KROHNE VFM 5090 EN - Manual (Page 60)

Range limit Calculation for Gases
To obtain the operating density r
x
at the operating temperature and pressure the following equation is used :
r
x
= r
n
´
Pn
Px
´
Tx
Tn
where rn, rx = density of the gas at normal and operating conditions respectively.
Pn, Px = pressure of the gas in absolute units at normal and operating conditions respectively.
Tn, Tx = temperature of the gas in Kelvin at normal and operating conditions respectively.
To obtain operating volumetric flow from normalized volumetric flow and vice-versa, following equation is used.
Qn = Qx ´
Pn
Px
´
Tx
Tn
Where : Qn and Qx are the normalized and operating volumetric flow rates respectively. The ratio of
compressibility factors is assumed to be equal to 1.
The operating data of the process medium must be known in order to calculate the measuring range.
Density rx of medium at flowing conditions, in kg/m 3 ·
·
·
·
·
Dynamic (absolute) viscosity of medium at flowing condition, h in mPA *s (or in centipoise)
Maximum volumetric flowrate Q
max
in m 3 /hr.
Minimum volumetric flowrate Q
min
in m 3 /hr.
Refer to page 64,65 & 66 for dimension of meter inside diameter.
The following limits apply to operation
Reynolds number at min flow, Re > 20 000 for linear measurements. ·
·
·
Minimum detectable flow velocity Vmin (See page 59 Diagram III)
Maximum flow velocity Vmax = 135.7306 /
Ö r
op
…………for sizes DN25 to 200( 1” to 8”)
= 83.87146 / Ö r
op
………...for sizes DN10S to 20( 3/8”S to 3/4”)
In both the cases Vmax is clamped at 75 m/s. For densities at & above 50 Kg/m3, Vmin is fixed to 0.94192 m/s
for all sizes.
calculating of Reynolds number
Re = 353.67 * Qmin(m 3 /h) * rx(kg/m 3 )
f(mm) * h(mPa*s)
Calculating of minimum flow
Qmin(m
3 /hr) = 1 * Vmin(m/s) * f 2 (mm) 2
353.67
Calculating of maximum flow
Qmax (m
3 /hr) = 1 * Vmax(m/s) * f 2 (mm) 2
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.
56