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Valve Techbook 2017

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Page 28 of 35 | September 2017 | 27 VALVE TECHBOOK: CASE STUDIES fly valves) achieved with three "offsets": 1. The shaft is placed behind the plane of the sealing surface; 2. The shaft is placed to one side of the pipe/ valve centerline; and 3. The seat and seal cone centerlines are inclined in respect to the pipe/ valve centerline. On these premises, the seal ring (and not the disc itself, as is in the case of other butterfly valve designs) is the critical component as it represents the key flexi- ble element necessary to perform sealing against a seat that is integral to the body and overlaid with Stellite grade 21. A dynamic spiral wound gasket ensures an adequate expansion and contraction of the seal ring during opening and closing. Isolation applications For centuries, linear (multi-turn) movement valves such as gate valves have been used in process isolation based on a simple design principle (i.e., a stem pushing a wedge into a seat to achieve shutoff.) Although this design is quite safe, over the past decades there has been a surge in the use of soft- seated quarter-turn valves. Ball valves in particular have gained popularity in view of their tighter shut- off capabilities because of the use of soft sealing materials, their functional versatility and a more compact design (which is a feature common to all other quarter-turn valves). Recently, end users have come to face tighter Health, Safety and Environment (HSE) regulation. As a result, revisions to the design concept behind the idea of "positively" sectioning a line (i.e., with no admissible leakage downstream) are continuously made. This function ensures that equipment can be safely removed and maintenance/ inspection requirements can be addressed without shutting down the entire facility. Based on their inher- ent characteristics and zero-leakage shutoff capability, Vanessa TOVs have been adopted for both positive and double positive isolation set-ups in lieu of both gate and ball valves, providing significant benefits in terms of operability, performance and overall cost of ownership. On-off applications An on-off process functionality implies initiating/ blocking the flow of a medium to manage the startup, operation and shutdown of a plant. From a process functional perspective, such actions are carried out to shutdown pieces of equipment, blow off/blow down media and redistribute flow to different lines (also known as switching). Usually handled by ball valves, these functions are subject to several challenges, mainly correlated to the degree of severity of the service and the conse- quent need for valve maintenance which generates important down time costs. Ball valves are typically pressure seated valves with a cavity that requires a relief system--which is non-standard and varies from manufacturer to manufacturer. Such cavity relief requires installation on the appropriate side, depend- ing on pressure differential, to be effective. Vanessa TOVs are capable of overcoming all these issues with a low maintenance solution combined with reduced weight and footprint. Emergency shutdown applications The IEC 61508/61511 standards set up key provisions Vanessa TOVs are capable of achieving zero leakage by using a non-rubbing design across the 90° rotation capable of delivering a single, instantaneous contact between sealing elements only when a closed position is achieved. Vanessa TOVs can be adopted for both positive and double positive isolation setups in place of a gate valve for their zero-leakage capabilities and reduced footprint.

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