KROHNE VFM 1091 - Manual (Page 34)

Range limit Calculation for Gases
To obtain the operating density rx at the operating temperature and pressure the following
equation is used
r x = r n x Px x Tn
Pn Tx
r x , r n
= density of the gas at normal & operating conditions respectively.
Pn, Px = pressure of the gas in absolute units at normal operating conditions respectively.
Tn, T x = temperature of the gas in Kelvin at normal & operating conditions respectively.
To obtain operating volumetric flow from normalized volumetric flow and vice-versa the following
equation is used:
Qn = Qx x Px x Tn
Pn Tx
where: Qn & Qx are normalized and operating volumetric flow rates respectively the ratio of compressibility
factors is assumed to be equal to 1.
The following operating data of the process medium must be known in order to calculate the measuring
range
· Density r x of product at flowing conditions, in kg/m3
· Dynamic (absolute) viscosity of medium at flowing conditions, in mPA *s (or in centipoise)
· Maximum volumetric flowrate Q
max
in m3/hr.
· Minimum volumetric flowrate Q
min
in m3/hr.
· Refer to page 4 for dimension of meter inside diameter.
The following limits apply to operation
· Reynolds number at min flow, Re > 20000 for linear measurements.
· Minimum detectable flow velocity Vmin (See Page 36 Diagram IV)
· Maximum flow velocity
Vmax =135.7306/ (
r
op) ...For sizes DN25 to 200(1”to 8”)
= 83.87146/ (
r
op)... For sizes DN10S to 20(3/8”S to 3/4”)
In both the cases,Vmax is clamped at 75m/s. For densities,at and above 50kg/m3, Vmin is fixed to 0.94192 m/s for all sizes
· Calculating Reynolds number
Re = 353. 67 Q min * (m 3 /h) * (kg/m 3 )
Ø(mm)* h(mPa *s)
· Calculating maximum flow
Q (m 3 /hr) = 1 * Vmin(m/s) * Ø 2 (mm) 2
mm
353.67
· Calculating maximum flow
Q (m 3 /hr) = 1 * Vmax(m/s) * Ø 2 (mm) 2
mm
353.67
Range limit Calculation for Saturated Vapour
In the same way as for gases, use the above equations to establish the range limits for the
volume flowrate. Refer to vapour tables for the appropriate density, and calculate the volume
flowrate from the mass flowrate. Check the range limits if operating parameters change.
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