Product category:
Process Industry Pumps
News Release from: Robbins and Myers | Subject: RM6000 screen
Edited by the Engineeringtalk Editorial
Team on 22 August 2000
Efficient screens save UK water
companies money
The onset of AMP3 means that greater screening efficiencies will be required for aesthetic pollutants such as faeces, tissue paper and sanitary products, as Robbins and Myers explains
Recent independent reseach has shown that the increased use of efficient screens could save UK water companies a significant amount of money The onset of AMP3 means that greater screening efficiencies will be required for aesthetic pollutants such as faeces, tissue paper and sanitary products
This article was originally published on Engineeringtalk on 8 Mar 2001 at 8.00am (UK)
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The water industry will be expected to meet these requirements, but testing in the field and at test sites has shown that simply using a combined sewer overflow (CSO) chamber is not sufficient.
In order to quantify the efficiency of such screens, performance evaluation tests were carried out on 17 screens culminating in May 1999, by the UK Water Industry Research Group (UKWIRG), at the National CSO Test Facility in the north of England.
Two performance evaluation criteria were assessed.
Firstly, how effective the screen was at retaining total solids, and secondly how well the screen met the regulation solids retention requirement of 6mm, when measured in 2 dimensions.
Among the screens tested was the RM6000, manufactured by Robbins and Myers, the Hampshire based fluid management specialists.
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The RM6000 performed consistently during the two-day series of tests, and achieved an impressive screenings retention value of over 60%, giving it a lead of 15% over most of its rivals.
Will Hooker, general manager at Robbins and Myers, said "We are delighted that the hard work put into the design of the screen has put it ahead of the competition.
We are currently talking to a variety of different water companies which have expressed interest." The UKWIRG is supported by representatives from each of the 10 water companies of England and Wales.
At its testing facilities in the north of England where part of the site is designated for screen testing, the wastewater flow can be fully controlled through this area and can be monitored to provide variations in inlet and outlet velocities and the type of throughput and waste used.
Thus the site is ideal for testing screens.
Robbins and Myers applied to the team to have its RM6000 screen evaluated.
Upon acceptance, the company made a screen to fit the dimensions of the prefabricated test area.
CSO Screen Testing It has been shown that aesthetic particles are neutrally buoyant.
Equally, when such particles flow through a CSO they divide according to the ratio of the flow split.
Hence the requirement to incorporate a screening device into CSO chambers during the AMP3 period.
The series of tests culminating in May 1999 tested the effectiveness of 17 screens incorporated into side weir, stilling pond and hydrodynamic separator chambers, and as stand alone devices.
The screens were tested at three flow rates, 45, 60 and 100 litres/second.
The continuation flow/inflow ratios were also varied at 0.1, 0.2 and 0.4 respectively, and three tests were carried out at each point.
During all nine evaluations, the different characterisics for flow and performance were measured for each screen.
The retained solids from the screened flow were collected in a mesh sack for later examination.
Several conclusions were drawn from the tests: - The inclusion of a screening device will significantly improve the total solids retention performance of the conventional FR 0488 CSO chamber.
- The position of the screen to the weir of the chamber may have a significant influence on the spill flow/head discharge relationship.
- The performance of all chamber types can be improved by installing a screen.
- The performance improvement for aesthetic solids offered by the screen is independent of chamber size.
- Total solids retention of mesh screens is generally better than that of raked bar screens.
- Total solids retention of a mesh screen is improved as the aperture size is reduced.
- Scumboards and screens in the same CSO chamber do not always complement each other.
RM6000 Belt Screen Performance The RM6000, which has a coated and reinforced urethane belt, with 6mm perforated holes as standard, and which rotates within a stainless steel frame.
The modular RM6000 screen has an automatic reversing sequence to aid self-cleaning, which is triggered by a PLC (programmable logic controller) and current sensor.
The screenings are returned to the flow by being transported to the downstream end of the screen, passing through two doctor blades and a rotary cleaning device; or they can be removed via a separate handling device.
The RM6000 screen is available in lengths up to 3m and heights of up to 1m which make it ideal for applications such as the high-sided overflow weirs at pumping stations.
According to the test results report, the total efficiency of the screen increased as the ratio of continuation flow and inflow increased.
The total efficiency performance remained relatively constant as the magnitude of the flow increased.
This indicates that the screen system offers a consistent and predictable level of performance over a range of steady inflow conditions.
At all flow rates, no blinding of the screen was observed to occur.
Considering Parameters When designing the CSO chamber, which, together with a screen, would be able to meet the AMP3 requirements for aesthetic control, several parameters need to be considered.
The performance of the screen depends on the flow pattern and hydraulic loading to the screen, the solids load retained by the screen and the effectiveness of any screen cleaning mechanism fitted.
The key to optimum screen performance is seen to be the direction and magnitude of the approach velocity.
These are typically between 0.8 m/s and 1.6 m/s.
The RM6000 was observed to consistently outperform the other screens tested within these parameters.
The correct position of the screen within the chamber is essential to optimising the design of a CSO chamber.
Where screens are mounted vertically on the CSO weir, the head above the weir may increase.
This is due to a reduced flow area on the weir, created by the screen.
The size and position of the screen is dictated by the hydraulic loading to the screen and the effectiveness of the screen to retain solids, without detriment to the design objectives for the chamber.
The screen needs to be well fitted to make sure there are no protruding edges or gaps.
Ideally, for large CSO chambers a scumboard should be considered to retain large rogue solids and prevent their discharge to the screen.
However, if the scumboard hinders how the screened solids are returned to the continuation flow, it is recommended that the scumboard should be omitted.
Engineers need to view the findings of the screen evaluation tests along with other factors when selecting a suitable screen.
Whole life cost, solids retention, operational performance and the maintenance requirements must also be considered when making the decision.
The Robbins and Myers RM6000 belt screen meets these requirements and more.
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