Process measurement / transducers--part 4: Flow meters

In many industrial applications it’s convenient and useful to measure flow and so a large percentage of transmitter sales are for measuring flow. As a result, there is a huge range of flowmeters to suit a variety of applications. The operation of these may conform to one of two approaches.

Energy-extractive flowmeters

This is the older of the two approaches, and uses flow measurement devices that reduce the energy of the system. The most common of these are the differential pressure producing flowmeters, such as the orifice plate, flow nozzle and venturi tube.

Orifice plate

A standard orifice plate is simply a smooth disk with a round, sharp-edged inflow aperture and mounting rings. In the case of viscous liquids, the upstream edge of the bore can be rounded. The shape of the opening and its location do vary widely, and this is dependent on the material being measured. Most common are concentric orifice plates with a round opening in the center. They produce the best results in turbulent flows when used with clean liquids and gases.

Turbulence Sensor (pressure) points

++++Flow patterns with an orifice plate

When measuring liquids the bore can be positioned at the top of the pipeline to allow the passage of gases. The same applies when allowing suspended solids to pass by positioning the bore at the bottom and gaining a more accurate liquid flow measurement.

Standard orifice meters are primarily used to measure gas and vapor flow. Measurement is relatively accurate; however because of the obstruction of flow there is a relatively high residual permanent pressure loss. They are well-understood, rugged and relatively inexpensive for large pipe sizes and are suited for most clean fluids and aren't influenced by high temperatures.

Transducer advantages:

• Simple construction

• Inexpensive

• Easily fitted between flanges

• No moving parts

• Large range of sizes and opening ratios

• Suitable for most gases and liquids

• Well understood and proven

• Price does not increase dramatically with size.

Transducer limitations or disabilities:

• Inaccuracy, typically 1%

• Low rangeability, typically 4:1

• Accuracy is affected by density, pressure and viscosity fluctuations

• Erosion and physical damage to the restriction affects measurement accuracy

• Cause some unrecoverable pressure loss

• Viscosity limits measuring range

• Require straight pipe runs to ensure if accuracy is maintained

• Pipeline must be full (typically for liquids)

• The inaccuracy with orifice-type measurement is due mainly to process conditions and temperature and pressure variations

• They are also affected by ambient conditions and upstream and downstream piping, as this affects the pressure and continuity of flow.

Turbine or rotor flow transducer:

Turbine meters have rotor-mounted blades that rotate when a fluid pushes against them.

They work on the reverse concept to a propeller system. In a propeller system, the propeller drives the flow; in this case the flow drives and rotates the propeller. Since it’s no longer propelling the fluid, it's now called a turbine. The rotational speed of the turbine is proportional to the velocity of the fluid.

Different methods are used to convey rotational speed information. The usual method is by electrical means where a magnetic pick-up or inductive proximity switch detects the rotor blades as they turn. As each blade tip on the rotor passes the coil, it changes the flux and produces a pulse. The rate of pulses indicates the flow rate through the pipe.

Turbine meters require a good laminar flow. In fact 10 pipe diameters of straight line upstream, and no less than 5 pipe diameters downstream from the meter are required.

They are therefore not accurate with swirling flows.

Turbine meters are specified with minimum and maximum linear flow rates that ensure the response is linear and the other specifications are met. For good rangeability, it’s recommended that the meter be sized such that the maximum flow rate of the application be about 70-80% of that of the meter. Density changes have little effect on the meter's calibration.

Transducer advantages:

• High accuracy, repeatability and rangeability for a defined viscosity and measuring range

• Temperature range of fluid measurement: -220 t o +350 °C

• Very high pressure capability: 9300 psi

• Measurement of non-conductive liquids

• Capability of heating measuring device

• Suitable for very low flow rates.

Transducer limitations or disabilities:

• Not suitable for high viscous fluids

• Viscosity must be known

• 10D upstream and 5D downstream of straight pipe is required

• Not effective with swirling fluids

• Only suitable for clean liquids and gases

• Pipe system must not vibrate

• Specifications critical for measuring range and viscosity

• As turbine meters rely on the flow, they do absorb some pressure from the flow to propel the turbine

• The pressure drop is typically around 20-30 kPa at the maximum flow rate and does vary depending on flow rate

• It’s a requirement in operating turbine meters that sufficient line pressure be maintained to prevent liquid cavitation

• The minimum pressure occurs at the rotor; however the pressure recovers substantially at the turbine

• If the back-pressure is not sufficient, then it should be increased or a larger meter chosen to operate in a lower operating range - this does have the limitation of reducing the meter flow range and accuracy.

