Apollo Tyres plant located in Durban, South Africa, for a long time had problems with controlling of steam supply to moulding press chambers used for production of truck tyres.
The root of the problem was in the need for fast chambers filling with steam when any excess pressure resulted in goods defects. Steam supply pressure was 17 bars and the maximum output pressure might not exceed 0.5 bar. Steam leaked from the chamber through a special hole of a defined size.
The old system comprised by several control ball valves was too bulky and did not ensure the required control accuracy.
Schubert & Salzer gate valves were selected as an excellent replacement for the existing valves. The controlling element of a gate valve consists of two discs positioned perpendicularly to the flow with equally located cuts and a gland between them. One of the discs is fixed and the second one moves in parallel to the first one. Therefore, by changing the discs position (and, accordingly, position of the cuts) relatively to each other, the flow is controlled. Because of the difference between the input and output pressure, the discs are pressed to each other and run in improving the sealing. Valves of this type ensure that leakage does not exceed 0.0001% of Kv value.
The secret of success of using valves of this type in the given conditions is in their response speed and accuracy. The accuracy of Schubert & Salzer gate valves is ensured by their design and the fact that the controlling gate is positioned perpendicularly to the flow means that the requirements to the drive capacity are minimal, i. e. small-size drives can be installed without any loss in the controlling speed and quality. Another feature is small valve rod travel (less than 8 mm) and precise digital positioner.
After the calculations it was decided to install valves of 8043 series, 15 mm in diameter, with digital positioner. Due to the high throughput capacity of the valve the Kv value had to be reduced significantly. From the very beginning of the system operation with the new valves it became obvious that they vastly surpass the old ones. Ashraff Sheik, automation engineer, confirmed this by saying that “the new valves respond much faster to changes in parameters and have significantly improved the presses’ performance.”