Control Valves – How It Works
Conventional globe style control valves have to strike a balance between maximum flow capability at full lift whilst maintaining stable control of low flow rates at the bottom end of the range. Achieving a high turndown ratio usually results in the control plug having to operate very close to the valve seat, which leads to wire drawing and rapid erosion wear of the trim. To maintain set point control at these small openings requires very small valve stem movements which the actuator may not be able to match. The valve begins to "hunt" and the whole control feed back loop becomes unstable.
Northvale's new control valve with the unique PARAGON high turndown ratio trim significantly improves the control characteristic at the bottom end of the flow range. This increases the valve's flexibility and reduces the number of trims required for each line size of valve. These improvements are achieved by the dual action PARAGON trim with its specially profiled plug and two stage compound seat.
Valve shut off is achieved on the lower seat. The upper controlling seat is not in contact with the plug.
Fluid entering the valve under the plug passes over the bottom seat and expands into the seat cavity before being throttled through the control annulas around the top seat. Pressure drop is taken across each seat which ensures a smooth transition between upstream and downstream conditions. This two stage feature allows the valve plug to lift well away from the bottom (closing) seat whilst remaining in close proximity to the upper controlling seat.
Beyond 50% valve lift the PARAGON trim begins to act like a conventional profiled plug valve with control being exercised solely by the top seat.