Triple-offset Valve Hits Safety, Uptime Home Run

"Triple-offset valves offer a lower-cost and lighter solution compared to ball valves. They ensure versatility and optimal torque/ tightness performance with two separate valves with distinct trims." Emerson’s Luca Vertova explained how triple-offset valves provide “double positive” isolation.Closing part of a line with double-positive isolation instead of shutting down an entire process unit or plant is a potentially huge deal. The question is: Can it be done safely and efficiently?

Luca Vertova, engineering director, for Emerson’s Vanessa technologies, reported "yes" because he knows that triple-offset valves (TOVs) can do double-positive isolation with none of the leaks or degradation that can affect single, soft-seated ball valves in high-risk and/or safety applications. TOVs typically have a non-symmetric design that maximizes their inherent safety.

Are you positive?

“Positively isolating a line means there's no admissible leakage downstream, so equipment can be safely removed, and operators and technicians can save time because their maintenance and inspections can be carried out without shutting down the entire facility," said Vertova. "Positive isolation evolved from linear, multi-turn movement valves, such as gate valves, which have been used in process isolation for centuries. Over the past few decades, there's been a surge in use of soft-seated, quarter-turn ball valves for 'positive' isolation. But environment, health and safety (EH&S) regulations have recently become more stringent, and all of these have revised the concept behind ‘positively’ isolating a line."

 Vertova presented "Achieving safer, more efficient double positive isolation at a lower cost" this week at the 2017 Emerson Global Users Exchange in Minneapolis.

To achieve positive isolation and safety, Vertova reported that:

  • Single block-and-bleed method, using one closed block valve and a vent or bleed valve, is less safe;
  • Double block-and-bleed and single-valve trim method is safer, but isolation can be compromised if the valve fails; and,
  • Double block-and-bleed with two independently driven valve trims is safest for critical operations because it establishes redundant isolation.

Choosing triple offset

To achieve its safer design, Vertova reported that TOVs use a sealing system consisting of a stationary seat and a rotating sealing surface that share an identical shape—an inclined conic section—which minimizes wear of sealing surfaces, while its cone angles don't compromise operability. Its three offsets include:

  • A shaft placed behind the plane of the sealing surface;
  • A shaft placed to one side of the pipe/valve centerline; and,
  • The seat and seal cone centerlines inclined with respect to the pipe/valve centerline.

"The geometry of a TOV is similar to a globe valve, but its characteristics are to provide long-lasting zero leakage," added Vertova. "This is accomplished by its torque seating, and the compression of it that resists high differential pressure, as well as the metal-to-metal seating of its seal ring and seat that also withstands high pressure. Again, there's no soft seal, so there there's no chance for degradation, event at very low or high temperatures."

TOVs also achieve zero leaks over the long term by employing quarter-turn rotations that minimize any type of wear or degradation. Plus, their non-rubbing rotation from 0° to 90° means there's no contact or chance for wear on their seats and surfaces. "TOVs also have a shaft side that supports seating of the valve, which further enables positive, shaft-side isolation. We also support this with a bolted-seat design in the same direction as the pressure of the material in the application.

Exercise with end user

Working in cooperation with a major oil-and-gas company, Vertova reported that Emerson recently used two TOVs in combination in a double-positive isolation experiment to study if it could be a lower-cost alternative. This combination has been officially approved by its end users.

"Double-positive isolation may not be feasible with ball valves due to costs, footprint and maintenance requirements," said Vertova. "TOVs offer a lower-cost and lighter solution compared to ball valves. They ensure versatility and optimal torque/ tightness performance with two separate valves with distinct trims. Absolute tightness performance is provided by using two independent bodies."

Both valves are installed with their shafts facing upstream to ensure double-positive isolation. The differential pressure supports the first valve's tightness. Unlike ball valves, the higher the cavity pressure is between the two TOVs, the more support is provided to the second valve seating, even in the remote case of torque application failure. "Whenever pressure comes from one side only, maybe due to leakage from the first seat, the second seat will provide sealing," added Vertova. "In fact, even in case of torque application failure and cavity-bleed valve failure, the valve cavity pressure itself will push the second seal against the seat.

"Though double positive isolation has recently become a key topic in the oil and gas industry because of its high pressures combined with explosive, potential environmental pollution and health-adverse risks, other industries may also start considering this approach to isolation."