Boiler feedwater control valves must achieve a smooth start-up and maintain the required drum level for safe, reliable and efficient plant operation. The high pressure differential at start-up or low-load and the sensitive control requirement call for a high performance severe service control valve.
Feedwater control valves, as with many other types on a boiler, experience very different conditions during start-up compared with normal running conditions. Not only are flow rates lower, but the pressure drop is usually much greater. This results in cavitation damage close to the seat, as well as poor control. One solution used in existing plants is to install a sacrificial valve in parallel to the control valve, only to be used during start-up.
Mitech has developed a control valve with a special trim to prevent cavitation damage. At the bottom of the retainer a series of restrictions eliminates cavitation by gradually dropping the pressure. This is called the ZZ design. As the plug opens, the flow passes through passageways with a smaller number of restrictions. In the top half of the retainer are single-hole restrictions which can handle the high flow, low pressure drop condition that the valve experiences in normal running mode. Typically this type of trim would give turndown ratios of up to 200:1. The valves ensure an improved performance during start-up, better control and longer life.
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