Cancel Cavitation
Mark Nord, Control Valve Solution Architect for Emerson and Steve Zinda, Key Account Manager at Novaspect, recently published an article in the Summer 2022 issue of Valve. It is titled “New Technologies Solve Severe Cavitation Problems,” and it describes how an advanced anti-cavitation control valve design enabled by 3D metal printing solved a power plant’s severe cavitation issue and improved their bottom line. A summary of the article follows.
Applications with very high pressure drops often create control valve problems, requiring innovative valve designs and advanced metallurgy. These issues are even more pronounced in a power plant where magnetite (iron oxide) is often present in the boiler feedwater (BFW). Valves in these applications face cavitation damage, as well as erosion and plugging caused by the magnetite.
The BFW pump (Figure 1) supplies high pressure water to the boiler. Due to the high temperatures and head pressures of the pump, a certain amount of water flow must continuously pass through the pump to avoid damage. During startup conditions, the boiler flow requirements are very low, so a boiler feedwater recirculation valve dumps water back to the separator to maintain pump throughput.
Figure 1: A boiler feedwater recirculation valve is installed on the discharge of the boiler feedwater pump, and it opens during plant startup to maintain pump flow.
The recirculation valve is a very difficult application, often taking a 2500 PSI drop when media is flowing, and it must then seal completely once the boiler water flow is adequate.
Anti-cavitation trim to the rescue?For this type of service, valve internals typically employ an anti-cavitation trim with many tiny holes (Figure 2 – left). The series of holes limits valve damage by minimizing bubble formation in the trim, and it channels the collapsing bubbles to the center of the flow stream where they can do the least damage.
Figure 2: A typical anti-cavitation design (left) employs a number of small holes in the trim to take the pressure drop in small steps. However, in this application magnetite was plugging the holes (right), reducing capacity.
In normal applications, this works well, but in BFW applications, the magnetite tends to plate out in the trim and plug the holes (Figure 2 right). The authors describe the problem.
While cavitating conditions tend to damage valve internals, the problem is made much worse by magnetite in the feedwater, which further erodes the valve internals, and plugs the small passages inside a typical drilled hole or torturous path anti-cavitation valve trim.
As the holes plug, the flow capacity of the valve is reduced, risking damage to the high-pressure pump during low flow conditions. Particulate erosion also damages the seat, and once it is damaged, it will start leaking, which damages the seat still more. Leaking water also wastes energy and robs the boiler of water flow, reducing plant output.
The plant tried installing mesh strainers upstream of the valve, but these plugged and had to be pulled and cleaned regularly, creating a maintenance problem. Clearly, a better solution was needed.
Innovative design enabled by 3D printingAfter evaluating the process conditions, Emerson engineers recommended a recently introduced anti-cavitation control valve design called Fisher dirty service trim (DST). Unlike the standard anti-cavitation trim (Figure 3 – left), the Fisher DST trim (Figure 3 – right) can pass entrained particulates up to ¾” in size. The new design also protects the seat from the major flow path, reducing erosion and ensuring tight shut off.
Figure 3: CAV III Anti-cavitation trim (left) has a large number of very small holes. The Fisher Dirty Service Trim (right) has much larger flow paths, allowing up to ¾” particulates to pass through.
The DST design was made possible by utilizing a new method of manufacturing called 3D metal printing. Lasers fuse metal powders into shapes that could not be economically produced using standard machining methods. Mark and Steve describe the process:
Historically very intricate parts could not be made of high-hardness materials because they were too brittle. However, 3D metal printing allows parts to be created from very high-grade, high-hardness materials, regardless of the level of intricacy.
In this case, the new DST valve was manufactured using R31233 cobalt chrome, so the new internals were much harder and erosion resistant than the valve trim originally installed.
Remarkable resultsThe new valve was installed and immediately generated significant savings for the plant. The DST trim eliminated the leakage and erosion issues, generating the following financial benefits:
Needless to say, plant personnel were pleased with the result, as was company management.
Figures all courtesy of Emerson
About the Author
Mark Nord is a Control Valve Solution Architect, Global Industry Sales at Emerson’s Flow Controls Business Unit. He has a BS in Mechanical Engineering from the University of North Dakota, and over 30 years of power industry experience, including over 25 years of control valve experience across all major industries
Steve Zinda is a Key Account Manager for Novaspect, an Emerson Impact Partner. He has a BS Mechanical Engineering and an MBA, both from UW-Madison. Zinda has 30 years of power industry experience, and he has 20 years of experience providing Emerson solutions to improve plant performance and reliability.