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A traditional valve positioner serves only the purpose of maintaining a valve an opening, that corresponds to the control signal. Digital valve controllers can also collect and analyze valve position data, valve operating characteristics and valve performance trending. They can also provide two-way digital communication to enable diagnostics of the entire valve assembly ( see http://www.metsoautomation.com/ and http://www.emersonprocess.com/fisher/products/fieldvue/dvc/index.html for details).
The potentials of smart valves are likely to be further increased and better appreciated in the next decade. Smart valve should be able to measure their own:
Smart valves should also be able to eliminate conversion errors (D/A and A/D) and should be able to update their inputs about 10 times per second. In addition, they should be provided with filters to remove errors caused by turbulence.
Valves Can Also Measure the Flow
A control valve is a variable area flow meter with a variable pressure drop. Just as the float position in a rotameter (a constant ?P flowmeter) can indicate the flow, so the stem position of a control valve can do the same, if the ?P is detected. Naturally, in order to do this, the accurate knowledge of the valve characteristics and of the process properties of the flowing fluid must be provided (click the Download Now button below for a pdf version of the figure mentioned here).
In order for the smart valves of the future to be able to accurately measure their own flow, they must be provided with sufficient intelligence to identify the sizing equation, which is applicable. Therefore, they will have to be able to detect laminar flow condition in viscous, choking condition in cavitating liquid or sonic flow in gas flow applications. Naturally, the other requirement is to be able to accurately measure the variables that are required to for solving the applicable equations. The required measurements include valve stem position, inlet, outlet and vena contracta pressures, flowing temperature, etc.
Yet, the potential advantages of such smart valves much outweigh the required investment of time and money. The savings include the elimination of both the initial cost of purchasing, installing and the energy cost of operating a separate flow sensor. An added advantage can be the increased rangeability of the flow measurement obtained from the valve. This is because the rangeability of traditional flow detectors is usually between 3:1 to 10:1, while control valves can provide rangeabilities of 25:1 to over 100:1, because of their variable area orifices.
Béla Lipták, PE, process control consultant, is also editor of the Instrument Engineers' Handbook and is seeking new co-authors for the forthcoming new edition of that multi-volume work.
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