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cpp SPECIAL Rotary gate valve with pivoted discs Controls even the smallest amounts Exact control of low flow rates is a special challenge for the design and the production accuracy of control valves. The 8C/6H micro-flow control valve from Arca Regler consists of two discs with lapped surfaces. This system of pivoted discs guarantees zero valve leakage without any limitation due to manufacturing tolerances or different thermal expansions. The discs can be made of any material and are therefore suitable for critical fluids and high differential pressures. Pictures: Arca Regler The class of so-called micro-flow valves covers the rated flow coefficient range from 1 m 3 /h down to 10 -5 m 3 /h. . As is generally known, the rated flow coefficient K Vs indicates the flow rate of water at 1 bar differential pressure when the valve is in the fully open position. This definition always assumes turbulent flow in the valve. Under the above-mentioned conditions, therefore, a control valve with a K Vs of 0.01 m 3 /h has a flow rate of 10 l/h (of water) when fully opened. Besides the K Vs value there are two other significant parameters for evaluating The 8C/6H rotary gate valve fulfils even the highest leak tightness demands the performance of control valves. The first is the so-called rangeability, which is the ratio of the rated flow coefficient K Vs to the lowest controllable flow rate K Vmin and thus a characteristic number for the valve’s control range. If the rangeability is 25:1, the control valve with a K Vs of 0.01 m 3 /h referred to here will be able to control a minimum flow of 0.4 l/h of water. The second parameter is the valve characteristic, which specifies the K V value as a function of the valve stroke (or the opening angle in the case of quarter-turn valves). The gain of the control loop should preferably be as constant as possible, in other words the response (e. g. the flow change) to a reference variable (e. g. the input signal of the positioner) must be largely independent of the respective valve opening. In addition to the basic characteristics (linear and equal percentage) described by international stand ards, inherent valve-specific characteristics are also tolerable and widely used. Control performance Globe-style micro-flow valves are normally manufactured in nominal sizes from DN 6 to 25 with a parabolic plug diameter of 3 mm or less. Alternatively, a V-port plug (a cylindrical plug with an engraved V notch) can be used. The rangeability, and hence the control performance, is mainly determined by the tolerance between the plug and the seat ring. Assuming the valve is fully open, a free flow area of 0.28 mm 2 is required for a flow rate coefficient K Vs of 0.01 m 3 /h and a specified rangeability of 25:1 (based on the empirical formula where a flow rate coefficient K Vs of 1 m 3 /h meets a free cross section of 28 mm 2 ); however, the figure for K Vmin is only 0.011 mm 2 . With a parabolic or V-port plug with a seat diameter of 3 mm, the latter value results in a maximum tolerance of only 3 μm between the valve plug and the seat ring. Apart from the challenge of producing such high-precision parts, it is a fact that this kind of trim can only be used for clean process fluids and, due to the thermal expansion of the plug and the seat ring, only within a restricted temperature range. 24 cpp 03-2018
Control valve with pivoted discs In view of the favourable fluidic geometry of V-port plugs, the 8C/6H trim from Arca Regler (patent pending) consists of two discs with lapped surfaces, which are pressed together by a spring as well as by the differential pressure. The bottom disc is fixed in the valve housing while the top disc is connected to the shaft and rotated by the actuator in the range from 0° (valve closed) to 60° (valve fully open). Both discs have one bore for the flow; however, the (fixed) bottom disc has an additional radial flow channel, the profile of which increases towards the bore (similar to a V-port plug). Depending on the position of the rotating top disc, a specific cross section of the V-notch is released and determines the flow rate through the valve. This system of pivoted discs guarantees zero valve leakage without any limitation due to manufacturing tolerances or different thermal expansions. The discs can be made of any material (even ceramic) and are therefore suitable for critical fluids and high differential pressures. Precise information about the process fluid and its chemical and physical properties is necessary in order to select the right material, because the discs slide against each other at fairly high pres - sure and in combination with the process fluid build up a tribological system. The inherent characteristic of the rotary gate valve is between linear and ideal equal-percentage Control functions of the rotary gate valve (represented by a replica made of acrylic glass) Triangular flow channel One further benefit, especially with polluted fluids, results from the flow channel cross section, which due to the tools that are used corresponds to an equilateral triangle along the entire length. The maximum particle size passing via this flow channel is roughly equivalent to the inside diameter of the triangle, which for a K Vs of 0.01 m 3 /h or a free area of 0.28 mm² is 0.46 mm. If the same free flow area were to be realised using a parabolic plug with a seat diameter of 3 mm, only particles less than 30 μm in size would be able to pass the throttling element. The flow channel defines the inherent characteristic of the rotary gate valve and is between a linear and an ideal equal-percentage characteristic. The new design is thus also suitable for applications where the pressure drop decreases rapidly as the flow rate increases. Possible applications The first practical application for a rotary gate valve with the above-mentioned characteristics is gland steam conditioning for the low-pressure side of a steam turbine. Hot steam is cooled down for this purpose from 500 to 300 °C by means of an injection nozzle with a downstream swirl unit. The rotary gate valve controls the cooling water flow to the injection nozzle. In addition to the high quantity requirements (the maximum flow rate is more than 90 kg/h for the required minimum flow capacity of 5 kg/h), the differential pressure at the valve varies between 5 and 15 bar as a result of the injection nozzle characteristic. A K V value of 0.034 m 3 /h was calculated for the maximum flow capacity and 0.0008 m 3 /h for the minimum flow capacity; 0.04 m³/h was chosen as the K Vs value. A control ratio of 50:1 was therefore required. This value was slightly exceeded under real operating conditions on the test bench. To ensure a long service life, the valve trim is made completely of hardened stainless steel, because cavitation occurs with the stipulated combination of cooling water pressure and temperature at differential pressures of more than 10 bar. Other upcoming applications comprise modified valves for metering high-pressure hydrogen (with traces of catalysts) in a petrochemical plant. This project can only be realised because the rotary gate valve features a standard series 8C or 6H housing, even though it is actually a quarter-turn valve. Field retrofits for existing valves are not a problem, in other words. www.cpp-net.com Online search: cpp0318arca Hall 4, Booth A2 AUTHOR: LOTHAR GRUTESEN Technical Director, Arca Regler cpp 03-2018 25
