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Anti-cavitation Trims Require Attention to Minimum Operating Point
Most anti-cavitation trims are cage style and utilise either drilled holes in the cage wall or a series of flow passages with right angle turns. Both these technologies are designed to provide pressure staging.
	Incidentally, they require a certain minimum number of holes or flow passages to be open in order to realize anti-cavitation benefits. If the valve is allowed to throttle below this minimum operating point, pressure staging will not take place. Instead, the entire pressure drop will be taken directly across the seating surfaces of the valve plug and seat ring. This is detrimental to the plug and seat ring as localised cavitation and erosion damage will occur. At the same time, this affects the tight shut-off capability of the valve. All control valves have a minimum operating point, which is usually 10% opening (as a general rule of thumb). However, for valves with anti-cavitation trims, the minimum operating point varies with trim design and is specified exclusively for each cage design.
It is important for both control valve selection engineers and users to pay attention to this key aspect during valve configuration and operation. One of the ways is to use the Fisher FIELDVUE® Digital Valve Controller as the valve stem positioning device. When the ‘Low Travel Cutoff’ feature in the FIELDVUE® DVC is enabled, it prevents the valve from throttling below the recommended minimum operating point.
For additional information, contact a local Emerson/Fisher sales office. About Emerson Process Management Emerson Process Management (www.emersonprocess.com), an Emerson business, is a leader in helping businesses automate their production, processing and distribution in the chemical, oil and gas, refining, pulp and paper, power, food and beverage, pharmaceutical and other industries. A division of Emerson, Fisher Controls International LLC (www.emersonprocess.com/fisher) is a global leader in the design, manufacture and application of final control systems. Fisher’s technology and innovation in process control play a key role in Emerson’s combining of superior products and technology with industry-specific engineering, consulting, project management and maintenance services. Emerson brands include: PlantWeb®; Fisher®; Micro Motion®; Rosemount®; DeltaV^TM;Ovation®; and AMS About Emerson St. Louis-based Emerson (www.gotoemerson.com) is a global leader in bringing technology and engineering together to provide innovative solutions to customers in process control; electronics and telecommunications; industrial automation; heating, ventilating and air conditioning; and appliance and tools. Sales in fiscal 2002 were $13.8 billion. # # # Fisher, PlantWeb, Rosemount, Micro Motion and DeltaV are marks of one of the Emerson Process Management family of companies. Other marks are the property of their respective owners.

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Last Updated 1/11/2006