A major North American oil extraction facility was experiencing the effects of unexpected downtime caused by the failure of isolation valves. Replacement of these failed isolation valves required shutting down half of the plant for approximately 12 hours at each occurrence. The financial impact of these unexpected shutdowns was tremendous, as each occurrence resulted in a loss of production worth hundreds of thousands of dollars.
Volatile process environment
The problem was located at the front end of two solvent recovery units. This is a volatile process environment in which two redundant hydroheater valves are used to control flow of the sand slurry to the solvent recovery vessel. Just upstream from the valves is a hydroheater where direct steam injection raises the temperature of the media, a high velocity slurry of fine sand, from 86° F to 194° F (30° C to 90° C). The combination of abrasion, turbulence and cavitation create the perfect environment for hydroheater valve failure.
Just in front of the solvent recovery units, existing 14-inch, metal-seated double block and bleed ball valves were used to isolate the critical hydroheater valves in each of the process trains. The purpose of isolation valves is to allow a safe replacement of the hydroheater valves without requiring the entire unit to be shut down and without any loss of production.
Isolation valves wearing too quickly
When a hydroheater valve requires replacement, an isolation valve can simply be closed to allow the removal of the hydroheater valve. The feed is then routed through the redundant hydroheater valve so that the entire process is not interrupted. In this particular facility, because of the aggressive nature of the media, the hydroheater valves had historically been replaced on the fly at unscheduled intervals, ranging from 12 hours to 2 months.
The engineers at this plant wanted to maximize their uptime by keeping the plant operating continuously for a full year. They wished to coordinate replacement of the isolation valves with the annual scheduled replacement of the slurry feed pipe, which required the entire unit to be shut down. Unfortunately, the isolation valves were wearing out at much shorter intervals of three to six months, making it impossible to achieve the desired yearly shutdown interval.
Designing for robustness
To address this problem, the plant’s engineers collaborated with Metso to develop a longer lasting, more robust alternative to the existing isolation valves. Based on experience gathered through testing, a special erosion-resistant coating bellows seat was provided for the original supplier’s valve design. Additionally, the new proposal called for a different design using a seat, which is spring energized and protects the area behind the seat. The coating was also extended to the entire valve, including the seat.
As a result of this new design, any wetted parts that touch the process would be protected. The facility approved the new design and began replacing all failed isolation valves with the new version. By 2008, all four of the plant’s isolation valves for the hydroheater valves featured this more robust design.
Eliminating lost production
After the valves had been in service for approximately seven months, plant operators had an opportunity to inspect the valves during an outage caused by an unrelated failure in a different part of the process. Upon inspection, it was concluded that the valves were enduring well and that they would likely last until the end of the year when they could then be changed in coordination with the planned annual replacement of the slurry feed pipe.
The new and more robust isolation valves provide numerous benefits to this facility. The ability to replace them on a planned basis reduces unscheduled labor costs. Also ownership costs are lower because the valves can be shipped to a nearby Metso Service Center for repair when needed. These factors, however, are dwarfed by the savings realized through the elimination of lost production due to unscheduled shutdowns.
Written by Sari Aronen. For additional information on the topic, please contact firstname.lastname@example.org