The Pressure's Off at Secunda

Sasol recently faced a challenge when one of the valves installed at its Secunda plant began experiencing problems. The plug on one of the existing valves kept separating; this caused rotating and damaging of the plug, the seal ring, and jamming of the valve. The pin holding the stem also came loose, requiring an efficient replacement at Secunda.

Understanding why a selected valve is not performing satisfactorily is not always known and in many cases is accepted as the norm. Many attempts throughout the industry aim to resolve the problems by addressing the effects of the damage. For example, the use of harder materials when erosion and cavitation damage occur. These solutions only marginally prolong the time at which failure will take place. Problems like erratic control, noise, mechanical vibration, cavitation, erosion, and short life are best solved by controlling the cause of damage.

To overcome Secunda’s challenges, CCI provided a resolution that would address the root cause of the problem with a customized specification for the site’s steam pressure application. Sasol received two, 16-in. x 16-in. (400-mm x 400-mm) control valves with design temperatures of 570 F (300 C). Incorporating DRAG® technology, the disk stacks have pressure equalizing ring (PER) grooves built into them and each disk is made with a land on the inside diameter so that localized pressure from each disk’s outlet is equalized around the plug. This eliminates radial forces on the plug which might otherwise cause binding in under-the-plug applications and/or radial vibration and buffeting in over-the-plug applications.

The DRAG® disk stack incorporates a pressure equalizing ring on its inside diameter to ensure equal pressure acting radially on the valve plug at all times. This eliminates the vibration that could occur because of rapid plug radial movement.

Published in SOLUTIONS Summer 2002