Output of PV vs Flow

[ChemEng84]

i'm having a problem that's driving me crazy, we have a PV that controls the pressure in a a vessel through allowing gaseous nitrogen into the vessel to increase pressure whenever needed. we can view the output of the valve on the DCS, however we can't track the flow of nitrogen into the vessel.
i need to calculate the flow of nitrogen through the valve, what i'm told is that i need to make a relation between the output and Cv, then a relation between Cv and flow. i don't have enough data to make the relations, i have 3 points for the relation between Output and Cv, ( @ 0, 100, middle value).
and just 2 points for the relation between Cv and Flow (@ 0 and middle value).
what can i do?
P.S the valve characteristic is equal percentage.

 

[JLSeagull]

One DCS supplier refers to the DCS output indication as implied valve position. Lacking a positioner on the valve there would be little chance that the relationship between the output indication and the actual valve travel could reflect the flow rate. If the control valve has a positioner adjusted with a linear proportional relationship between the input signal and output position - then you might be able to calculate the flow rate within about 10% - perhaps.

Consider adding a nitrogen flow meter. Check to see if GE Panametrics or Siemens Controlotron will rent a clampon ultrasonic meter for a two-week application trial. If successful, buy an ultrasonic meter and wire the signal to a spare pair within the existing junction box. You have to be lucky for spare pairs to exist, etc.

 

[gerhardl]

It is always more simple to give advice if more details of the application are known, rather than a purely theoretical advice.

I have an inkling you are either after the actual amount of nitrogen necessary or a check on the valve size to control the inlet and uphold necessary pressure.

Blanketing continously at low pressure? Batchwise? Higher pressure? Perhaps your problem could be soølved in a practical way rather than a perhaps uncertain calculation?

If blanketing at low pressures there are special valves adapted for this, and suppliers can advice.

 

[roydm]

If you are adding gas on a batch basis and you know the volume of the padded area you can work out the quantity of gas from the pressure increase (is it Boyle's law, I don't remember) otherwise a thermal mass flow-meter from Brooks or similar would work very well.
Like Gerhardt says, we need some more info.
Regards
Roy

 

[ChemEng84]

thanks for your replys, i'll try to add more information.
the vessel has 2 PC with 1 PV each.
one PC controls the low pressure inside the vessel by allowing nitrogen to flow in when the pressure is low while the other controls the high pressure by flaring excess gases at high pressure.
we are tracking the nitrogen consumption throughout the plant to invistigate the reason for high nitrogen consumption latly, one of the easons is that the PV controlling the low pressure of the vessel opened for a long time, so i need to know whether it did affect the nitrogen consumption in a way that's worth looking in the situition and spending time on that valve.
unfortuanatly getting a flow transmitter isn't applicable as this problem will rise again with different valves and we/none can afford to buy flow transmitters for all PV valves.
as i said , i have the output of the valve all the time, Cv and output at 5Nm3/h and completly shutoff, and Cv, output but no information on flow at 100% openned.