Over 25 years ago, Schubert & Salzer Control Systems took a new approach when it developed the sliding gate control valve. Handy, light and accurate, the valve is based on a principle that excited Leonardo Da Vinci and even today, satisfies the most exacting requirements that can be placed on a control valve.
The alternative when the demands are high
The sliding gate valve series controls liquid, vapour and gaseous media precisely, quickly and economically. A sealing plate fixed in the body at right angles to the flow direction has a certain number of crossways slots of equal height. A rotationally fixed disc with the same arrangement of slots is moved at right angles to this, thereby changing the flow cross section. The prevailing differential pressure presses the moving disc against the fixed disc and seals it. The advantages of sliding gate valves include:
* Compact design – minimum use of space and easy installation.
* Excellent leak tightness – due to pressure from the media against the sealing disc, uses a surface instead of an annular seal.
* Outstanding positioning ratio – 40:1 to 80:1.
* Significantly reduced energy consumption – short stroke requires very little actuation energy.
* Optimal flow control – avoids cavitation problems in the valve and operates quietly by lowering turbulence.
* Minimal wear – related to the effect of the force which is applied at right angles to the direction of flow and minimised by the highly effective pairing of the materials used for the moving and fixed discs.
* Maximum differential pressures – using its unique compact design and low energy consumption, the GS valve gives accurate control of high differential pressures up to 160 bar.
Efficiency and cavitation
The outstanding feature of the sliding gate valve is the actuating force which is only about 10% of that needed to actuate a seat globe valve of the same nominal size and the same differential pressure. This permits the use of much smaller actuators even though both designs of the same nominal size have about the same flow rate. This beneficial feature stems from the fact that, in the sliding gate valve, closure is transverse to the direction of flow and not against it, as with the seat globe unit.
A high rate of flow through the narrowest cross section of a valve will lower the local pressure below the vapour pressure of the liquid. Vapour bubbles form which then break in the regions of higher pressure. When they come into contact with solid boundaries (valve body), the imploding bubbles can cause damage. In the case of a sliding gate valve, these dangerous cavitation zones are external, or more accurately, they are located about 1-2 m beyond the valve. The cavitation bubbles then collapse around the centre of the pipeline where they can cause no harm.
Sliding gate valves are significantly ‘faster’ than conventional control valves. This can be shown using the frequency response when the set-point value to an installed positioner is taken as the input and the resulting stroke amplitude as the output value. The progression of the frequency response affects the standard of control of the entire control circuit.
For more information contact Rowan Blomquist, Macsteel Fluid Control, +27 (031) 581 7803, [email protected]
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