Flow Meters-1

INTRODUCTION TO FLOW METERS

A flow meter is a device used to measure the flow rate or quantity of a gas or liquid passing through a pipe.

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They are wide variety of flow meters using in the industrial applications. Some of them are,

1) Coriolis Flow Meter.

2) Vortex Flow Meter.

3) Differential Pressure Flow Meter.

4) Turbine Flow Meter.

5) Electro Magnetic Flow Meter.

6) Ultrasonic Flow Meter.

Selecting A Flow Meter:

The basis of flow meter selection is a clear understanding of the requirement of the particular application. Therefore time should be invested in fully evaluating the nature of the process fluid and of the overall installation. Here are some key questions which we need to answer before selecting the flow meter.

1) What is the fluid being measured by the flow meter?

2) Do you require rate measurement and/or totalization from the flow meter?

3) If the liquid is not water, Then what is the Viscosity of liquid?

4) Is the fluid clean?

5) What is the minimum and maximum flow rate of the flow meter?

6) What is the minimum and maximum process pressure?

7) What is the minimum and maximum process temperature?

8) Do you require a local display on the flow meter or do you need a electronic signal output?

9) Is the fluid chemically compatible with the flow meter wetted parts?

10) Is this is a process application, what is the size of pipe?

Flow meter selection guide

DIFFERENTIAL PRESSURE FLOW METER:

In differential pressure transmitter, The measurement of the flow of fluid in a pipe is based on the Bernoullis Equation. Differential pressure flow meter introduces a constriction in the pipe that creates a pressure drop across the flow. When the flow increases more pressure drop is created. The DP transmitter measures the difference between upstream pressure and downstream pressure of the constriction.

ΔP=P1-P2

  

The velocity(V), Volumetric flow(Q) and the Mass flow(W) can be calculated using the following formulas,
                                                  V = k (h/D)^0.5
                                                  Q = kA(h/D)^0.5
                                                   W = kA(hD)^0.5

Where,
A=Cross sectional area of pipe,
h=Differential pressure,
D=Density of flowing fluid,
k= Discharge coefficient of the element.

Bernoullis equation states that the pressure drop across the constriction is proportional to the square of flow rate. using this relationship, the 10% of the full scale flow produces only 1% of the full scale differential pressure. At 10% of full scale flow, the differential pressure flow meter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure.

Different geometrics are used in the differential pressure flow meter. Some of them are,
1) Orifice plate,
2) Flow Nozzle,
3) Venturi Tube, Etc

ORIFICE PLATE:
Orifice plates are the most widely used type of flow meter in the world. They offer significant cost benefits over other types of flow meter, especially in larger line sizes, and have proved to be rugged, effective and reliable over many years. Where a need exists for a rugged, cost effective flow meter which has a low installation cost and a turn down of not more than 4:1, The orifice plate continues to offer a very competitive solutions.

An orifice plate installed in a line creates a differential pressure as the fluid flows through it. This differential pressure is measured via impulse lines by a differential pressure transmitter which converts it into an Analog or Digital signal which can be processed to provide a display of the instantaneous rate of flow. The relationship between the rate of flow and the differential pressure produced is very well understood and is fully covered by comprehensive national standards. The relevant standards are BS1042 and ISO5167.

Advantages:
1) Use on Liquid, Gas & Steam.
2) Suitable for extreme temperature & pressure.
3) High pressure loss.
4) No moving parts.
5) Low material costs.
6) Inexpensive, When compared to other flow meter.

Disadvantages:
1) Limited rangeability.
2) Affected by changes in Density, Pressure & Viscosity.
3) Expensive to install.
4) Low accuracy
5) Pressure drops can be large.
6) Contraction of flow reduces accuracy of downstream pressure measurement.


FLOW NOZZLE:

Flow nozzles have a smooth elliptical inlet leading to a throat section with a sharp outlet. This restriction in the fluid flow causes a pressure drop which relates to the flow rate by applying Bernoulli's Equation.
Due to its rigidity the flow nozzle is more stable at higher temperature and flow rate than Orifice.
The flow nozzle is generally used for measuring the flow of steam, non-viscous, erosive and high velocity media. Flow nozzles are typically used to measure steam, air, gas, vapour and chemical substances and liquids with a small degree of suspended solids.
The design and manufacture of the flow nozzle complies with the requirement of ISO5167 standard. Thus an accuracy of "+ (or) -1.0%" is assured. By means of a calibration a higher accuracy of upto "+(or) - 0.25%" can be achieved.

Advantages:
1) For same flow rate, It has significantly lower pressure drop than an orifice plate.
2) May be mounted Horizontally and Vertically.
3) Less suspectible to erosion and corrosion than an orifice plate.

Disadvantages: 
1) Unsuitable for Viscous flow and Sticky materials.
2) Requires consistently smooth flow rate, Cna't handle Eddies and Vortices.

VENTURI TUBE:

 Venturi tube consists of a convergent cone section, cylindrical bore and divergent cone section. The cylindrical bore restricts the fluid flow and results in the pressure drop. This differential pressure relates to the flow rate using Bernoullis Equation.
Due to its simplicity and dependability, The venturi tube is used in applications where its necessary with higher Turndown rates, Lower pressure drops than the orifice plates can provide.

In the venturi tube, The fluid flow rate is measured by reducing the cross sectional flow area in the flow path and generating a pressure difference. The design and manufacture of the Venturi Tube is complies with the ISO5167 standard.

.The angled inlet and outlet cones help control the pressure recovery, making venturi the most efficient of all the differential meters available. This results in lower permanent pressure loss and greater capacity than other differential meters of the same size. Permanent pressure loss is typically 5% to 15% of the differential pressure depending on the bore size selected.

Advantages:
1) Handle large flow volumes at lower pressure drops.
2) Accommodate liquids with high solid content.
3) May be mounted horizontally or vertically.
4) No moving parts, Low maintainance.

Disadvantages:
1) Require four or more pressure taps to accurately measure pressure differential.
2) Accuracy depends on consistent flow rate.
3) Less accurate at low flow rate.

In my next post i will go through Vortex & Ultrasonic Flow Meters.
if i skipped any main points in this post, Then please let me know through ur valuable comments.
Thanks For Reading,
Have a good day :)