Introduction to Flow Measurement Technologies

Flow Measurement Technologies

• Variable Area (rotameters)
• Rotating Vane (paddle & turbine)
• Positive Displacement
• Differential Pressure
• Vortex Shedding
• Thermal Dispersion
• Magnetic
• Thermal Mass

Variable Area (Rota Meter) Flow Meters

Some Facts About Variable Area Flowmeters Called “float type”, “rotameter”, or “variable area” flowmeters By far THE most common specified, purchased, and installed flowmeter in the world Low Cost Solution with prices starting under $200.00

Flow Network Variable Area Models

• KSV                        Low Volume Variable area Flowmeter
• KDG/KDF              Low Volume Variable area Flowmeter with flow switch contacts
• KFR                        Acrylic Flowmeter for caustic media
• KMI                         Low Volume All Metal Flowmeter with Analog Output and Flow Switch Contact
• KSK/KSM               Low Volume All Plastic Variable Area Flowmeter
• KDM                       Armored Variable Area FLowmeter / Flow Transmitter / Flow Switch,  Flanged
• SV                           Brass or Stainless Steel Variable Area Flowmeter, Flow Switch
• SM series              Brass or Stainless with Analog Indicator and Flow Switch output

Variable Area Flowmeters

How They Work Fluid flow moves the float upward against gravity. Float will find equilibrium when area around float = column flow of the fluid. Some types have a guide rod to keep float stable

Variable Area Flowmeters with V Notch Technology

Variable Area

Rather than a conical shaped tube, this technology uses a straight tube with conical slots to achieve the variable area This allows for a higher flow in a smaller package and gives us full control over the turndown

FLOW NETWORK Variable Area Models with V Notch Technology

SV Series – Glass Tube BVO Series – Glass Tube OEM S Series – High Pressure All Metal Switch SM Series SMV – High Pressure Vertical Up SMF – High Pressure Vertical Up with Dampening Spring SMW – High Pressure Horizontal SMO – High Pressure Vertical Down VKP /  VKG – Viscosity Compensated – Mount in Any Orientation – Flowmeter and Flow Switch

Variable Area

Pros:

• Low Cost (pricing usually starts < $100)
• Simple Reliable Design
• Can Measure Liquid or Gas Flows
• Tolerates Dirty Liquids or Solids in Liquid

Cons:

• Usually Limited to a 10:1 Turndown
• Electronic Output Difficult and Expensive
• Coating Liquids a Problem
• Must Calibrate for Viscous Liquids and Compressed Gases
• Errors due to Changing Viscosity for Liquids and Line Pressure for Gases

Paddle Wheel/Turbine

Paddle Wheel Flow Sensor

Theory of operation

• An in-line rotor or paddle spins as the media flows
• The rotational speed of this rotating paddle wheel or turbine is a direct function of the flowrate
• The paddle wheel or turbine rotation is detected electronically
• The two primary methods of sensing rotation are magnetic pick-up and inferred (optical) detection.
• Magnetic pick-up main consideration is magnetic particles in the flow stream (will stick to the paddle or turbine and retard or impede the output).
• Inferred (optical) sensing main consideration is dark media. Dark media will block the inferred signal.
• This frequency output is either used directly or is converted to a proportional analog signal
• Paddle-style is like a riverboat paddlewheel, turbine-style rotates on the longitudinal axis in a corkscrew fashion

Paddle Wheel Flow Sensor

Application Guidelines

• Paddle wheel flow sensors are commonly called rotating vane style flowmeters
• Repeatable, easy maintenance, low ΔP
• Universal mounting (but, avoid having paddle face up)
• Not bi-directional, flow arrow applies in most cases
• Over-ranging causes excessive wear - leading to erratic readings

Paddle Wheel Flow Sensor

• Direct, accurate volumetric flow measurement (Often with visual indicator)
• Universal mounting
• Fast response with good repeatability
• Material flexibility (good compatibility)
• Coupled with modern electronics (Various options to choose from)
• Easy installation and maintenance (rebuild kits available on some models
• Minimal pressure drop
• Sense very low flow rates

Cons:

• Primarily clean media
• Low viscosity media
• Ferrous particles cannot be tolerated – must be filtered out with
• magnetic coupled electronics
• Up & downstream straight pipe run requirements for most models
• Pipe must be full for accurate readings
• Over-ranging flowrate (>20%) causes excessive bearing wear

DP Flow Measurement

Differential Pressure Flowmeters

What is a Differential Pressure Flowmeter?

• A flowmeter which creates a pressure drop across a calibrated restriction in the supply pipe diameter.
• The pressure loss created by the restriction behaves according to the well studied empirical equations based on the Bernoulli Principle.
• Since, according to Bernoulli, the pressure loss is proportional to the square of the flow rate, it is simple to determine any flow rate once the meter has been calibrated at a single point.

How do DP Flowmeters Work?

The principle is simple:

For any fixed flow rate (V), the pressure change experienced in moving from a pipe of cross-section A1, to cross-section A2, is fixed.

In fact, the pressure change is:

Flow Rate -Pressure Loss Relationship

The flow rate versus pressure loss relationship is very easily seen graphically:

Differential Pressure Flowmeters

Pros:

• No moving parts in the flow stream.
• Not greatly affected by viscosity changes.

Note: Because viscosity affects the flow profile through the meter, viscous effects can be calibrated into the meter.

• Density dependencies can be calibrated into the meter as well.
• Flow measurement for liquids and gasses.
• 10 PSIG minimum line pressure requirement
• 10 pipe diameter inlet piping requirement
• Limited turndown

Vortex Shedding Flowmeters

How Vortex Shedding and Fluid Flowmeters Work Vortex shedding occurs when a blunt of flat-faced body, called a bluff body, is positioned in a flowing stream of gas or liquid. As the fluid passes the bluff body, vortices are created. The vortices form on each side of the bluff body and break away in an alternating pattern. The alternating shedding of vortices is the basis of the meter operation. Pressure decreases when a vortex is formed and increases when it is shed. This is true on both sides of the bluff body. Vortex shedding causes pressure pulsations The frequency of the pressure pulsations is directly proportional to flowrate.

Vortex Shedding Flowmeters

• A piezoelectric element senses the pressure pulsations of vortices as they shift from side to side behind the bluff body.
• The signal is amplified and converted to a 4-20 ma signal
• The frequency of the vortex generation is directly proportional to the fluid velocity.

Vortex Shedding Flowmeters

Pros: 

• Low to medium initial set up cost  
• Not much maintenance needed when
• used in clean flow conditions   

Cons: 

• Low to medium pressure drop due to
• the obstruction in the flow path