Microsoft Word Course Control valves R. doc

Upstream Pressure Increase

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Control Valves Basics - Sizing & Selection
FMD3x00 98 DB Initialize (5), configuration-and-evaluation-software-pi-9045582-en-gb, configuration-and-evaluation-software-pi-9045582-en-gb, Document, colour present

Upstream Pressure Increase
If a control valve is sized incorrectly and is too small, fluid velocity upstream of the valve will be slow, causing an increase in pressure (much like when the end of a garden hose is partially obstructed. This increase in upstream pressure can be detrimental to certain processes, i.e. membrane filtration processes, where a large pressure difference across the membrane is desired. If the valve increases the pressure on the permeate side of the membrane, the driving force for separation will be reduced.
Hysteresis
Hysteresis, regarding control valves, is the principle that a control valve is dependent on the prior valve position. A common valve opening may correlate to different flow rates depending on if the valve was opened or closed to get to that position. This shows that the direction a valve is being changed may need to be accounted for in a

control system to obtain a desired flow rate. If hysteresis becomes too large, it can cause the control architecture to force a system to oscillate around a desired point.

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