I am trying to model the steam consumption of different unit operations in the chemical plant; as a verification, I plan to make the steam consumption measurements for selected unit operations; I want to install the steam flowmeter into the pipeline, but since this is pretty tedious and risky task, I am thinking about calculating the steam flow from the control valve-opening percentage values, which are accesible from the main control system (knowing steam pressure, pipe diameter and estimating the Cv of the valve)
does anybody have practical experience with this? some quantitative information (delta measurement-estimation) would be of great help.

What you are suggesting is technically possible. In practice it won't be a very accurate measure, but is will be fairly reproducible unless there is a problem at the valve (erosion, partial plugged, wet steam, etc). I have a full open valve now only passing 8ton/hr instead of the expected 18ton/hr- without steam measurement I would struggle to know that something was wrong with the valve station.

I see you left downsteam (chest or destination) pressure out of your list. Perhaps in many cases your control valve is running with choked flow, but you can see the calculational challenge of this discontinuity in the flow relation.

As an alternative to measuring the steam flow directly consider an energy balance using data on the process side. Also some cases maybe you can measure condensate flow.

It >is< "Possible" to measure flow with a control valve.  THe Valtek Starpak does exactly this by measuring P1, P2, and T all inside the valve body,  Each Individual valve plug is calibrated for Cv vs travel and that curve is loaded into the electronics, along with the sizing program. Thus it is possible to read and control flow all in one component.   It works well but it is not cheap.  

You might accomplish your goal with an energy balance, possibly of the process fluid instead of the steam.  If you know the flowrate and the Delta-T, of your process fluid, you can look up the specific heat and calculate BTU per hour. Divide by the steam difference in energy at the statepoints entering and leaving the HX (It'll be close to 1000 BTU/lb) and there's your steam flow rate.  If the HX is uninsulated, add abot 15% to the steam flow needed. 

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