This will be used as the primary measured variable during the investigation and analysis of the vacuum system problem. The performance of filtration plants will be measured by the combined moisture content of the discharged phosphate product.
Literature review Filtration Plants
Pressure: The pressure difference measured from the supply to the other side of the filter. The filtration process's performance and efficiency are dependent on the filter medium used in the application.
Filtration Processes
The efficiency and filtration rate of a filtration process will decrease as the process progresses; this is due to the increased resistance created by the build-up of the filter cake. The filter feed is at atmospheric pressure and the remote side is either at a negative pressure (vacuum pressure) or a positive pressure depending on the filter process.
Process Importance
Horizontal Belt filter Process Description
The conveyor belt has edges on its sides that act as an air seal between the sides of the filter fabric and the conveyor belt. The liquid moving through the filter fabric and conveyor belt flows into the drainage system in the vacuum box.
Process/Operation Control Systems
Maintenance
Areas of Concern and Observations
Vacuum system
Process/Operation Description
The vacuum pumps under consideration are liquid ring displacement pumps, which displace water to achieve a vacuum inside the pump. Maximum achievable vacuum = (-) [(Atmospheric pressure at measured altitude (kPa)) x (Maximum rated vacuum pump level (kPa)) / Absolute vacuum (kPa))].
Design
The vacuum pump should be inspected for any leaks in the hull, water pipes, manifolds and valves. Barometric inspections of the seal tank should be made a high priority task if holes appear in the vacuum pump.
Thickeners
The thickener uses rakes at the bottom, which are equipped with blades that further increase the density of the slurry by compressing it. The feed rate, feed tons and pulp densities (Pd) of the process can be read from the system. The main area of concern for the thickeners is the density of the slurry being discharged.
When the pulp density of the slurry produced by the thickener is too high, the filter cloth of the horizontal belt filter will either be blinded or the filter cake will be too dense and will disable the filtration process. When the density of the pulp pulp produced by the thickener is too low, the filter cake will not be able to form on the horizontal belt filter, and the vacuum losses will be too large, as there will be no air seal, so the filtration process will not be achieved. This Pd range was set considering the performance characteristics of the filter fabric.
Quality and Process Control Tools and Techniques
Do not add too much flocculent to the thickener as this will increase the Pd of the liquid at a rate that the rakes cannot handle and the thickener will choke. Choking the thickener is an expensive mistake, as the thickener will be a shutdown that needs to be cleaned. The control charts will be used to analyze the efficiency of the moisture content of the product discharged by the plant and the pulp density at which the thickeners operate. Cards will be evaluated daily and out-of-control points will be related to activities or irregularities that may occur during these out-of-control periods.
Root cause analysis will be used in many problem identification activities, and this activity will be aided by drawing cause and effect diagrams. These meetings will be used to gather information and data that is critical to identifying problem areas and to gain insight into the process from the people who work on the process on a daily basis. The concepts of change outlined in Chapter 10 of Quality Management, 3rd Edition will be implemented to further improve the process after the major problem areas are resolved.
Investigation Design & Analysis Method
Standard operating procedures (SOP’s) at Filtration
Best practices for Horizontal belt filter operations
Observation of filtration process
Interviews
Daily observations
Information gathering and documentation
Observation of the three main filtration areas
Sample gathering and operation observations
Historic and current database product moisture content analysis
Analysis of current plant operation
Vacuum system focused investigation
Analysis of Plant Operation
Investigation of plant inspections and maintenance
Results
Standard operating procedure and best practices for the horizontal belt filter plant
The conveyor belt is responsible for draining the liquid separated from the filter cake, which drains the liquid into drainage holes in the middle of the belt and into the vacuum box below the conveyor belt. The maximum achievable vacuum of – 78 kPa should be the measurement achieved when the valve is closed to the rest of the vacuum system if the pump is in good working order. The pressure reading at the gauge on the filter manifold will show a difference of between 2% and 5% which is the result of frictional head losses in the vacuum system piping;.
The filtrate vessel is designed to remove liquid and dissolved solids so that only air moves through the rest of the vacuum system, since this function is not always achieved, two intermediate tanks with a barometric leg are installed between the filtrate vessel and the vacuum pump. and a barometric seal vessel that acts as a liquid air separator for intermediate tanks. Liquids and dissolved solids should never enter the vacuum pump through the system as this creates friction in the vacuum pump which is destructive to the vacuum pump and over time the efficiency of the vacuum pump decreases and the vacuum pump can develop over time. The vacuum pump will inevitably be replaced after a leak occurs, as it will become ineffective.