Summary:

Turbine meters provide excellent accuracy, repeatability and rangeability for a defined viscosity and measuring range, and are commonly used for custody transfer applications of clean liquids and gases.

Energy-additive flow meters

A common example of the energy-additive approach is the magnetic flowmeter. This device is used to make flow measurements on a conductive liquid. A charged particle moving through the magnetic field produces a voltage proportional to the velocity of the particle. A conductive liquid consisting of charged particles will then produce a voltage proportional to the volumetric flow rate.

Turbulent velocity flow profile; Laminar velocity flow profile or Magnet coil B V

++++ Schematic representation of a magnetic flowmeter

The magmeter:

The advantages of magnetic flowmeters are:

• They have no obstructions or restrictions to flow

• No pressure drop or differential

• No moving parts to wear out

• They can accommodate solids in suspension

• No pressure sensing points to block up

• They measure volume rate at the flowing temperature independent of the effects of viscosity, density, pressure or turbulence

• Another advantage is that many magmeters are capable of measuring flow in either direction.

Most industrial liquids can be measured by magnetic flowmeters; these include acids, bases, water and aqueous solutions. However some exceptions are most organic chemicals and refinery products which have insufficient conductivity for measurement.

Also pure substances, hydrocarbons and gases cannot be measured.

In general the pipeline must be full, although with newer models, level sensing takes this factor into account when calculating a flow rate.

Accuracy:

Magnetic flowmeters are very accurate and have a linear relationship between the output and flow rate. Alternatively, the flow rate can be transmitted as a pulse per unit of volume or time.

The accuracy of most magnetic flowmeter systems is 1% of full-scale measurement.

This takes into account both the meter itself and the secondary instrument. Because of its linearity, the accuracy of low flow rates exceeds that of such devices as the Venturi tube.

The magnetic flowmeter can be calibrated to an accuracy of 0.5% of full scale and is linear throughout.

Selection, sizing and liners:

Sizing of magmeters is done from manufacturer's nomographs to determine suitable diameter meters for flow rates.

The principle of operation of the magmeter requires the generation of a magnetic field and the detection of the voltage across the flow.

If the pipe is made of a material with magnetic properties, then this will disrupt the magnetic field and effectively short circuit the magnetic field. Likewise if the inside of the pipe is conductive, then this will short circuit the electrodes used to detect the voltage across the flow.

The meter piping must be manufactured from a non-magnetic material such as stainless steel in order to prevent short circuiting of the magnetic field.

The lining of the meter piping must also be lined with an insulating material to prevent short circuiting of the electric field.

The liner has to be chosen to suit the application, particularly the resistance it has to the following:

• Chemical corrosion

• Erosion

• Abrasion

• Pressure

• Temperature.

Liner materials

Teflon (Polytetrafluoroethylene (PTFE) resin)

• Widely used due to its high temperature rating

• Anti-stick properties reduce problems with build-up

• Approved for food and beverage environments

• Resistant to many acids and bases.

Neoprene

• Good abrasion resistance

• Good chemical resistance.

Soft rubber

• Relatively inexpensive

• High resistance to abrasion

• Used mainly for slurry applications.

Hard rubber

• Inexpensive

• General-purpose applications

• Used mainly for water and soft slurries.

Ceramic

• High abrasion resistance

• High corrosion resistance

• High temperature rating

• Less expensive to manufacture

• Also suited to sanitary applications

• Strong compressive strength, but poor tensile strength

• Brittle

• May crack with sudden temperature changes, especially downward

• Cannot be used with oxidizing acids or hot concentrated caustic.

Installation techniques:

For correct operation of the magmeter, the pipeline must be full. This is generally done by maintaining sufficient back-pressure from downstream piping and equipment. Meters are available that make allowance for this problem, but are more expensive and are specialized. This is mainly a problem in gravity feed systems.

Magmeters are not greatly affected by the profile of the flow and are not affected by viscosity or the consistency of the liquid. It’s however recommended that the meter be installed with 5 diameters of straight pipe upstream and 3 diameters of straight pipe downstream from the meter.

