Selecting correct specs of mass flow meters is the base for safe operation and
accuracy measurement. Follow these selecting directions, could help you right use mass flow meters. Before selecting, should know parameters needed to select: flow range (maximum flow rate, rating flow rate, common use flow rate, minimum flow rate), line size (inner diameter, outside diameter), operation pressure, permission pressure drop, the temperature of the fluid, density (specific gravity) and viscosity of fluid, permission velocity of flow, accuracy. Further should know the demands of real-world installation: such as length of cable, signal format, demand for the explosion-proof, environment, and so on.
（1） Primary select a nominal line size of mass flowmeters
User or engineer primary select a spec of mass flow meters base on flow range of
fluid and inner diameter, referring to Table 3.
Example: such as flow range: 8～20t/h, inner diameter: Φin = 50mm, viscosity of
fluid: μ = 25cP, specific gravity: S = 0.8. So primary select DN50 mass flow meters
referring to Table 3, nominal line size: 50mm, maximum flow rate: 80t/h. (if the user has known motive viscosity value mm2/s of fluid, could obtain absolute viscosity value μ with multiply by fluid density or specific gravity.)
（2） Check pressure drop
Figure 15 is curves of flow rate/pressure drop with different viscosity based on
water (μ=1) of multi-specs of flow meters. In this example, find the point of 20t/h in the abscissa of Figure 15f, draw a parallel with the ordinate through this point
intersect with the curve of μ= 25, got point Q, draw a parallel with the abscissa through the point Q intersect with the ordinate, got point P, we can read out point P pressure drop 0.08MPa.
This pressure drop called figure pressure drop, sign as
ΔPF = 0.08MPa.
（3） Calculate real pressure drop using fluid specific gravity S
Real pressure drop of fluid: ΔPR = ΔPF / S = 0.08MPa / 0.8 = 0.1MPa
Commonly, this pressure drop would meet the demand for design and
engineering. Next, check if tube pressure, temperature range meets with demands.
（4） Check velocity of flow
If there is a limit flow velocity by design and engineering, you have to
calculate flow velocity using this formula:
V = Kv × Qm / C.
Herein: maximum flow rate Qm = 20 (t/h);
Velocity parameter (see Table 1) Kv = 0.197 (h m/t s);
Ratio of density of fluid by water (or specific gravity) C = S = 0.8.
Then, got velocity: V = 20 ×0.197 / 0.8 = 4.925 m/s.
For flammability and explosive chemical fluid, maximum velocity must lower
the limit by safety and engineering, to avoid static produced.
（5）If viscosity more than 25cP, such as 500cP, then find pressure drop 0.8MPa
at the same flow rate reference to Figure 15f. If this pressure drop could not permit by design or engineering, must select a large line size of mass flow meters, such as DN80.
Then repeat above（1）（2）（3）（4）to calculate and check.
（ 6 ） After selecting the right line size of mass flow meters, should check if
measurement accuracy met the demands.
Accuracy = ±[0.2% +（zero stability /flow rate×100%）]
（7）Another two figures attached below, for convenience to calculate and select.
Figure 16 is curves of flow rate/pressure drop based on water of multi-specs of
flow meters. It is easy to check and calculate pressure drop referring to Figure 16 if
the density and viscosity of fluid close to water.
Figure 17 is the curves of pressure drop and measurement accuracy. The abscissa is
the flow rate percentage over full-scale, the ordinates are measurement accuracy and pressure drop. This figure shows the influence of accuracy and pressure drop base on the water of multi-specs flowmeters with flow rate. Because the figure is not exacted, so do not refer to this figure to check and calculate. But you could directly see the characteristics of mass flow meters. Any spec of mass flow meters has a wide flow range, but more errors at the lower limit and more pressure drop at the upper limit.
