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Going with the flow

Going with the flow With so many different designs of flowmeter to choose from, and so much riding on the correct design, where do you start? David Almond of PVL looks at the various different flowmeter types available.

Without a background in flow monitoring, trying to select the most appropriate technology for a given application can seem a daunting task. Indeed, it can appear that there are almost as many different measuring technologies as there are media to measure. Once you start to break down the application requirements and fluid characteristics, however, it soon becomes apparent that some solutions are more appropriate than others.

A good place to begin is to look at what sort of flow information is required. Is it continuous or totalised? After that, how will the flow be read - visually, via electronic outputs or a combination of the two? What are the characteristics of the medium - gas or liquid, clear or opaque, viscous, aggressive, corrosive, etc? What about temperature variations, acceptable pressure drop, and even issues such as ease of maintenance? All of these factors need careful consideration.

Once these questions are answered, the engineer can then look at parameters such as the required meter range, the required accuracy, and the required repeatability. With all of that in mind, let's look at some of the more common flowmeter technologies, their principles of operation, their advantages, and the sorts of applications where they excel.

Piston valve flowmeters
In the piston valve flowmeter, a magnet-equipped piston rests in the valve seat of the housing and is moved vertically by the flow of the medium. The movement of the piston is directly proportional to the flow rate in a linear relationship. Since the piston works against the force of a supporting spring, the device can be installed in any orientation and reset safely when the volume flow diminishes. Piston valve flowmeters enable simple and precise user adjustment of the switching point.
The advantages of piston valve flowmeters can be summed up as:
  • Low pressure drop
  • Good repeatability
  • Resistant to dirt
  • Precise, continuous setting of switch value
  • Good for dark or contaminated liquids

Typical applications for piston valve flowmeters include industrial metering and monitoring, cooling systems, lubrication circuits, and explosive environment applications.

Piston inline flowmeter
A variation on the piston valve flowmeter, the piston inline design features a magnet-equipped and spring-supported piston in a linear shaped body, whereby the piston moves in the direction of the flow of the flowing medium. The movement of the piston is directly proportional to the flow rate. Also, like the piston valve flowmeter, the device can be installed in any orientation and can be reset safely when the volume flow diminishes.

The inline design means the flowmeter can be manufactured from a solid block to form a linear shaped body to withstand extremely high pressure ranges. The design also provides a very compact form factor for applications where space is a key consideration. The switches are arranged on the outside of the body, with the switch units horizontally adjustable. Extremely accurate device setting can be easily achieved. Applications in which piston inline flowmeters excel include industrial metering and monitoring, and oil monitoring. And the advantages of the piston inline flowmeter can be summed up as:
  • Good repeatability
  • A wide range of indicator options
  • Resistant to dirt
  • Precise, continuous setting of switch value
  • Good for dark or contaminated liquids

Variable area float meters
The float metering principle is a tried and trusted method of accurately measuring liquid and gaseous media. In operation, a float moves vertically in a transparent tube, indicating the current flow rate according the position of the float against a scale on the housing. This direct indication of position without any further functional components is an important advantage, allowing measurement to an accuracy of 1% of full scale.

In addition to providing visual indication of flow, the flowmeter can be equipped with threshold contacts or sensors on the outside of the metering tube. This enables the flowmeter to produce a linear continuous output signal. The advantages of the variable area float meter can be summed up as:
  • Good accuracy
  • Good repeatability
  • Suitable for use with liquids and gases

Typical applications for variable area float meters include display of small flows, biogas plants, inert gas monitoring in welding equipment, and flow display of spindles.

Paddle flowmeters
Here a spring-supported paddle is situated within the flow and moves a distance proportional to the flow rate. The paddle can be connected to a pointer indicating metering units, and/or to a permanent magnet which in turn is coupled to an external threshold contact. When the selected threshold is reached, the contact is triggered. Changing the position of the contact in relation to the magnetic field enables flexible adjustment of the switching point.

Alternatively, the paddle can provide mechanical triggering of microswitches. Here the paddle movement is guided outwards via bellows and acts on the contact via a spring-supported lever arrangement.

Paddle devices are mainly used with liquid metering materials. An advantage over piston types is that they are less influenced by the viscosity of the liquid. In addition, the minimal intrusion of the free pipe diameter results in good resistance to dirt and minimum pressure losses. Screw-in type devices can be used independent of nominal diameters. The advantages of paddle flowmeters can thus be summed up as:
  • Low pressure drop
  • Good repeatability
  • Resistant to dirt
  • Precise, continuous setting of switch value
  • Good for dark and contaminated liquids

Typical applications include flow switching in transformers or heat exchangers, flow monitoring in hot water installations, and flow monitoring in explosive environments.

