Brandon Bell, product manager for Fisher rotary flow control products, recently published an article in the March 2021 issue of Oil & Gas Engineering. The article described a new rotary valve anti-cavitation trim, and a recent refinery application where the trim solved significant operations problems. The article is titled Next-gen Trim Cuts Rotary Valve Cavitation and is summarized below.
Rotary-style control valves are common across many industries because they’re relatively inexpensive for a given line size and have high flow capacities. However, the low recovery factor of these designs makes them prone to cavitation. Brandon describes the problem:
Cavitation poses serious problems for control valves. .As a liquid passes through the valve restriction, increased velocity creates a low-pressure zone that effectively boils the liquid and creates vapor bubbles in the flow stream. As the fluid passes beyond the trim and velocity slows, the pressure returns and the vapor bubbles collapse creating shockwaves that can severely damage valve components and piping.
Equipment damage is amplified if the process fluid contains erosive particles, as the following case study illustrates. Brandon continues:
A refinery in Louisiana encountered a situation that constituted a worst-case combination of cavitation and erosion. The application involved a control valve passing large quantities of river water laden with fine silt. The flow rates and line size demanded a rotary valve, and the pressure drop across the valve was high. The valve typically exhibited high cavitation noise and vibration.
The cavitating river water with entrained silt exited the 10” rotary globe valve and impinged on the wall of the downstream piping, eventually causing it to breach (Figure 1).
Figure 1: Downstream piping fails due to extreme cavitation and the erosive effects of river silt. Note the river water spraying out of the pipe at the bottom left of the photo. Courtesy: Emerson
To avoid costly downtime, the control valve was proactively replaced every 24 months, and the downstream piping was replaced every 6 to 12 months. This situation continued for years.
Historically, anti-cavitation trim options for rotary valves have been limited, with most manufacturers only offering some type of attenuator integrated into the ball of the valve. These devices are at best partially successful for reducing cavitation, providing varying effectiveness at differing degrees of rotation, and tending to divert the process fluid toward the valve body and piping, damaging those areas.
Additive manufacturing options
Fortunately, recent advances in additive manufacturing enabled improvements in anti-cavitation trim designs. This manufacturing technique enables specialized trim designs that would have been impossible or prohibitively expensive just a few years ago. One of these new designs is the Cavitrol Hex anti-cavitation trim offered for the Fisher Vee-ball rotary control valve (Figure 2).
Figure 2: Anti-cavitation trim embodies a design enabled by additive manufacturing. The trim can be retrofitted to existing valves and works well at any throttling position. Flow enters the above diagram from the left. Courtesy: Emerson
Unlike previous designs, this rotary valve anti-cavitation trim is inserted into the valve from the downstream side and is not connected to the rotating ball in any way. This allows the trim to perform as desired regardless of the degree valve opening, and it can be retrofitted into any existing Fisher Vee-ball valve.
The specialized flow channels reduce cavitation within the valve and direct the process fluid straight downstream, eliminating wall impingement and pipe damage.
Successful river trial
At the previously mentioned refinery, the existing rotary valve was replaced with a Fisher Vee-Ball valve with Cavitrol Hex anti-cavitation trim. Cavitation noise and vibration abatement was immediately apparent. After a year in service, the valve was pulled and inspected (Figure 3).
Figure 3: After a year in service there was no sign of downstream piping damage. Courtesy: Emerson
No damage to the downstream piping was detected, and damage to the valve was limited to erosion of the lower section of the anti-cavitation trim face. After the initial inspection, Emerson offered an upgraded trim composed of higher hardness R31233 alloy. The valve was returned to service for another year.
The next annual inspection found small rocks trapped in the trim, but there was no damage to the downstream piping or the valve body, and minimal erosion damage to the Cavitrol Hex trim (Figure 4).
Figure 4: The anti-cavitation trim was replaced with a higher hardness R31233 alloy and returned to service for another year. The next inspection found a few small rocks stuck in the trim, but no damage to downstream piping, no damage to the valve body, and greatly reduced erosion damage of the Hex trim. Courtesy: Emerson
The valve remains in service to this day, and the refinery remains extremely pleased with the valve’s dramatically improved performance.
Author Bio: Brandon Bell is a product marketing manager for Fisher rotary flow control products based in Marshalltown, Iowa. He has been with Emerson for over 20 years with prior experience in the areas of test & evaluation, product engineering, and new product development. Brandon is a prolific inventor with nine US patents and has a Bachelor of Science in Mechanical Engineering from Kansas State University.
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