Problem identification
A value driver tree and root cause analysis were developed using information in the manuals and information gathered from observing plant operations.
Filtration moisture content trend 2008 – 2011
By June 2011, they had managed to reach 8.5% moisture content, but this was short-lived and the Filtration Department has since then performed at its worst in the last four years.
Filtration samples and MES database analysis for January 2012 and March 2012
The Pd values for January 2012 were within the prescribed range of 1.7 -1.8 for an overall percentage of 11% for new filters and 8% for old filters. The thickeners for new and old filters were rarely used in the prescribed range of 1.7 – 1.8.
Pulp densities for New & Old filters January 2012
Pulp densities for New & Old filters March 2012
The vacuum pump measurements for January 2012 focused primarily on vacuum pumps 184, 186 and 190, as filter 12, which uses vacuum pump 181, was shut down for maintenance and there was an instrument problem with the valve on vacuum pump 183. Taking care of the vacuum system New filters allows an additional vacuum pump (vacuum pump 183) to run together with vacuum pump 181 for filter 12 or vacuum pump 184 for filter 13. In the first half of December 2011 it was observed that when filter 12 was still running and pump 183 was being used on filter 12 that was very weak vacuum pressures close to – 30 kPa were achieved on all three pumps for new filters, namely vacuum pumps.
During the shutdown of filter 12, vacuum pump 183 ran together with vacuum pump 184 and vacuum pressures in the vicinity of – 70 kPa were observed on many of the inspections. The process controller was asked to turn off the back-up vacuum pump for filter 13 and the vacuum pressure reading for vacuum pump 184 dropped to the vicinity of – 55 kPa and a visible increase in moisture content of product from filter 13 could be seen on the conveyor in the form of silt sections. The loss in vacuum pressure for vacuum pump 184 once the back-up is turned off may contribute to the possibility of leaks in the vacuum system, which would have destroyed the back-up vacuum pump.
Vacuum pressures for January 2012
The difference in vacuum pressure at the vacuum pump and filter manifold for filter 13 could not be read as the filter manifold pressure gauge was painted red. The vacuum system for old filters does not provide for the use of a backup vacuum pump. The difference in vacuum between the vacuum pump and the filter manifold was close to 20 kPa, indicating large vacuum losses in the vacuum system.
Vacuum pressures for vacuum pump 190 were near -55 kPa for most of December 2011. The difference in vacuum pressures between the vacuum pump and the filter manifold was close to 5 kPa during the observation periods. The difference in vacuum at the vacuum pump and filter manifold for filter 13 could not be read because the filter manifold pressure gauge was painted red.
Vacuum pressures for March 2012
The vacuum pump readings for March 2012 showed zero readings for vacuum pump 181 which was still under repair and down. The best practices in terms of operation and maintenance of the vacuum system have been neglected. The vacuum obtained by both vacuum pump 186 and 190 worked continuously around the – 50 kPa mark.
The vacuum should be brought up to the -70 kPa range by eliminating the possible causes of vacuum loss, as shown in the root cause analysis.
Moisture content (%) for January 2012
During this period the operation of the plant was not according to the Standard Operating Procedures, as can be seen from the Thickener and Vacuum System output readings; these issues were mentioned until the end of January 2012. This 9% could have been close to the desired target of 8.5% if the product of both conveyors could have been low. The product on conveyor belt 51 could most likely be from the output of Old Filters as during the same period their thickeners and vacuum systems were operated closer to specification and the New filters' thickeners and vacuum systems were not operated within specification.
The readings from week 3, 4 and 5 of March 2012 gave a good indication of when the plant is operating according to the design specification; achieving the 8.5% target is possible.
Moisture content (%) for March 2012
Process Control Analysis
Observations
This photo shows the discharge line of the discharge vacuum box disconnected and resting on top of the moving conveyor belt. Due to the lack of control measures with regard to the moisture content and the thickness of the cake, there is a possibility that irregularities in the functioning of the plant go unnoticed.
Downstream effects of high moisture content product
Proposed Implementations
Conclusion
Appendix