Dynamic flap flowmeters
In the dynamic flap flowmeter, a thin, spring-mounted shutter that covers the entire flow cross-section is displaced by the flow of the liquid. The shutter has a magnet that creates a changing magnetic field when the shutter is displaced. This field is detected by an analogue Hall-sensor. The spring properties of the shutter and a moulded stop mean that even strong liquid impacts won't damage the flowmeter. In addition, the relatively small number of parts that come into contact with the medium guarantees low soiling and reliable operation. Benefits of dynamic flap flowmeters include:
  • Low pressure loss
  • Large overload security
  • Wide measurement range
  • Fast response time

The design is ideal for use in industrial metering and monitoring, starting systems for high pressure cleaners, machine tools for emulsion control, and laser coolant monitoring where rapid reaction time is essential.

Rotor flowmeters
The primary component here is a rotor situated in the liquid chamber, with the flow of the medium driving the rotor. The rotor blade tips are equipped with metal or magnets, and their passing is detected by a proximity sensor - in the form of a Hall sensor, inductive sensor or optical sensor depending on the housing - with flow speed proportional to the detected frequency. The turn of the revolving flowmeter also provides a visual indication of flow rate. Benefits include:
  • Accuracy to ±3% full scale
  • Self safe function

In addition, the devices can be built around numerous different materials and sensing options to address the requirements of a wide range of industrial metering applications covering aggressive media, high pressure ranges, and liquids with variable viscosity.

Turbine flowmeters
Like rotor flowmeters, turbine flowmeters are ideal for both flow indication and continuous flow metering. The two designs have much in common, with the medium causing a rotary element - in this case a turbine - to revolve directly in proportion to flow rate. This allows visual indication of the flow rate, while a linear output signal can be produced by activating sensors. Other advantages include:
  • Accuracy to ±1% of full scale
  • Self safe function
  • Large measuring range
  • Compact design

As with rotor flowmeters, turbine designs can meet the needs of applications involving aggressive media, high pressure ranges, and media with variable viscosity.

Gear wheel/screw flowmeters
In the gear wheel flowmeter, the medium fills a defined space between two gear wheels, turning these toothed wheels at a speed dependent on the volume flow. The revolving gear wheels provide a visual indication of flow, whilst Hall sensors can be used to detect the revolutions to provide a linear output signal proportional to flow rate. Typical applications include position monitoring, metering of present value, lubrication applications, totalisation, consumption metering and dry-run protection.

A variation is the screw flowmeter which meets the needs of position monitoring, filling applications, lubrication applications, hydraulics, consumption metering and dry-run protection. Both designs are only suitable for use with liquid media, and their advantages include:
  • Accuracy to ±3% of full scale
  • Self safe function
  • Viscosity compensated operation

Calorimetric flowmeters
These provide electronic flow control with no moving parts. These devices work on the principle that the difference in temperature between two sensors within the medium is proportional to flow rate. One sensor is permanently heated such that a constant temperature difference between the two sensors is established. The flow of the liquid alters this temperature difference, and the modification is proportional to the flow.

Different liquid viscosities and sensor housing materials affect the response time, and that has to be taken into consideration. However, there are a number of advantages to the design, including:
  • Compact design
  • Increased reliability and longevity (no moving parts)
  • Very low pressure drop
  • One sensor for a wide range of pipe diameters

Typical applications include flow monitoring, dry-run protection, cooling water control, continuous mixing processes, and continuous monitoring of very low liquid quantities.

Inductive flowmeters
Like calorimetric flowmeters, inductive flowmeters also have no moving parts. In this case, a conductive fluid generates a flow proportional-proportional voltage as it passes through a magnetic coil. Advantages include:
  • No moving parts
  • Very low pressure drop
  • High grade materials
  • One sensor for most pipe diameters

Inductive flowmeters are ideal for metering of present value, batch counting and filling applications, totalisation, consumptive metering and dry-run protection.

Vortex flowmeters
A third class of flowmeter with no moving parts, the vortex flowmeter is built around a narrow, triangular body that goes through the entire cross section of the meter tube, creating a vortex when a flow is present. This is known as the Kármáns vortex effect, and the frequency of the vortex, detected by a piezo sensor lying behind the triangular body, is proportional to flow. Advantages include:
  • No moving parts
  • Large overload security
  • High accuracy

Vortex flowmeters are ideal for metering of present values, totalisation, filling applications, consumption metering and dry-run protection.

Sight glass flowmeter
Where visual monitoring of dark or contaminated liquids is required, the sight glass flowmeter is the ideal solution. With no moving parts the design provides excellent long term reliability. The liquid medium entering the sight glass can be quickly checked optically for quality and consistency. A double-windowed design provides excellent visibility, and the sight glass flowmeter typically offers a high temperature rating. Applications include visual flow control of fluids, test equipment and filling plants.

Variations on the sight glass flowmeter include the flap flowmeter, which adds the ability to monitor flow rate by the position of the flap. Then there is the sphere flowmeter which includes a glass dome with a sphere inside it, with the position of the sphere inside the dome giving an indication of the intensity of flow.

Flowmeters are vital components in a multitude of applications, and product selection is a far from trivial task. As ever, it pays to have discussions with experts such as the engineering team at PVL at the earliest stages of a system design, ensuring the right component is chosen for the medium, the environment and the required levels of accuracy and repeatability.
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