Roof tiles are a modern, high-tech material. They are extremely resistant and even defy difficult weather conditions such as hail and storms. As a result of their material and production process, roof tiles are available in various colours and finishes and are manufactured using natural minerals, or they are cement-based.
The conveying, accurate dosing, mixing and processing of materials that are extremely abrasive such as sand, cement and iron oxide-based colour pigment, makes the utilisation of extremely precise yet robust control valves essential. Consequently, it is not only essential that these fittings ensure a clean cut-off and control the abrasive media flow; they must also offer long service lives and be cleaned in a proper and quick manner.
In order to optimise the process sequences at its German production location, one of the world‘s leading manufacturers of roof systems, Braas, has utilised a Schubert & Salzer Control Systems ball control valve for the first time. This valve with a nominal size of DN 250 has now been in operation beneath the mixer cone for over three years.
The ball sector
The decisive core piece of this control and stop valve is the ball sector. One part of the ball sector serves as a cut-off and the other part has a drill hole with a diameter that usually corresponds to approximately 80% of the valve nominal size. When closing, the shape of the opening cross-section changes from completely round to elliptical. This shape reduces the risk of the valves getting blocked in the event of small valve openings. The ball sector also seals the medium under the most difficult of operating conditions via an easily-replaceable seat ring located in the housing.
Excellent regulation characteristics
The characteristic curve of the ball sector valve is almost of an equal percentage. This shape is ideal for the majority of operating conditions, especially if the differential pressure on the valve disproportionately drops in the event of an increasing degree of opening during operation. This is in addition to an outstanding rangeability of 300:1 which also supports the aforementioned characteristics in terms of the control behaviour.
Special structural features
The ball sector is centrally mounted and, as a result, deposits on the ball surface are wiped off by the seat ring. Furthermore, no solid materials can get jammed between the seat ring and the ball sector when closing the valve. As a result of the position of the O-rings upstream of the bearing position, it is not possible for any abrasive materials to enter the bearing. The friction and, as a result, the control accuracy of the valve cannot be impaired. In order to avoid tension in the event of thermal or mechanical loads, the bearings and the shaft seal are stored in pendulum-mounted bearing bushes within the housing.
In the Braas abrasive slurry application, the ball sector valve from Schubert & Salzer was required to perform approximately 100 switches per shift and has been put under immense load in two-shift operation for the past three years. With their extraordinary robustness, the valve has made a significant contribution towards minimising maintenance and operating costs, as well as downtimes.
Reiner Wolf, head of construction at Schubert & Salzer Control Systems explains: “The ball sector valve is designed for utilisation with particularly abrasive media but, thanks to the selection of the sealing materials, we are able to address the particular requirements of our customers in an extremely individual manner. We can also equip the valves with special actuators, for instance with an ultra-high resolution of 8192 control steps at a 90° valve opening. Other extremely challenging applications for the ball sector valve include steam up to a temperature of 230°C, wastewater or abrasive suspensions of iron ore, coal, lime and fly ash.”
For more information contact Keith Hedges, Macsteel Fluid Control, +27 (0)82 575 9354, [email protected], www.macsteel.co.za
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