need for operators to make complex and difficult decisions and hence eliminates operator error. The operators are simply required to set the flow rates required from each pumping station and the control system will automatically determine the number of pumps and pump speeds required and the degree of opening of the PCV.


Optimized pumping and hence lower use of energy providing savings in power costs as well as the environmental benefit of minimizing the carbon footprint of the project.


Avoidance of air entrainment in the pipeline and the disadvantages noted above.


Following construction, hydraulic validation was carried out in order to set the system up for use. The pumps, pipeline and PCV were tested at different set points to refine the lookup table and ensure that the system was functioning correctly. During the hydraulic validation a large difference was found between the theoretical values for the eccentric plug valve (PCV) and the actual calculated values. This necessitated adjustments in the lookup table during commissioning. The ease with which you can make these updates needs to be considered during design and a sensitivity analysis conducted to see if variations such as these have a large effect on the hydraulics of the pipeline.

Completion: Construction was completed in mid 2008. The scheme has been successfully operating since then. A project to convert the pipeline to transfer recycled effluent to customers has been constructed and the pipeline will supply secondary treated effluent to customers who will further treat it for use in non-potable applications.

Conclusion: This control philosophy has been operating successfully for two years now and has provided an easy and user friendly solution to the problem of how to control the complex operating parameters of this pipeline. Care must be taken in setting up a similar system in relation to reliance on theoretical head loss coefficients for valves and to ensure that the system is capable of being readily updatable.
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Keywords: Control System; Energy Saving; Hydraulics; Odor Control; Operating Philosophy; Sewerage Force Main

Document Type: Research Article

Publication date: 2011-01-01

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