For steam measurement leaves us below technologies to do the job

– Orifice plate flow meter

-Turbine Flow Meter(Bypass flowmeter)

-Variable Area Flow-meter

– Pitot tube flow meter

-Vortex flow meter

Each of the above flow meters has its own advantages and disadvantages. In order to ensure the performance and accuracy of the steam flow meter, the most important thing is that the flow meter should meet all working conditions.

Now we’ll make introduce and discuss their characteristics and their respective advantages and disadvantages, typical applications and installations.

Orifice flowmeter

Orifice flow meter belongs to differential pressure flow meter family, The fluid flows through a contracted section and measures the differential pressure across the section. The velocity of the fluid flowing through the orifice is proportional to the square of its pressure loss.

Orifice flowmeter, restricting flow by inserting a throttling element with a small hole in the pipe, in order to measure the differential pressure generated when the fluid flows, there is an induction hole on the upper and lower sides of the orifice, and the pressure sensing is connected to the differential pressure transmitter. Signal from the differential pressure transmitter will transfer to flow computer or with temperature & pressure signal delivery to system for temperature and pressure compensation. For horizontal pipes that measure steam, condensate may accumulate upstream. To prevent this, a small hole is required at the bottom of the orifice.

 Install:

Under different β factor recommendations straight pipeline in upstream

<0.32

<0.45 <0.55 <0.63 <0.70 <0.77

<0.84

A ball valve in upstream

18

20 23 27 32 40

49

Dual curve  in upstream

15

18 22 28 36 46

47

Curve in upstream

10

13 16 22 29 44

46

Straight pipeline required:

Required at least 5D downstream to reduce the distribution of flow velocity caused by pipe effect.

The upstream straight pipeline required is kind of difference, we should take below points into consider.

β=d(diameter of Orifice plate)/Ddiameter of pipe

 The advantages of Orifice plate flowmeter

  • Simple structure
  • High accuracy
  • Low cost

The disadvantages of Orifice plate flowmeter

  • Turndown is 4:1 or 5:1 due to velocity of the fluid flowing through the orifice is proportional to the square of its pressure loss.
  • Infected by water hammer will cause Orifice plate flowmeter deformation or even blockage the pipeline if incorrect in station.
  • Need to maintenance and replace parts in time to make sure the accuracy.
  • Required very long straight pipeline especially when pipe size is big.

The application of Orifice plate flowmeter

Flow range changes in 5:1 or 4:1;  In boiler system output or steam branch system, when part of device shut down but total flow wouldn’t change a lot.

Turbine Flow Meter

The main components consist of a multi-impeller rotor that is mounted in a direction perpendicular to the incoming flow through a freely rotatable shaft. The diameter of the rotor is smaller than the diameter of the flowmeter cavity, and the rotational speed of the rotor is proportional to the volumetric flow of the fluid.

Due to turbine flow meter including some moving parts so there few factors need to included when put into operating:

  • The temperature, pressure and viscosity of medium;
  • Fluids lubrication;
  • Change in the dimensions of the impeller;
  • Input speed segmentation and vortex effects;
  • Pressure drop after flow though the meter;

The advantages of turbine flow meter:

  • Turndown can be 10:1 if place in right site;
  • High accuracy;
  • Low cost;

The disadvantages of turbine flow meter:

  • Calibrated under a preset pressure value so steam pressure changes will bring error results, unless by density compensation;
  • Flow conditioner is essential during working;
  • If the flow fluctuates, the impeller may run at high speed or low speed, resulting in errors due to hysteresis;
  • Wet steam can break the impeller and then influence the accuracy;
  • Can’t reading the low flow;
  • Can’t working in high viscosity conditions;
  • Medium measured must be clean and Particles need <100um;

The application of turbine flow meter:

  • Superheated steam;
  • Liquid flow measurement;

 

Variable Area Flow-meter

Variable area flow meter often said rotameter. A tapered reducer with a small hole at the bottom, a float that can move freely in the fluid, when flow goes by the pipe it will influence by below factor to break down the balance of float:

  • Upward power generated by the fluid;
  • Downward gravity due to the mass of the float;

Therefore we can know the flow range by track the position of float.

The advantages of turbine flow meter:

  • Turndown 10:1;
  • Simple structure;
  • Linear output;
  • Low pressure drop;

 The disadvantages of turbine flow meter:

  • Must be installed vertically;
  • Low accuracy;
  • Influence by medium temperature and pressure a lot;

The application of turbine flowmeter: Gas flow measurement in small pipes pr laboratory;

Pitot tube flow meter

In large-diameter steam pipes, both the flowmeter itself and the installation cost are high. The cost of the pitot tube is relatively low, the flow meter itself is relatively cheap, and the installation cost is low. One flow meter can be used in multiple occasions.

In a simple structure pioto tube flow meter the static pressure and total pressure hole install in the same position sampling only a single point, due to changes in the flow distribution (flow rate distribution) within the pipe, it is critical to accurately determine the location of the sampling point. And The flow rate is also proportional to the square of the differential pressure, which limits its turndown ratio.

u1=velocity of medium;

= total pressure – static pressure;

ρ=density;

The advantages of pioto tube flow meter:

  • Low resistance to fluid;
  • Cheap price;
  • Simple structure suitable variables pipe size;

The disadvantages of pioto tube flow meter:

  • Turndown ratio is 4:1;
  • Easy to blocked in wet steam measurement;
  • The insertion position inside the pipe is very critical

The application of pioto tube flowmeter: normally used for flow indicator and can for steam measurement if turndown ratio is suitable.

Vortex flow meter

Vortex flow measurement technology is specialize to measure of flow like compressed air,steam& even fuel oil. Vortex meters are reliable & most accurate flow measurement innovation technology available & widely accepted across the industries.

Vortex flow meters measurement principle according to “Karman vortex” working theory designed for gas flow, steam or liquid’s volume flow, standard volume flow or mass flow measurement.

Operational theory:

vortex flow meter designed according to fluid oscillation principle, flow will alternately generate vortices when passing by the shedding body. The frequency of the vortices is related to flow velocity and the area of shedding body:

f=Stv/d;

f: the frequency of vortices, unit is Hz;

v: the average speed of flow, unit is m/s;

d: the width of vortex shedding body, unit is m;

St: Strouhal number, range is 0.14~0.27;

St is the function of Re value, St=f(1/Re)

when Re in 102~105  St is 0.2, in this time, f=0.2V/d;

the average speed of flow v can be calculated by the value of vortices frequency,

q=vA, A is the vortex shedding body sectional area;

then we could know the flow rate q;

The advantages of vortex flow meter:

  • Wide turndown ratio;
  • No moving parts;
  • Low pressure drop;

The disadvantages of vortex flow meter:

  • It is not advisable to generate vortices when the flow rate is low, and the flowmeter reading is low;
  • Vibration will cause fake readings;
  • Required right position to install and straight pipeline;

The application of vortex flow meter:

  • Boiler system steam flow measurement;
  • Natural gas measurement in boiler system;

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