Intelligent measurements in water treatment applications
July 2012, Control Systems Design
In the spring of 2010, the largest nanofiltration plant in Germany was officially put into operation in a municipal water works at the city of Dinslaken.
In future, Dinslaken will be supplied with high quality drinking water from the Voerde-Löhnen water works, a plant that became necessary because coal mining in the catchment area is expected to affect the flow of underground water. The result: Rhine bank filtrate and a number of trace materials could get into the raw water of the extraction wells. To remove these unwanted materials from the drinking water, Voerde-Löhnen installed a state-of-the-art membrane filtration plant. The pressure drop across the membranes filters out ions and particles down to the nanometer range.
As things stand, the contaminants in the Rhine have not yet reached the water catchment area: full treatment will start as soon as the Rhine bank filtrates contaminate the extraction wells. Nobody knows when this will happen because the advance of the Rhine water cannot be predicted with any accuracy. To avoid any risk, the municipal works has set up measuring stations close to the Rhine that will warn the operators when the Rhine water penetrates the groundwater aquifers (underground layers that carry groundwater).
Siemens Industry Solutions was awarded the contract to implement the automation and electrical engineering by Voerde-Löhnen water works, including the nanofiltration. Siemens also renewed the entire power supply and distribution and installed a backup network to provide maximum protection against failure. Medium voltage switchgear and a low voltage main and secondary distribution centre were also commissioned.
Better safe than sorry
The municipal works decided to modernise and expand the existing process control technology for the entire plant using the Simatic PCS 7 process control system. The redundantly designed system ensures totally integrated operator control and a uniform visualisation of the entire plant. The control technology was designed so that the control centre can monitor and control individually the important plant parts, such as the pressure intensifier system, water storage tanks and clean water pumps, using an Ethernet communication backbone. The special challenge: all the work had to be performed while the water works remained fully operational because the water supply had to remain available.
Diagnostics made easy
To pressure the raw water into the filter blocks, an average operating pressure of 8,2 bar is set, and to achieve this, eleven 150 kW conveyor pumps were installed.
Solids can block or damage the membranes of the filter blocks. To remove sand and particles from the raw water, two pre-filters are installed in series before each nanofiltration block. The first filter contains filter elements with a 5 μm mesh aperture and the second pre-filter has a mesh aperture of 1 μm. The performance of the filters is monitored by SitransI P300 relative pressure transmitters before and after the pre-filters. If either of the filters is contaminated the differential pressure increases considerably, allowing easy detection of blockages.
Trust is good, control is better
The plant consists of 11 identically designed filter blocks; each has two filter stages. The first filter stage contains 10 and the second stage five pressure pipes. 60% of the raw water leaves the filter block as cleaned permeate and 40% as concentrate containing the filtered materials. This concentrate is cleaned again in the second filter stage. The total yield is 80%.
Each filter block is equipped with five conductivity measurements from Hach Lange installed directly in the pipe using weld-on supports. To achieve the best possible accuracy, two-pole contacting electrodes were selected. For the lower conductivities (permeate), a sensor made of stainless steel with a cell constant K=0,1. The sensor for the higher measured values (raw water and concentrate) is made of graphite and has a cell constant of K=1,0. Temperature compensation is performed automatically by the integrated temperature sensor Pt100.
Measuring the conductivity allows the state of the raw water to be monitored. A typical measured value would be 840 ìS/cm. The measurement in the permeate outflow following the first filter stage is used to monitor the filtering performance of the first membrane. The conductivity measurement is used to detect a break in the membrane following which the measured value increases tenfold or more. A sampling point allows the defective pressure pipe to be identified.
The concentrate from the first filter is cleaned again in the second stage and monitored by a conductivity measurement in the permeate outflow. Finally, the two permeates are mixed and re-measured. The concentrate from the second filter stage flows back into the Rhine. The display of an SC1000 controller allows all five measured conductivity values of a filter block to be displayed simultaneously.
Measuring technology with intelligence
An ideal conveyor pressure for the filter membrane has a significant effect on the control of the process. If the pressure is too low, the filter performance is reduced, if it is too high, energy consumption is increased unnecessarily. To achieve high measurement accuracy, relative pressure transmitters of the type Sitrans P300 are used, these are outstanding for their extremely low characteristic curve deviation (≤0,075 %) and low long-term drift of (≤0,25 % in five years.
Two Sitrans P300 transmitters monitor the conveyor pressure in the inflow of stage 1 and 2 of each filter block. If there is a membrane break, there is a jump not only in the conductivity but also in the pressure after the filters. For this reason, additional pressure measurements are installed in the permeate outflow and the collected concentrate outflow. The measuring transmitter has comprehensive diagnostics functions that inform the control system of any problems.
An additional aim of the monitoring is to detect blockage of the membranes by measuring the difference in pressure between the individual stages.
If it becomes necessary to clean the membranes, the cleaning pump injects cleaning chemicals to circulate in the filter blocks. The deposits flushed out of the membrane filters are removed from the cleaning solution via an intermediate filter. The monitoring of this ‘police filter’ is also handled by the Sitrans P300. The pressure measuring transmitter also monitors the pneumatic pressure required for controlling the pneumatic armatures.
The fill levels in the raw water, permeate and concentrate containers, and in the pump shafts, are measured by the hydrostatic pressure transmitters of the type Sitrans P MPS. This simple-to-use pressure transmitter consists of a piezo resistance sensor flush mounted on the front with a housing and measuring membranes of stainless steel.
Successful overall completion
In the nanofiltration plant in the Voerde-Löhnen water works, tried and tested measurement techniques such as pressure measurements from Siemens and conductivity measurements from Hach Lange ensure reliable plant operation.
Up to now, the customer is completely satisfied with the operation of the plant. The use of modern control technology opens up numerous new possibilities: when the water works are unmanned (for example during the night), disruptions are signalled and standby personnel informed, employees can then connect to the plant and operate and monitor it remotely using a standby client.
For more information contact Keshin Govender, Siemens Southern Africa, +27 (0)11 652 2412, email@example.com, www.siemens.co.za