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Controls even the smallest amounts

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.

The class of so-called micro-flow valves covers the rated flow coefficient range from 1 m3/h down to 10-5 m3/h.. As is generally known, the rated flow coefficient KVs 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 KVs of 0.01 m3/h has a flow rate of 10 l/h (of water) when fully opened. Besides the KVs value there are two other significant parameters for evaluating the performance of control valves. The first is the so-called rangeability, which is the ratio of the rated flow coefficient KVs to the lowest controllable flow rate KVmin and thus a characteristic number for the valve’s control range. If the rangeability is 25:1, the control valve with a KVs of 0.01 m3/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 KV 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 standards, 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 mm2 is required for a flow rate coefficient KVs of 0.01 m3/h and a specified rangeability of 25:1 (based on the empirical formula where a flow rate coefficient KVs of 1 m3/h meets a free cross section of 28 mm2); however, the figure for KVmin is only 0.011 mm2. 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.

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 pressure and in combination with the process fluid build up a tribological system.

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 KVs of 0.01 m3/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 KV value of 0.034 m3/h was calculated for the maximum flow capacity and 0.0008 m3/h for the minimum flow capacity; 0.04 m³/h was chosen as the KVs 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.

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Author: Lothar Grutesen

Technical Director,

Arca Regler

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