Boiler feedwater valves
March 2013
Valves, Actuators & Pump Control
As with many valves on a boiler, feedwater valves experience very different conditions during start-up compared with the normal running condition. Not only are flow rates lower, but the pressure drop is usually much greater. This results in cavitation and flashing 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 to be used only during start-up.
PSV Mitech has developed a control valve with a special trim to handle both conditions. At the bottom of the retainer there are a series of restrictions that eliminate cavitation by gradually dropping the pressure. This is 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. This type of trim gives turndown ratios of up to 200:1. The valves are 50, 80 and 100 mm in size and rated to ANSI 2500#.
For more information contact Greg Walker, PSV Mitech, +27 (0)11 657 6000, gregoryw@mitech.co.za, www.mitech.co.za
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