Simple solution - use feedwater flow minus you extractions (steam/water) to figure out your steam flow. It is usually more accurate.
Extractions could be - blowdown, sootblower steam, attemperation (only if a sweetwater condensor is not used)
Okay - long winded explaination:
A boiler generally operates at a constant pressure (okay a somewhat constant pressure). The boiler operating pressure will (or should) remain fairly constant at all steam loads. There is really no way to measure steam flow from boiler pressure alone. That is where the DP transducer is used. If the boiler is operating at a minimum load, you flow meter may be out of its range. If you have a low pressure boiler that is subject to swings in load, your flow measurement will be all over the place as well (due to the change in specific volume of the steam).
All that being said look at:
flow rate = C Y A [(2g rho (P1-P2)/(1-beta^4)]^(1/2)
Where
C=dimensionless, coeff. of discharge of specific device
Y=compressibility factor (usually 1.0 for most liquids and gases where pressure drop across the device is less than 20%
A=cross sectional area of the throat
g = gravity
P1=upstream pressure
P2=downstream pressure
Beta=ratio of throat to pipe diameter
rho=desity at upstream temperature and pressure (lb/ft^3)
good luck - I don't know if this clarifies anything but feel free to ask more.