Applications requiring reduction in the pipe diameter for the meter installation need to allow for the extra length of reducing pipe. It’s also recommended that in those applications, the reducing angle not be greater than 8º, although manufacturer's data should be sought.

Grounding is another important aspect when installing magmeters, and manufacturer's recommendations should be adhered to. Such recommendations would require the use of copper braid between the meter flange and pipe flange at both ends of the meter. These connections provide a path for stray currents and should also be grounded to a suitable grounding point. Magmeters with built-in grounding electrodes eliminate this problem, as the grounding electrode is connected to the supply ground.

Transducer advantages:

• No restrictions to flow

• No pressure loss

• No moving parts

• Good resistance to erosion

• Independent of viscosity, density, pressure and turbulence

• Good accuracy

• Bi-directional

• Large range of flow rates and diameters.

Transducer limitations or disabilities:

• Expensive

• Most require a full pipeline

• Limited to conductive liquids

• As mentioned earlier, a magnetic flowmeter consists of either a lined metal tube, usually stainless steel because of its magnetic properties, or an unlined non-metallic tube. The problem can arise if the insulating liners and electrodes of the magnetic flowmeter become coated with conductive residues deposited by the flowing fluid.

• Erroneous voltages can be sensed if the lining becomes conductive

• Maintaining high flow rates reduces the chances of this happening. However, some manufacturers do provide magmeters with built-in electrode cleaners

• Block valves are used on either side of AC-type magmeters to produce zero flow and maintain full pipe to periodically check the zero. DC units don’t have this requirement.

Ultrasonic flow measurement:

There are two types of ultrasonic flow measurement:

1. Transit-time measurement, used for clean fluids

2. Doppler effect, used for dirty, slurry-type flows.

Transit-time ultrasonic flow measurement:

The transit-time flowmeter device sends pulses of ultrasonic energy diagonally across the pipe. The transit time is measured from when the transmitter sends the pulse to when the receiver detects the pulse.

Each location contains a transmitter and receiver. The pulses are sent alternatively upstream and downstream and the velocity of the flow is calculated from the time difference between the two directions.

Transit-time ultrasonic flow measurement is suited for clean fluids. Some of the more common process fluids consist of water, liquefied gases and natural gas.

Doppler effect ultrasonic flow measurement;

The Doppler effect device relies on objects with varying density in the flow stream to return the ultrasonic energy. With the Doppler effect meter a beam of ultrasonic energy is transmitted diagonally through the pipe. Portions of this ultrasonic energy are reflected back from particles in the stream of varying density. Since the objects are moving, the reflected ultrasonic energy will have a different frequency. The amount of difference between the original and returned signals is proportional to the flow velocity.

General summary:

Most ultrasonic flowmeters are mounted on the outside of the pipe and as such operate without coming in contact with the fluid. Apart from not obstructing the flow, they are not affected by corrosion, erosion or viscosity. Most ultrasonic flowmeters are bi-directional and sense flow in either direction.

Advantages:

• Suitable for large diameter pipes

• No obstructions, no pressure loss

• No moving parts, long operating life

• Fast response

• Installed on existing installations

• Not affected by fluid properties.

Transducer limitations or disabilities:

• Accuracy is dependant on flow profile

• Fluid must be acoustically transparent

• Errors caused by build-up in pipe

• Only possible in limited applications

• Expensive

• Pipeline must be full

• Turbulence or even the swirling of the process fluid can affect the ultrasonic signals

• In typical applications the flow needs to be stable to achieve good flow measurement, and typically this is done by allowing sufficient straight pipe up and downstream of the transducers

• The straight section of pipe upstream would need to be 10-20 pipe diameters with the downstream requirement of 5 pipe diameters

• For the transit time meter, the ultrasonic signal is required to traverse across the flow, therefore the liquid must be relatively free of solids and air bubbles

• Anything of a different density (higher or lower) from the process fluid will affect the ultrasonic signal.

Summary:

Doppler flowmeters are not high accuracy or high performance devices, but do offer an inexpensive form of flow monitoring. Their intended operation is for dirty fluids, and find applications in sewage, sludge and waste water processes.

Being dependent on sound characteristics, ultrasonic devices are dependent on the flow profile and are also affected by temperature and density changes.

NEXT: Level transmitters

PREV: part 3

All related articles   Top of Page   Home