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Flow Meter

How to select a Vortex Flow meter ?

Vortex Flow meter can be used to measue liquid , gas or steam, it is widely used ,we also need to pay attention to below factors when selecting a proper type vortex flow meter.

Precautions before Selection

1.Measuring Range

Vortex flow transducer cannot be used to measure fluid with low Reynolds’ Number (Re≤2×10). Since Strouhal Number changes with low Reynolds Number, Meter accuracy will become less, and high viscosity fluid will affect or even prevent eddies generated. Therefore, should select product according to flowrate range in this catalog. For stable operation, flowrate would be better at 30 ~ 70% of vortex flow transducer measuring span.

2.High Temperature Type

When fluid temperature ≥250℃,must use high temperature type of vortex flow transducer.

3.Max. Temperature for Inserted Type Vortex Flow meter

Inserted Vortex transducer is just used at location with fluid temperature≤120℃.

Temperature & Pressure Compensation

Normally, Vortex Flow meter does not need compensation when measure liquid volumetric flow. When measure mass flow of overheated steam and volumetric flow of gas, Vortex Flow meter needs temperature & pressure compensation, and pressure (or temperature) compensation for mass flow of saturated steam.

4.Dirty of Fluid

Vortex flow transducer is applicable to a wide range of fluid, however, should pay special attention to dirty of fluid. Because fluid with solid particles eroding bluff body will cause noise and wear off bluff body. Short fiber intertwining bluff body will change meter factor.

Vortex flow transducer can be used to measure gas-liquid two phase fluid mixed with little dispersive even bubbles and volumetric gas content ≤2%; also can be used to sporadic, even solid particles, but volumetric solid content should be less than 2%.

Measuring range of Vortex Flow Transducer:

When measure liquid, upper limit is affected by pressure loss and cavitation erosion, velocity is 0.7~5.5m/s;

When measure gas, upper limit is affected by fluid compressibility, down limit by Reynolds’ Number

and sensitivity of transducer, velocity is 7~55m/s (7~50m/s for DN≥200).

Selection modes

It is very important to select vortex flow meter nominal diameter and specification. Selection should abide by certain principles. Nominal diameter selection should accord to the following three points to ensure flow transducer keep good working status:

(1) When select flow transducer, nominal diameter should converge properly to get bigger velocity and proper flowrate span;

(2) When measure liquid, should confirm that for smaller nominal diameter, process pipe should have enough back pressure to avoid causing cavitation. When measure liquid, gas and saturated steam, pressure loss caused by vortex flow transducer should affect production process least.

(3) For convenient selection, can select according to simple calculation and checkup relevant parameter table in this catalog.

Nominal diameter selection of vortex flow transducer is as below:

Firstly, should make sure the following process parameters:

(1) Fluid name, ingredients

(2) Max., normal and min. flowrate under operating condition

(3) Max., normal and min. pressure and temperature under operating condition

(4) Viscosity of measured fluid under operating condition

Nominal diameter selection for liquid flowrate

Select proper nominal diameter according to max., normal, min. flowrate under operating condition and calculate the down limit Reynolds’ Number when fluid flows to ensure Reynolds’ Number ≥2×104; meanwhile, calculate the pressure loss and back pressure.

Nominal diameter selection for gas flowrate

Select proper nominal diameter according to max., normal, min. flowrate under operating condition, and calculate the down limit Reynolds’ Number when fluid flows to ensure Reynolds’ Number ≥2×104; meanwhile, calculate the pressure loss.

Because the output signal of vortex flow transducer is proportional to volumetric flowrate under operating condition, if volumetric flowrate under standard state or mass flowrate is known, should change it into volumetric flowrate under operating condition.

Thereinto: qv-----Volumetric flowarte under operating condition(m3/h)

qn-----Volumetric flowrate under standard condition(m3/h)

P------Absolute pressure under operating condition(Pa)

Pn-----Absolute pressure under standard condition(Pa)

T------Thermodynamic temperature under operating condition(K)

Tn------Thermodynamic temperature under standard condition(K)

Fluid Density р and Volumetric Flowrate qv under Operating Condition

Thereinto: ρ-----Flow density under operating condition(kg/m3)

ρn-----Flow density under standard condition(kg/m3)

Other symbols are the same as above.

Thereinto: qv-----Volumetric flowrate under operating condition(m3/h)

Qm-----Mass flowrate under operating condition(m3/h)

2. Calculation of Reynolds’ Number, Pressure Loss and Back Pressure

When select nominal diameter of vortex flow transducer, should check up Reynolds’ Number for down limit of flowrate, which should meets two conditions: smallest Reynolds’ Number should be no less than down limit of Reynolds’ Number; For stress type vortex flow transducer, vortex strength of down limit flowrate should be no less than design value (relationship between vortex strength is proportional to lift force, depends on the design of bluff body). When measure liquid, should check up whether min. operating pressure is higher than saturated steam pressure under operating temperature, that is to say, whether cause cavitation.

Reynolds’ Number: It is a dimensionless number of fluid of inertia force to viscosity force.

Thereinto: Q-----Volumetric flowrate (m3/h)

D-----Inside diameter(mm)

υ------ Kinematic viscosity (m/s)2

Pressure Loss: △P

Thereinto: △P -----Pressure loss (kg/cm2)

ρ-----Fluid density

V------ Velocity(m/s)

Back Pressure: Pipe Pressure of Flow Transducer Downstream

Thereinto: P -----Min pipe pressure at flow transducer downstream 3.5~7.5D (kg/cm2abs)

P0-----Saturated steam pressure of liquid(kg/m2abs )

△P -----Pressure loss (kg/cm2)

3. Selection of Pressure & Temperature Resistance

Pressure resistance of transducer is selected according to 1.2 times or more of Max. operating pressure under process condition;

Stress type vortex flow transducer adopts piezo crystal as detecting element. Because limited by temperature, -40~250℃ is standard type (-40~120℃ for inserted type), 100~350℃ is for high temperature type.

4. Selection of Protection Class

Select according to process protection requirement on site.

5. Selection of Explosion Proof

Select according to process explosion proof requirement on site. There are three types of explosion proof: no, flame proof, intrinsical safe to select.

6. Selection of Output Signal

Can select 4~20mA, pulse, RS-485, Hart, ModBus according to measuring and control requirement.

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