For a valve, zero leakage is defined as no visible leakage when tested at high pressure with water and low pressure with air according to existing international standards. Some of these seat leakage standards for control and isolation valves include ANSI/FCI 70-2/IEC 60534-4, API 598, MSS SP-61 and ISO 5208. ANSI/FCI 70-2 and IEC 60534-4. Emerson’s Vanessa triple offset valves (TOVs) were the first to provide bidirectional zero-leakage performance, creating a new industry category using a technology not previously available with other quarter turn valves.
The three offsets in these metal-to-metal, torque-seated with quarter-turn non-rubbing rotation Vanessa valves are:
- The shaft is placed behind the plane of the sealing surface to provide a continuous seat path.
- The shaft is placed to one side of the pipe/valve centerline to allow the displacement of the seal from the seat during the 90° opening.
- The seat and seal cone centerlines are inclined in respect to the pipe/valve centerline. This third offset completely eliminates rubbing.
In ethylene production plants, molecular sieves are used in the cracked-gas dehydration process.
Sergio and Marco explain this process and the challenges:
…the cracked gas is saturated with water before compression and after each intercooler stage, therefore it needs to be dried be-fore fractionation to prevent the presence of ice at low temperatures. This is typically done by chilling and by adsorption using molecular sieves. The latter process involves several operational challenges that put valves under severe strain, potentially causing damage and service disruptions.
This application is challenging to valves due to the:
…high number of open/close cycles, the extreme temperatures and the presence of particulate in the flow…
The frequent open/close requirements for these valves are because they are used:
…to perform frequent ON-OFF functions to switch one or more vessels from gas purification to adsorbent regeneration modes…
Switching valve reliability in the process is critical since:
…failure of molecular sieve can lead to significant plant downtime, product loss, potential environmental pollution and other safety issues.
Unlike conventional ball valves, TOVs such as the Vanessa Series 30,000 triple offset valve, provide a:
…solution for molecular sieving because of their design and ability to perform reliably in such extreme conditions. Using TOVs minimizes the need for maintenance while also offering significant footprint/weight reduction as compared with conventional ball valves…
Read the article for more on how the triple offset design works for molecular sieve and other challenging applications. You can also connect and interact with other chemical industry and control & isolation valve experts in the Chemical and Valves groups in the Emerson Exchange 365 community.
