GESTRA BA 46 - Determining the Flowrate

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A = (Q × S) / (K – S)
whereby:
A = quantity of boiler water to be drained [kg/h]
Q = boiler capacity [kg/h]
S = feedwater conductivity [㎲/cm]
K = admissible boiler water conductivity [㎲/cm]
Example
Differential pressure: 15 bar with nominal size
DN 20
Boiler capacity Q = 10,000 kg/h
Conductivity S = 100 ㎲/cm
Admissible conductivity K = 3,000 ㎲/cm
Quantity of boiler water to be drained
A = approx. 345 kg/h
The required blowdown rate A1 is the quantity of
boiler water to be drained A minus 10 % for
intermittent blowdown.
Blowdown rate A1 = 310 kg/h
The required control lever position can be seen in
the relevant flow diagram, with value A1.
 Use the flow diagram with which the required
blowdown rate is achieved at the named
differential pressure.
In the example shown here, you would use the flow
diagram for DN 15 to DN 32 with a capacity range
up to 1,020 kg/h.
 Determine the intersection of the curve for the
given differential pressure with the required
flowrate.
 Drop a perpendicular onto the axis of the control
lever position.
In this example, the required control lever position
is 41 %.
Determining the flowrate
The possible flowrate as a function of the
differential pressure is presented in the following
diagrams.
To improve clarity, the flow diagrams for DN 15 to
DN 32 and those for DN 40 and DN 50 are each
shown separately over three pages.
The abbreviations in the diagrams have the
following meanings:
A Flowrate [kg/h]
B
Position of control lever (dial position)
C
Differential pressure [bar]